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  • 1.
    Abbas, Ghulam
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Johansson, Gustav
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Alay-e-Abbas, Syed Muhammad
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad 38040, Pakistan.
    Shi, Yijun
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, J. Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Quasi Three-Dimensional Tetragonal SiC Polymorphs as Efficient Anodes for Sodium-Ion Batteries2023Inngår i: ACS Applied Energy Materials, E-ISSN 2574-0962, Vol. 6, nr 17, s. 8976-8988Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the present work, we investigate, for the first time, quasi 3D porous tetragonal silicon–carbon polymorphs t(SiC)12 and t(SiC)20 on the basis of first-principles density functional theory calculations. The structural design of these q3-t(SiC)12 and q3-t(SiC)20 polymorphs follows an intuitive rational approach based on armchair nanotubes of a tetragonal SiC monolayer where C–C and Si–Si bonds are arranged in a paired configuration for retaining a 1:1 ratio of the two elements. Our calculations uncover that q3-t(SiC)12 and q3-t(SiC)20 polymorphs are thermally, dynamically, and mechanically stable with this lattice framework. The results demonstrate that the smaller polymorph q3-t(SiC)12 shows a small band gap (∼0.59 eV), while the larger polymorph of q3-t(SiC)20 displays a Dirac nodal line semimetal. Moreover, the 1D channels are favorable for accommodating Na ions with excellent (>300 mAh g–1) reversible theoretical capacities. Thus confirming potential suitability of the two porous polymorphs with an appropriate average voltage and vanishingly small volume change (<6%) as anodes for Na-ion batteries.

    Fulltekst (pdf)
    fulltext
  • 2.
    Ahmed, Shahbaz
    et al.
    Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad, 38040, Faisalabad, Pakistan.
    Zulfiqar, Waqas
    Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad, 38040, Faisalabad, Pakistan.
    Javed, Farrukh
    Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad, 38040, Faisalabad, Pakistan.
    Arshad, Hurriya
    Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad, 38040, Faisalabad, Pakistan.
    Abbas, Ghulam Gilani
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Laref, Amel
    Department of Physics and Astronomy, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia.
    Alay-e-Abbas, Syed Muhammad
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Computational Materials Modeling Laboratory, Department of Physics, Government College University, Faisalabad, 38040, Faisalabad, Pakistan.
    Accurate First-Principles Evaluation of Structural, Electronic, Optical and Photocatalytic Properties of BaHfO3 and SrHfO3 Perovskites2022Inngår i: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 892, artikkel-id 162071Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A reliable first-principles account of experimentally observed physical properties of perovskite oxides is crucial for realizing their employment in electronic and optical devices. In this context, SCAN meta-GGA functional of DFT offers good approximation for the exchange-correlation energy; facilitating accurate determination of structural and energetic properties. However, SCAN is unable to reproduce electronic and optical properties of wide bad gap materials. In the present study, we report systematic DFT calculations to show that structural, energetic, electronic and optical properties of hafnium based BaHfO3 and SrHfO3 perovskite oxides can be accurately determined through a combine application of SCAN and Tran-Blaha modified Becke-Johnson (TB-mBJ) meta-GGAs. The structural and energetic properties computed using SCAN functional for both BaHfO3 and SrHfO3 are found to be in good agreement with experimental data; achieving a level of accuracy comparable to computationally expansive hybrid DFT calculations. On the other hand, TB-mBJ calculated band gaps computed using the SCAN optimized lattice parameters provide better agreement with experimental data at a low computational cost. The optical properties, band edge potentials and effective masses of the charge carriers in BaHfO3 and SrHfO3 are also computed to examine the combined application of SCAN and TB-mBJ meta-GGAs in predicting the photocatalytic performance of these wide band gap materials. Our results clearly show that the combination of the two meta-GGAs provide a computationally economical route for evaluating the photocatalytic performance of alkaline-earth metal hafnates.

  • 3.
    Ahmer, Muhammad
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. AB SKF.
    Intelligent fault diagnosis and predictive maintenance for a bearing ring grinder2023Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Predicting the failure of any structure is a difficult task in a mechanical system. However complicated and difficult the prediction might be, the first step is to know the actual condition of the system. Given the complexity of any machine tool, where a number of subsystems of electro-mechanical structures interact to perform the machining operation, failure diagnostics become more challenging due to the high demand for performance and reliability. In a production environment, this results in maintenance costs that the management always strives to reduce. Condition-based machine maintenance (CBM) is considered to be the maintenance strategy that can lead to failure prediction and reducing the maintenance cost by knowing the actual condition of the asset and planning the maintenance activities in advance.

    Grinding machines and grinding processes have come a long way since the inception of the centuries old grinding technique. However, we still have a number of challenges to overcome before a completely monitored and controlled machine and process can be claimed. One such challenge is to achieve a machine level CBM and predictive maintenance (PdM) setup which is addressed in this thesis. A CBM implementation framework has been proposed which combines the information sampled from sensors installed for the purpose of the process as well as condition monitoring. Accessing the machine's controller information allows the data to be processed with respect to different machine states and process stages. The successful implementation is achieved through a real-time and synchronized data acquisition setup that allows data from multiple sources to be acquired, stored, and consolidated. The dataset thus generated is used in a significant part of this project and is also published in Swedish National Data Service (SND).

    The thesis also presents the failure diagnostic model based on two step classification approach using benchmarked random forest models. The binary classifier predicts if there is a fault present in the machine based on crucial sensors data from the Idle segment of the grinding cycle. Multi-class random forest classifier diagnosis the fault condition. PdM, knowing when to trigger maintenance action, is achieved through predicting the overall quality of the produced parts from the feature set extracted from sensor data of the Spark-out segment of the grinding cycle. Combining fault diagnosis with the predicted quality information resulted in reliable and actionable maintenance decisions for the bearing ring grinder. The demonstrated setup, based on a production bearing ring grinder, is adaptable to similar machines in production.

    Fulltekst (pdf)
    fulltext
  • 4.
    Ahmer, Muhammad
    et al.
    AB SKF, Gothenburg, Sweden.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    AB SKF, Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    A unified approach towards performance monitoring and condition-based maintenance in grinding machines2020Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The process controller in a precision grinder for bearing rings puts high performance demands on the machine to achieve desired quality in production. This paper presents a unique approach of adding additional sensors for machine condition monitoring for the purpose of learning and using high fidelity condition indicators. The consolidation of real-time sensor data and the process control signals yields high-dimensional dataset. Automatic segmentation helps optimize the amount of data for processing and data mining ahead of fault diagnosis. The proposed setup is state of the art for prognostics as part of condition-based maintenance in a production machine.

