Turbulent mixing by breaking gravity waves

This study investigates the charactericstics of turbulence caused by three-dimensional breaking of internal gravity waves beneath a critical level using large-eddy simulations. The flow evolves in three stages. In the first one the flow is two-dimensional: internal gravity waves propagate vertically upwards and create a convectively unstable region beneath the critical level. Convective instability leads to a first turbulent breakdown in the second stage. The developing three-dimensional mixing region is organised in shear-driven overturning rolls in the plane of wave propagation and in counter-rotating streamwise vortices in the spanwise plane. The production of turbulent kinetic energy by shear is maximum. The last stage is characterized by a quasisteady state balance between shear production and mechanical dissipation of turbulent kinetic energy. The whole flow evolution depends on external (wave amplitude, stratification) and on internal (Reynolds, Prandtl number) parameters. Here, only implications of the viscosity on the instability structure and evolution in terms of the Reynolds number #DELTA#UH/#nu# are considered. Smaller viscosity leads to an earlier convective instability of the overturning wave. However, the viscous damping does not only damp the onset of the smaller-scale three-dimensional instabilities but also leads to a reduced momentum transfer to the mean flow below the critical level. Hence, the generation of secondary overturning rolls is enforced by a lower viscosity. The budget of total kinetic and potential energy is calculated. Although the domain-averaged turbulent kinetic energy is less than 1% of the total kinetic energy it is strong enough to form a violent turbulent mixing layer below the critical level. The mixing efficiency of the breaking event is calculated to be about 20%. (orig.)36 refs.Available from TIB Hannover: RR 6341(66) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman