Characteristics of turbulent Taylor-Couette flow of low-viscosity fluid on plastron-covered superhydrophobic surface

Abstract

This study introduces a newly developed Taylor-Couette (TC) flow system designed to investigate flow dynamics in low-viscosity fluids, such as water, under fully turbulent conditions. To ensure precise drag measurements, the system accounts for mechanical friction from bearings and von Ka<acute accent>rma<acute accent>n torque (the torque generated by fluid motion in the gap between the end-plates of the cylinders), enabling accurate evaluation of TC torque. Utilizing exact counter-rotation conditions that produce featureless turbulence, we explored the drag reduction capabilities of a hybrid superhydrophobic surface (SHS) mounted on the inner cylinder, alongside visualizing the resultant plastron formations. For the first time, two-dimensional particle image velocimetry (2D PIV) was used near the wall to quantify drag reduction based on total shear stress derived from flow visualization on SHS in a TC flow system. The plastron-induced slip conditions led to significant shifts in bulk velocity within the TC gap. A detailed analysis of Reynolds stresses revealed substantial modifications in flow dynamics, including reduced peak Reynolds stress and increased near-wall Reynolds stress, while total shear stress decreased across the gap. Additionally, simultaneous visualization and assessment of the plastron provided novel insights into its role in enhancing drag reduction. These findings underscore the importance of accounting for bearing mechanical friction in torque measurements when using low-viscosity fluids and confirm the effectiveness of SHS in modifying turbulence for drag reduction. The results highlight the TC-PIV system's robust capability for detailed fluid dynamics investigations and its potential to inform hydrodynamic drag reduction strategies.