Numerical Study of the Interaction of Bidisperse Turbidity Currents with Complex Topography

Turbidity currents contain grains of many sizes. The influence of complex bottom topography on the settling patterns of the particles depends on their size distribution. In this work, we report the effect of topography, represented by a Gaussian bump, on a bidisperse turbidity current.

We employ a lock-exchange configuration in which initially a heavier suspension is separated from clear fluid through a membrane. As the membrane is removed, a turbidity current starts to propagate as an underflow of heavy particle-laden fluid into lighter fluid. We study the resultant flow using our in-house code TURBINS that performs accurate three-dimensional numerical simulation of the governing Boussinesq Navier-Stokes and particle concentration transport equations. Complex topography is represented by means of an immersed boundary approach in TURBINS.

Simulations were performed using Direct Numerical Simulation (DNS) in which all the relevant scales of the motion are resolved. The heavy suspension includes two particle sizes with a settling velocity ratio of 10. The Reynolds number (Re) based on the lock height and the buoyancy velocity for the resultant turbidity current is 2000. As the current travels over the bottom topography, we record instantaneous deposit profiles and wall shear stress contours. As the current impinges on the obstacle, it becomes strongly three-dimensional. Comparison of the final deposit profiles near the Gaussian bump against the case of a flat surface shows a smaller influence of the topography on the fine particles than on the coarse ones. Due to lateral deflection, deposition generally decreases near the bump, while increasing away from it. Some distance downstream of the obstacle, the deposit profiles lose their memory of the bump and become nearly uniform again. Instantaneous wall shear stress profiles are employed in order to estimate the critical conditions at which bedload transport and/or particle resuspension can occur in various regions.

Simulations were also performed using Large-Eddy Simulation (LES) in which only the energy carrying large-eddies are resolved while the effect of unresolved or “subgrid” scale motion is modeled. LES enable us to simulate a much higher Reynolds number current, representative of laboratory and field scale flows at an affordable computational cost. Results from LES at higher Reynolds number will be compared to the lower Reynolds number current computed using DNS.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California