Effect of roughness on density currents

Turbulent density currents are important class of fluid flows encountered in environment, including oil-spill in the ocean, transport of pollution to inland, pollution due to burning buildings. To date, our current understanding of these buoyancy driven density currents are restricted to flow over smooth-wall. In this study, numerical tool was developed to simulate density currents over rough-walls in lock-exchange mechanism. A fundamental analysis has been performed to understand the effect of surface-roughness on the front evolution and dynamics. The focus of this study is to develop scaling laws of the front velocity and locations in terms of the spacing between the roughness elements. Roughness parameter lambda/k is an important parameter that plays role in the study of turbulent flow with rough surface. Effect of this roughness parameter lambda/k and the roughness shape was studied. Two dimensional numerical simulation was conducted with flat bottom surface, bottom mounted sinusoidal roughness (wavy roughness) elements and bottom mounted cylindrical roughness elements. For flat bottom surface, evolution of front location with time shows distinct phases, which is in good agreement with literature. Different regimes of front location evolution with time are identified for sinusoidal shaped roughness element cases. Flow structures for sinusoidal roughness cases revealed that the wake generated at the valley region for sinusoidal or space between the cylinders plays role on turbulent mixing which leads to reduction in frontal velocity. An important conclusion of the present study is different scaling exist for the k-type and d-type sinusoidal roughness.