| 21. |
- Sahlin, Fredrik, et al.
(författare)
-
2D CFD-analysis of micro-patterned surfaces in hydrodynamic lubrication
- 2004
-
Ingår i: Proceedings of the ASME/STLE international joint tribology conference 2004. - New York. ; , s. 1637-1645, s. 1637-1645
-
Konferensbidrag (refereegranskat)abstract
- Results of a numerical study of the influence of micro-patterned surfaces in hydrodynamic lubrication of two parallel walls are reported. Two types of parameterized grooves with the same order of depth as the film thickness are used on one stationary wall. The other wall is smooth and is sliding with a specified tangential velocity. Isothermal incompressible two dimensional full film fluid flow mechanics is solved using a Computational Fluid Dynamics method. It is shown that, by introducing a micro-pattern on one of two parallel walls, a net pressure rise in the fluid domain is achieved. This produces a load carrying capacity on the walls which is mainly contributed by fluid inertia. The load carrying capacity increases with Reynolds number. The load carrying capacity is reported to increase with groove width and depth. However, at a certain depth a vortex appears in the groove and near this value the maximum load carrying capacity is achieved. It is shown that the friction force decreases with deeper and wider grooves. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance are reported.
|
|
| 22. |
- Sahlin, Fredrik, et al.
(författare)
-
A cavitation algorithm for arbitrary lubricant compressibility
- 2007
-
Ingår i: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 40:8, s. 1294-1300
-
Tidskriftsartikel (refereegranskat)abstract
- A general cavitation algorithm that accommodates for an arbitrary density-pressure relation is presented. It is now possible to model the compressibility of the lubricant in such a way that the density-pressure relation is realistic throughout the contact. The algorithm preserves mass continuity for cavitation caused by bearing geometry and surface topography. It is a commonly accepted physical assumption that the contribution of the pressure driven flow is negligible in the cavitated region. This phenomenon is adopted in the present algorithm, which is similar to that of Elrod, and is modeled by a switch function that terminates the pressure gradient at the cavitation regions. Results with this algorithm for different density-pressure relations are presented and discussed. The effects of inlet conditions, such as surface roughness and starvation, on the load carrying capacity of the contact are analyzed.
|
|
| 23. |
- Sahlin, Fredrik, et al.
(författare)
-
A cavitation algorithm for arbitrary lubricant compressibility
- 2005
-
Ingår i: Proceedings of the World Tribology Congress III - 2005. - New York : American Society of Mechanical Engineers. ; , s. 541-542, s. 541-542
-
Konferensbidrag (refereegranskat)abstract
- A general cavitation algorithm is presented that accommodates for an arbitrary density-pressure relation. Here it is possible to model the compressibility of the lubricant in such way the density-pressure relation is realistic from sub-cavity to high pressure regions respectively. The algorithm preserves mass continuity which is of importance when inter-asperity cavitation of rough surfaces is considered. Results with this algorithm for different density-pressure relations are presented and discussed.
|
|
| 24. |
- Sahlin, Fredrik, et al.
(författare)
-
A mixed lubrication model incorporating measured surface topography. Part 1: Theory of flow factors
- 2010
-
Ingår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology. - 1350-6501 .- 2041-305X. ; 224:4, s. 335-351
-
Tidskriftsartikel (refereegranskat)abstract
- A mixed lubrication model that permits real three-dimensional surface topography as input is developed. The theory of computing flow factors within the model is presented, and with a following paper (Part 2) the method of measuring and adapting the surface roughness, and model validation through flow measurements and application to a bearing is shown. A contact mechanics model is used to calculate the elastoplastic displacement of a periodic topography signal. A method based on homogenization is used to calculate flow factors for all lubrication regimes. The flow factors are compared with the Patir and Cheng method. Results indicate that the two methods compare well for longitudinal roughness lay, but differ significantly for a cross-patterned surface roughness due to the more complete flow description of the current model.
|
|
| 25. |
- Sahlin, Fredrik, et al.
