Rpj Rudie Kunnen

Breakdown of large-scale circulation in turbulent rotating convection

Turbulent rotating convection in a cylinder is investigated both numerically and experimentally at Rayleigh number Ra=109 and Prandtl number σ=6.4. In this letter we discuss two topics: the breakdown under rotation of the domain-filling large-scale circulation (LSC) typical for confined convection, and the convective heat transfer through the fluid layer, expressed by the Nusselt number. The presence of the LSC is addressed for several rotation rates. For Rossby numbers Ro1.2 no LSC is found (the Rossby number indicates relative importance of buoyancy over rotation, hence small Ro indicates strong rotation). For larger Rossby numbers a precession of the LSC in anticyclonic direction (counter to the background rotation) is observed. It is shown that the heat transfer has a maximal value close to Ro=0.18 being about 15% larger than in the non-rotating case Ro=∞. Since the LSC is no longer present at this Rossby value we conclude that the peak heat transfer is independent of the LSC.

Experimental and numerical investigation of turbulent convection in a rotating cylinder

The effects of an axial rotation on the turbulent convective flow because of an adverse temperature gradient in a water-filled upright cylindrical vessel are investigated. Both direct numerical simulations and experiments applying stereoscopic particle image velocimetry are performed. The focus is on the gathering of turbulence statistics that describe the effects of rotation on turbulent Rayleigh–Be nard convection. Rotation is an important addition, which is relevant in many geophysical and astrophysical flow phenomena.

A model for vortical plumes in rotating convection

Buoyant convection and the Coriolis force caused by the rotation of our Earth are important forces in the flows in the atmosphere and the oceans. A convenient model for such flows, although not fully compatible, is the rotating Rayleigh‐Benard setting: A horizontally infinite layer of fluid is vertically confined by solid walls rotating around a vertical axis, the bottom wall being at a higher temperature than the top wall. Although the lack of a top wall in the geophysical flows makes the model not directly applicable, the general behavior of the model flow shows considerable similarities to real flow in the atmosphere. Furthermore, in the atmosphere the tropopause can be regarded as a “top wall” to a certain extent. Especially for the large-scale flows in the atmosphere, the effect of the rotation is dominant. The Rossby number, the ratio between inertial and Coriolis forces, is rather small O0.1. A well-known theorem valid in rotation-dominated flows was formulated by Proudman 1 and experimentally proven by Taylor; 2 it is known as the Taylor‐Proudman theo

Transitions in turbulent rotating convection: A Lagrangian perspective

Using measurements of Lagrangian acceleration in turbulent rotating convection and accompanying direct numerical simulations, we show that the transition between turbulent states reported earlier [e.g., S. Weiss et al., Phys. Rev. Lett. 105, 224501 (2010)PRLTAO0031-900710.1103/PhysRevLett.105.224501] is a boundary-layer transition between the Prandtl-Blasius type (typical of nonrotating convection) and Ekman type.

Infotaxis in a turbulent 3D channel flow

In this paper, the infotaxis-based search algorithm is tested in several simulated turbulent channel flows. The algorithm is adapted to detect plumes of high concentration instead of independent particles. Direct numerical simulation is used to test this adapted search algorithm by detection of high concentration levels in turbulent channel flows with a Schmidt number Sc of 1.0 and Reynolds numbers Re of 5600 and 28000.For the direct numerical simulation with the adapted algorithm, there is a positive relation between the initial distances to the source and the running time, which holds for Re = 5600 but which is not observed at Re = 28000 . This is caused by the low Schmidt number and the high velocity, which leads the searcher to the source very fast after the first detection of a high concentration level.The search algorithm is also tested in reverse to detect whether a fluid is well-mixed. The time required for a detection of a too high or low concentration and the number of detections are used as measures for success. By applying the algorithm to some prescribed concentration distributions in two dimensions, it is found that the method is very sensitive to the threshold values for the mixing indicators.