  • 5.
    Ahmer, Muhammad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. AB SKF, Gothenburg, Sweden.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    AB SKF, Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    An implementation framework for condition-based maintenance in a bearing ring grinder2022Inngår i: Leading manufacturing systems transformation – Proceedings of the 55th CIRP Conference on Manufacturing Systems 2022 / [ed] Emanuele Carpanzano, Claudio Boër, Anna Valente, Elsevier, 2022, Vol. 107, s. 746-751Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The demand on a bearing ring grinder, as any other machine tool, is to produce parts as per the specification and desired quality. A failure to achieve the quality can be due to functional issues or mechanical failure modes. Ultimately this results in lower productivity and higher production costs. Despite the increased emphasis on practicing condition-based machine maintenance (CBM) in manufacturing applications, it is still considered a challenge to fully deploy CBM in production machines due to diversity in equipment and variety in machine configurations as well as complex characteristics of failure modes. Although there exists extensive literature on CBM for machine tools and subsystems, the issue remains with realization of a technically capable and cost effective CBM system, specifically for a bearing ring grinder. Therefore, sensor(s) selection, data acquisition setup, data processing and analysis are the essential factors considered in the proposed framework to ensure a systematic and organized CBM implementation. The CBM setup is evaluated against production of bearing rings under different process and failure conditions. A machine type independent data acquisition system is designed to capture both machine and process dynamics. The data gathered from sensors at strategic locations exhibits its effectiveness in capturing the process and condition variations in relation to time and operating modes. The presented results of data analysis support the capability and effectiveness of the proposed framework. The utility of this framework can be extended for any number of scenarios including predictive maintenance or adaptive process optimization where solutions using machine learning and artificial intelligence tools can benefit from high dimensional structured dataset. The proposed framework provides a strong foundation to fast track the adaption of CBM in other production machines having similar subsystems.

  • 6.
    Ahmer, Muhammad
    et al.
    AB SKF, Gothenburg, Sweden.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    AB SKF, Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Integration of process monitoring and machine condition diagnostics to improve quality prediction in grinding2021Inngår i: 9th CIRP Conference on High Performance Cutting / [ed] Erdem Ozturk; David Curtis; Hassan Ghadbeigi, Elsevier, 2021, s. 170-173Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Bearing ring grinding incorporates sensors to control the grinding cycle in real time. Prediction of output quality is difficult due to the complex combination of process settings and machine characteristics. Causal relationship of machine performance with varying operating conditions was studied with reference to the produced quality by adding condition monitoring setup to the machine. Data driven diagnostics of machine condition through integration of condition and process monitoring sensor data at the completion of the grinding cycle improves quality cognisance. This can be used to tune control parameters to achieve more predictable quality in successive cycles.

  • 7.
    Ahmer, Muhammad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. AB SKF, Gothenburg, Sweden.
    Sandin, Fredrik
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    AB SKF, Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Dataset Concerning the Process Monitoring and Condition Monitoring Data of a Bearing Ring Grinder2022Dataset
    Abstract [en]

    In the manuscript, we have investigated the effective use of sensors in a bearing ring grinder for failure classification in the condition-based maintenance context. The proposed methodology combines domain knowledge of process monitoring and condition monitoring to successfully achieve failure mode prediction with high accuracy using only a few key sensors. This enables manufacturing equipment to take advantage of advanced data processing and machine learning techniques.

    The grinding machine is of type SGB55 from Lidköping Machine Tools and is used to produce functional raceway surface of inner rings of type SKF-6210 deep groove ball bearing. Additional sensors like vibration, acoustic emission, force, and temperature sensors are installed to monitor machine condition while producing bearing components under different operating conditions. Data is sampled from sensors as well as the machine's numerical controller during operation. Selected parts are measured for the produced quality.

  • 8.
    Ahmer, Muhammad
    et al.
    Manufacturing and Process Development, AB SKF, Gothenburg, Sweden.
    Sandin, Fredrik
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    Manufacturing and Process Development, AB SKF, Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Failure mode classification for condition-based maintenance in a bearing ring grinding machine2022Inngår i: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 122, s. 1479-1495Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Technical failures in machines are major sources of unplanned downtime in any production and result in reduced efficiency and system reliability. Despite the well-established potential of Machine Learning techniques in condition-based maintenance (CBM), the lack of access to failure data in production machines has limited the development of a holistic approach to address machine-level CBM. This paper presents a practical approach for failure mode prediction using multiple sensors installed in a bearing ring grinder for process control as well as condition monitoring. Bearing rings are produced in a set of 7 experimental runs, including 5 frequently occurring production failures in the critical subsystems. An advanced data acquisition setup, implemented for CBM in the grinder, is used to capture information about each individual grinding cycle. The dataset is pre-processed and segmented into grinding cycle stages before time and frequency domain feature extraction. A sensor ranking algorithm is proposed to optimize feature selection for failure classification and the installation cost. Random forest models, benchmarked as best performing classifiers, are trained in a two-step classification framework. The presence of failure mode is predicted in the first step and the failure mode type is identified in the second step using the same feature set. Defining the feature set in the failure detection step improves the predictor generalization with the classifiers’ performance accuracy of 99%99% on the test dataset. The presented approach demonstrates an efficient failure mode classification by selecting crucial sensors resulting in a cost-effective CBM implementation in a bearing ring grinder.

  • 9.
    Ahmer, Muhammad
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement. Manufacturing and Process Development, AB SKF, 415 50 Gothenburg, Sweden.
    Sandin, Fredrik
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Gustafsson, Martin
    Manufacturing and Process Development, AB SKF, 415 50 Gothenburg, Sweden.
    Berglund, Kim
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Using Multivariate Quality Statistic for Maintenance Decision Support in a Bearing Ring Grinder2022Inngår i: Machines, E-ISSN 2075-1702, Vol. 10, nr 9, artikkel-id 794Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Grinding processes’ stochastic nature poses a challenge in predicting the quality of the resulting surfaces. Post-production measurements for form, surface roughness, and circumferential waviness are commonly performed due to infeasibility in measuring all quality parameters during the grinding operation. Therefore, it is challenging to diagnose the root cause of quality deviations in real-time resulting from variations in the machine’s operating condition. This paper introduces a novel approach to predict the overall quality of the individual parts. The grinder is equipped with sensors to implement condition-based maintenance and is induced with five frequently occurring failure conditions for the experimental test runs. The crucial quality parameters are measured for the produced parts. Fuzzy c-means (FCM) and Hotelling’s T-squared (T2) have been evaluated to generate quality labels from the multi-variate quality data. Benchmarked random forest regression models are trained using fault diagnosis feature set and quality labels. Quality labels from the T2 statistic of quality parameters are preferred over FCM approach for their repeatability. The model, trained from T2 labels achieves more than 94% accuracy when compared to the measured ring disposition. The predicted overall quality using the sensors’ feature set is compared against the threshold to reach a trustworthy maintenance decision.

