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• 201.
Luleå tekniska universitet.
Linear and non-linear growth of disturbances in channel flows1996Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
• 202.
Luleå tekniska universitet.
Three-dimensional disturbances: considering starting profiles and optimal profiles in Couette and Poiseuille flow1996Ingår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 8, nr 5, s. 1149-1158Artikel i tidskrift (Refereegranskat)

The influence of the velocity profile on transient (algebraic) growth of three-dimensional disturbances in channel flow is investigated. Only streamwise-independent perturbations are considered and the effect of a single forcing Orr-Sommerfeld mode is studied. This restriction is motivated by previous investigations, which show that the largest possible (global optimal) energy growth of a disturbance is generally obtained for streamwise-independent perturbations. Analytic solutions for the energy growth on starting profiles for plane Couette and plane Poiseuille flow are deduced and the findings reveal that the most favorable profiles for transient growth are indeed the fully developed ones. An isoperimetric method to compute optimal profiles is then presented with the purpose to increase the energy growth compared to the fully developed profiles. These profiles are dependent on the spanwise wave number, as well as the perturbation growth time. A numerical investigation show that significant energy magnification can take place on such a profile, even at short growth times of the perturbation. It is further established that these profiles are stable only for low Reynolds number.

• 203.
Luleå tekniska universitet.
Nonlinear interaction of laminar three-dimensional disturbances in plane Poiseuille flow1996Ingår i: Advances in turbulence VI: proceedings of the Sixth European Turbulence Conference, held in Lausanne, Switzerland, 2-5 July 1996 / [ed] Spyros Gavrilakis; Luc Machiels; Peter A. Monkewitz, Kluwer Academic Publishers, 1996, s. 333-334Konferensbidrag (Refereegranskat)

Results are presented of an experimental study of localized disturbances in a plane Poiseuille flow, with emphasis on the nonlinearities of localized (laminar) disturbances. It is found that the nonlinear interaction (NI) of the center of the structure in general appears in connection with large velocity gradients in the streamwise and spanwise directions of the disturbances. At the outer regions of the composite disturbance, the NI follows the negative amplitudes of the velocity

• 204.
Department of Design Fundamentals and Fluid-Flow Machinery, Wrocaw University of Technology, Wrocaw, Poland.
Department of Design Fundamentals and Fluid-Flow Machinery, Wrocaw University of Technology, Wrocaw, Poland. Department of Fundamentals of Machine Design and Tribology, Wrocaw University of Technology, Wrocaw, Poland. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
Experimental Study of Lubricating Grease Flow inside the Gap of a Labyrinth Seal Using Microparticle Image Velocimetry2018Ingår i: Tribology Transactions, ISSN 1040-2004, E-ISSN 1547-397X, Vol. 61, nr 1, s. 31-40Artikel i tidskrift (Refereegranskat)

In this study, the flow of lubricating greases in a labyrinth seal geometry is studied using microparticle image velocimetry (µPIV). The aim is to evaluate the grease velocity distribution inside the gap of a labyrinth seal and to find a relationship between the grease consistency and the transferred speed from the rotating ring in order to choose the correct grease as a sealing medium. In addition, the grease flow characteristics are important for the understanding of fracture due to grease layer displacement. For these purposes, four greases with different rheological properties were used in µPIV experiments. It was found that the grease consistency plays a crucial role in speed development as well as the grease composition and presence of a slip effect at the grease–rotating wall interface.

• 205.
Polytechnic University of Bucharest, Romania.
Polytechnic University of Bucharest, Romania. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Sensitivity analysis on flow rate estimation using design of experiments: Application to the pressure-time method2017Ingår i: 2017 10th International Symposium on Advanced Topics in Electrical Engineering, ATEE 2017, Piscataway, NJ: Institute of Electrical and Electronics Engineers (IEEE), 2017, s. 533-538, artikel-id 7905035Konferensbidrag (Refereegranskat)

The present work investigates the flow parameters influence over the accuracy of flow rate determination by means of Design of Experiments -a statistical procedure widely used for experiments. The method used for the estimation of the flow rate derives from the standard pressure-time method, modified and improved - the solution is obtained by solving the water hammer equations considering an unsteady friction factor. The influence of the following parameters over the final moment of the flow rate variation regime is determined: the time of valve closure, the Reynolds number of the flow and the type of the closure law. A 23 factorial design is chosen to test the manner the three parameters influence, separately and together, the time value that should be chosen for the end of the flow rate variation regime. The results will help understand the influence of these parameters and the way in which they should be modeled within the flow rate determination method.

• 206.
POLITEHNICA University of Bucharest.
POLITEHNICA University of Bucharest. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Hydro-Québec Research’s Institute. Hydro-Québec Research’s Institute.
Investigation of the pressure-time method with an unsteady friction2013Ingår i: Hydro 2013: International Conference and Exhebition : Promoting the Versalite Role of Hydro, 2013Konferensbidrag (Refereegranskat)

The pressure-time method is used to determine the flow rate in hydraulic turbines. Over the years, developments of the method to improve its accuracy have been proposed, such as a time dependent friction (Jonsson) or a different upper integration limit (Adamkowski). The present work investigates these developments on measurements performed on a full-scale machine for different flow rates. The discharge computed values using pressure measurements are compared to ultrasound transit time measurements performed simultaneously. The results point out the significance of the developments on the final results; variations of 0.5% are obtained when combining Jonsson friction formulation and Adamkowski upper integration limit.

