Joint paper in PRL: Finite-size effects lead to supercritical bifurcations in turbulent rotating Rayleigh-Benard convection by Stephan Weiss, Richard J.A.M. Stevens, Jin-Qiang Zhong, Herman J. H. Clercx, Detlef Lohse and Guenter Ahlers, Phys. Rev. Lett. 105, 224501 (2010)

Abstract: In turbulent thermal convection in cylindrical samples with an aspect ratio Γ≡D/L (D is the diameter and L the height), the Nusselt number Nu is enhanced when the sample is rotated about its vertical axis because of the formation of Ekman vortices that extract additional fluid out of thermal boundary layers at the top and bottom. We show from experiments and direct numerical simulations that the enhancement occurs only above a bifurcation point at a critical inverse Rossby number 1/Ro_c, with 1/Ro_c∝1/Γ. We present a Ginzburg-Landau–like model that explains the existence of a bifurcation at finite 1/Ro_c as a finite-size effect. The model yields the proportionality between 1/Ro_c and 1/Γ and is consistent with several other measured or computed system properties.

Paper in PRL: Resonant enhancement of turbulent dissipation by Ergun Cekli, Carl Tipton and Willem van de Water, Phys. Rev. Lett 105, 044503 (2010).

Abstract: We periodically modulate a turbulent wind-tunnel flow with an active grid. We find a resonant enhancement of the mean turbulent dissipation rate at a modulation frequency which equals the large-eddy turnover rate. Thus, we find the best frequency to inject energy into a turbulent flow. The resonant response is characterized by the emergence of vortical structures in the flow and depends on the spatial mode of the stirring grid.

Focus on Fluids: Formation of coherent structures by fluid inertia in three-dimensional laminar flows by Z. Pouransari, M. F. M. Speetjens and H. J. H. Clercx, J. Fluid Mech. (2010), 654, pp. 5–34.

Mixing under laminar flow conditions is key to a wide variety of industrial fluid systems of size extending from micrometres to metres. Profound insight into three-dimensional laminar mixing mechanisms is essential for better understanding of the behaviour of such systems and is in fact imperative for further advancement of (in particular, microscopic) mixing technology. This insight remains limited to date, however. The present study concentrates on a fundamental transport phenomenon relevant to laminar mixing: the formation and interaction of coherent structures in the web of three-dimensional paths of passive tracers due to fluid inertia. Such coherent structures geometrically determine the transport properties of the flow and thus their formation and topological structure are essential to three-dimensional mixing phenomena. The formation of coherent structures, its universal character and its impact upon three-dimensional transport properties is demonstrated by way of experimentally realizable time-periodic model flows. Key result is that fluid inertia induces partial disintegration of coherent structures of the non-inertial limit into chaotic regions and merger of surviving parts into intricate three-dimensional structures. This response to inertial perturbations, though exhibiting great diversity, follows a universal scenario and is therefore believed to reflect an essentially three-dimensional route to chaos. Furthermore, a first outlook towards experimental validation and investigation of the observed dynamics is made.

This paper  was selected for a Focus on Fluids in the same issue: Coherent structures and chaotic advection in three dimensions 
by Stephen Wiggins

Abstract: In the 1980s the incorporation of ideas from dynamical systems theory into theoretical fluid mechanics, reinforced by elegant experiments, fundamentally changed the way in which we view and analyse Lagrangian transport. The majority of work along these lines was restricted to two-dimensional flows and the generalization of the dynamical systems point of view to fully three-dimensional flows has seen less progress. This situation may now change with the work of Pouransari et al. (J. Fluid Mech., this issue, vol. 654, 2010, pp. 5–34) who study transport in a three-dimensional time-periodic flow and show that completely new types of dynamical systems structures and consequently, coherent structures, form a geometrical template governing transport.

Joint paper in PRL: Transitions between turbulent states in rotating Rayleigh-Benard convection, by Richard Stevens, Jin-Qiang Zhong, Herman Clercx, Guenter Ahlers and Detlef Lohse.