(författare)
-
A mixed lubrication model incorporating measured surface topography. Part 2: Roughness treatment, model validation, and simulation
- 2010
-
Ingår i: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology. - 1350-6501 .- 2041-305X. ; 224:4, s. 353-365
-
Tidskriftsartikel (refereegranskat)abstract
- A mixed lubrication flow factor model that permits real three-dimensional surface topography as input has been developed. Part 1 gives the theory of computing flow factors within the model. In this article, a method of adapting the measured surface topography signal to suit the numerical models is developed and presented in detail. The mixed lubrication model is validated through flow measurements for three different rough surface test specimens. Simulation of a hydrodynamic bearing was conducted and the results are presented in terms of pressure distributions and Stribeck curves covering all lubrication regimes. The results indicate that the model may be an efficient and accurate engineering design and research tool for tribological devices operating in all lubrication regimes.
|
|
| 26. |
- Sahlin, Fredrik
(författare)
-
Hydrodynamic lubrication of rough surfaces
- 2005
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interacting surfaces are frequently found in mechanical systems and components. A lubricant is often added between the surfaces to separate them from mechanical contact in order to increase life and performance of the contacting surfaces. In this work various aspects of hydrodynamic lubrication are investigated theoretically. This is where interacting surfaces are completely separated by a fluid film which is often the desired operating condition of machine components when wear and friction is to be reduced. Different flow regimes can be identified within the scope of hydrodynamic lubrication. If the surfaces are separated by a thick fluid film the influence from surface asperities is small and the surfaces can be treated as smooth. If the rate of change in film thickness with respect to the spatial directions is significantly large and if the flow velocity or Reynolds number is large, the ordinary fluid mechanical approach treating viscous flow with Computational Fluid Dynamics (CFD) has to be used. CFD is used to investigate influence from the use of an artificial microscopic surface pattern on one of the two interacting surfaces. The influence from the pattern is isolated from any other pressure generating effects by keeping the interacting surfaces parallel. Results are shown for different shapes of the micro-pattern. If the Reynolds number decreases, the system enters a regime called Stokes flow where the inertia effects are neglected. The full CFD approach is compared with the Stokes for various physical and geometrical cases. If the change in film thickness is small in the spatial directions, the thin film approximation is applicable and the full momentum equations describing fluid flow together with the mass continuity equation can be reduced to the Reynolds equation. Depending on boundary conditions, low pressures can occur at location of expanding fluid gap leading to tensile stress applied to the lubricant. However, a real liquid lubricant can only resist small tensile stresses until it cavitates into a mixture of gas and liquid. This often happens close to atmospheric pressure due to contamination and dissolved air into the liquid and occurs at higher pressures than the actual vaporization. To avoid pressures reaching too low levels, a general cavitation algorithm applied to the Reynolds equation is presented that accommodates for an arbitrary density-pressure relation. It is now possible to model the compressibility of the lubricant in such a way that the density-pressure relation is realistic through out the contact. The algorithm preserves mass continuity which is of importance when inter-asperity cavitation of rough surfaces is considered. For small film thicknesses the surface roughness becomes important in the performance of the lubricated contact. Even the smoothest of real surfaces is rough at a microscopic level and will influence the contact condition. The Reynolds equation still applies since the heights of the surface asperities are small compared to the spatial elongation. Treatment of the roughness of a real surface in a deterministic fashion is however beyond the scope of today's computers. Therefore other approaches need to be employed in order to take the surface roughness into account. In this work a homogenization method is used where the governing equation of the flow condition is formulated with a two-scale expansion, the global geometry and the roughness. Solutions are achieved for the limit of the roughness wavelength approaching zero and the method renders a possibility to treat the two scales separately. A method to generate dimensionless flow factors compensating for the surface roughness is developed. The flow factors, once solved for a particular surface, can be used to compensate for the surface roughness in any smooth global problem for any film thickness.
