Large-eddy simulations of convective shear flows

Abstract

Abstract The structure of the instantaneous flow fields and turbulence statistics in oceanic convection affected by wind-induced shear are analysed using a large-eddy simulation (LES) data set. A modified subgrid-scale (SGS) model has been developed in order to overcome known deficiencies of the Smagorinsky SGS model in near-wall flows. The SGS model has been formulated as a second-order moment approach. Three distinct convective flows driven by surface cooling are generated. One is the convectively mixed layer with negligible surface shear corresponding to calm wind conditions. The other two are convectively mixed layers affected by enlarged wind-generated shear stresses corresponding to a wind speed of 7 and 14 m s - 1 , respectively. The heat flux was held constant in order to provide equal thermal forcing. Surface gravity waves are excluded. Instantaneous flow fields reveal the ability of the mean shear to order temperature fluctuations into convective roll-like structures under severe wind conditions, whereas under calm to moderate wind conditions convective cells are formed. The ratio of friction velocity u * to Deardorff velocity w * controls the formation of either cell or roll structures. The well-known non-local effects due to turbulent and pressure transport of turbulent kinetic energy from the surface to the bulk of the convectively mixed layer are confirmed. The flows follow the “ 1 2 ” power law for the Nusselt–Rayleigh number relation (based on eddy viscosity and diffusivity) at high Rayleigh numbers. With increasing shear the heat flux decreases. Rotational effects clearly influence the flows despite the shallow mixed layer of only about 200 m depth. The unstable stratification significantly changes the volume transport of momentum compared to near-neutral stratified flows. Whereas for moderate wind conditions the transport is damped compared to the near-neutral case, the situation under strong wind conditions is completely different and volume transport of momentum is enhanced. As rotation as well as stratification tend to reduce the turbulent length scales, the role of the SGS model becomes more pronounced in the presence of rotation and/or stratification.