  • 10.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Gong, Guan
    RISE SICOMP AB, Fibervägen 2, 943 33 Öjebyn, Sweden.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Mechanical Performance of PE Reinforced with Graphene Nanoplatelets (GNPs): Effect of Composition and Processing ParametersManuskript (preprint) (Annet vitenskapelig)
  • 11.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Gong, Guan
    Rise Sicomp AB, Fibervägen 2, SE-941 26 Öjebyn, Sweden.
    Nyström, Birgitha
    Podcomp AB, Skylvägen 1, SE-943 33 Öjebyn, Sweden.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Characterization of Wood and Graphene Nanoplatelets (GNPs) Reinforced Polymer Composites2020Inngår i: Materials, E-ISSN 1996-1944, Vol. 13, nr 9, artikkel-id 2089Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper investigates the utilization of commercial masterbatches of graphene nanoplatelets to improve the properties of neat polymer and wood fiber composites manufactured by conventional processing methods. The effect of aspect ratio of the graphene platelets (represented by the different number of layers in the nanoplatelet) on the properties of high-density polyethylene (HDPE) is discussed. The composites were characterized for their mechanical properties (tensile, flexural, impact) and physical characteristics (morphology, crystallization, and thermal stability). The effect of the addition of nanoplatelets on the thermal conductivity and diffusivity of the reinforced polymer with different contents of reinforcement was also investigated. In general, the mechanical performance of the polymer was enhanced at the presence of either of the reinforcements (graphene or wood fiber). The improvement in mechanical properties of the nanocomposite was notable considering that no compatibilizer was used in the manufacturing. The use of a masterbatch can promote utilization of nano-modified polymer composites on an industrial scale without modification of the currently employed processing methods and facilities.

  • 12.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Jantel, Ugo
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Tribological Study on Wood and Graphene Reinforced High Density Polyethylene2022Inngår i: ECCM 2022 - Proceedings of the 20th European Conference on Composite Materials: Composites Meet Sustainability / [ed] Vassilopoulos, Anastasios; Michaud, Véronique, Lausanne: EPFL Lausanne, Composite Construction Laboratory , 2022, Vol. 1, s. 585-592Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Wear rate (WR) and coefficient of friction (COF) for high-density polyethylene (HDPE)and its composites of wood flour (WF) and/or graphene nanoplatelets (GNPs) are studied. Theinvestigation is performed by pin-on-disc test configuration on samples with different moisturecontents (dry, and samples saturated at RH of 33% and 79% in room temperature). The effect ofthe different scales of reinforcement (GNPs and WF) on these properties is discussed. Themorphological/microstructural changes in the materials induced by the motion in contact and/ormoisture content are investigated by differential scanning calorimetry (DSC). Results show thatreinforcing the polymer with WF or GNPs reduces the WR significantly, compared to neat HDPE.The hybrid reinforcements contribute to maximum improvement in wear resistance (>98%) andin the reduction of COF (>11%). The improvement in the tribological behavior of bio-basedmaterials has a significant impact on sustainable development through the improved design,durability, and environmental impact.

  • 13.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Synergistic Effect of Multiscale Reinforcement on Wear of Wood Polymer Composites2022Inngår i: PolyTrib 2022, 2022, s. 34-35Konferansepaper (Annet vitenskapelig)
  • 14.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Ouarga, Ayoub
    High Throughput Multidisciplinary Research Laboratory, Mohammed VI Polytechnic University (UM6P), Lot 660—Hay Moulay Rachid, 43150 Benguerir, Morocco.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Chouhan, Shailesh Singh
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, EISLAB.
    Landström, Anton
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Hajlane, Abdelghani
    Laboratory of Crystallography and Materials Sciences, National Graduate School of Engineering of Caen, 6 Boulevard Maréchal Juin, 14000 Caen, France.
    Conductive Regenerated Cellulose Fibers for Multi-Functional Composites: Mechanical and Structural Investigation2021Inngår i: Materials, E-ISSN 1996-1944, Vol. 14, nr 7, artikkel-id 1746Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Regenerated cellulose fibers coated with copper via electroless plating process are investigated for their mechanical properties, molecular structure changes, and suitability for use in sensing applications. Mechanical properties are evaluated in terms of tensile stiffness and strength of fiber tows before, during and after the plating process. The effect of the treatment on the molecular structure of fibers is investigated by measuring their thermal stability with differential scanning calorimetry and obtaining Raman spectra of fibers at different stages of the treatment. Results show that the last stage in the electroless process (the plating step) is the most detrimental, causing changes in fibers’ properties. Fibers seem to lose their structural integrity and develop surface defects that result in a substantial loss in their mechanical strength. However, repeating the process more than once or elongating the residence time in the plating bath does not show a further negative effect on the strength but contributes to the increase in the copper coating thickness, and, subsequently, the final stiffness of the tows. Monitoring the changes in resistance values with applied strain on a model composite made of these conductive tows show an excellent correlation between the increase in strain and increase in electrical resistance. These results indicate that these fibers show potential when combined with conventional composites of glass or carbon fibers as structure monitoring devices without largely affecting their mechanical performance.

    Fulltekst (pdf)
    fulltext
  • 15.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pupure, Liva
    Riga Technical University, Kalnciema Iela 6, Rīga, LV-1048, Latvia.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Analysis of long-term performance of wood polymer composites with added multifunctionality2022Inngår i: 80th International Scientific Conference of the University of Latvia - Advanced Composites and Applications: Book of Abstracts, Riga: University of Latvia , 2022, s. 9-Konferansepaper (Annet vitenskapelig)
  • 16.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pupure, Liva
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Time-dependent properties of graphene reinforced HDPE2019Inngår i: Proceedings of 9th International Conference on Composite Testing and Model Identification: Book of Abstracts / [ed] R. Joffe; L. Pupure; J. Varna; L. Wallström, 2019, artikkel-id 163Konferansepaper (Annet vitenskapelig)
  • 17.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pupure, Liva
    Department of Structural Engineering,Riga Technical University, Rīga, Latvia.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Time-dependent properties of high-density polyethylene with wood/graphene nanoplatelets reinforcement2023Inngår i: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 44, nr 1, s. 465-479Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of graphene nanoplatelets (GNPs) on the long-term performance of wood fiber/high-density polyethylene (HDPE) composite is investigated by using short-term creep tests with an efficient, faster data analysis approach. Previously, it was shown that the addition of GNPs at 15 wt% into HDPE reduces the viscoplastic (VP) strain developed during 2 h creep by ~50%. The current study shows that 25 and 40 wt% wood content in HDPE reduce the VP strains developed during 2 h creep time by >75% with no noticeable effect of the increased wood content. However, further addition of GNPs results in more than 90% total reduction in the VP strains. The current study shows that the development of the VP strains in the hybrid composites follows Zapas model. Viscoelastic (VE) response of these composites is nonlinear and thus is described by Schapery's model. Parameters for VP and VE models are obtained from the creep experiments and were validated in a separate loading-unloading test sequence. Results show a very good agreement between experiments and predictions for the studied materials as long as the micro-damage is not present.