Weakly-rotating turbulent Rayleigh-B\'enard convection was studied experimentally and numerically. With increasing rotation and large enough Rayleigh number an abrupt transition from a turbulent state with nearly rotation-independent heat transport to another turbulent state with enhanced heat transfer is observed at a critical inverse Rossby number 1/Ro_c \simeq 0.4. The strength of the large-scale convection-roll is either enhanced or essentially unmodified depending on parameters for 1/Ro < 1/Ro_c, but the strength increasingly diminishes beyond 1/Ro_c where it competes with Ekman vortices that cause vertical fluid transport and thus heat-transfer enhancement.

link to Physical Review Letters

Joint paper in PRL: "Prandtl-, Rayleigh-, and Rossby-Number Dependence of Heat transport in Turbulent Rotating Rayleigh-Benard Convection", by Jin-Qiang Zhong, Richard J. A. M. Stevens, Herman J. H. Clercx, Roberto Verzicco, Detlef Lohse, and Guenter Ahlers.

Experimental and numerical data for the heat transfer as a function of the Rayleigh, Prandtl, and Rossby numbers in turbulent rotating Rayleigh-Bénard convection are presented. For relatively small Ra=~10⁸ and large Pr modest rotation can enhance the heat transfer by up to 30%. At larger Ra there is less heat-transfer enhancement, and at small Pr <~ 0.7 there is no heat-transfer enhancement at all. We suggest that the small-Pr behavior is due to the breakdown of the heat-transfer-enhancing Ekman pumping because of larger thermal diffusion.

 link to Physical Review Letters

Paper in PRL:Enhanced vertical inhomogeneity in turbulent rotating convection", by Rudie Kunnen, Herman Clercx en Bernard Geurts.

Rotating Rayleigh-Bénard convection, a rotating layer of fluid heated from below and cooled from above, is a simple model for many geophysical flows. Using Stereoscopic Particle Image Velocimetry (SPIV) we have performed local measurements of velocity inside a convection cell. This novel technique measures full three-component velocity vectors simultaneously in many positions in a planar cross-section of the fluid. We have for the first time experimentally considered the anisotropy of rotating convection, using the invariants of the anisotropy tensor and the so-called Lumley triangle. It is found that rotation enhances anisotropy in the centre of the cell, while anisotropy is reduced near the bottom and top plates. There is enhanced vertical inhomogeneity. The inhomogeneity is surprising when considering the Taylor-Proudman theorem, which states that all vertical gradients must disappear under strong rotation.

link to Physical Review Letters 

Paper in PRL: Dispersion of Inertial particles in stratified turbulence, by Marleen van Aartrijk and Herman Clercx.

The effect of preferential concentration of heavy particles in a homogeneous stably stratified turbulent flow is studied by means of direct numerical simulations. Particle distributions show different clustering patterns in horizontal and vertical directions, thereby representing the anisotropy of the flow. Preferential concentration in stably stratified turbulence can be quantified using 2D and 3D radial distribution functions and the correlation dimension D₂. With increasing stratification strength, the effect of preferential concentration decreases. Furthermore, it is found that in stably stratified turbulence preferential accumulation is enhanced when gravitational forces act on heavy particles.

link to Physical Review Letters

Paper in PRL: Turbulence of a free surface, by Ralph Savelsberg and Willem van de Water.

We study the free surface of a turbulent channel flow, in particular, the relation between the statistical properties of the wrinkled surface and those of the velocity field beneath it. For an irregular flow shed off a vertical cylinder, surface indentations are strongly correlated with vortices in the subsurface flow. For fully developed turbulence this correlation is dramatically reduced. This is because the large eddies excite random capillary-gravity waves that travel in all directions across the surface. Both their predominant wavelength and their anisotropy are determined by the subsurface turbulence.

link to Physical Review Letters