Turbulence characteristics and mixing properties of gravity currents over complex topography

Abstract

Understanding gravity currents developing on complex topography, which involve turbulence and mixing processes on a wide range of spatial and temporal scales, is of importance for estimating near ground fluxes in oceanic and atmospheric circulation. We present experimental results, based on high resolution velocity and density measurements, of constant upstream buoyancy supply gravity currents flowing from a horizontal boundary onto a tangent hyperbolic shaped slope. The mean flow, turbulence characteristics, and mixing properties, the latter expressed in terms of mixing lengths and eddy coefficients, are determined, highlighting their dependency on topography. These mean flow and mixing characteristics are compared with the field measurements in katabatic winds by Charrondière et al. [“Mean flow structure of katabatic winds and turbulent mixing properties,” J. Fluid Mech. 941, A11 (2022)], which are gravity flows that develop over sloping terrain due to radiative cooling at the surface. The results obtained show that the mean katabatic flow structure is substantially different from that of the upstream buoyancy supply gravity current. However interestingly, dimensionless mixing lengths and eddy coefficients compare well despite the difference in the mean flow structure and a two order of magnitude difference in the Reynolds number.