|
|
| 27. |
- Sahlin, Fredrik
(författare)
-
Lubrication, contact mechanics and leakage between rough surfaces
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mechanical components with interacting surfaces are central in everyday life. Interactions between surfaces, such as the sliding, rolling and bouncing, on one another may be found everywhere. These surfaces are often required to withstand severe conditions resulting in wear, which may ultimately lead to failure. To reduce the risk of failure, a lubricant is often added between the surfaces that partly or completely separates the surfaces from direct mechanical contact. The physical conditions existing between two interacting surfaces are complex, with parameters such as colliding surface asperities, mechanical deformations, lubricant flow transport and chemical reactions. A combination of all these parameters will affect the output parameters of interest for the consumer, e.g., friction, wear rate and leakage a.k.a. energy efficiency, service life and environmental impact in other terminologies. To design components with improved performance, more knowledge of the tribological (interacting surfaces) interface is needed both from experimental and theoretical viewpoint. In this work, various aspects of the lubrication between surfaces were theoretically simulated, to gain a greater understanding of tribological interfaces and to develop tribological design tools. When the interacting surfaces are separated by a thick fluid film, the influence from surface asperities is small and the surfaces can be treated as if they were smooth. For this type of lubrication condition, Computational Fluid Dynamics (CFD) is used to investigate the influence from a surface pattern applied onto one of the two interacting surfaces. It is shown that parallel surfaces generate a pressure increase originating from fluid inertia between the surfaces as a result of introducing the micro-pattern. In some lubricant films low pressures may occur at region of an expanding gap between the interacting surfaces. A liquid lubricant can only resist small tensile stresses until it cavitates into a mixture of gas and liquid. Hence, a cavitation model is presented that accommodates for an arbitrary lubricant compressibility. It was found that the geometry and lubricant starvation at the inlet of the tribological interface, as well as the compressibility model, are significant factors for load carrying capacity of the lubricant when cavitation is considered. For thin lubricant films, surface roughness becomes important in the performance of the tribological interface. Direct numerical simulations of the interface with measured surface roughness requires too many degrees of freedoms to be accounted for in computations. Therefore, a homogenization method is used, where the gap between the surfaces in the tribological interface may be modeled by two scales; a global geometry scale and a local surface roughness scale, where the method enables the two scales to be treated separately. A method to generate dimensionless flow factors to compensate for the surface roughness is developed. The flow factors, once solved for a particular surface, can be used to compensate for the surface roughness in any smooth global problem for any film thickness. It is shown that the cumulative distribution of heights of the surface roughness (bearing ratio) completely determines the lubricating conditions for two-dimensional roughness and that the effects of the roughness increase as the film thickness decreases. By further decreasing the film thickness, into the mixed lubrication regime, the surface asperities will start to collide and take over some of the load carried by the fluid. The surface roughness has a crucial influence on the performance in this regime. A model to simulate the linear elastic perfectly plastic deformation of rough surfaces is developed. The model is based on FFT to improve the computational efficiency. Thus, the model is suitable to accept periodic input, which is a demand for the homogenization method previously mentioned. To consider both the asperity collisions and the hydrodynamic effects, a mixed lubrication model is developed capable of using three-dimensional measured surface roughness as input. The model is based on computing flow factors that carry the effects of a specific surface roughness in all regimes from completely separated surfaces to dry contact and full asperity deformation. An efficient simulation procedure is described, from importing the roughness measurement data to the simulation of a complete application. Linear elastic perfectly plastic displacement is considered and a homogenization method for fluid transport is used. The mixed lubrication model is validated through experiments with good correlation. It is shown that real deterministic surface roughness measurements may be efficiently imported and used in the model. Also, any global geometry may be simulated in any regime once the rough surface flow factors have been calculated. An important property of a tribological interface, especially in seals, is the flow or leakage through the contact. In many applications, leakage is crucial in terms of environmental impact. By using the mixed lubrication model previously developed, leakage through a range of measured elastomer and seal surfaces is investigated (see cover figure). It is found that the mixed lubrication model permits an efficient analysis of the leakage, even though real measured surface roughness was used as input. Moreover, the valley roughness parameters are shown to be important in characterizing the leakage and the peak roughness parameters are important for the percolation threshold.
|
|
| 28. |
- Sahlin, Fredrik, et al.