• 207.
POLITEHNICA University of Bucharest.
POLITEHNICA University of Bucharest. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. POLITEHNICA University of Bucharest.
Discharge evaluation from pressure measurements by a genetic algorithm based method2015Ingår i: Flow Measurement and Instrumentation, ISSN 0955-5986, E-ISSN 1873-6998, Vol. 46, s. 49-55Artikel i tidskrift (Refereegranskat)

This paper presents an alternative to the Gibson method, for discharge estimation of a turbine inside a hydropower plant. The method proposes a genetic algorithm which includes an integration procedure for the water hammer equations using the method of characteristics. The decision variables are represented by the steady-state regime discharge before the valve closure and the pipe friction factor. The recordings of pressure/differential pressure in measuring sections are used.

• 208.
POLITEHNICA University of Bucharest.
POLITEHNICA University of Bucharest. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
On The Use of the Water Hammer Equations with Time Dependent Friction During a Valve Closure, for Discharge Estimation2016Ingår i: Journal of Applied Fluid Mechanics, ISSN 1735-3572, E-ISSN 1735-3645, Vol. 9, nr 5, s. 2427-2434Artikel i tidskrift (Refereegranskat)

The paper presents a new method for in site discharge estimation in pressured pipes. The method consists in using the water hammer equations solved with the method of characteristics with an unsteady friction factor model. The differential pressure head variation measured during a complete valve closure is used to derive the initial flow rate, similarly to the pressure-time (Gibson) method. The method is validated with a numerical experiment, and tested with experimental laboratory measurements. The results show that the proposed method can reduce the discharge estimation error by 0.6% compared to the standard pressure-time (Gibson) method for the flow rate investigation.

• 209.
POLITEHNICA University of Bucharest.
POLITEHNICA University of Bucharest. Pöyry. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Discharge measurements using the Pressure-Time Method: Different evaluation procedures2013Konferensbidrag (Refereegranskat)

This paper analyzes the pressure-time method and its developments. The pressure-time method used to determine the discharge in hydraulic turbines is described and applied in a generic test case in well controlled laboratory conditions. Developments of the method are presented: a time dependent friction factor (proposed by Jonsson) and a different upper integration limit (proposed by Adamkowski). Laboratory experiments are used to compute the discharge using the pressure-time method in the standard and modified versions and the results are compared. The precision of the methods is verified by comparing the computed discharge values to the values measured with a magnetic flowmeter.

• 210.
POLITEHNICA University of Bucharest.
POLITEHNICA University of Bucharest. Pöyry SwedPower AB. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Discharge measurements using the Pressure-Time Method: Different evaluation procedures2014Ingår i: Polytechnical University of Bucharest. Scientific Bulletin. Series D: Mechanical Engineering, ISSN 1454-2358, Vol. 76, nr 4, s. 195-202Artikel i tidskrift (Refereegranskat)

This paper analyzes the pressure-time method and its developments. The pressure-time method used to determine the discharge in hydraulic turbines is described and applied in a generic test case in well controlled laboratory conditions. Developments of the method are presented: a time dependent friction factor (proposed by Jonsson) and a different upper integration limit (proposed by Adamkowski). Laboratory experiments are used to compute the discharge using the pressure-time method in the standard and modified versions and the results are compared. The precision of the methods is verified by comparing the computed discharge values to the values measured with a magnetic flowmeter.

• 211. Ebermark, Sofia
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Pressning av SMC: numeriska tryckfältsberäkningar med hjälp av inversmodellering2007Ingår i: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, s. 74-Konferensbidrag (Övrigt vetenskapligt)
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• 212.
Luleå tekniska universitet.
Cleaning up diesel fuelled vehicles by improving the engine fuel and by exhaust aftertreatment1992Ingår i: DProceedings: Florence, Italy, 1st - 5th June 1992: Dedicated Conference on Zero Emission Vehicles - the Electric, Hybrid and Alternative Fuel Challenge, Croydon: Automotive automation , 1992, s. 359-Konferensbidrag (Refereegranskat)
• 213.
Luleå tekniska universitet.
Avgasemissioner från biogasdrivna fordon1995Rapport (Övrigt vetenskapligt)
• 214.
Luleå tekniska universitet.
Undersökning av hur dieselbränslets sammansättning påverkar avgasemission1991Rapport (Övrigt vetenskapligt)
• 215. Egebäck, Karl-Erik
Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
Avgasnorm för snöskotrar: utredning för Statens Naturvårdsverk år 19941995Rapport (Övrigt vetenskapligt)
• 216.
Luleå tekniska universitet.
Luleå tekniska universitet.
Bedömning av miljöpotentialen hos de alternativa drivmedlen biogas, etanol, metanol, naturgas, rapsmetylester och dimetyleter1997Rapport (Övrigt vetenskapligt)
• 217.
Luleå tekniska universitet.
Luleå tekniska universitet. Luleå tekniska universitet.
The use of methanol as a fuel for transportation1997Rapport (Övrigt vetenskapligt)