    Fulltekst (pdf)
    fulltext
  • 18.
    Al-Maqdasi, Zainab
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pupure, Liva
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap. Riga Technical University, Institute of Construction and Reconstruction, Riga, Latvia.
    Gong, Guan
    RISE SICOMP AB, Composite materials and product development, Piteå, Sweden.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Joffe, Roberts
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Time‐dependent properties of graphene nanoplatelets reinforced high‐density polyethylene2021Inngår i: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 138, nr 30, artikkel-id 50783Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The deformation of polymers at constant applied stress is one of their major drawbacks, limiting their use in advanced applications. The study of this property using classical techniques requires extensive testing over long periods of time. It is well known that reinforced polymers show improved behavior over time compared to their neat counterparts. In this study, the effect of adding different amounts of graphene nanoplatelets (GNPs) on the time‐dependent properties of high‐density polyethylene (HDPE) is investigated using short‐term creep tests and load/unload recovery tests. The results are discussed in terms of the test profile and the influence of loading history. Viscoplasticity/viscoelasticity analysis is performed using Zapas model and by comparing creep, creep compliance and pure viscoelasticity curves. The results show that the reinforcement of 15 wt% GNP have the most significant effect on the time‐dependent behavior, reducing the strain by more than 50%. The creep compliance curves show that nano‐reinforced HDPE behaves nonlinearly viscoelastically even at very low stresses. In addition to demonstrating the effect of nano‐reinforcement, the discussion of the results concludes that the influence of loading history can be quite significant and should not be neglected in the design and evaluation of material behavior.

    Fulltekst (pdf)
    fulltext
  • 19.
    Almeida, Nuno A.F.
    et al.
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Rodrigues, Joana
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Silva, Patricia
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Emami, Nazanin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Soares, Manuel J.
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Monteiro, Teresa
    Physics Department & I3N, University of Aveiro, Campus Universitário de Santiago.
    Lopes-da-Silva, José A.
    Chemistry Department & QOPNA, University of Aveiro, Campus Universitário de Santiago.
    Marques, Paula A.A.P.
    Mechanical Engineering Department & TEMA, University of Aveiro, Campus Universitário de Santiago.
    Pressure dependent luminescence in titanium dioxide particles modified with europium ions2016Inngår i: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 234, s. 137-144Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Particles of titanium dioxide were prepared in the presence of europium ions (TiO2:Eu) by a solvothermal method and thermal annealed in air at 500 °C. The spectroscopic properties of TiO2:Eu particles were analyzed indicating that the Eu3+ ions are likely distributed at the surface or near the surface of the titanium dioxide particles. The photoluminescence analysis showed that the intraionic emission was strongly sensitive to reduced pressure conditions, as seen by its absence under vacuum conditions. The ion emission was re-established as soon as the atmosphere was restored. Additionally, the ion integrated emission intensity follows a linearly dependence with pressure in the range of 150 to 800 mbar revealing a high sensitivity to small variations in pressure, which is an unprecedented result. This innovation will allow the study of new technologies in the area of low vacuum sensors where TiO2:Eu may act as the active element of an optical sensor for a pressure device.

  • 20.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fundamentals of Physics-Informed Neural Networks Applied to Solve the Reynolds Boundary Value Problem2021Inngår i: Lubricants, E-ISSN 2075-4442, Vol. 9, nr 8, artikkel-id 82Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents a complete derivation and design of a physics-informed neural network (PINN) applicable to solve initial and boundary value problems described by linear ordinary differential equations. The objective with this technical note is not to develop a numerical solution procedure which is more accurate and efficient than standard finite element- or finite difference-based methods, but to give a fully explicit mathematical description of a PINN and to present an application example in the context of hydrodynamic lubrication. It is, however, worth noticing that the PINN developed herein, contrary to FEM and FDM, is a meshless method and that training does not require big data which is typical in machine learning.

  • 21.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Homogenization of the Reynolds equation governing hydrodynamic flow in a rotating device2011Inngår i: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 133, nr 2, s. 021705-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, a method facilitating the analysis of the effects of surface roughness on the lubrication of a rotating device is presented. The analysis utilizes homogenization—a suitable technique for averaging the effects of roughness as modeled by the Reynolds equation. The originality of this work lies in a novel way of deriving the so called local problems, also known as microbearing problems. It is clearly shown how this increases the computational efficiency by eliminating the dependence of the global coordinates on the formulation of these local problems. This does not only speed up the computation, it also means that the derived flow factors or flow tensors require less storage space. To provide for good usability, alongside the flow factors for the averaged Reynolds equation, the correction factors for the averaged friction torque (and force) and the expression for averaged load carrying capacity are presented here.

