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Session: Geophysical and astrophysical turbulence 2
Session starts: Thursday 27 August, 15:00
Presentation starts: 15:45
Room: Room C

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Robert Ecke (Los Alamos National Laboratory)
Philippe Odier (Ecole Normale Superieure Lyon)

Abstract:
Stratified shear flows are ubiquitous in geophysical systems such as oceanic overflows, wind-driven thermoclines, and atmo- spheric inversion layers. The stability of such flows is governed by the Richardson Number Ri which represents a balance between the stabilizing influence of stratification and the destabilizing influence of shear. For a shear flow with velocity difference U, density difference ∆ρ and characteristic length H, one has Ri = g(∆ρ/ρ)H/U^2 which is often used when detailed information about the flow is not available. A more precise definition is the gradient Richardson Number Rig = N^2/S^2 where the buoyancy frequency N = ((g/ρ)∂ρ/∂z)^{1/2}, the mean strain S = ∂U/∂z in which z is parallel to gravity and suitable ensemble or time averages define the gradients. We explore the stability and mixing properties of a wall-bounded shear flow over a range 0.1< Rig <1 using simultaneous planar measurements of density and velocity fields using Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), respectively. The flow, confined from the top by glass horizontal boundary, is a lighter alcohol-water mixture injected from a nozzle into quiescent heavier salt-water fluid with velocity between 5 and 10 cm/s and with a relative fractional density difference of 0.0026 or 0.0052. The injected flow is turbulent with Taylor Reynolds number between 50 and 100. We compare a set of length scales that characterize the mixing properties of our turbulent stratified shear flow including the Thorpe Length L_T, the Ozmidov Length L_o, the Ellison Length L_E, and turbulent mixing lengths L_m and L_ρ.