The aim of the project was to collect and report international experiences concerning the use of methanol as an automotive fuel. The method has been to study the literature which covers the subject and most of the information has been collected that way. The project started with a participation in a conference and a visit to people who have been involved in activities concerning the use of automotive alcohols. Car manufacturers, environmental authorities and users of alcohol fuels i.e. representatives of bus companies, were interviewed. The different applications for the use of methanol as an automotive fuel has been described in the report as well as the production of methanol. Some results, mostly in form of emission data and other experiences derived from the use of alcohol fuels, have also been presented. The use of ethanol and methanol has been compared and based on information from engine manufacturers and users of alcohol fueled vehicles there seems to be a preference for the use of ethanol. However, the question 'methanol or ethanol' has not been answered as the decision which of the two is to be used seems to depend more on economic factors, such as cost of the production of the fuel etc., than on other factors

• 218.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater. Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
A balanced incomplete block design experiment for ranking criticality of stimuli for listening testsManuskript (preprint) (Övrigt vetenskapligt)
• 219.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater. Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
Criticality of audio stimuli for listening tests: listening durations during a ranking task2014Ingår i: 136th Audio Engineering Society Convention 2014, Red Hook, NY: Curran Associates, Inc., 2014, s. 114-122Konferensbidrag (Refereegranskat)

The process of selecting critical audio stimuli for listening tests is known from the literature to be both labor-intensive and time-consuming, and has been described as more of art than science. Explicit accounts of systematic procedures are not the most commonly encountered. In a previous study a ranking-by-elimination method was investigated, resulting in a rank order that could be used as a guide for critical stimuli selection. This paper presents a further exploratory analysis of data on the subjects’ listening durations, both as a function of number of stimuli left on screen and individually per stimulus. A strong negative correlation was found between the rank order of criticality and playing duration.

• 220.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater. Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
Ranking criticality of stimuli for listening tests2016Manuskript (preprint) (Övrigt vetenskapligt)
• 221.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater.
Luleå tekniska universitet, Institutionen för konst, kommunikation och lärande, Medier ljudteknik och upplevelseproduktion och teater. Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Drift, underhåll och akustik.
Selection of audio stimuli for listening tests2011Ingår i: 130th Audio Engineering Society convention 2011: London, United Kingdom, 13 - 16 May 2011, Red Hook, NY: Curran Associates, Inc., 2011, Vol. 2, s. 1211-1217Konferensbidrag (Refereegranskat)

Two listening test methods in common use for the subjective assessment of audio quality are the ITU-R recommendations BS.1116-1 for small impairments and BS.1534-1 (MUSHRA) for intermediate quality. They stipulate the usage of only critical audio stimuli (BS.1116-1) to reveal differences among systems under test, or critical audio stimuli which represents typical audio material in a specific application context (MUSHRA). A poor selection of stimuli can cause experimental insensitivity and introduce bias, leading to inconclusive results. At the same time this selection process is time-consuming and labour-intensive, and is difficult to conduct in a systematic way. This paper reviews and discusses the selection of audio stimuli in listening test-related studies.

• 222.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
Flöde i slitsränna av komposit2017Självständigt arbete på avancerad nivå (yrkesexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)

På grund av allt hårdare miljökrav från EU och nedläggningen av kärnkraften så kommer vattenkraft, vilket redan spelar en stor roll för dagens energiproduktion, behöva utökas. På grund av detta behövs en hållbarare och billigare lösning på tekniska fiskvägar, vilka idag oftast gjuts i betong. En av lösningarna som företaget Composite Service Europe AB föreslår är att göra detta i komposit, då det är lättare att underhålla och specialdesigna. För att enklare kunna specialdesigna och se så den uppfyller alla krav som ställs så kommer detta examensarbete presentera en simulering av en dessa fiskrännor samt validering av denna simulering.

En ränna modellerades upp i Siemens NX vilken sedan simulerades med ANSYS CFX. Från denna simulering togs sedan flödesfältet och hastighetskonturen från den tredje bassängen ut och jämfördes mot experimentella flöden. De experimentella flödena är framtagna i kursen Experimentella Metoder och gav en indiktation för hur flödet borde se ut i bassängen.

Då det experimentella arbetet var gjort på ett sådant sätt att inga hastigheter eller andra kvantiteter för flödet fanns, så kan inte flödet räknas som helt validerat utan vidare experimentellt arbete krävs. Men det som jämförelsen tyder på är att det simulerade flödet följer det experimentella. Vidare måste simuleringstiden sänkas då denna simuleringen tog 21 dagar, vilket inte är gångbart ute i industrin. Några sätt att göra det på som föreslås är att göra en nätstudie och se om det går att simulera en bassäng.

