Viscoelastic thin film lubrication in finite width channels

TL;DR

This paper develops simplified models to analyze how viscoelasticity influences load capacity in thin film lubrication within finite channels, revealing significant spanwise effects and the importance of channel aspect ratio.

Contribution

It introduces reduced-order models for viscoelastic thin film lubrication that account for spanwise effects and channel aspect ratio, extending previous approaches.

Findings

Viscoelasticity increases the net force on channel walls compared to Newtonian fluids.

The net force varies strongly with Deborah number and channel aspect ratio.

Spanwise effects are significant when the channel aspect ratio is around one.

Abstract

Lubricant viscoelasticity arises due to a finite polymer relaxation time ($\lambda$) and can provide beneficial effects. In applications, such as bearings, gears, biological joints, etc., where the height-to-length ratio is small ($H_0 / \ell_x$) and the shear due to the wall velocity ($U_0$) is high, a simplified two-dimensional computational analysis across the channel length and height reveals a finite increase in the load carrying capacity of the film purely due to polymer elasticity. In channels with a finite length-to-width ratio, $a$, the spanwise effects can be significant, but the resulting mathematical model is computationally intensive. In this work, we propose simpler reduced-order models, namely via a (i) first-order perturbation in the Deborah number ($\lambda U_0 / H_0$), and the (ii) viscoelastic Reynolds approach extended from \textit{Ahmed, H., \& Biancofiore, L. (2021). A new approach for modeling viscoelastic thin film lubrication. Journal of Non-Newtonian Fluid Mechanics, 292, 104524}. We predict the variation in the net vertical force exerted on the channel walls (for a fixed film height) versus increasing viscoelasticity, and the channel aspect ratio. The models predict an increase in the net force, which is zero for the Newtonian case, versus both the Deborah number and the channel aspect ratio. Interestingly, for a fixed $De$, this force varies strongly between the two limiting cases (i) $a << 1$; an infinitely wide, and (ii) $a >> 1$; an infinitely short channel channel, implying a change in the polymers response. Furthermore, we observe a different trend (i) for a spanwise varying channel, in which a peak is observed between the two limits, and (ii) for a spanwise uniform channel, where the largest load value is for $a << 1$. When $a$ is O($1$), the viscoelastic response varies strongly and spanwise effects cannot be ignored.