Exploring the geostrophic regime of rapidly rotating convection with experiments

Rapidly rotating Rayleigh–Benard convection is studied using time-resolved particle image velocimetry and three-dimensional particle tracking velocimetry. Approaching the geostrophic regime of rotating convection, where the flow is highly turbulent and at the same time dominated by the Coriolis force, typically requires dedicated setups with either extreme dimensions or troublesome working fluids (e.g., cryogenic helium). In this study, we explore the possibilities of entering the geostrophic regime of rotating convection with classical experimental tools: a table-top conventional convection cell with a height of 0.2 m and water as the working fluid. In order to examine our experimental measurements, we compare the spatial vorticity autocorrelations with the statistics from simulations of geostrophic convection reported earlier in [D. Nieves et al., “Statistical classification of flow morphology in rapidly rotating Rayleigh-Benard convection,” Phys. Fluids 26, 086602 (2014)]. Our findings show that we hav...

Rotating turbulent Rayleigh–Bénard convection subject to harmonically forced flow reversals

The characteristics of turbulent flow in a cylindrical Rayleigh–Bénard convection cell which can be modified considerably in case rotation is included in the dynamics. By incorporating the additional effects of an Euler force, i.e., effects induced by non-constant rotation rates, a remarkably strong intensification of the heat transfer efficiency can be achieved. We consider turbulent convection at Rayleigh number Ra = 109 and Prandtl number σ = 6.4 under a harmonically varying rotation, allowing complete reversals of the direction of the externally imposed rotation in the course of time. The dimensionless amplitude of the oscillation is taken as 1/Ro* = 1 while various modulation frequencies 0.1 ≤ Roω ≤ 1 are applied. Both slow and fast flow-structuring and heat transfer intensification are induced due to the forced flow reversals. Depending on the magnitude of the Euler force, increases in the Nusselt number of up to 400% were observed, compared to the case of constant or no rotation. It is shown that a large thermal flow structure accumulates all along the centreline of the cylinder, which is responsible for the strongly increased heat transfer. This dynamic thermal flow structure develops quite gradually, requiring many periods of modulated flow reversals. In the course of time, the Nusselt number increases in an oscillatory fashion up to a point of global instability, after which a very rapid and striking collapse of the thermal columnar structure is seen. Following such a collapse is another, quite similar episode of gradual accumulation of the next thermal column. We perform direct numerical simulation of the incompressible Navier–Stokes equations to study this system. Both the flow structures and the corresponding heat transfer characteristics are discussed at a range of modulation frequencies. We give an overview of typical time scales of the system response.

Structure functions in rotating Rayleigh-Benard convection

A combined numerical–experimental investigation on the scaling of velocity structure functions in turbulent rotating Rayleigh–B´enard convection is carried out. Direct numerical simulations in a cylindrical domain and a horizontally periodic domain are compared with experiments using a cylindrical tank in which stereoscopic particle image velocimetry is employed. The turbulent length scales that govern the scaling of the structure functions are evaluated directly in the numerical simulations. They provide a framework for the interpretation of the structure functions. The composition of the domain (cylinder/periodic) has a quantitative effect on the length scales even in the fluid bulk. At lower rotation rates an additional scaling range due to rotation is found. At higher rotation rates a direct transition is observed from dissipation-range scaling at small separations to an uncorrelated state at larger separations.

DNS of convective heat transfer in a rotating cylinder

The effects of rotation on the convective heat transfer and flow structuring in a cylindrical volume of fluid is investigated with direct numerical simulation (DNS). A formulation of the discrete equations of motion in cylindrical coordinates is solved with finite-difference approximations. At constant Rayleigh (Ra; dimensionless temperature gradient) and Prandtl (Ro, the ratio of buoyancy and Coriolis forces, is varied between runs with 0.045 Ro For Ro 1.2 we find the so~called large-scale circulation. At Ro 1.2 slender columnar vortical plumes are found. In a range of Ro heat transfer is increased by Ekman pumping in the vortical plumes. plumes.

Anisotropy in turbulent rotating convection

A simple model for many geophysical and astrophysical flows, such as oceanic deep convection and the convective outer layer of the Sun, is found in rotating Rayleigh.BeLenard convection: a horizontal fluid layer heated from below and cooled from above is rotated about a vertical axis. Three dimensionless parameters characterise this flow: the Rayleigh number Ra describes the strength of the destabilising temperature gradient, the Prandtl number relates the diffusion coefficients for heat and momentum of the fluid, and the Rossby number Ro is the ratio of buoyancy and Coriolis forces ( \(Ro = \infty \) when rotation is absent and Ro 1 for rotation-dominated flow). We investigate the effect of rotation on the flow anisotropy in turbulent convection in an upright cylinder of equal height and diameter, with experiments and numerical simulations.