Browsing by Subject "Hydrodynamic lubrication"

Now showing 1 - 8 of 8

Open Access
A numerical algorithm to model wall slip and cavitation in two-dimensional hydrodynamically lubricated contacts
(Elsevier Ltd, 2023-03-23) Çam, Mert Yusuf; Giacopini, M.; Dini, D.; Biancofiore, Luca
Hydrodynamic lubrication takes a fundamental role in mechanical systems to reduce energy losses and prevent mechanical breakdown. The analytic instrument usually adopted to describe hydrodynamic lubrication is the Reynolds equation, which in its simplest statement for monophase lubricants and with assuming no fluid slip at the walls, is a linear equation in the hydrodynamic pressure. However, this classical linear Reynolds equation cannot reflect all the lubricant characteristics in engineered surfaces (e.g. superhydro(oleo)phobic surfaces and textured surfaces). In these cases, the effect of two critical factors, such as wall slip and cavitation, need to be considered, introducing non-linearities in the system. In order to tackle this issue, a modified two-dimensional Reynolds equation is introduced, able to capture both the cavitation presence, via a complementary mass-conserving model, and wall slippage, starting from the multi-linearity description introduced by Ma et al. (2007). In addition, an alternative model for the slippage at the wall is proposed by modifying the multi-linearity wall slip model to improve accuracy and computational cost. In this new model, the possible slip directions are limited to three, separated by equal angles, with the slip occurring only along the first direction, and the other directions, then, used to iteratively adjust the direction of slippage, until a suitable convergence criterion is satisfied. The proposed mathematical model is validated versus results available in literature with tests performed on (i) journal bearings, (ii) slider bearings, (iii) squeeze dampers, and (iv) surface textured bearings. By conducting these tests, the proposed alternative wall slip model is proved to be up to one order of magnitude more computational efficient than the original multi-linearity wall slip model.
Open Access
Active lubrication interfaces with tunable micro-textures
(2023-07) Pekol, Sena
This thesis investigates a homogenization-based space-time optimization framework in the context of hydrodynamic lubrication in order to design micro-textures which can be actively controlled through external stimuli. The response at the interface is established via the Reynolds equation to describe the physics of the lubrication for a small film thickness. Subsequently, the interface is subjected to multiscale analysis and effective macroscopic parameters are derived via homogenization method. In order to calculate the macroscopic parameters, Finite Element (FE) formulation is employed and the implementation of the parameters in the in-house FE code is demonstrated. For the suboptimality problem due to typically employed fixed unit cell in FE analysis, a geometry optimization scheme is developed. Thereafter, a sensitivity based topology optimization framework is introduced with the aim of identifying the spatial distribution and temporal variation of the micro-texture, and the shape of the unit cell which together help achieve the targeted lubrication response. The performance of the employed framework is assessed through objectives which ultimately determine the macroscopic flux at the interface as well as the frictional traction that is associated with the macroscopic dissipation at the interface. Finally, three-dimensional realizations are constructed for active micro-textures by adopting a readily deployable experimental architecture.
Open Access
The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation
(Elsevier, 2021-03-27) Gamaniel, Samuel Shari; Dini, D.; Biancofiore, Luca
In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.
Open Access
Homogenization in hydrodynamic lubrication: microscopic regimes and re-entrant textures
(American Society of Mechanical Engineers (ASME), 2018) Yildiran, I. N.; Temizer, I.; Çetin B.
The form of the Reynolds-type equation which governs the macroscopic mechanics of hydrodynamic lubrication interfaces with a microscopic texture is well-accepted. The central role of the ratio of the mean film thickness to the texture period in determining the flow factor tensors that appear in this equation had been highlighted in a pioneering theoretical study through a rigorous two-scale derivation (Bayada and Chambat, 1988, "New Models in the Theory of the Hydrodynamic Lubrication of Rough Surfaces," ASME J. Tribol., 110, pp. 402-407). However, the resulting homogenization theory still remains to be numerically investigated. For this purpose, after a comprehensive review of the literature, three microscopic regimes of lubrication will be outlined, and the transition between these three regimes for different texture types will be extensively demonstrated. In addition to conventional textures, representative re-entrant textures will also be addressed. Copyright
Open Access
Homogenization-based design of surface textures in hydrodynamic lubrication
(John Wiley and Sons Ltd, 2016) Waseem, A.; Temizer, İ.; Kato, J.; Terada, K.