  • 22.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Modelling and simulation elastohydrodynamic lubrication of tilted-pad bearings2018Konferansepaper (Fagfellevurdert)
  • 23.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    On the effects of surface roughness in lubrication2006Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Tribology is a multidisciplinary field defined as the science and technology of interacting surfaces in relative motion, and embraces the study of friction, wear and lubrication. A typical tribological application is the rolling element bearing. Tribological contacts may also be found in other types of bearings, cam-mechanisms, gearboxes and hydraulic systems. Examples of bearings inside the human body are the operation of the human hip joint and the contact between teeth during chewing. To fully understand the operation of this type of application one has to understand the couplings between the lubricant fluid dynamics, the structural dynamics of the bearing material, the thermodynamical aspects and the resulting chemical reactions. This makes modeling tribological applications an extremely delicate task. Because of the multidisciplinary nature, such theoretical models lead to mathematical descriptions generally in the form of non-linear integro-differential systems of equations. Some of these systems of equations are sufficiently well posed to allow numerical solutions to be carried out, resulting in accurate predictions on performance. In this work, the influence on performance of a surface microscopical nature, the surface roughness, in contact interfaces between different types of machine element components is the subject of study. An example is the non-conformal lubricated contact between one of the rollers and the inner ring in a rolling element bearing. The tribological contact controlling the operation of the human hip joint is also very similar to this. Another example of a non-conformal contact occurs when driving on rainy roads, where the hydrodynamic action of the water separates the tire. To enable investigations of these types of problems, different theoretical models were studied; for the selected model, a numerical solution technique was developed within this project. This model is based on the Reynolds equation coupled with the film thickness equation. The numerical solution technique involves a multilevel technique to facilitate the solution process. Results presented in this thesis, utilizing this approach, study elementary surface features such as ridges and indentations passing each other inside the lubricated conjunction. The Reynolds equation is derived under the assumptions of thin fluid film and creeping flow, and considers in its most general form shear thinning of the lubricant. This type of equation describes the hydrodynamic action of the lubricant flow and may be used when the interfaces consist of either conformal or non-conformal conjunctions. Examples of applications having conformal interfaces are thrust- and journal- bearings or the contact between the eye and a (optical) contact lens. In such types of applications the load carried by the interface is distributed over a fairly large area that under certain circumstances helps to prevent mechanical deformation of the contacting surfaces. Such applications are said to operate in the hydrodynamic lubrication (HL) regime. Lubricant compressibility and cavitation are important aspects and have received some attention. However, the main objective when modeling HL has been to investigate and develop methods that enable the influence of surface roughness to be to be studied efficiently. Homogenization is a rigorous mathematical concept that when applied to a certain problem may be regarded as an averaging technique as well as it provides information about the induced effects of local surface roughness. Homogenization inflicts no restrictions on the surface roughness representation other than the representative part of the chosen surface roughness being assumed periodically distributed and of course the assumptions of thin film flow made through the Reynolds equation. The homogenization process leads to a two sets of equations one for the local scale describing surface roughness, scale and one for the global scale describing application geometry. The unequivocally determined coefficients of the global problem, which may be regarded as flow factors, are obtained through the solution of local problems. This makes homogenization an eminent approach to be used investigating the influence of surface roughness on hydrodynamic performance. In the present work, homogenization has been used to derive computationally feasible forms of problems originating from incompressible and compressible Reynolds type equations that describe stationary and unstationary flows in both cartezian and cylindrical co-ordinates. This technique enables simulations of surface roughness induced effects when considering surface roughness descriptions originating from measurements. Moreover, the application of homogenization facilitates the interpretation of results. Numerical investigations following the homogenization process have been carried out to verify the applicability of homogenization in hydrodynamic lubrication. Homogenization has also been shown here to enable efficient analysis of rough hydrodynamically lubricated problems. Also of note, in connection to the scientific contribution within tribology, collaboration with a group in applied mathematics has lead to the development of novel techniques in that area. These ideas have also been successfully applied, with some results presented in this thesis. At start-ups, the contact in a rolling element bearing could be both starved and drained from lubricant. In this case the hydrodynamic action becomes negligible in terms of load carrying capacity. The load is carried exclusively by surface asperities, the tribo film, or both. This is hereby modeled as the unlubricated frictionless contact between rough surfaces, i.e. a contact mechanical approach. A variational principle was used in which the real area of contact and the contact pressure distribution minimize the total complementary potential energy. The material model is linear elastic-perfectly plastic and the energy dissipation due to plastic deformation is accounted for. The numerics of this contact mechanical approach involve the fast Fourier transformation (FFT) technique in order to facilitate the solution process. Investigation results of the contact mechanics of realistic surfaces are presented in this thesis. In this investigation the variation in the real area of contact, the plasticity index and some surface roughness parameters due to applied load were studied.

    Fulltekst (pdf)
    FULLTEXT01
  • 24.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    On the effects of surface roughness in lubrication2009 (oppl. 2)Bok (Annet vitenskapelig)
  • 25.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Preface2021Inngår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 235, nr 12, s. 2504-2505Artikkel i tidsskrift (Annet vitenskapelig)
  • 26.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Rough surface elastohydrodynamic lubrication and contact mechanics2004Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    In the field of tribology, there are numerous theoretical models that may be described mathematically in the form of integro-differential systems of equations. Some of these systems of equations are sufficiently well posed to allow for numerical solutions to be carried out resulting in accurate predictions. This work has focused on the contact between rough surfaces with or without a separating lubricant film. The objective was to investigate how surface topography influences contact conditions. For this purpose two different numerical methods were developed and used. For the lubricated contact between rough surfaces the Reynolds equation were used as a basis. This equation is derived under the assumptions of thin fluid film and creeping flow. In highly loaded, lubricated, non- conformal contacts of surfaces after running-in, the load concentration no longer results in plastic deformations, however large elastic deformations will be apparent. It is the interaction between the hydrodynamic action of the lubricant and the elastic deformations of the surfaces that, in certain applications, enable the lubricant film to fully separate the surfaces. This is commonly referred to as full film elastohydrodynamic (EHD) lubrication. Typical machine elements that operates in the full film EHD lubrication (FL) regime include rolling element bearings, cams and gears. Unfortunately, a cost effective way of machining engineering surfaces seldom results in a surface topography that influence contact conditions in the same way as a surface after running-in. Such topographies may prevent the lubricant from fully separating the surfaces because of deteriorated hydrodynamic action. In this case the applied load is carried in part by the lubricant and in part by surface asperities and/or surface active lubricant additives. This could also be the case in lubricant starved contacts, which is a common situation in not only grease lubricated contacts but also in many liquid lubricated contacts, such as high speed operating rolling element bearings. The load sharing between the highly compressed lubricant and the surface and/or surface active lubricant additives is the reason why this lubrication regime is most commonly referred to as mixed EHD lubrication (ML). Machine elements that while running operate in the FL regime may experience a transition into the ML regime at stops or due to altered operating conditions. It is not possible to simulate direct contact between the surfaces using a numerical method based on Reynolds equation. A parameter study, of elementary surface features passing each other inside the EHD lubricated conjunction, was performed. The results obtained, even though no direct contact could be simulated, does indicate that a transition from the FL to the ML regime would occur for certain combinations of the varied parameters. At start-ups, the contact in a rolling element bearing could be both starved and drained from lubricant. In this case the hydrodynamic action becomes negligible in terms of load carrying capacity. The load is carried exclusively by surface asperities and/or surface active lubricant additives. This regime is referred to as boundary lubrication (BL). Operation conditions could also make both FL and ML impossible to achieve, for example, in the case in a low rpm operating rolling element bearing. The BL regime is in this work modeled as the unlubricated frictionless contact between rough surfaces, i.e., a dry contact approach. A variational principle was used in which the real area of contact and contact pressure distribution are those which minimize the total complementary energy. A linear elastic-perfectly plastic deformation model in which energy dissipation due to plastic deformation is accounted for was used. The dry contact method was applied to the contact between four different profiles and a plane. The variation in the real area of contact, the plasticity index and some surface roughness parameters due to applied load were investigated. The surface roughness parameters of the profiles differed significantly.

