15:30 〜 16:00
[MIS01-01] Transport by turbulence and waves: suspened load of stratified plane Poiseuille flow and gravity currents
★Invited Papers
キーワード：sediment transport, turbulence, internal waves, gravity currents
The transport dynamics of negatively buoyant sediment are the critical control for particle-laden density driven flow: turbidity currents. Here we discuss the dynamics of particle suspension in such stratified flows. Thermally stratified plane Poiseuille flow are studied, enabling high-fidelity numerical simulations that quantify key controls of suspension. In particular we establish that large-scale coherent structures, inherent to stratified flows, have a principle role in sediment transport processes.
Sediment concentration fields are passively solved with settling velocities of Vs=0.005, Vs=0.01 and Vs=0.02, made dimensionless by the friction velocity. It is shown that strong buoyancy forces in the core predominately arise from up-gradient mass and momentum transport. These processes have a profound impact on sediment transport, leading to two-layer sediment concentration profiles (gravity wave elasticity) with concentration gradients increasing with sediment settling velocity, Vs. When compared to unstratified flow, stratification leads to considerably larger differences in concentration profiles and higher order statistics between the different settling velocities. When appropriately scaled by either the thermal shear temperature or the vertically varying mean sediment concentration, scalar statistics collapse to near common vertical profiles.
The collapse of scalar statistics is explained by revisiting the classical gradient diffusion hypothesis, which links scalar fluxes to respective mean gradients. However, collapse of scalars is poor in the channel core where large concentration gradients coincide with large-scale mixing events. Here the differences between vertical turbulent diffusivities of sediment and temperature increase with increasing Vs, reaching 20% for Vs=0.02. Discrepancies arise due to a breakdown of the linear gradient diffusion hypothesis. Predictions using the gradient diffusion hypothesis are expected to worsen with increasing settling velocity, indicating that classical models poorly predict dilute particle transport in strongly stratified flows, which are common to a range of gravity currents in both environmental and industrial settings.
Sediment concentration fields are passively solved with settling velocities of Vs=0.005, Vs=0.01 and Vs=0.02, made dimensionless by the friction velocity. It is shown that strong buoyancy forces in the core predominately arise from up-gradient mass and momentum transport. These processes have a profound impact on sediment transport, leading to two-layer sediment concentration profiles (gravity wave elasticity) with concentration gradients increasing with sediment settling velocity, Vs. When compared to unstratified flow, stratification leads to considerably larger differences in concentration profiles and higher order statistics between the different settling velocities. When appropriately scaled by either the thermal shear temperature or the vertically varying mean sediment concentration, scalar statistics collapse to near common vertical profiles.
The collapse of scalar statistics is explained by revisiting the classical gradient diffusion hypothesis, which links scalar fluxes to respective mean gradients. However, collapse of scalars is poor in the channel core where large concentration gradients coincide with large-scale mixing events. Here the differences between vertical turbulent diffusivities of sediment and temperature increase with increasing Vs, reaching 20% for Vs=0.02. Discrepancies arise due to a breakdown of the linear gradient diffusion hypothesis. Predictions using the gradient diffusion hypothesis are expected to worsen with increasing settling velocity, indicating that classical models poorly predict dilute particle transport in strongly stratified flows, which are common to a range of gravity currents in both environmental and industrial settings.