HR: 17:00h
AN: H14B-04    [Abstracts]
TI: Coherent turbulence structures in lateral separation eddies
AU: * Schmeeckle, M W
EM: schmeeckle@asu.edu
AF: Arizona State University, P.O. Box 870104, Tempe, AZ 85287, United States
AU: Akahori, R
EM: rakahori@people.kobe-u.ac.jp
AF: Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan
AB: An abrupt expansion in channel width generally produces lateral flow separation. A zone of recirculating flow is produced downstream of the point of separation that is termed a lateral separation eddy. Often the abrupt expansion is preceded upstream by a rapid constriction of the channel width. Lateral separation eddies are common in the Colorado River in Grand Canyon downstream of tributary debris fans. They are also produced by spur dikes, which are engineering structures designed to promote lateral sedimentation and eliminate lateral channel migration. Accurate prediction of the sediment transport field in a lateral separation eddy is difficult because of the presence of large and energetic turbulence structures. We have produced a three-dimensional numerical model of turbulence employing the large eddy simulation (LES) technique, in which large scale turbulence is directly calculated by integration of the spatially-filtered Navier-Stokes equations. Our model employs a moving boundary-fitted coordinate system to capture the irregular geometry of the channel and the moving free water surface. The model was used to simulate the flow and large-scale turbulence structures downstream of a single spur dike and in the Thirty Mile lateral separation eddy in Grand Canyon. The lambda-2 technique was used to extract and visualize the large-scale vortices. In both simulations, the upstream channel constriction produces strong secondary circulation resulting in a large vortex core in the main channel flow that is oriented downstream. Past the point of separation, vertical Kelvin-Helmholtz vortex cores are periodically produced along the free shear layer in both simulations. In the spur-dike simulation, the vertical Kelvin-Helmholtz vortex cores interact with the downstream-directed vortex core to produce regularly-shaped vortex cores which periodically give rise to strong, near-bed, cross-stream velocities directed toward the zone of flow reattachment. In the Grand Canyon simulation, the vertical cores and downstream core interact to produce highly irregularly- shaped vortex cores and episodic strong near-bed velocities directed into the lateral separation zone. Presumably, these periodic and episodic, near-bed, cross-stream velocities are important for the flux of sediment into a lateral separation eddy.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
DE: 4568 Turbulence, diffusion, and mixing processes (4490)
SC: Hydrology [H]
MN: 2007 Fall Meeting