Direct numerical simulation of Taylor-Couette flow with grooved walls: torque scaling and flow structure

TL;DR

This study uses direct numerical simulations to analyze how grooved walls affect torque scaling and flow structures in Taylor-Couette flow, revealing boundary layer interactions, flow transitions, and torque increases due to grooves.

Contribution

It provides new insights into the impact of grooved walls on flow dynamics and torque scaling in Taylor-Couette systems, especially at high Reynolds numbers.

Findings

Torque remains unchanged when groove height is below boundary layer thickness.

Flow transitions involve plume ejection and secondary circulations at high Ta.

Torque increases with larger grooves, even after reaching the ultimate regime.

Abstract

We present direct numerical simulations of Taylor-Couette flow with grooved walls at a fixed radius ratio $\eta=r_i/r_o=0.714$ with inner cylinder Reynolds number up to $Re_i=3.76\times10^4$, corresponding to Taylor number up to $Ta=2.15\times10^9$. The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of $90^\circ$. Results are compared to the smooth wall case in order to investigate the effects of grooves on Taylor-Couette flow. We focus on the effective scaling laws for the torque, flow structures, and boundary layers. It is found that, when the groove height is smaller than the boundary layer thickness, the torque is the same as that of the smooth wall cases. With increasing $Ta$, the boundary layer thickness becomes smaller than the groove height. Plumes are ejected from the tips of the grooves and secondary circulations between the latter are formed. This is associated to a sharp increase of the torque and thus the effective scaling law for the torque vs. $Ta$ becomes much steeper. Further increasing $Ta$ does not result in an additional slope increase. Instead, the effective scaling law saturates to the "ultimate" regime effective exponents seen for smooth walls. It is found that even though after saturation the slope is the same as for the smooth wall case, the absolute value of torque is increased, and the more the larger size of the grooves.