Turbulence is omnipresent in Nature and technology, governing the transport of heat, mass, and momentum on multiple scales. For real-world applications of wall-bounded turbulence, the underlying surfaces are virtually always rough; yet characterizing and understanding the effects of wall roughness for turbulence remains a challenge, especially for rotating and thermally driven turbulence. By combining extensive experiments and numerical simulations, here, taking as example the paradigmatic Taylor-Couette system (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. If only one of the walls is rough, we reveal that 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 thoroughly eliminated in the boundary layers and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of transport, whose existence had been predicted by Robert Kraichnan in 1962 (Phys. Fluids 5, 1374(1962) and in which the scalings laws can be extrapolated to arbitrarily large Reynolds numbers.

In the maritime industry, the injection of air bubbles into the turbulent boundary layer under the ship hull is seen as one of the most promising techniques to reduce the overall fuel consumption. However, the exact mechanism behind bubble drag reduction is unknown. Here we show that bubble drag reduction in turbulent flow dramatically depends on the bubble size. By adding minute concentrations (6 ppm) of the surfactant Triton X-100 into otherwise completely unchanged strongly turbulent Taylor-Couette flow containing bubbles, we dramatically reduce the drag reduction from more than 40% to about 4%, corresponding to the trivial effect of the bubbles on the density and viscosity of the liquid. The reason for this striking behavior is that the addition of surfactants prevents bubble coalescence, leading to much smaller bubbles. Our result demonstrates that bubble deformability is crucial for bubble drag reduction in turbulent flow and opens the door for an optimization of the process.Theoretical, numerical and experimental studies on drag reduction (DR) of a solid body moving in a turbulent flow have been performed for more than three decades [1][2][3][4][5][6]. A few volume percent (≤ 4%) of bubbles can reduce the overall drag up to 40% and beyond [7][8][9][10][11][12][13][14]. However, the exact physics behind this drag reduction mechanism is unknown, thus hindering further progress and optimization, and even the dependence of the effect on the bubble size is controversial [15][16][17], though it is believed to be independent of the bubble size [1].In this Letter, we experimentally investigated the mechanism behind bubble drag reduction in a TaylorCouette (TC) system, i.e. the flow between two independently rotating coaxial cylinders. The TC system can be seen as "drosophila" of physics of fluids, with many concepts in fluid dynamics being tested therewith, ranging from instabilities, to pattern formation, to turbulence, see the reviews [18,19]. Here we inject bubbles into the system, which due to the density difference to water experience a centripetal force towards the inner cylinder, mimicking the upwards gravitational force acting on bubbles under a ship hull.The experiments are performed in the Twente Turbulent Taylor-Couette facility (T 3 C) [20], with the inner one strongly rotating, corresponding to very large Reynolds number of Re ∼ 10 5 − 10 6 . The setup has an inner cylinder with a radius of r i = 200 mm and an outer cylinder with a radius of r o = 279 mm, resulting in a radius ratio of η = r i /r o = 0.716. The inner cylinder rotates with a frequency up to f i = 20 Hz, resulting in Reynolds numbers up to Re = 2πf i r i (r o − r i )/ν α = 2 × 10 6 , in which ν α is kinematic viscosity of water-bubble mixture. The outer cylinder is at rest. The cylinders have a height of L = 927 mm, resulting in an aspect ratio of Γ = L/(r o − r i ) = 11.7. The flow is cooled through both endplates to prevent viscous heating through the viscous dissipation. The torque τ is measured with a co-axial torque transducer (Ho...

Practically all flows are turbulent in nature and contain some kind of irregularly-shaped particles, e.g. dirt, pollen, or life forms such as bacteria or insects. The effect of the particles on such flows and vice-versa are highly non-trivial and are not completely understood, particularly when the particles are finite-sized. Here we report an experimental study of millimetric fibers in a strongly sheared turbulent flow. We find that the fibers show a preferred orientation of −0.38π ± 0.05π (−68 ± 9 • ) with respect to the mean flow direction in high-Reynolds number Taylor-Couette turbulence, for all studied Reynolds numbers, fiber concentrations, and locations. Despite the finite-size of the anisotropic particles, we can explain the preferential alignment by using Jefferey's equation, which provides evidence of the benefit of a simplified point-particle approach. Furthermore, the fiber angular velocity is strongly intermittent, again indicative of point-particle-like behavior in turbulence. Thus large anisotropic particles still can retain signatures of the local flow despite classical spatial and temporal filtering effects.

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 Re i = 5 × 10 5 ). 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 15 Wo 114. 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.

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 (Rei = 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.Turbulence is a phenomenon far from equilibrium: Turbulent flow is driven in one or the other way by some energy input and at the same time energy is dissipated, predominantly (but not exclusively) at the smaller scales. For statistically stationary turbulence, this balance is reflected in the famous picture of the Richardson-Kolmogorov energy cascade [1,2]. While the driving on large scales clearly is non-universal, depending on the flow geometry and stirring mechanism, the energy dissipation mechanism has been hypothesized to be self-similar [3][4][5][6][7][8].How exactly is the energy taken out of the system? A good way to find out is to turn off the driving and follow the then decaying turbulence, as then all scales are probed during the decay process. This has been done in various studies over the last decades for homogeneous isotropic turbulence (HIT). Experimentally, the focus of attention was on grid-induced turbulence [8][9][10][11][12][13][14][15], whereas in numerical simulations periodic boundary conditions were used [16][17][18][19]. To what degree the decay of the turbulence depends on the initial conditions [20][21][22] and whether or not it is selfsimilar has controversially been debated [5,11,16,[23][24][25][26][27]. We note that for HIT, already from dimensional analysis one obtains power laws for the temporal evolution of the vorticity and kinetic energy in decaying turbulence, namely ω(t) ∝ t −3/2 and k(t) ∝ t −2 , respectively, in good agreement with many measurements [10,12,28]. These scaling laws are also obtained [29] when employing the 'variable range mean field theory' of Ref. [30], developed for HIT. In that way, the late-time behavior, when the flow is already viscosity dominated, can also be calculated, allowing for the calculation of the lifetime of the decaying turbulence [29].However, real turbulence is neither homogeneous nor isotropic, but it has anisotropies and is wall-bounded, with a considerable fraction of the dissipa...

We report on the modification of drag by neutrally buoyant spherical finite-sized particles in highly turbulent Taylor-Couette (TC) flow. These particles are used to disentangle the effects of size, deformability, and volume fraction on the drag, and are contrasted with the drag in bubbly TC flow. From global torque measurements we find that rigid spheres hardly decrease or increase the torque needed to drive the system.The size of the particles under investigation have a marginal effect on the drag, with smaller diameter particles showing only slightly lower drag. Increasing the particle volume fraction shows a net drag increase, however this increase is much smaller than can be explained by the increase in apparent viscosity due to the particles. The increase in drag for increasing particle volume fraction is corroborated by performing laser Doppler anemometry where we find that the turbulent velocity fluctuations also increase with increasing volume fraction. In contrast with rigid spheres, for bubbles the effective drag reduction also increases with increasing Reynolds number. Bubbles are also much more effective in reducing the overall drag.

In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at 5.0 × 10 5 Re 1.7 × 10 6 over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder -coated with a rough, hydrophobic material -is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction α present in the working fluid. For small volume fractions (α < 4 %), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air (α 4 %), the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at place: on the one hand the roughness invokes an extension of the log-layer -resulting in an increase in drag -and on the other hand there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration α = 6 % we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.

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 α = 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.