Decaying quasi-two-dimensional turbulence in a rectangular container: laboratory experiments

Abstract

Results from a new series of experiments on quasi-two-dimensional turbulence decaying in a rectangular container are presented. The flows are generated electromagnetically in a thin layer of conducting fluid when different configurations of permanent magnets are used to specify the initial characteristics of the flow. The particle image velocimetry method is used to determine the velocity and vorticity fields. These fields are further used to determine the global characteristics of the flow such as the energy, enstrophy and the net angular momentum with respect to the center of the container. The experimental results demonstrate that variations of net angular momentum occur due to the interactions of coherent vortex dipoles/jets with the walls of the container. The variation of the angular momentum of the system, which occurs due to the action of stresses at the solid boundary, can be obtained quantitatively if the initial linear impulse exerted by the magnets on the fluid is known. An analysis of external forces and torques allows one to predict that during the intermediate state of the flow evolution the pattern of the flow will be a rotating quadrupole. The energy spectra of the flow demonstrates upscale energy transfer and corresponding growth of the energy-weighted mean scale. Power law exponents are obtained for both the low and high wave number regions of the spectra. Growth of the Reynolds number of the flow was observed during the intermediate phase of the flow evolution.