Comparison of turbulent flow structures over rough permeable and impermeable sediment beds using Direct Numerical Simulation*

Pore-resolved direct numerical simulations (DNS) of turbulent flow over a sediment bed are performed to test the hypothesis that the structure and dynamics of turbulence over a porous sediment bed can be significantly different than that over an impermeable rough wall. Simulations are carried out at Re_τ= 180 and roughness Reynolds number of Re_k∼2.56, corresponding to stream flow over a randomly packed permeable gravel bed, and compared to impermeable bed with roughness elements similar to the top layer.

Comparison of flow statistics confirms that bed permeability can significantly increase the skin friction coefficient (~30% greater) due to enhanced flow resistance. Data shows that form-induced/dispersive turbulent stresses induce deeper penetration of turbulence into the sediment (~43% greater).  Results show that a negative streamwise dispersive stress accompanies a positive wall-normal dispersive stress behind bed protrusions, resulting in ejection motions and vice versa in trough regions, causing sweep motions. The stream flow statistics of porous bed as well as surface pressure fluctuations are well captured by half-layer sediment roughness element; however, turbulence penetration and ejection-sweep events are altered in presence of porous bed