Viscoelastic effects in lubricated contacts in the presence of cavitation
Author
Gamaniel, Samuel Shari
Advisor
Biancofiore, Luca
Date
2021-01Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
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.
Keywords
TribologyThin films
Hydrodynamic lubrication
Viscoelasticity
Cavitation
Journal bearings
Journal bearings
Surface texture
Squeezing