(författare)
-
Rough surface flow factors in full film lubrication based on a homogenization technique
- 2007
-
Ingår i: Tribology International. - : Elsevier BV. - 0301-679X .- 1879-2464. ; 40:7, s. 1025-1034
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes a method to compute the flow factors that compensate for an arbitrary surface roughness in a compressible hydrodynamic lubrication based on a homogenization technique. The Reynolds equation is used as the governing equation and the two-scale expansion involved in the homogenization process enables the local roughness scale to be treated separately from the global geometry scale. With this method, it is possible to compute the flow factors for any deterministic roughness. Measured two-dimensional surface profiles are used as examples. Profiles having the same Abbot curve are also shown to have the same flow factors, providing an efficient classification of surfaces in hydrodynamic two-dimensional contacts. Flow factors are computed for the rough surface profiles, and solutions for global bearing geometry are obtained and compared with the corresponding solutions from a smooth geometry.
|
|
| 29. |
- Sahlin, Fredrik, et al.
(författare)
-
Two-Dimensional CFD-Analysis of Micro-Patterned Surfaces in Hydrodynamic Lubrication
- 2005
-
Ingår i: Journal of tribology. - : ASME International. - 0742-4787 .- 1528-8897. ; 127:1, s. 96-102
-
Tidskriftsartikel (refereegranskat)abstract
- Results of a numerical study of the influence of micro-patterned surfaces in hydrodynamic lubrication of two parallel walls are reported. Two types of parameterized grooves with the same order of depth as the film thickness are used on one stationary wall. The other wall is smooth and is sliding with a specified tangential velocity Isothermal incompressible two dimensional full film fluid flow mechanics is solved using a Computational Fluid Dynamics method. It is shown that, by introducing a micro-pattern on one of two parallel walls, a net pressure rise in the fluid domain is achieved. This produces a load carrying capacity on the walls which is mainly contributed by fluid inertia. The load carrying capacity increases with Reynolds number The load carrying capacity is reported to increase with groove width and depth. However at a certain depth a vortex appears in the groove and near this value the maximum load carrying capacity is achieved. It is shown that the friction force decreases with deeper and wider grooves. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance are reported.
|
|
| 30. |
- Sahlin, Johan, 1972-
(författare)
-
Om kyrklundheten : Värde, kunskap och skrivande i Willy Kyrklunds Om godheten
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The quest for certainty is a constant theme in the works of Swedish-Finnish author Willy Kyrklund. This quest is communicated both explicitly in essayistic prose articles and implicitly as a theme in his works of fiction. The late essay hybrid Om godheten (”On Goodness”, 1988) is a key text in Kyrklund’s oeuvre in this regard as it consists of a mediation between these strategies of communication, the philosophical and the literary. With Om godheten as a starting-point it is the purpose of this study to bring the problem of knowledge to the fore by acknowledging its importance for Kyrklund’s strategies of writing and for his treatment of philosophical questions regarding for example morals and identity. The purpose is also to identify Kyrklund’s use of literature as a cognitive tool with which the reader is lead to reflect on the premises for knowledge and to view the act of reading as parallel to the constructing of knowledge. Furthermore the purpose is to acknowledge the importance of analytical philosophy and natural science in this literary strategy. I consider Finnish philosopher Eino Kaila’s version of logical empiricism an important source of influence for Kyrklund. The gap between empirical (the appeal to experience) and rationalistic (the attempt to generalize) strategies in the strive for knowledge is pointed out as the root problem for logical empiricism by Kaila and this problem has also been touched upon explicitly by Kyrklund. While Kyrklund seems to subscribe to general empiricistic or logical-empiricistic virtues such as empiristic scrutiny, the appeal to science and the rejection of metaphysics, his work does not, however, offer any solutions of how to overcome this gap in any specific logical-empiricistic way. I recognize this gap, as well as the parallel gap between a subjective and an objective standpoint, as a figure of thought in Kyrklund’s literary thinking. Examined in three thematic chapters concerning value, knowledge and writing respectively, the mediation between empiristic and rationalistic tendencies can be found on different levels in Om godheten: in evolutionary approaches to morals versus the teleological approach to morals, in descriptive meta-ethical naturalism versus normative naturalistic ethics, in observations versus laws of nature, in individual expression versus common language, in instances versus generalizing examples and in explorative essayistic modes of writing versus a sovereign metafictive authoral position, to name a few. In putting together traditional text types, genres and modes of writing in new contexts of thought Kyrklund enables a certain way of seeing: combined with several intertextual relations the literary process thus stands out as a parallel to the cognitive process and the epistemological inferiority, where no fundament for certain knowledge is possible, is communicated as an existential problem.
|
|
