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Publications (10 of 94) Show all publications
Ràfols, F. P. & Almqvist, A. (2019). Generating randomly rough surfaces with given height probability distribution and power spectrum. Tribology International, 131, 591-604
Open this publication in new window or tab >>Generating randomly rough surfaces with given height probability distribution and power spectrum
2019 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 131, p. 591-604Article in journal (Refereed) Published
Abstract [en]

In this work we present a simple method to generate surface topography. The main advantage of the presented method as compared with those available in the literature is that the power spectrum and the height probability distribution can be specified independently. In this article we present the method and show its versatility by generating surface topographies with three different height probability distributions: the Weibull distribution, a bimodal distribution and a distribution containing a delta function that represents worn surfaces. The MATLAB-code we used to generate the numerical examples are also provided to the reader.

Keywords
Roughness, Non-Gaussian, Power-spectrum, Generated surfaces
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-71894 (URN)10.1016/j.triboint.2018.11.020 (DOI)2-s2.0-85057500827 (Scopus ID)
Note

Validerad;2019;Nivå 2;2018-12-05 (svasva)

Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-13Bibliographically approved
Ràfols, F. P. & Almqvist, A. (2018). An Enhanced Stochastic Two-Scale Model for Metal-to-Metal Seals. Lubricants, 6(4), Article ID 87.
Open this publication in new window or tab >>An Enhanced Stochastic Two-Scale Model for Metal-to-Metal Seals
2018 (English)In: Lubricants, ISSN 2075-4442, Vol. 6, no 4, article id 87Article in journal (Refereed) Published
Abstract [en]

Leakage in static metal-to-metal seals is predominantly determined by the topography of the contacting surfaces. The topography consists of features that span the entire range from its carefully engineered geometry down to micro-sized surface asperities. The mesh density necessary to fully resolve all the features, in this large span of length scales, generates too many degrees of freedom for a direct numerical approach to be applicable. Some kind of sophistication, either incorporated in the mathematical model or in the numerical solution procedure or even a combination of both is therefore required. For instance, in a two-scale model, the geometrical features can be addressed in the global-scale model, while the features belonging to length scales smaller than a given cut-off value are addressed in the local-scale model. However, the classical two-scale approaches do not explicitly address the stochastic nature of the surfaces, and this has turned out to be a requirement in order to obtain quantitative predictions of leakage in metal-to-metal seals. In this work, we present a continued development of an already existing two-scale model, which incorporates a stochastic element. The novelty lies in the way we characterise the permeability at the local scale and how this is used to build a more efficient and useful approach

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
leakage, contact mechanics, reynolds equation, two-scale modelling, stochastic, metal-to-metal seal
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-71294 (URN)10.3390/lubricants6040087 (DOI)2-s2.0-85056378906 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-10-22 (svasva)

Available from: 2018-10-22 Created: 2018-10-22 Last updated: 2018-11-23Bibliographically approved
Vakis, A., Almqvist, A. & Ciavarella, M. (2018). Modeling and simulation in tribology across scales: An overview. Tribology International, 125, 169-199
Open this publication in new window or tab >>Modeling and simulation in tribology across scales: An overview
2018 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 125, p. 169-199Article in journal (Refereed) Published
Abstract [en]

This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.

Place, publisher, year, edition, pages
Elsevier, 2018
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-67668 (URN)10.1016/j.triboint.2018.02.005 (DOI)000435747800018 ()2-s2.0-85044579364 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-06-11 (rokbeg)

Available from: 2018-02-15 Created: 2018-02-15 Last updated: 2018-07-19Bibliographically approved
Almqvist, A. (2018). MULTISCALE MODELLING OF ELASTOHYDRODYNAMIC TILTED-PAD BEARINGS: A METAMODEL APPROACH. In: : . Paper presented at Joint ECCOMAS conference ECCM 6 and ECFD 7 2018.
Open this publication in new window or tab >>MULTISCALE MODELLING OF ELASTOHYDRODYNAMIC TILTED-PAD BEARINGS: A METAMODEL APPROACH
2018 (English)Conference paper, Oral presentation with published abstract (Refereed)
Abstract [en]

6th European Conference on Computational Mechanics (ECCM 6)7th European Conference on Computational Fluid Dynamics (ECFD 7)11 – 15 June 2018, Glasgow, UK 