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• 223.
Luleå tekniska universitet.
Simulation methods for hydropower flows: modeling and validation experiments2001Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)

My research project aims for more accurate modeling of the flow in hydropower plants. By accurate modeling, it is possible to try different modifications on existing plants in an affordable manner. Also, entirely new designs can be tested before realization. Hydropower delivers almost 50% of Sweden's electricity demand with a production of 74 TWh. Hydropower plants are very efficient, but even improvements of 0.1% in efficiency corresponds to great values for the hydropower industry. Modeling hydropower plants in computer simulations gives the possibility to try new designs or modifications to existing designs that may increase the efficiency, without expensive full-scale tests. The flow in hydropower applications is very complex, putting high demands on the modeling tool. The main tool is Computational Fluid Dynamics (CFD) software. Due to the highly turbulent flow, special models have to be implemented to account for these effects. Many such models exist, but in the complex geometries and flow conditions of hydropower flows better models have to be developed. To be able to improve existing models or to develop new models, we study the flow and models in experiments and computer simulations. By generic experiments in our lab, using a water tunnel with different geometries, we can measure the real flow field and compare it to different models in simulations. In this way, we are able to isolate different phenomena in the hydropower flow (such as pressure gradients, streamline curvature, swirl, separation and unsteadiness), and to see their impact on the simulations. In this thesis (in Paper C), we present measurements in a diffuser, which is a part of the draft tube in a hydropower plant. The diffuser imposes an adverse pressure gradient on the flow, which makes modeling more difficult. The experimental equipment includes a high standard Laser Doppler Velocimetry system (Dantec) for accurate velocity measurements. Detailed measurements have been made in the near-wall region down to y+=1. This study will be a validation case for CFD simulations. The simulation part consists of evaluating existing turbulence models and developing new models. The flow region close to solid walls is an area of special interest. This is where existing models seem to fail. The near-wall region is characterized by large gradients and difficult boundary conditions for turbulent quantities. There are a number of different ways to handle this problem. The standard way, often used in industrial CFD, uses a very simplified model near the wall (wall functions). More sophisticated models use different correction functions, or even a different turbulence model (two-layer models) in the near-wall region. Our focus is on two-layer models and enhanced wall functions. There are also models that do not need special treatment near walls, which we will study as well. Two different simulation studies are presented in papers A and B. The first is an investigation of the gyroscopic effects that are present when the swirling flow in a hydropower draft tube is forced through a bend. These effects cause an asymmetrical flow distribution after the bend, which is thought to induce losses. A new design method is considered that tries to minimize the losses. In paper B, a sensitivity analysis is performed on three input parameters in a complex CFD simulation, the Turbine-99 Draft Tube test case. A two-level factorial design is used to assess the influence of surface roughness, radial velocity profile and dissipation length scale (last two at the inlet) on some engineering quantities such as the pressure recovery. Factorial design is shown to be an efficient, quantitative tool to recognize the important input parameters. It can suggest what experiments that should be carried out in order to get sufficient input data for simulations.

• 224.
Luleå tekniska universitet.
Simulations and experiments of turbulent diffuser flow with hydropower applications2003Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)

Most hydropower plants in Sweden were built more than 50 years ago. The design tools were model tests and one-dimensional analytical methods. Since then the field of Computational Fluid Dynamics has evolved into an important engineering tool. To use CFD in a design or redesign stage of hydropower waterways would be very attractive due to its flexibility and cost-effectiveness. The hydropower plants operate under different circumstances today, since the electricity market has been deregulated and the nuclear power will be phased out. Turbines are thus run in a different manner than before with high power output during peak demand hours when the electricity price is high. Hence, efficiency improvements over a wide range of operating conditions are of interest. Predictions of runner flow have been established since long. Turbulence is of secondary importance which makes flow modeling relatively easy. The draft tube, however, is responsible for a large portion of the total hydraulic losses for low and medium head turbines and it is also the most challenging part of the waterways to predict numerically. The draft tube flow is characterized by the interaction of many complex flow features such as diffuser effects, unsteadiness, swirl, streamline curvature, separation and impingement. The Turbine-99 workshops assessed the ability of CFD to predict the flow features and engineering quantities of a model draft tube flow. It was found that many different problems contribute to the overall uncertainty in the results. The implementation of unknown boundary conditions and the post processing of the results were identified as error sources. The performance of the different turbulence models could not be evaluated, due to the numerical uncertainties. In the present work, factorial design was applied to investigate the sensitivity of the numerical solution to the unknown boundary conditions. It was shown to objectively give valuable information about the importance of different input parameters and to simplify the assessment of different turbulence models. To validate turbulence models for draft tubes, carefully selected validation cases need to be developed. This work presents two experimental validation cases that incorporates some of the complex phenomena in the draft tube, but in a well-controlled environment where detailed near-wall turbulence data can be measured by laser Doppler velocimetry. The effect of a 3-D straight asymmetric diffuser on a developing duct flow is investigated both for steady and pulsating flows. It was found that the average flow field is unaffected by the pulsation, while the oscillating turbulent quantities were influenced depending in their frequency.