    Fulltekst (pdf)
    FULLTEXT01
  • 27.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Burtseva, Evgeniya
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rajagopal, K.
    Department of Mechanical Engineering, Texas AM University, Texas, United States.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On lower-dimensional models in lubrication, Part B: Derivation of a Reynolds type of equation for incompressible piezo-viscous fluids2021Inngår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 235, nr 8, s. 1703-1718Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Reynolds equation is a lower-dimensional model for the pressure in a fluid confined between two adjacent surfaces that move relative to each other. It was originally derived under the assumption that the fluid is incompressible and has constant viscosity. In the existing literature, the lower-dimensional Reynolds equation is often employed as a model for the thin films, which lubricates interfaces in various machine components. For example, in the modelling of elastohydrodynamic lubrication (EHL) in gears and bearings, the pressure dependence of the viscosity is often considered by just replacing the constant viscosity in the Reynolds equation with a given viscosity-pressure relation. The arguments to justify this are heuristic, and in many cases, it is taken for granted that you can do so. This motivated us to make an attempt to formulate and present a rigorous derivation of a lower-dimensional model for the pressure when the fluid has pressure-dependent viscosity. The results of our study are presented in two parts. In Part A, we showed that for incompressible and piezo-viscous fluids it is not possible to obtain a lower-dimensional model for the pressure by just assuming that the film thickness is thin, as it is for incompressible fluids with constant viscosity. Here, in Part B, we present a method for deriving lower-dimensional models of thin-film flow, where the fluid has a pressure-dependent viscosity. The main idea is to rescale the generalised Navier-Stokes equation, which we obtained in Part A based on theory for implicit constitutive relations, so that we can pass to the limit as the film thickness goes to zero. If the scaling is correct, then the limit problem can be used as the dimensionally reduced model for the flow and it is possible to derive a type of Reynolds equation for the pressure.

  • 28.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Burtseva, Evgeniya
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rajagopal, Kumbakonam
    J. Mike Walker’66 Department of Mechanical Engineering, Texas A&M University, 100 Mechanical Engineering, Office Building, 3123 TAMU, College Station, TX 77843-3123, TX, USA.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On flow of power-law fluids between adjacent surfaces: Why is it possible to derive a Reynolds-type equation for pressure-driven flow, but not for shear-driven flow?2023Inngår i: Applications in Engineering Science, ISSN 2666-4968, Vol. 15, artikkel-id 100145Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Flows of incompressible Navier–Stokes (Newtonian) fluids between adjacent surfaces are encountered in numerous practical applications, such as seal leakage and bearing lubrication. In seals, the flow is primarily pressure-driven, whereas, in bearings, the dominating driving force is due to shear. The governing Navier–Stokes system of equations can be significantly simplified due to the small distance between the surfaces compared to their size. From the simplified system, it is possible to derive a single lower-dimensional equation, known as the Reynolds equation, which describes the pressure field. Once the pressure field is computed, it can be used to determine the velocity field. This computational algorithm is much simpler to implement than a direct numerical solution of the Navier–Stokes equations and is therefore widely employed by engineers. The primary objective of this article is to investigate the possibility of deriving a type of Reynolds equation also for non-Newtonian fluids, using the balance of linear momentum. By considering power-law fluids we demonstrate that it is not possible for shear-driven flows, whereas it is feasible for pressure-driven flows. Additionally, we demonstrate that in the full 3D model, a normal stress boundary condition at the inlet/outlet implies a Dirichlet condition for the pressure in the Reynolds equation associated with pressure-driven flow. Furthermore, we establish that a Dirichlet condition for the velocity at the inlet/outlet in the 3D model results in a Neumann condition for the pressure in the Reynolds equation.

    Fulltekst (pdf)
    fulltext
  • 29.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Burtseva, Evgeniya
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rajagopal, Kumbakonam
    Department of Mechanical Engineering, Texas A&M University, Texas, USA.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On lower-dimensional models in lubrication, Part A: Common misinterpretations and incorrect usage of the Reynolds equation2021Inngår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 235, nr 8, s. 1692-1702Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Most of the problems in lubrication are studied within the context of Reynolds’ equation, which can be derived by writing the incompressible Navier-Stokes equation in a dimensionless form and neglecting terms which are small under the assumption that the lubricant film is very thin. Unfortunately, the Reynolds equation is often used even though the basic assumptions under which it is derived are not satisfied. One example is in the mathematical modelling of elastohydrodynamic lubrication (EHL). In the EHL regime, the pressure is so high that the viscosity changes by several orders of magnitude. This is taken into account by just replacing the constant viscosity in either the incompressible Navier-Stokes equation or the Reynolds equation by a viscosity-pressure relation. However, there are no available rigorous arguments which justify such an assumption. The main purpose of this two-part work is to investigate if such arguments exist or not. In Part A, we formulate a generalised form of the Navier-Stokes equation for piezo-viscous incompressible fluids. By dimensional analysis of this equation we, thereafter, show that it is not possible to obtain the Reynolds equation, where the constant viscosity is replaced with a viscosity-pressure relation, by just neglecting terms which are small under the assumption that the lubricant film is very thin. The reason is that the lone assumption that the fluid film is very thin is not enough to neglect the terms, in the generalised Navier-Stokes equation, which are related to the body forces and the inertia. However, we analysed the coefficients in front of these (remaining) terms and provided arguments for when they may be neglected. In Part B, we present an alternative method to derive a lower-dimensional model, which is based on asymptotic analysis of the generalised Navier-Stokes equation as the film thickness goes to zero.

  • 30.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Burtseva, Evgeniya
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Rajagopal, Kumbakonam
    Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    On lower-dimensional models of thin film flow, Part C: Derivation of a Reynolds type of equation for fluids with temperature and pressure dependent viscosity2023Inngår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 237, nr 3, s. 514-526Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper constitutes the third part of a series of works on lower-dimensional models in lubrication. In Part A, it was shown that implicit constitutive theory must be used in the modelling of incompressible fluids with pressure-dependent viscosity and that it is not possible to obtain a lower-dimensional model for the pressure just by letting the film thickness go to zero, as in the proof of the classical Reynolds equation. In Part B, a new method for deriving lower-dimensional models of thin-film flow of fluids with pressure-dependent viscosity was presented. Here, in Part C, we also incorporate the energy equation so as to include fluids with both temperature and pressure dependent viscosity. By asymptotic analysis of this system, as the film thickness goes to zero, we derive a simplified model of the flow. We also carry out an asymptotic analysis of the boundary condition, in the case where the normal stress is specified on one part of the boundary and the velocity on the remaining part.

