Ruben A. Verschoof

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.

Self-similar decay of high Reynolds number Taylor-Couette turbulence.

We study the decay of high-Reynolds-number Taylor-Couette turbulence, i.e., the turbulent flow between two coaxial rotating cylinders. To do so, the rotation of the inner cylinder (Re i =2×10 6 , the outer cylinder is at rest) is stopped within 12 s, thus fully removing the energy input to the system. Using a combination of laser Doppler anemometry and particle image velocimetry measurements, six decay decades of the kinetic energy could be captured. First, in the absence of cylinder rotation, the flow-velocity during the decay does not develop any height dependence in contrast to the well-known Taylor vortex state. Second, the radial profile of the azimuthal velocity is found to be self-similar. Nonetheless, the decay of this wall-bounded inhomogeneous turbulent flow does not follow a strict power law as for decaying turbulent homogeneous isotropic flows, but it is faster, due to the strong viscous drag applied by the bounding walls. We theoretically describe the decay in a quantitative way by taking the effects of additional friction at the walls into account.

Affecting drag in turbulent Taylor-Couette flow

Turbulent flows are omnipresent in nature and technology. The majority of flows encountered in daily life and in industrial applications deal with rough walls and transient effects. Furthermore, many flows can be regarded as multiphase flows, i.e. the flow consisting of multiple phases of liquids, gasses and solids. Surprisingly maybe, the understanding of these flows is still limited, and many studies focus on idealised situations, which do not take the aforementioned phenomena into account. To study these types of flow, we used a Taylor-Couette system, i.e. the flow between 2 con- centric independently rotating cylinders. This system is one of the canonical flow setups in which the physics of fluids is studied, and it has been used to study a.o. pattern formation, instabilities, viscosity measurements, turbulence and multiphase flows. Taylor-Couette flow is known to be mathematically similar to Rayleigh-B´enard convection. That is, written in the correct dimensionless form, the relevant scaling laws are identical for both systems. In that sense, one can learn about Rayleigh- Benard convection by studying Taylor-Couette flow, and vice versa. In this thesis, we chose to specifically study transient effects, rough walls and air lubrication in turbulent flows, not only to increase our fundamental understanding of these of ubiquitous flows, but also to address highly relevant questions in collaboration with industrial partners. In maritime industry, the use of air lubrication is seen as a promising method to reduce the overall friction between a ship and the surrounding water, and thus the fuel consumption. However, the relevant parameters optimizing air lubrication are not yet well understood. Wall roughness is known to increase the drag, but given the enormous variety of roughness types, many open questions remain to be unanswered. The thesis is divided in three parts, i.e. Part 1: Transient turbulence, Part 2: Roughness in turbulence, and Part 3: Air lubrication in turbulent flows.

Periodically driven Taylor-Couette turbulence

We study periodically driven Taylor-Couette turbulence, i.e. the flow confined between two concentric, independently rotating cylinders. Here, the inner cylinder is driven sinusoidally while the outer cylinder is kept at rest (time-averaged Reynolds number is

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

Re_i = 5 \times 10^5

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

). Using particle image velocimetry (PIV), we measure the velocity over a wide range of modulation periods, corresponding to a change in Womersley number in the range

Video: Freezing supersonic flow by LED based Schlieren imaging

15 \leq Wo \leq 114

Bubble Drag Reduction Requires Large Bubbles

. To understand how the flow responds to a given modulation, we calculate the phase delay and amplitude response of the azimuthal velocity. In agreement with earlier theoretical and numerical work, we find that for large modulation periods the system follows the given modulation of the driving, i.e. the system behaves quasi-stationary. For smaller modulation periods, the flow cannot follow the modulation, and the flow velocity responds with a phase delay and a smaller amplitude response to the given modulation. If we compare our results with numerical and theoretical results for the laminar case, we find that the scalings of the phase delay and the amplitude response are similar. However, the local response in the bulk of the flow is independent of the distance to the modulated boundary. Apparently, the turbulent mixing is strong enough to prevent the flow from having radius-dependent responses to the given modulation.

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

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

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

DR=33\%