Dennis Bakhuis

Wall roughness induces asymptotic ultimate turbulence

Turbulence governs the transport of heat, mass and momentum on multiple scales. In real-world applications, wall-bounded turbulence typically involves surfaces that are rough; however, characterizing and understanding the effects of wall roughness on turbulence remains a challenge. Here, by combining extensive experiments and numerical simulations, we examine the paradigmatic Taylor–Couette system, which describes the closed flow between two independently rotating coaxial cylinders. We show how wall roughness greatly enhances the overall transport properties and the corresponding scaling exponents associated with wall-bounded turbulence. We reveal that if only one of the walls is rough, the bulk velocity is slaved to the rough side, due to the much stronger coupling to that wall by the detaching flow structures. If both walls are rough, the viscosity dependence is eliminated, giving rise to asymptotic ultimate turbulence—the upper limit of transport—the existence of which was predicted more than 50 years ago. In this limit, the scaling laws can be extrapolated to arbitrarily large Reynolds numbers.Turbulence is seldom confined by boundaries that are perfectly smooth, but wall roughness is usually ignored. A study of flows between rotating cylinders suggests that roughness enhances turbulent transport and alters its scaling behaviour.

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

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

Rough-wall turbulent Taylor-Couette flow: The effect of the rib height

DR=33\%

Mixed insulating and conducting thermal boundary conditions in Rayleigh-Bénard convection

and

Air cavities at the inner cylinder of turbulent Taylor–Couette flow

DR=23\%

Finite-sized rigid spheres in turbulent Taylor-Couette flow: Effect on the overall drag

, respectively, for a void fraction of

Finite-sized rigid spheres in turbulent Taylor-Couette flow.

\alpha=6\%

Anisotropic particles in highly turbulent Taylor-Couette flow

. However, with ribs mounted on both cylinders or on the outer cylinder only, the drag reduction is weak, less than

Turbulent Taylor-Couette flow

DR=11\%

Rigid spherical particles in highly turbulent Taylor-Couette flow

, 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.