• 225.
Luleå tekniska universitet.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
Gyroscopic design of swirling flow diffusers1999Ingår i: Proceedings of ASME/JSME FEDSM'99: ASME/JSME Fluids Engineering Division Summer Meeting, American Society of Mechanical Engineers , 1999Konferensbidrag (Refereegranskat)

The flow in the draft tube of a hydro power plant is often swirling when the turbine is operating outside its best efficiency point. The swirl gives rise to gyroscopic effects when the flow is forced through the bend in the draft tube. The resulting complex flow field causes losses.The idea in this paper is to investigate the possibility of using a simple model to calculate a new geometry of the draft tube that avoids distortion of the vortex core. Simulations are carried out using the CFD code CFX. A Reynolds stress model, with wall functions, is used to model turbulence.A loss factor is calculated and it was found that the new design draft tube shows approximately the same loss as a non-modified draft tube. The explanation to the somewhat surprising result is that the flow through and after the bend is dominated by the centrifugal effects from streamline curvature. It is therefore concluded that the most important loss mechanism appears to be triggered by streamline curvature.

• 226.
Luleå tekniska universitet.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Vattenfall Research & Development.
Proceedings of Turbine-99 - Workshop 2: The second ERCOFTAC Workshop on Draft Tube Flow, Älvkarleby, Sweden, June 18-20 20012002Bok (Övrigt vetenskapligt)
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• 227.
Luleå tekniska universitet.
Using TV holography with phase modulation to determine the deflection phase in a baritone guitar1998Ingår i: Proceedings International symposium on musical acoustics 1998: leeping Lady Conference Center, Leavenworth, Washington, United States of America 26 June - 1 July 1998 / [ed] Douglas Keefe, Woodbury: Acoustical Society of America (ASA), 1998, s. 103-108Konferensbidrag (Refereegranskat)
• 228.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
3D analysis of deformation and porosity of dry natural snow during compaction2019Ingår i: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, nr 6, artikel-id 850Artikel i tidskrift (Refereegranskat)

The present study focuses on three-dimensional (3D) microstructure analysis of dry natural snow during compaction. An X-ray computed microtomography (micro-CT) system was used to record a total of 1601 projections of a snow volume. Experiments were performed in-situ at four load states as 0 MPa, 0.3 MPa, 0.6 MPa and 0.8 MPa, to investigate the effect of compaction on structural features of snow grains. The micro-CT system produces high resolution images (4.3 μm voxel) in 6 hours of scanning time. The micro-CT images of the investigated snow volume illustrate that grain shapes are mostly dominated by needles, capped columns and dendrites. It was found that a significant number of grains appeared to have a deep hollow core irrespective of the grain shape. Digital volume correlation (DVC) was applied to investigate displacement and strain fields in the snow volume due to the compaction. Results from the DVC analysis show that grains close to the moving punch experience most of the displacement. The reconstructed snow volume is segmented into several cylinders via horizontal cross-sectioning, to evaluate the vertical heterogeneity of porosity distribution of the snow volume. It was observed that the porosity (for the whole volume) in principle decreases as the level of compaction increases. A distinct vertical heterogeneity is observed in porosity distribution in response to compaction. The observations from this initial study may be useful to understand the snow microstructure under applied stress.

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• 229. Eriksson, Martin
Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system. RISE - Swedish Institute of Computer Science. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Datavetenskap. Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system.
Monitoring and Modelling Open Compute Servers2017Ingår i: Proceedings IECON 2017: 43rd Annual Conference of the IEEE Industrial Electronics Society, Piscataway, NJ: Institute of Electrical and Electronics Engineers (IEEE), 2017, s. 7177-7184Konferensbidrag (Refereegranskat)

Energy efficient control of server rooms in modern data centers can help reducing the energy usage of this fast growing industry. Efficient control, however, cannot be achieved without: i) continuously monitoring in real-time the behavior of the basic thermal nodes within these infrastructures, i.e., the servers; ii) analyzing the acquired data to model the thermal dynamics within the data center. Accurate data and accurate models are indeed instrumental for implementing efficient data centers cooling strategies. In this paper we focus on a class of Open Compute Servers, designed in an open-source fashion and currently deployed by Facebook. We thus propose a set of methods for collecting real-time data from these platforms and a control-oriented model describing the thermal dynamics of the CPUs and RAMs of these servers as a function of both manipulable and exogenous inputs (e.g., the CPU utilization levels and the air mass flow produced by the server's fans). We identify the parameters of this model from real data and make the results available to other researchers.

• 230.
Faculty of Engineering, Amol University of Special Modern Technologies, Amol, Iran.
Faculty of Engineering, Amol University of Special Modern Technologies, Amol, Iran. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Advanced Institute of Manufacturing with High-tech Innovations and Department of Mechanical Engineering, National Chung Cheng University, 168 University Rd., Minhsiung Township, 62102, Chiayi, Taiwan, ROC.
Model based water management diagnosis in polymer electrolyte membrane fuel cell2020Ingår i: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487Artikel i tidskrift (Refereegranskat)

