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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #75772
Title: TURBULENCE MODULATION AND PARTICLE VELOCITIES OVER FLAT SAND BEDS AT LOW TRANSPORT RATES

Author
item BEST, JIM - UNIVERSITY OF LEEDS
item Bennett, Sean
item BRIDGE, JOHN - BINGHAMTON UNIVERSITY
item LEEDER, MIKE - UNIVERSITY OF LEEDS

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: The presence of sand moving at low transport rates over a flat bed modifies the production of fluid turbulence when compared to a clear-water flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases fluid mixing within the lower flow and generates an increase in the near-wall velocity gradient when compared with the clear-water values. This increased gradient is associated with a greater roughness height and near-bed turbulence intensities and lower mixing lengths. Several mechanisms of turbulence modification in the presence of mobile sediment may be invoked to explain these changes, including increased roughness, eddy shedding from large grains, added mass effects and particle-particle and particle-wall interactions. Since mobile sediment modifies the flow, this suggests that existing theories of sediment suspension need to be re-evaluated.

Technical Abstract: The presence of sand moving at low transport rates over a plane bed modulates the production of turbulence when compared to clear-water flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the carrier fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases fluid mixing within the lower boundary layer and generates an increase in the near-wall velocity gradient when compared with the clear-water values. This increased shear velocity is associated with a greater roughness height and near-bed turbulence intensities and lower mixing lengths. Quantification of slip velocities between the fluid and sediment phases reveals particle Reynolds numbers that range from 1 to 30. Several mechanisms of turbulence modulation in the presence of mobile sediment may be invoked to explain these changes, including increased grain/bedload roughness, eddy shedding from large grains, added mass effects and particle-particle / particle-wall interactions. Since mobile sediment modulates the carrier fluid turbulence, this suggests the need for both modification to existing theories of sediment suspension and caution when interpreting velocity profiles that are obtained without discriminating the fluid and sediment phases.