    Fulltekst (pdf)
    fulltext
  • 31.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Burtseva, Evgeniya
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ràfols, Francesc Pérez
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    New insights on lubrication theory for compressible fluids2019Inngår i: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 145, artikkel-id 103170Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The fact that the film is thin is in lubrication theory utilised to simplify the full Navier–Stokes system of equations. For incompressible and iso-viscous fluids, it turns out that the inertial terms are small enough to be neglected. However, for a compressible fluid, we show that the influence of inertia depends on the (constitutive) density-pressure relationship and may not always be neglected. We consider a class of iso-viscous fluids obeying a power-law type of compressibility, which in particular includes both incompressible fluids and ideal gases. We show by scaling and asymptotic analysis, that the degree of compressibility determines whether the terms governing inertia may or may not be neglected. For instance, for an ideal gas, the inertial terms remain regardless of the film height-to-length ratio. However, by means of a specific modified Reynolds number that we define we show that the magnitudes of the inertial terms rarely are large enough to be influential. In addition, we consider fluids obeying the well-known Dowson and Higginson density-pressure relationship and show that the inertial terms can be neglected, which allows for obtaining a Reynolds type of equation. Finally, some numerical examples are presented in order to illustrate our theoretical results.

  • 32.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Campañá, C
    University of Ottawa, Department of Chemistry, Ottawa, Canada K1N6N5.
    Prodanov, N
    Sumy State University, 2 Rimskii-KorsakovStr., 40007 Sumy, Ukraine.
    Persson, B N J
    IFF, Jülich.
    Interfacial separation between elastic solids with randomly rough surfaces: Comparison between theory and numerical techniques2011Inngår i: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 59, nr 11, s. 2355-2369Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the distribution of interfacial separations at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for the distribution of interfacial separations using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) a Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.

  • 33.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Dasht, Johan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The homogenization process of the Reynolds equation describing compressible liquid flow2006Inngår i: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 39, nr 9, s. 994-1002Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper summarizes the homogenization process of rough, hydrodynamic lubrication problems governed by the Reynolds equation used to describe compressible liquid flow. Here, the homogenized equation describes the limiting result when the wavelength of a modeled surface roughness goes to zero. The lubricant film thickness is modeled by one part describing the geometry/shape of the bearing and a periodic part describing the surface topography/roughness. By varying the periodic part as well as its wavelength, we can try to systematically investigate the applicability of homogenization on this type of problem. The load carrying capacity is the target parameter; deterministic solutions are compared to homogenized by this measure. We show that the load carrying capacity rapidly converges to the homogenized results as the wavelength decreases, proving that the homogenized solution gives a very accurate representation of the problem when real surface topographies are considered

  • 34.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Dasht, Johan
    Glavatskih, Sergei
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Marklund, Pär
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sahlin, Fredrik
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Homogenization of the Reynolds equation2005Rapport (Annet vitenskapelig)
  • 35.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Essel, Emmanuel Kwame
    Department of Mathematics and Statistics, University of Cape Coast.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Multiscale homogenization of a class of nonlinear equations with applications in lubrication theory and applications2011Inngår i: Journal of Function Spaces and Applications, ISSN 0972-6802, E-ISSN 1758-4965, Vol. 9, nr 1, s. 17-40Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We prove a homogenization result for monotone operators by using the method of multiscale convergence. More precisely, we study the asymptotic behavior as epsilon -> 0 of the solutions u(epsilon) of the nonlinear equation div a(epsilon)(x, del u(epsilon)) = div b(epsilon), where both a(epsilon) and b(epsilon) oscillate rapidly on several microscopic scales and a(epsilon) satisfies certain continuity, monotonicity and boundedness conditions. This kind of problem has applications in hydrodynamic thin film lubrication where the bounding surfaces have roughness on several length scales. The homogenization result is obtained by extending the multiscale convergence method to the setting of Sobolev spaces W-0(1,p)(Omega), where 1 < p < infinity. In particular we give new proofs of some fundamental theorems concerning this convergence that were first obtained by Allaire and Briane for the case p = 2.

  • 36.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Essel, Emmanuel Kwame
    Fabricius, John
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Reiterated homogenization applied in hydrodynamic lubrication2008Inngår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 222, nr 7, s. 827-841Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work is devoted to studying the combined effect that arises due to surface texture and surface roughness in hydrodynamic lubrication. An effective approach in tackling this problem is by using the theory of reiterated homogenization with three scales. In the numerical analysis of such problems, a very fine mesh is needed, suggesting some type of averaging. To this end, a general class of problems is studied that, e.g. includes the incompressible Reynolds problem in both artesian and cylindrical coordinate forms. To demonstrate the effectiveness of the method several numerical results are presented that clearly show the convergence of the deterministic solutions towards the homogenized solution.Moreover, the convergence of the friction force and the load carrying capacity of the lubricant film is also addressed in this paper. In conclusion, reiterated homogenization is a feasible mathematical tool that facilitates the analysis of this type of problem.

    Fulltekst (pdf)
    FULLTEXT01
  • 37.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Essel, Emmanuel Kwame
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Reiterated homogenization of a nonlinear Reynolds-type equation2008Rapport (Annet vitenskapelig)
  • 38.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Essel, Emmanuel Kwame
    Department of Mathematics and Statistics, University of Cape Coast.
    Fabricius, John
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Variational bounds applied to unstationary hydrodynamic lubrication2008Inngår i: International Journal of Engineering Science, ISSN 0020-7225, E-ISSN 1879-2197, Vol. 46, nr 9, s. 891-906Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper is devoted to the effects of surface roughness in hydrodynamic lubrication. The numerical analysis of such problems requires a very fine mesh to resolve the surface roughness, hence it is often necessary to do some type of averaging. Previously, homogenization (a rigorous form of averaging) has been successfully applied to Reynolds type differential equations. More recently, the idea of finding upper and lower bounds on the effective behavior, obtained by homogenization, was applied for the first time in tribology. In these pioneering works, it has been assumed that only one surface is rough. In this paper we develop these results to include the unstationary case where both surfaces may be rough. More precisely, we first use multiple-scale expansion to obtain a homogenization result for a class of variational problems including the variational formulation associated with the unstationary Reynolds equation. Thereafter, we derive lower and upper bounds corresponding to the homogenized (averaged) variational problem. The bounds reduce the numerical analysis, in that one only needs to solve two smooth problems, i.e. no local scale has to be considered. Finally, we present several examples, where it is shown that the bounds can be used to estimate the effects of surface roughness with very high accuracy.

  • 39.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Essel, Emmanuel Kwame
    Persson, Lars-Erik
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Homogenization of the unstationary incompressible Reynolds equation2007Inngår i: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 40, nr 9, s. 1344-1350Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper is devoted to the effects of surface roughness during hydrodynamic lubrication. In the numerical analysis a very fine mesh is needed to resolve the surface roughness, suggesting some type of averaging. A rigorous way to do this is to use the general theory of homogenization. In most works about the influence of surface roughness, it is assumed that only the stationary surface is rough. This means that the governing Reynolds type equation does not involve time. However, recently, homogenization was successfully applied to analyze a situation where both surfaces are rough and the lubricant is assumed to have constant bulk modulus. In this paper we will consider a case where both surfaces are assumed to be rough, but the lubricant is incompressible. It is also clearly demonstrated, in this case that homogenization is an efficient approach. Moreover, several numerical results are presented and compared with those corresponding to where a constant bulk modulus is assumed to govern the lubricant compressibility. In particular, the result shows a significant difference in the asymptotic behavior between the incompressible case and that with constant bulk modulus.

