Low shear turbulence structures beneath stress-driven interface with neutral and stable stratification

Abstract

This study deals with turbulent flow below a stress-driven interface with stable density stratification, as a simplified configuration of a turbulent flow below a calm gas-liquid interface with wind forcing. A simple shear stress, which is constant in time and uniform at the interface, is enforced, hence only turbulence below the interface is studied. A flat interface is assumed to simplify transport phenomena beneath the interface by limiting the strength of the enforced wind stress and avoiding violent wave motions. A direct numerical simulation technique is employed to obtain three-dimensional turbulence structures below the interface. Results from the present study indicate two major changes of turbulence structures below the sheared interface in the presence of stable density stratification; one is a reduction of the Reynolds stress at the near-interface region, which leads to a coincidental increase of mean velocity gradient. The other effect of stable stratification is the quantitative variation of the surface-streak structures and associated microscale turbulence structures. Examination of the autocorrelation coefficients of the streamwise velocity fluctuations in the subinterface region exhibited that the surface-streak structures in the velocity field are established in both neutral and stably stratified fluid. These streak structures are, however, partially disorganized in the region very close to the interface when strong stable stratification is imposed. The spacing between the streaks tends to decrease with increasing Richardson number in the subinterfacial region −30<x3+<−20, where the disorganization of the streaks is not encountered. A flow visualization technique verifies the establishment of these streaks in every Richardson number case and their partial disorganization under the effect of strong density stratification. A statistical three-dimensional relation between the streaks and the vortical structures are explored by applying a two-point correlation technique to instantaneous turbulent flow realizations. The presence of a counter-rotating vortex pair below the high-momentum fluid at the interface can be found in these examinations. Such presence of the counter-rotating vortex pair are confirmed by the visualization of the vortical structures in instantaneous turbulent flow realizations, whose three-dimensional shape is rather a hairpin-like configuration than two separated vortex tubes.