Colliding Dipolar Vortices in a Stratified Fluid

International audienceThe photographs illustrate experiments that have been performed on the collapse of a three-dimensional turbulent patch in a linearly stratified fluid. The turbulence was gener­ated by horizontal injection of a small volume of fluid during a short time interval. A transition to two-dimensional flow occurs when the turbulent patch collapses under gravity, as can be observed from the spectral flux of kinetic energy to larger scales. The collapsed fluid eventually gets organized in a dipolar flow structure that moves slowly forward along a straight line. The robustness of this dipolar coherent struc­ture is demonstrated in experiments on head-on collisions of two dipoles with approximately identical characteristics. Consecutive stages of a head-on collision are shown by the (plan view) photographs, taken (a) 42 sec, (b) 70 sec, (c) 120 sec, and (d) 225 sec after the injections were stopped. The asymmetry in the observed flow patterns is due to a slight misalignment of the initial dipoles. Nevertheless, the experiment shows nicely that the original dipoles exchange partners, and that two new dipoles emerge, moving along straight lines away from the collision area. Further experi­mental details are described elsewhere

The role of Stewartson and Ekman layers in turbulent rotating Rayleigh-B\'enard convection

When the classical Rayleigh-B\'enard (RB) system is rotated about its vertical axis roughly three regimes can be identified. In regime I (weak rotation) the large scale circulation (LSC) is the dominant feature of the flow. In regime II (moderate rotation) the LSC is replaced by vertically aligned vortices. Regime III (strong rotation) is characterized by suppression of the vertical velocity fluctuations. Using results from experiments and direct numerical simulations of RB convection for a cell with a diameter-to-height aspect ratio equal to one at $Ra \sim 10^8-10^9$ ($Pr=4-6$) and $0 \lesssim 1/Ro \lesssim 25$ we identified the characteristics of the azimuthal temperature profiles at the sidewall in the different regimes. In regime I the azimuthal wall temperature profile shows a cosine shape and a vertical temperature gradient due to plumes that travel with the LSC close to the sidewall. In regime II and III this cosine profile disappears, but the vertical wall temperature gradient is still observed. It turns out that the vertical wall temperature gradient in regimes II and III has a different origin than that observed in regime I. It is caused by boundary layer dynamics characteristic for rotating flows, which drives a secondary flow that transports hot fluid up the sidewall in the lower part of the container and cold fluid downwards along the sidewall in the top part.Comment: 21 pages, 12 figure

Machine learning for closure models in multiphase flow applications

Multiphase flows are described by the multiphase Navier-Stokes equations. Numerically solving these equations is computationally expensive, and performing many simulations for the purpose of design, optimization and uncertainty quantification is often prohibitively expensive. A simplified model, the so-called two-fluid model, can be derived from a spatial averaging process. The averaging process introduces a closure problem, which is represented by unknown friction terms in the two-fluid model. Correctly modeling these friction terms is a long-standing problem in two-fluid model development. In this work we take a new approach, and learn the closure terms in the two-fluid model from a set of unsteady high-fidelity simulations conducted with the open source code Gerris. These form the training data for a neural network. The neural network provides a functional relation between the two-fluid mo

Dipole formation and collisions in a stratified fluid

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Stable and unstable monopolar vortices in a stratified fluid

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Effects of rotation and stratification: an introduction

Large-scale flows in the natural environment can be influenced by the planetary rotation and also by density differences. This chapter aims to provide an informal introduction into the effects of background rotation and stratification