Water management diagnosis in polymer electrolyte membrane fuel cell is of great importance. The water produced in the fuel cell affects its performance and lifetime through the current distribution and the two phase flow pattern in the channel. The aim of the present study is to modify a segmented model in order to investigate a model-based water management diagnosis at different operating conditions. Simulations are conducted in three current densities: low (0.2 A/cm2), medium (0.6 A/cm2) and high (1 A/cm2), four temperatures ranged from 40 to 70, two stoichiometries (2 and 3) and four inlet humidities (25%, 50%,75% and 100%). The results show that at fully saturation inlet condition, there is a uniform local current density for all three considered current densities. Also, two-phase pressure drop and output voltage have similar trends. Hence two phase pressure drop can be considered as a suitable criterion for water management diagnosis. At inlet humidities less than 50%, non-uniformity of local current density increases that leads to reduction of output voltage, especially at high current density. Generally, for non-saturated inlet condition, two phase pressure drop and output voltage may show different trends. Therefore, two-phase pressure drop can be used only as a criterion for the formation of water and not for water management diagnosis.

• 231.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
Darcy's law for flow in a periodic thin porous medium confined between two parallel plates2016Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 115, nr 3, s. 473-493Artikel i tidskrift (Refereegranskat)

We study stationary incompressible fluid flow in a thin periodic porous medium. The medium under consideration is a bounded perforated 3D-domain confined between two parallel plates. The distance between the plates is $$\delta$$, and the perforation consists of $$\varepsilon$$-periodically distributed solid cylinders which connect the plates in perpendicular direction. Both parameters $$\varepsilon$$, $$\delta$$ are assumed to be small in comparison with the planar dimensions of the plates. By constructing asymptotic expansions, three cases are analysed: (1) $$\varepsilon \ll \delta$$, (2) $$\delta /\varepsilon \sim \text {constant}$$ and (3) $$\varepsilon \gg \delta$$. For each case, a permeability tensor is obtained by solving local problems. In the intermediate case, the cell problems are 3D, whereas they are 2D in the other cases, which is a considerable simplification. The dimensional reduction can be used for a wide range of $$\varepsilon$$ and $$\delta$$ with maintained accuracy. This is illustrated by some numerical examples.

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• 232.
Luleå tekniska universitet, Institutionen för system- och rymdteknik.
Influence of Reynolds Number and Heat Flux when Cooling an Array of Cylinders2020Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)

When batteries are used, it is of interest to know how high the temperature will become and if the cooling is efficient enough. In this thesis project, the geometry used are batteries placed in an array shape, inside a box. This gives the possibility of assuming porosity conditions when looking at the box from the outside. With the use of the commercial program ANSYS CFX, simulations of convective cooling of cylinders was performed. The goal was to find how large influence the Reynolds number and heat flux  from the batteries have on the fluid flow and if there is a possibility to find a prediction equation to determine certain temperatures even before simulating. To determine the influence, the variables Reynolds number, heat flux and type of fluid was varied one at a time. The Reynolds number change the inlet velocity of the fluid and a higher Reynolds number will transport the heat away faster. The heat flux was changed to see the influence on how the temperature of the fluid changed and how the velocity profiles varied due to the amount of power generated by the batteries. Increasing the power output would increase the temperature of the fluid and change the velocity profile difference. Varying the fluid was done to see how the thermal conductivity influenced the fluids behaviour while heated up. The three fluids used in this thesis was water, engine oil and air. There was a clear difference between the three fluids with engine oil being the most efficient while air was considerably worse.

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• 233.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Performance improvement of a solar air heater by covering the absorber plate with a thin porous material2020Ingår i: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 190, artikel-id 116437Artikel i tidskrift (Refereegranskat)

The effect of covering the absorber plate of a solar air heater with a thin porous media is investigated in the present study. Simulations are carried out for turbulent flow and heat transfer in the solar heater and within the porous layer. The effects of different parameters such as Reynolds number, Darcy number and solid to fluid thermal conductivity ratio on the thermal and thermo-hydraulic performances of a solar air heater are studied. It is observed that the implementation of a thin porous layer over the absorber plate significantly increases the thermal and thermo-hydraulic performances of the solar air heater. The maximum increase in the thermal and thermo-hydraulic performances is more than 5 times of those obtained in a solar heater without porous medium. Meanwhile, the maximum increase in the frictions factor of the porous solar heater is 2 times of that in a solar heater without porous media at the maximum Reynolds number studied. The proposed configuration also reduces the risk of hot spots since no entrapped eddies are formed over the absorber plate. It is shown that the turbulence produced at the porous-fluid interface penetrates into the thin porous layer and enhances the heat transfer from the absorber plate. The results also reveal that the conduction heat transfer within the porous layer highly affects the thermal and thermo-hydraulic performances of the solar heater.