An optimization framework is developed for surface texture design in hydrodynamic lubrication. The microscopic model of the lubrication interface is based on the Reynolds equation, and the macroscopic response is characterized through homogenization. The microscale setting assumes a unilateral periodic texture but implicitly accounts for the bilateral motion of the surfaces. The surface texture in a unit cell is described indirectly through the film thickness, which is allowed to vary between prescribed minimum and maximum values according to a morphology variable distribution that is obtained through the filtering of a design variable. The design and morphology variables are discretized using either element-wise constant values or through first-order elements. In addition to sharp textures, which are characterized by pillars and holes that induce sudden transitions between extreme film thickness values, the framework can also attain a variety of non-standard smoothly varying surface textures with a macroscopically isotropic or anisotropic response. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
Open Access
Homogenization-based microscopic texture design and optimization in hydrodynamic lubrication
(2016-08) Waseem, Abdullah
The aim of this thesis is to develop an optimization framework for the texture optimization in hydrodynamic lubrication using multi-scale homogenization technique. In hydrodynamic lubrication the asperities do not come into contact due to fluid film present between the surfaces and normal load is carried by the viscous fluid. The Reynolds equation can be used with confidence for such problems. For two-scale separation, a basis for optimizing the surface textures is established through an asymptotic expansion based homogenization scheme, which delivers a macroscopic Reynolds equation containing homogenized coefficients. These homogenized coefficients depend on the fluid film thickness directly and by controlling these coefficients a desired macroscopic response can be obtained. Design variables are introduced to control the fluid film thickness indirectly through an intermediate filtering stage. Both microscopic and macroscopic objectives are defined for texture optimization. The quality of the designed textures are evaluated numerically as well as aesthetically and optimization parameters are selected accordingly. Isotropic and anisotropic textures can be designed by using the proposed optimization scheme. For both microscopic and macroscopic objectives optimization surface textures are reconstructed as a sanity check. Texture optimization for prescribed load bearing capacity and maximum load bearing capacity in temporal and spatial variations are then carried out for squeeze film flow and wedge problem, respectively. Finally, to reduce the computational cost, Taylor’s expansion is proposed for the optimization problem. Overall, the methodology developed in this thesis froms a basis for a comprehensive micro-texture design framework for computational tribology.
Open Access
A numerical algorithm to model wall slip and cavitation in two-dimensional hydrodynamically lubricated contacts
(2022-01) Çam, Mert Yusuf
Hydrodynamic lubrication takes a fundamental role in mechanical systems to reduce energy losses and prevent mechanical breakdown. In order to model hydrodynamic lubrication in thin films the solution of the Reynolds equation is required. However, the Reynolds equation cannot re ect all the lubricant characteristics in thin films. The effects of two critical factors, wall slip and cavitation, need to be considered. A new numerical solution of the Reynolds equation is presented to model two-dimensional hydrodynamic lubrication, including the linear complementary mass-conserving cavitation model and multi-linearity wall slip model. In addition, a new wall slip model has been proposed by modifying the multi-linearity wall slip model to make it more computationally affordable. The proposed mathematical model has been validated against available models in literature with the tests performed on journal bearings, slider bearings, squeeze dampers, and surface textured bearings. The proposed novel wall slip model is up to 9 times more computational affordable than the original multi-linearity wall slip model.
Open Access
Viscoelastic effects in lubricated contacts in the presence of cavitation
(2020-11) Gamaniel, Samuel Shari
A model is proposed to study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Previous studies on viscoelastic lubricants did not consider the presence of cavitation, rather reported negative pressures in regions where cavitation was expected to occur. The proposed model uses the Oldroyd-B constitutive model to describe the presence of cavitation and assumes that the Deborah number (De), the ratio between polymer relaxation time and flow time scale, is small. In doing so, the viscoelastic thin film equations can be linearised in a similar approach to what was pioneered by ”Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model.” The zeroth order solution in De corresponds to the Reynolds equation and has been modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We model several bearing configurations for the flow of viscoelastic lubricants using (i) a cosine/parabolic profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. Introducing viscoelasticity to the cavitating journal bearing decreases the length of the non-active (cavitation) region due to an increasing pressure distribution in the lubricant film. This results in an increase to the load carrying capacity with increasing De corroborating the beneficial influence of the polymers in fluid film bearings. The pocket profile is shown to either increase or decrease the load carrying capacity with increasing viscoelastic effects, depending on the location of surface texturing at the contact. An oscillating squeeze flow problem is modeled for viscoelastic lubricants between two flat plates with motion only at the top surface. A reduction in the load carrying capacity at larger values of De is observed as film reformation is seen to be retarded with increasing viscoelastic effects. As viscoelastic effects become stronger, the nonactive region is grows continuously until reaching a value of De beyond which a full film reformation does not occur upon the inception of cavitation. The study is extended to a direct numerical simulations using the openFoam toolbox. A model that couples a solver for incompressible, isothermal, two phase flow with interaction between the phases and a solver for viscoelastic fluids is proposed. However, DNS are only valid for lower values of De as instabilities occur as a result of the non-linear coupling.