MULTISCALE MODELLING OF ELASTOHYDRODYNAMIC TILTED-PAD BEARINGS: A METAMODEL APPROACH 

G. N. de Boer1, A. Almqvist2, L. Gao3, R. W. Hewson4 and H. M. Thompson

1 School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK, g.n.deboer@leeds.ac.uk, https://engineering.leeds.ac.uk/staff/1036/Dr_Greg_de_Boer 

2 Division of Machine Elements, Luleå University of Technology, E837a Luleå, Sweden, a.almqvist@ltu.se, https://www.ltu.se/staff/a/almqvist 

3 Department of Aeronautics, Imperial College London, London, SW7 2AZ, UK, leiming.gao@imperial.ac.uk, http://www.imperial.ac.uk/people/leiming.gao 

4 Department of Aeronautics, Imperial College London, London, SW7 2AZ, UK, r.hewson@imperial.ac.uk, https://www.imperial.ac.uk/people/r.hewson 

5 School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK, h.m.thompson@leeds.ac.uk, https://engineering.leeds.ac.uk/staff/140/Professor_Harvey_Thompson 

Key Words: Elastohydrodynamic Lubrication; Surface Topography; Metamodelling; Moving Least Squares; Evolving Design of Experiments. 

Elastohydrodynamic Lubrication (EHL) refers to the contact of two surfaces in relative motion under fully flooded conditions, pressure generated in the lubricant generates deformation of the bodies and this is coupled to determine a total load carrying capacity. In such contacts the size of surface topography and film thickness are of a similar order of magnitude and this therefore has a role in describing the phenomena. However the length scales associated with surface topography and the contact region are disparate and in order to model such effects authors have developed homogenisation based methods. 

Recently the Heterogeneous Multiscale Methods (HMM) have been employed to study the problem. This has allowed the effects of micro-EHL to be explored and coupled into the macro-scale EHL problem. Fundamental to this is the separation of scales and periodicity applied at to simulations describing surface topography. de Boer [1] outlines a method for coupling the scales of the problem using Moving Least Squares metamodels to calculate flow factors. This was further used to optimise surface topographical features to produce the minimum possible coefficient of friction in an EHL contact [2]. This research focuses on the metamodelling approach of [1, 2] to explore more complex 3D titled-pad bearing geometries than have previously been investigated. The means by which the scales of the problem are coupled is complicated by an increase in the number of design variables. Additionally the choice of Design of Experiments and how this evolves with the solution procedure is vital to the accuracy of the approach. 

REFERENCES 

[1] de Boer G, Gao L, Hewson R, Thompson H. Heterogeneous Multiscale Methods for modelling surface topography in EHL line contacts. Tribol Int 2017, 113:262-278. 

[2] de Boer G, Gao L, Hewson R, Thompson H, Raske N, Toropov V. A multiscale method for optimising surface topography in elastohydrodynamic lubrication (EHL) using metamodels. Struct Mulidisc Optim 2016, 54(3):483-497. 

National Category
Mechanical Engineering Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-71742 (URN)
Conference
Joint ECCOMAS conference ECCM 6 and ECFD 7 2018
Funder
Swedish Research Council
Available from: 2018-11-23 Created: 2018-11-23 Last updated: 2018-11-23
Pérez-Ràfols, F., Larsson, R., van Riet, E. J. & Almqvist, A. (2018). On the flow through plastically deformed surfaces under unloading: A spectral approach. Proceedings of the Institution of mechanical engineers. Part C, journal of mechanical engineering science, 232(5), 908-918
Open this publication in new window or tab >>On the flow through plastically deformed surfaces under unloading: A spectral approach
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part C, journal of mechanical engineering science, ISSN 0954-4062, E-ISSN 2041-2983, Vol. 232, no 5, p. 908-918Article in journal (Refereed) Published
Abstract [en]

This study considers flow through the gap left between two surfaces during unloading, in other words, when an applied load is gradually reduced after loading to a state where plastic deformation occurs. In particular, the permeability of the gap is studied. It was found that a substantial reduction of the applied load is required before the permeability starts to increase significantly. The explanation for this phenomenon is given by the combination of components with different wavelengths present in the surface. Components with long wavelengths deform elastically and those with shorter wavelengths may also deform plastically. We found that plastic deformation acts to keep the permeability nearly constant at the beginning of the unloading and elastic spring-back is responsible for the rapid increase at lower loads. This principle constitutes a basis for the strategy that was developed in order to predict the load at which the rapid increase of permeability starts.