• 234.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical Engineering, Tarbiat Modares University. Department of Mechanical Engineering, Tarbiat Modares University, Faculty of Engineering Technologies, Amol University of Special Modern Technologies, Amol.
Numerical Computation of Macroscopic Turbulent Quantities in a Porous Medium: an Extemsion to a macroscopic Turbulent model2016Ingår i: Journal of Porous Media, ISSN 1091-028X, E-ISSN 1934-0508, Vol. 19, nr 6, s. 497-513Artikel i tidskrift (Refereegranskat)

A numerical study is conducted using a standard numerical model for a porous medium consisting of a staggered arrangement of square cylinders. Fully developed macroscopic turbulent kinetic energy and dissipation rate are derived and analyzed for different porosities of the medium at different Reynolds numbers. The results obtained are used to extend the applicability range of an existing macroscopic turbulence model in porous media to low-Reynolds-number turbulent flows. It is shown that the levels of normalized macroscopic turbulent kinetic energy and dissipation rate are not constant over the entire range of Reynolds number. These quantities increase from lower levels at low Reynolds numbers up to an asymptotic value being independent of Reynolds number. The constants in the closure expression of the macroscopic turbulence equations are modified using the present results. Finally, in order to highlight the importance of the present modifications, the results of the macroscopic turbulence model before and after the modifications are compared for two cases.

• 235.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media2020Ingår i: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 81, artikel-id 108523Artikel i tidskrift (Refereegranskat)

In the present study, the importance of the thermal dispersion and the turbulent heat flux in porous media and their effects on the macroscopic distribution of thermal energy are investigated. To this end, turbulent flow and heat transfer within five unit-cells mimicking porous media are solved using large eddy simulation. It is shown that the thermal dispersion and the turbulent heat flux are negligible as compared to the convection term in the macroscopic energy equation. When further scrutinizing this equation, it is revealed that except for the longitudinal components of the thermal dispersion, the other components of thermal dispersion and turbulent heat flux may be neglected away from the boundaries as compared to the interfacial heat transfer. Visualizations of vortices show that the size of the turbulence structures within the cells is of the same order as the size of the pores; therefore, the turbulent heat flux is limited to the intra-pore level. Finally, a discussion is provided on the accuracy of the gradient type diffusion model commonly used for turbulent heat flux in porous media in the absence of macroscopic turbulence. It is shown that the intra-pore turbulence does not affect the macroscopic transport of thermal energy within the porous media studied.

• 236.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
A Subgrid-Scale Model for Turbulent Flow in Porous Media2019Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 129, nr 3, s. 619-632Artikel i tidskrift (Refereegranskat)

Given the analogy between the filtered equations of large eddy simulation and volume-averaged Navier–Stokes equations in porous media, a subgrid-scale model is presented to account for the residual stresses within the porous medium. The proposed model is based on the kinetic energy balance of the filtered velocity field within a pore; hence, when using the model, numerical simulations of the turbulent flow in the pores are not required. The accuracy of the model is validated with available data in the literature on turbulent flow through packed beds and staggered arrangement of square cylinders. The validation yields that the model successfully captures the effect of the pore-scale turbulent motion. The model is then used to study turbulent flow in a wall-bounded porous media to assess its accuracy.

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• 237.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media2020Ingår i: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 131, nr 2, s. 739-765Artikel i tidskrift (Refereegranskat)

The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

• 238.
Department of Mechanical Engineering, UPC - Technical University of Catalonia.
Department of Mechanical Engineering, UPC - Technical University of Catalonia. Department of Graphic Expression, UPC - Technical University of Catalonia. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
The use of Rapid Prototyping techniques (RPT) to manufacture micro channels suitable for high operation pressures and µPIV2016Ingår i: Rapid prototyping journal, ISSN 1355-2546, E-ISSN 1758-7670, Vol. 22, nr 1, s. 67-76Artikel i tidskrift (Refereegranskat)

Purpose– This paper aims to present a new methodology to manufacture micro-channels suitable for high operating pressures and micro particle image velocimetry (μPIV) measurements using a rapid-prototyping high-resolution 3D printer. This methodology can fabricate channels down to 250 μm and withstand pressures of up to 5 ± 0.2 MPa. The manufacturing times are much shorter than in soft lithography processes. Design/methodology/approach– The novel manufacturing method developed takes advantage of the recently improved resolution in 3D printers to manufacture an rapid prototyping technique part that contains the hose connections and a micro-channel useful for microfluidics. A method to assemble one wall of the micro-channel using UV curable glue with a glass slide is presented – an operation required to prepare the channel for μPIV measurements. Once built, the micro-channel has been evaluated when working under pressure and the grease flow behavior in it has been measured using μPIV. Furthermore, the minimum achievable channels have been defined using a confocal microscopy study. Findings– This technique is much faster than previous micro-manufacturing techniques where different steps were needed to obtain the micro-machined parts. However, due to current 3D printers ' resolutions (around 50 μm) and according to the experimental results, channels smaller than 250-μm2 cross-section should not be used to characterize fluid flow behaviors, as inaccuracies in the channel boundaries can deeply affect the fluid flow behavior. Practical implications– The present methodology is developed due to the need to validate micro-channels using μPIV to lubricate critical components (bearings and gears) in wind turbines. Originality/value– This novel micro-manufacturing technique overcomes current techniques, as it requires less manufacturing steps and therefore it is faster and with less associated costs to manufacture micro-channels down to 250-μm2 cross-section that can withstand pressures higher than 5 MPa that can be used to characterize microfluidic flow behavior using μPIV.