  • 40.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A new approach for studying cavitation in lubrication2014Inngår i: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 136, nr 1, artikkel-id 11706Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The underlying theory, in this paper, is based on clear physical arguments related to conservation of mass flow and considers both incompressible and compressible fluids. The result of the mathematical modeling is a system of equations with two unknowns, which are related to the hydrodynamic pressure and the degree of saturation of the fluid. Discretization of the system leads to a linear complementarity problem (LCP), which easily can be solved numerically with readily available standard methods and an implementation of a model problem in matlab code is made available for the reader of the paper. The model and the associated numerical solution method have significant advantages over today's most frequently used cavitation algorithms, which are based on Elrod-Adams pioneering work

  • 41.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Reynolds equation flow factor estimates by means of homogenization2010Inngår i: ASIATRIB 2010: Frontiers in tribology - knowledge & friendship . proceedings of the fourth Asia International Conference on Tribology, 5-9 December 2010, Perth, Western Australia, 2010, s. 185-Konferansepaper (Fagfellevurdert)
    Fulltekst (pdf)
    FULLTEXT01
  • 42.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Lundström, Staffan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Flow in thin domains with a microstructure: Lubrication and thin porous media2017Inngår i: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1798, artikkel-id 020172Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper is devoted to homogenization of different models of flow in thin domains with a microstructure. The focus is on applications connected to the effect of surface roughness in full film lubrication, but a parallel to flow in thin porous media is also discussed. Mathematical models of such flows naturally include two small parameters. One is connected to the fluid film thickness and the other to the microstructure. The corresponding asymptotic analysis is a delicate problem, since the result depends on how fast the two small parameters tend to zero relative to each other. We give a review of the current status in this area and point out some future challenges.

  • 43.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Spencer, Andrew
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Similarities and differences between the flow factor method by Patir and Cheng and homogenization2011Inngår i: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 133, nr 3, s. 031702-1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Different averaging techniques have proved to be useful for analyzing the effects of surface roughness in hydrodynamic lubrication. This paper compares two of these averaging techniques, namely the flow factor method by Patir and Cheng (P&C) and homogenization. It has been rigorously proved by many authors that the homogenization method provides a correct solution for arbitrary roughness. In this work it is shown that the two methods coincide if and only if the roughness exhibits certain symmetries. Hence, homogenization is always the preferred method.

  • 44.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Homogenization of a Reynolds equation describing compressible flow2011Rapport (Annet vitenskapelig)
  • 45.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Fabricius, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Homogenization of a Reynolds equation describing compressible flow2012Inngår i: Journal of Mathematical Analysis and Applications, ISSN 0022-247X, E-ISSN 1096-0813, Vol. 390, nr 2, s. 456-471Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We homogenize a Reynolds equation with rapidly oscillating film thickness function hε, assuming a constant compressiblity factor in the pressure-density relation. The oscillations are due to roughness on the bounding surfaces of the fluid film. As shown by previous studies, homogenization is an effective approach for analyzing the effects of surface roughness in hydrodynamic lubrication. By two-scale convergence theory we obtain the limit problem (homogenized equation) and strong convergence in L2 for the unknown density ρε. By adding a small corrector term we also obtain strong convergence in the Sobolev norm.

  • 46.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Glavatskih, Sergei
    Larsson, Roland
    Marklund, Pär
    Sahlin, Fredrik
    Dasht, Johan
    Persson, Lars-Erik
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Homogenization of Reynolds equation2005Rapport (Annet vitenskapelig)
  • 47.
    Almqvist, Andreas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Hardell, Jens
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Prakash, Braham
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Selected papers from those presented at the 3rd International Tribology Symposium of IFToMM2014Inngår i: Lubrication Science, ISSN 0954-0075, E-ISSN 1557-6833, Vol. 26, nr 5, s. 273-275Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    Selected papers presented at the 3rd International Tribology Symposium of the International Federation for the Promotion of Mechanism and Machine Science (IFToMM) are discussed. The 3rd International Tribology Symposium of IFToMM was organized by the Division of Machine Elements of Luleå University of Technology in Sweden from March 19 21, 2013. The symposium was held in the university campus, providing a perfect forum for the tribologists for presenting their latest research and exchange of ideas. The participants were also given the opportunity to experience the snowy landscape during a memorable outdoor dinner. The symposium attracted 146 participants from 26 countries along with 92 papers spread over 32 sessions. These papers covered the fundamental and applied aspects of wide ranging topics such as friction, wear, lubrication, lubricants, tribology in hostile environment, tribomaterials, solid lubricants, surface engineering, and tribotesting.

  • 48. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    The effect of two-sided roughness in rolling/sliding EHL line contacts2004Inngår i: Transient processes in tribology: proceedings of the 30th Leeds-Lyon Symposium on Tribology / [ed] Gerard Dalmaz, Amstersam: Elsevier, 2004Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In most theoretical studies carried out to date on the effect of surface roughness in elastohydrodynamic lubrication (EHL) one surface is considered smooth and one as being rough. In real tribological contacts however, both surfaces normally have similar roughness heights. When modelling a rolling contact it is possible to simply sum the roughness of the two contact surfaces but in a sliding EHL contact, a continuously changing effective surface roughness occurs. The aim of this work was to investigate the influence of elementary surface features such as dents and ridges on the film thickness and pressure. This was done numerically using transient non-Newtonian simulations of an EHL line contact using a coupled smoother combined with a multilevel technique. Four different "overtaking" phenomena were investigated; ridge-ridge, dent-ridge, ridge-dent, and dent-dent. It was shown that the minimum film-thickness produced by a ridge is further reduced in a dent-ridge overtaking event. The squeeze effect seen in the ridge-ridge case resulted in large deformations and film-thickness heights comparable to the corresponding smooth case just before the overtaking event occurred. These local effects arising from simulating two-sided roughness were compared to simulations using a traditional "one-sided rough surface contacting a perfectly smooth surface.".

  • 49. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Sahlin, Fredrik
    Development of a lubrication simulation model2009Inngår i: Svenska mekanikdagarna: Södertälje 2009, Stockholm: Svenska nationalkommittén för mekanik , 2009, s. 74-Konferansepaper (Annet vitenskapelig)
  • 50. Almqvist, Andreas
    et al.
    Larsson, Roland
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Wall, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The homogenization process of the time dependent Reynolds equation describing compressible liquid flow2006Rapport (Annet vitenskapelig)
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