GALCIT Colloquium

Buoyancy effects are ubiquitous in environmental flows. We will present results from two examples of flow in a background of stable density stratification that illustrate the interplay between buoyancy and turbulence. The first problem concerns flow past a bluff body where DNS and LES are performed to examine the evolution of the flow from near the body into the far wake. A non-standard power law is found for the velocity deficit in the unstratified case of this moderate-Reynolds number wake. The stratified case exhibits fluctuations with a complex pattern of vortical motions, internal gravity waves and small-scale turbulence. Surprisingly, the dependence of turbulent kinetic energy in the near wake on stratification is found to be non-monotone. A new instability mode and enhanced turbulence is found when stratification increases beyond a threshold. The drag coefficient also increases with increasing stratification. The second problem concerns the ocean where the energy converted from the oscillating tide to internal waves at deep, rough topography is known to be a key source of turbulence and mixing. I will summarize results from our numerical simulations regarding nonlinear pathways from topographic internal gravity waves to turbulence.