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Synchronous high-speed measurement of near-bed turbulent flow and particle entrainment from a granular bed. -- 31/10/03
Mark W. Schmeeckle, (Arizona Sate University) schmeeckle@asu.edu
R. L. Shreve, (University of Washington)
J. M. Nelson, (US Geological Survey)
Generally, the entrainment of particles from the bed of a turbulent flow is calculated using methods that relate entrainment with a modified boundary shear stress. However, these methods work poorly when the near-bed turbulent flow structure is different from that over a flat bed in a uniform flow, such as downstream of separated flow. Some examples where the turbulent flow is significantly altered and, hence, affects entrainment include: the motion of sand through fixed gravel or boulders; transport of sand and gravel over dunes, ripples, and bedload sheets; and the entrainment of grains into suspension when there is simultaneous motion of bedload particles. If the coupling between the grain entrainment and near-grain fluid velocity is well understood, entrainment can be calculated in any flow where the temporally- and spatially-variable near-bed turbulent structure can be measured or modeled. Results will be presented from recent water flume experiments in which the entrainment of a spherical particle from a granular bed was photographed using a high-speed video camera at a rate of 200 frames per second. A laser sheet, aligned in the vertical and downstream plane and centered on the spherical particle, illuminated small, nearly-neutrally-buoyant tracer particles in the flow. Particle imaging velocimetry (PIV) of the tracer particles was used to calculate the 2-dimensional velocity field surrounding the spherical bed grain between successive video frames. Thus, the two-dimensional velocity field surrounding the particle was measured synchronously to the initiation of motion of the particle. Preliminary results suggest that particle entrainment occurs when the downstream velocity near the middle and top of the particle is significantly higher than the average for a sustained period of time. Earlier experiments have shown that the instantaneous downstream force can be as much as three to five times the time-averaged downstream force. The high instantaneous force has to be maintained long enough for the particle to begin motion. The experimental results are incorporated in a three-dimensional, mixed-grain-size, discrete particle model of bedload transport that is driven by a spatially- and temporally- variable near-bed turbulent flow.