On turbulence and mixing at the top of a gravity current over a rough surface

This study investigates experimentally the effects of surface roughness on turbulence and vertical mixing at a sheared density interface forming at the top of a dense current flowing over a horizontal surface. The experiments were conducted in a water channel using the lock-exchange technique. Two cases were considered: one with a smooth channel bottom, and the other with the bottom made rough by means of a series of parallelepiped elements about one-sixth the height of the dense current. Feature Tracking and Planar Laser-Induced Fluorescence techniques were used to measure fluid velocity and density, respectively. Results show a general sensitivity of the mean flow and turbulence variables on surface characteristics. In the rough case, the density interface is thinner and some of the main parameters involved in the turbulence kinetic energy equation, e.g., buoyancy, production, and its dissipation rate, are higher than in the smooth case. Similarly, turbulent diffusivities of mass and momentum also depend on roughness, although their ratio remains practically unchanged going from the smooth to the rough surface. Finally, some of the results are consistent with field data and numerical simulations found in the literature.