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-62112 (URN)10.1177/0954406217690180 (DOI)000429913000015 ()
Note

Validerad;2018;Nivå 2;2018-02-26 (svasva)

Available from: 2017-02-22 Created: 2017-02-22 Last updated: 2018-04-27Bibliographically approved
Ràfols, F. P., Larsson, R., Van Riet, E. J. & Almqvist, A. (2018). On the loading and unloading of metal-to-metal seals: A two-scale stochastic approach. Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, 232(12), 1525-1537
Open this publication in new window or tab >>On the loading and unloading of metal-to-metal seals: A two-scale stochastic approach
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 232, no 12, p. 1525-1537Article in journal (Refereed) Published
Abstract [en]

During operation, the mating surfaces of a metal-to-metal seal typically undergo significant plastic deformation, which in turn can have beneficial effect on its performance. In previous studies, it has, for instance, been shown that plastic deformation can provide for better sealing during unloading. Those studies did, however, only consider flow through unrealistically small domains. Therefore, it is possible that this might be a size effect, which would not be apparent in a real situation with a much larger domain. In this paper, we develop a model which can handle real-sized seal domains at the same time as fine details of the surface topography. More precisely, we construct a two-scale model, in which the global scale represents the seal domain and where the influence of the fine details at the local scale are represented by a stochastic element. By means of this stochastic two-scale model, we show that the beneficial effect associated with the plastic deformation persists also when real-sized seal domains are considered.

Place, publisher, year, edition, pages
Sage Publications, 2018
Keywords
Contact mechanics, load cycle, Reynolds equation, seals, stochastic, two-scale modelling
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-67695 (URN)10.1177/1350650118755620 (DOI)000450295800004 ()2-s2.0-85044080716 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-11-19 (johcin)

Available from: 2018-02-20 Created: 2018-02-20 Last updated: 2018-12-03Bibliographically approved
de Boer, G. & Almqvist, A. (2018). On the Two-Scale Modelling of Elastohydrodynamic Lubrication in Tilted-Pad Bearings. Lubricants, 6(3), Article ID 78.
Open this publication in new window or tab >>On the Two-Scale Modelling of Elastohydrodynamic Lubrication in Tilted-Pad Bearings
2018 (Swedish)In: Lubricants, ISSN 2075-4442, Vol. 6, no 3, article id 78Article in journal (Refereed) Published
Abstract [en]

A two-scale method for modelling the Elastohydrodynamic Lubrication (EHL) of tilted-pad bearings is derived and a range of solutions are presented. The method is developed from previous publications and is based on the Heterogeneous Multiscale Methods (HMM). It facilitates, by means of homogenization, incorporating the effects of surface topography in the analysis of tilted-pad bearings. New to this article is the investigation of three-dimensional bearings, including the effects of both ideal and real surface topographies, micro-cavitation, and the metamodeling procedure used in coupling the problem scales. Solutions for smooth bearing surfaces, and under pure hydrodynamic operating conditions, obtained with the present two-scale EHL model, demonstrate equivalence to those obtained from well-established homogenization methods. Solutions obtained for elastohydrodynamic operating conditions, show a dependency of the solution to the pad thickness and load capacity of the bearing. More precisely, the response for the real surface topography was found to be stiffer in comparison to the ideal. Micro-scale results demonstrate periodicity of the flow and surface topography and this is consistent with the requirements of the HMM. The means of selecting micro-scale simulations based on intermediate macro-scale solutions, in the metamodeling approach, was developed for larger dimensionality and subsequent calibration. An analysis of the present metamodeling approach indicates improved performance in comparison to previous studies.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
tilted-pad bearings, elastohydrodynamic lubrication, surface topography, homogenization, heterogenous multiscale methods
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-71293 (URN)10.3390/lubricants6030078 (DOI)2-s2.0-85056411989 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-10-22 (svasva)

