Browsing by Subject "Cavitation"

Now showing 1 - 6 of 6

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
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
Interplay between wall slip and cavitation: a complementary variable approach
(Elsevier, 2019) Biancofiore, Luca; Giacopini, M.; Dini, D.
In this work a stable and reliable numerical model based on complementary variables is developed to study lubricated contacts characterised by slip at one or both surfaces and in the presence of cavitation. This model can be used to predict surface behaviour when cavitation induced by e.g. the presence of surface texture, slip, or a combination of the two is encountered, with varying surface parameters. For this purpose, two different algorithms are coupled to predict the formation of cavitation, through a mass-conserving formulation, and the presence of slip at the wall. The possible slippage is described by a limiting shear criterion formulated using a Tresca model. To show the flexibility of our model, several bearing geometries have been analysed, such as a twin parabolic slider, a cosine profile used to mimic a bearing, and a pocketed slider bearing employed to study the effect of surface texture. We observe that the lubrication performance (i.e. low friction coefficient) can be improved by using materials that promote slippage at the moving wall. The location of the slippage region can be optimised to find the lowest value of friction coefficient. Our theoretical developments and numerical implementation are shown to produce useful guidelines to improve and optimise the design of textured superoleophobic surfaces in the presence of lubricated contacts.
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
Numerical and experimental investigation of the effect of channel geometry on cavity formation in microfluidic channels
(2020-06) Özkazanç, Gökçe
Cavitation formation in low pressure regions of a ﬂow is a chaotic distortion in ﬂuid mechanics. Due to the complicated nature of multiphase ﬂows, its modelling is expensive in terms of time and computational power. Opensource softwares such as OpenFOAM reduce license expenses and provide a developer friendly environment to simulate these types of complicated ﬂows. In this thesis, by using OpenFoam software, several diﬀerent geometries that cause cavitation are investigated. Presented results are compared with both literature and supported by experimental results. Experiments are carried out in microﬂuidic chips that are fabricated with soft lithography technique; ﬂuorescent particles were introduced in the ﬂow and cavity formation was observed under a ﬂuorescent camera. Results showed that, OpenFOAM is well capable of predicting the cavitation formation in small scales. It was observed that increasing channel width reduces the pressure diﬀerence causing bubbles to form in higher input pressures. It was also seen that decreasing the channel width causes friction and viscous forces to dominate and reduce the velocity of ﬂuid preventing the cavitation to form. Overall the modelling of cavity formation in microchannels with varying width and cross sectional proﬁle were successfully accomplished and results were veriﬁed with experiments.
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.