• 239.
Department of Mechanical Engineering, UPC - Technical University of Catalonia.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical Engineering, UPC - Technical University of Catalonia.
New solution to preent excessive wear in wind turbine gears2014Ingår i: Society of Tribologists and Lubrication Engineers Annual Meeting and Exhibition 2014: Lake Buena Vista, United States, 18 - 22 May 2014, Society of Tribologists and Lubrication Engineers , 2014, Vol. 2, s. 548-550Konferensbidrag (Refereegranskat)
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• 240.
Department of Mechanical Engineering, UPC - Technical University of Catalonia, KIC InnoEnergy S.E. - Location Iberia, Barcelona.
Department of Mechanical Engineering, UPC - Technical University of Catalonia, KIC InnoEnergy S.E. - Location Iberia, Barcelona. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
New Lubrication Device to Minimize Wear at the Pitch Gear2015Konferensbidrag (Refereegranskat)
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• 241.
Department of Mechanical Engineering, UPC - Technical University of Catalonia, KIC InnoEnergy S.E. - Location Iberia, Barcelona.
Department of Mechanical Engineering, UPC - Technical University of Catalonia, KIC InnoEnergy S.E. - Location Iberia, Barcelona. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
New Lubrication Device to Minimize Wear at the Pitch Gear2015Konferensbidrag (Refereegranskat)
FULLTEXT01
• 242.
Department of Mechanical Engineering, UPC - Technical University of Catalonia.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Department of Mechanical Engineering, UPC - Technical University of Catalonia.
Embedded Micro-nozzles in the Pitch Gear Deddendum to Minimize Wear at Zero Degree Position2015Konferensbidrag (Refereegranskat)
• 243.
Mechanical Engineering Department, UPC - Technical University of Catalonia, Terrassa.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik. Mechanical Engineering Department, UPC - Technical University of Catalonia, Terrassa.
New method for lubricating wind turbine pitch gears using embedded micro-nozzles2017Ingår i: Journal of Mechanical Science and Technology, ISSN 1738-494X, E-ISSN 1976-3824, Vol. 31, nr 2, s. 797-806Artikel i tidskrift (Refereegranskat)

The increase of power generated by wind turbines has increased the stresses applied in all of its components, thereby causing premature failures. Particularly, pitch and yaw gears suffer from excessive wear mainly caused by inappropriate lubrication. This paper presents a novel method to automatically lubricate the wind turbine pitch gear during operation. A micro-nozzle to inject fresh grease continuously between the teeth in contact was designed, manufactured, and installed in a test bench of a 2 MW wind turbine pitch system. The test bench was used to characterize the fatigue behavior of the gear surface using conventional wind turbine greases under real cyclic loads. Measurements of wear evolution in a pitch gear with and without micro-nozzle show a decrease of 70 % of the wear coefficient after 2×104 cycles.

• 244.
Swerea SICOMP AB.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Fibre reinforced polymer composites based on nanostructured constituents2008Ingår i: White Book: Polymer Nanoscience and Nanotechnology, a European Perspective, 2008, s. 233-256Kapitel i bok, del av antologi (Övrigt vetenskapligt)
• 245.
Luleå tekniska universitet.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Particle Distribution Mechanisms in Infusion Moulded Composites2004Ingår i: From nano-scale interactions to engineering structures: ECCM 11, 11th European Conference on Composite Materials ; May 31 - June 3, 2004, Rhodes, Greece / [ed] Costas Galiotis, Rhodos, 2004Konferensbidrag (Refereegranskat)
• 246.
Swerea SICOMP AB, Box 271, 941 26, Piteå.
Swerea SICOMP AB, Box 271, 941 26, Piteå. Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Mechanisms controlling particle distribution in infusion molded composites2006Ingår i: Journal of reinforced plastics and composites (Print), ISSN 0731-6844, E-ISSN 1530-7964, Vol. 25, nr 1, s. 59-70Artikel i tidskrift (Refereegranskat)

This article presents results from an experimental investigation in which two grades of aluminatrihydroxide (ATH) particles are added to liquid resin and used in infusion molding experiments. Based on the results, potential key mechanisms controlling resin flow and hence also the final particle distribution are proposed. A pore doublet model is proposed to explain the seemingly random spatial distribution of particle-dense regions within the final material. These dense regions are found within flow channels, at locations where local shear strain rates are low. This suggests that they are formed as a consequence of a Bingham type of viscosity behavior observed for the suspension and/or due to filtering of particles during fiber bundle impregnation.

• 247.
Luleå tekniska universitet.
Luleå tekniska universitet.
A high speed photographic study of impact on fibres and woven fabrics1992Rapport (Övrigt vetenskapligt)
• 248.
Luleå tekniska universitet.
Luleå tekniska universitet.
Transverse impact of yarns: i. Direct measurement of longitudinal wave velocity : ii. Effect of pre-stress : iii. High-speed photography of failure1992Rapport (Övrigt vetenskapligt)
• 249.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Jet formation in liquid impact1984Licentiatavhandling, monografi (Övrigt vetenskapligt)
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• 250.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Strömningslära och experimentell mekanik.
Non-stokesian flows in thin porous media2020Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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