Experimental study of stratified turbulence forced with columnar dipoles

Abstract : We present a novel experimental setup aimed at producing a forced strongly stratified turbulent flow. The flow is forced by an arena of 12 vortex pair generators in a large tank. The continuous interactions of the randomly produced vortex pairs give rise to a statistically stationary disordered flow in contrast to previous experiments where the stratified turbulence is decaying. The buoyancy frequency N is set to its highest value N = 1.7 rad/s using salt as stratifying agent so that the horizontal Froude number F h = Ω/N is low, while the buoyancy Reynolds number R=ReFh2 , where Re = Ωa 2/ν is the classical Reynolds number, is as high as possible given the experimental constraints (Ω is the maximum angular velocity of the vortices, a their radius and ν the viscosity). PIV measurements show that the flow is not homogeneous in the horizontal plane and is organised into horizontal layers along the vertical. When R is increased, we observe a progressive evolution from the viscosity dominated regime with smooth layers to a regime with small scales superimposed on the layers and for which the vertical Froude number is of order one. The latter regime resembles the strongly stratified turbulent regime with a downscale cascade that has been predicted for large R . However, horizontal second order structure functions do not exhibit a clear inertial range for the largest R achieved R=310 . In addition, the corresponding turbulent buoyancy Reynolds number Rt=P/(νN2) based on an estimation of the injection rate of energy P is only of order unity Rt≃0.4 indicating that only the edge of the strongly stratified turbulent regime has been reached. However, these results suggest that sufficiently large turbulent buoyancy Reynolds numbers, Rt≃10 , could be achieved experimentally by scaling up five times this novel set-up.
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Pierre Augier, Paul Billant, M. E. Negretti, Jean-Marc Chomaz. Experimental study of stratified turbulence forced with columnar dipoles. Physics of Fluids, American Institute of Physics, 2014, 26 (4), pp.046603. ⟨10.1063/1.4870703⟩. ⟨hal-01048580⟩

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