The influence of wall roughness on bubble drag reduction in Taylor-Couette turbulence

Abstract

We experimentally study the influence of wall roughness on bubble drag reduction in turbulent Taylor-Couette flow, i.e.\ the flow between two concentric, independently rotating cylinders. We measure the drag in the system for the cases with and without air, and add roughness by installing transverse ribs on either one or both of the cylinders. For the smooth wall case (no ribs) and the case of ribs on the inner cylinder only, we observe strong drag reduction up to $DR=33\%$ and $DR=23\%$, respectively, for a void fraction of $\alpha =6\%$. However, with ribs mounted on both cylinders or on the outer cylinder only, the drag reduction is weak, less than $DR=11\%$, and thus quite close to the trivial effect of reduced effective density. Flow visualizations show that stable turbulent Taylor vortices --- large scale vortical structures --- are induced in these two cases, i.e. the cases with ribs on the outer cylinder. These strong secondary flows move the bubbles away from the boundary layer, making the bubbles less effective than what had previously been observed for the smooth-wall case. Measurements with counter-rotating smooth cylinders, a regime in which pronounced Taylor rolls are also induced, confirm that it is really the Taylor vortices that weaken the bubble drag reduction mechanism. Our findings show that, although bubble drag reduction can indeed be effective for smooth walls, its effect can be spoiled by e.g.\ biofouling and omnipresent wall roughness, as the roughness can induce strong secondary flows.

arXiv comment10 pages, 5 figures

References in corpus (12)

• High Reynolds number Taylor-Couette turbulence
• Multiple states in highly turbulent Taylor-Couette flow
• Roughness as a Route to the Ultimate Regime of Thermal Convection
• Exploring the phase diagram of fully turbulent Taylor-Couette flow
• Bubble drag reduction requires large bubbles
• The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow
• Optimal Taylor-Couette turbulence
• Microbubbly drag reduction in Taylor-Couette flow in the wavy vortex regime
• The Twente turbulent Taylor-Couette (T3C) facility: Strongly turbulent (multiphase) flow between two independently rotating cylinders
• Direct numerical simulation of Taylor-Couette flow with grooved walls: torque scaling and flow structure
• Wall roughness induces asymptotic ultimate turbulence
• Deformable ellipsoidal bubbles in Taylor-Couette flow with enhanced Euler-Lagrange tracking

Cited by in corpus (1)

• Bubbly drag reduction using a hydrophobic inner cylinder in Taylor-Couette turbulence