Change search
Link to record
Permanent link

Direct link
BETA
Publications (10 of 97) 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.

Place, publisher, year, edition, pages
Elsevier, 2019
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)000456766000058 ()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: 2019-04-23Bibliographically approved
Ernens, D., Ràfols, F. P., Van Hoecke, D., Roijmans, R. F. H., van Rie, E. J., Vande Voorde, J. B. E., . . . Pasaribu, H. R. (2019). On the Sealability of Metal-to-Metal Seals With Application to Premium Casing and Tubing Connections. SPE Drilling & Completion
Open this publication in new window or tab >>On the Sealability of Metal-to-Metal Seals With Application to Premium Casing and Tubing Connections
Show others...
2019 (English)In: SPE Drilling & Completion, ISSN 1064-6671, E-ISSN 1930-0204Article in journal (Refereed) Epub ahead of print
Abstract [en]

Metal-to-metal seals are used in connections of casing and tubing in oil and gas wells. This paper describes the mechanisms of sealing metal-to-metal seals as investigated using an experimental setup and a stochastic numerical sealing model. Experiments were conducted for a variety of thread compounds and applied pin/box surface coatings. The results were used to validate a stochastic numerical sealing model for sealability. The model couples a contact-mechanics model with a flow model and takes into account the influence of all the surface-topography features by introducing the concept of seal permeability. Once validated, the model was used together with the experimental results to better understand the sealing mechanisms of metal-to-metal seals.

The sealing configuration is a face seal with an 80-mm roundoff radius on one face pressing against a flat on the other face. The face-seal specimens were manufactured from P110 tubing to ensure material properties that are representative for casing or tubing. The test setup used is designed for investigating only the metal-to-metal seal of the connection. The setup can perform rotary sliding under constant load to simulate surface changes during makeup and subsequently perform a leakage test. The sealing limit is determined by applying 700-bar fluid pressure and then gradually reducing the normal force until leakage is observed. The data are subsequently used to validate the previously published stochastic numerical sealing model.

The results indicate a strong dependence on the type of thread compound used for the onset of leakage. The thread compound affects the amount of wear and thus changes the surface topography of the interacting surfaces. It is shown that the stochastic numerical sealing model is capable of predicting the onset of leakage within the experimental accuracy. The model shows further that certain surface topographical features improve the sealing performance. In particular, a surface manufactured by turning on a lathe that is in contact with, for instance, a smooth shot-blasted surface topography leads to highly localized contact areas, which in turn yield the best sealing performance.

Place, publisher, year, edition, pages
Society of Petroleum Engineers, 2019
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-75204 (URN)10.2118/194146-PA (DOI)
Available from: 2019-07-03 Created: 2019-07-03 Last updated: 2019-07-03
Ernens, D., Ràfols, F. P., Van Hoecke, D., Roijmans, R. F. H., van Riet, E. J., Voorde, J. V., . . . Pasaribu, H. R. (2019). On the Sealability of Metal-to-Metal Seals with Application to Premium Casing Connections. In: : . Paper presented at SPE/IADC Drilling International Conference and Exhibition held in The Hague, The Netherlands, 5-7 March 2019..
Open this publication in new window or tab >>On the Sealability of Metal-to-Metal Seals with Application to Premium Casing Connections
Show others...
2019 (English)Conference paper (Other (popular science, discussion, etc.))
Abstract [en]

Metal-to-metal seals are used in connections of casing in oil and gas wells. This paper describes the mechanisms of sealing of metal-to-metal seals as investigated using an experimental set-up and a sealability model. Experiments were conducted for a variety of thread compounds and applied pin/box surface coatings. The results were used to validate a numerical model for sealability. The stochastic model couples a contact mechanics model with a flow model and takes the influence of all the surface topography features into account. Once validated, the model was used together with the experimental results to explain the sealing mechanisms of metal-to-metal seals. The sealing configuration is a face seal with an R=80 mm round-off radius pressing against a flat. The face seal specimens were manufactured from P110 tubing. The used test set-up is designed for investigating only the metal-to-metal seal of the connection. The set-up can carry out rotary sliding under constant load to simulate surface evolution during make-up and subsequently perform a leakage test. The sealing limit is determined by applying 700 bar fluid pressure and then gradually reducing the normal force until leakage is observed. The data is subsequently used to validate a previously published model. The results indicate a strong dependence of the type of thread compound used on the onset of leakage. The thread compound affects the amount of wear and thus changes the surface topography of the interacting surfaces. It is shown that the sealability model is capable to predict the onset of leakage within the experimental accuracy. The model shows further that certain surface topographical features improve the sealing performance. Namely, a turned against a flat surface topography leads to highly localized contact areas, which in turn yields the best sealing performance. The combination of experimental data with the validated model leads to much deeper insights for the sealing mechanisms than what could be obtained using either on their own.

National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-73329 (URN)
Conference
SPE/IADC Drilling International Conference and Exhibition held in The Hague, The Netherlands, 5-7 March 2019.
Available from: 2019-03-27 Created: 2019-03-27 Last updated: 2019-03-27Bibliographically approved
Almqvist, A. & Ràfols, F. P. (2019). Scientific Computing with Applications in Tribology: A course compendium.
Open this publication in new window or tab >>Scientific Computing with Applications in Tribology: A course compendium
2019 (English)Other (Other academic)
Abstract [en]

This compendium comprises models and numerical solution procedure for tribological interfaces. It describes the tribological contact and the classical lubrication regimes. A thorough derivation of the Reynolds equation, governing the fluid pressure, from the Navier-Stokes momentum equations and the continuity equation for conservation of mass, is presented along with its analytical solution for the infinitely wide linear slider bearing.

The compilation of the compendium was conducted by the first author during his tenure as Professor at the Division of Machine Elements, Department of Engineering Sciences and Mathematics, Luleå University of Technology and by the second author during his tenure as a postdoctoral researcher at the same division.

Although the compilation of this text is the work solely of the authors, the models and solution procedure presented herein is joint development of many good colleagues and co-authors. Our sincere gratitude is extended towards them all.

National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-72934 (URN)
Available from: 2019-02-18 Created: 2019-02-18 Last updated: 2019-05-24Bibliographically 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)000455618500003 ()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: 2019-02-01Bibliographically 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
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7029-1112

Search in DiVA

Show all publications