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 Fh = ω/N is low, while the buoyancy Reynolds number R = ReFh2, where Re = ωa2/v 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 v 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 largestRachievedR = 310. In addition, the corresponding turbulent buoyancy Reynolds number Rt = P/(vN2) based on an estimation of the injection rate of energy P is only of order unityRt ≃ 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. © 2014 AIP Publishing LLC.