Available from: 2018-10-22 Created: 2018-10-22 Last updated: 2018-11-23Bibliographically approved
Söderfjäll, M., Larsson, R., Marklund, P. & Almqvist, A. (2018). Texture-induced effects causing reduction of friction in mixed lubrication for twin land oil control rings. Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, 232(2), 166-178
Open this publication in new window or tab >>Texture-induced effects causing reduction of friction in mixed lubrication for twin land oil control rings
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 232, no 2, p. 166-178Article in journal (Refereed) Published
Abstract [en]

Piston rings are responsible for a major part of the frictional losses in a heavy-duty diesel engine. Such losses can be reduced by applying texture, such as dimples, on the cylinder liner surface. This paper investigates the effect of such texture on the friction between a land of the oil control ring and a textured cylinder liner via numerical simulation. A simulation model considering inertia and mixed lubrication together with a mass-conserving cavitation model is developed. The model is used to determine the dimple parameters that yield the lowest amount of friction for a specific oil control ring of a heavy-duty diesel engine

Place, publisher, year, edition, pages
Sage Publications, 2018
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-63889 (URN)10.1177/1350650117709152 (DOI)000424781600008 ()2-s2.0-85042864633 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-02-12 (svasva)

Available from: 2017-06-12 Created: 2017-06-12 Last updated: 2018-03-14Bibliographically approved
Philip, C., Almqvist, A. & Larsson, R. (2018). Thermal Turbulent Flow in Leading Edge Grooved and Conventional Tilting Pad Journal Bearing Segments: A Comparative Study. Lubricants, 6(4), Article ID 97.
Open this publication in new window or tab >>Thermal Turbulent Flow in Leading Edge Grooved and Conventional Tilting Pad Journal Bearing Segments: A Comparative Study
2018 (English)In: Lubricants, ISSN 2075-4442, Vol. 6, no 4, article id 97Article in journal (Refereed) Published
Abstract [en]

A comparative study between a conventional- and leading edge grooved (LEG) tilting pad journal bearing (TPJB) segment is performed. The developed model uses the Shear Stress Transport (SST) turbulence model, coupled with the energy equation and a partial differential equation for the fluid domain mesh displacement to predict the thermal flow characteristics. Instead of using an effective boundary condition to determine the inlet temperature of the LEG pad and excluding the additional LEG portion, as is common practice, the whole geometry of the LEG is modeled. Several sizes of the LEG portion is investigated and it is shown to have quite a small influence on pressure, temperature, film thickness and turbulence intensity. Moreover, the results also show that the conventional pad gives rise to higher levels of turbulence in the mid plane compared to its LEG counterpart, while the latter has a marginally higher value of turbulence when the volume average value is computed. The maximum value of turbulence is however present in the conventional model. 

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
tilting pad journal bearing, leading edge groove, turbulence, thermal flow, hydrodynamics, lubrication
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-71732 (URN)10.3390/lubricants6040097 (DOI)2-s2.0-85056402242 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-11-23 (svasva)

Available from: 2018-11-23 Created: 2018-11-23 Last updated: 2018-11-23Bibliographically approved
Shirzadegan, M., Larsson, R. & Almqvist, A. (2017). A low degree of freedom approach for prediction of friction in finite EHL line contacts. Tribology International, 115, 628-639
Open this publication in new window or tab >>A low degree of freedom approach for prediction of friction in finite EHL line contacts
2017 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 115, p. 628-639Article in journal (Refereed) Published
Abstract [en]

A low degree of freedom (LDOF) approach for expedient prediction of friction in finite EHL line contact is presented. Friction prediction are in good agreement with existing results for all of the reference cases tested. With the present LDOF approach, friction predictions can be obtained thousands of times faster than if using a fully coupled FE-based model. It can, therefore be used to simulated the tribological contacts in multi-body dynamic (MBD) systems with millions of degrees of freedoms. The model was utilised to study edge stress concentrations due to roller tilt. The results confirmed that increasing the tilting angle increases the maximum pressure and shifts its location towards the edge of roller. An increase in total friction was also observed.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-64193 (URN)10.1016/j.triboint.2017.06.019 (DOI)000407187900061 ()2-s2.0-85021720586 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-07-06 (rokbeg)

Available from: 2017-06-19 Created: 2017-06-19 Last updated: 2018-07-10Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-7029-1112

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