AEOLIAN ENTRAINMENT THRESHOLDS IN A DEVELOPING BOUNDARY LAYER
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The aeolian threshold condition is identified as defining the critical state between a static sediment bed and active aeolian transport. Disparities in reported mean threshold shear velocities (U*T) are attributed to variation in: (a) flow regimes; (b) entrainment criteria; (c) particle characteristics and exposures; and (d) techniques for measurement of 1*T The relative significance of these factors is assessed for strips and beds of ballotini, aeolian sand, salt crystals and sugar crystals using the controlled, reproducible flow conditions of the developing boundary layer over a flat plate. Such a plate was covered with a non-erosive layer of grains to simulate a flat sediment bed and was positioned in a wind tunnel. values were calculated using the momentum integral method. In a separate study, threshold conditions on impervious and permeable beds were compared directly. Analysis of entrainment from strips and beds on the plate shows that the threshold condition is principally determined by a critical degree of overlap between the probability distributions of local shear velocity, P ( U ), and of grain threshold shear velocity, P(U*T). Characteristic P(U*T) distributions for test materials were determined and two objective definitions of U*T were devised. Experimental *T values agree well with published data and are used to resolve the data scatter associated with both 'restricted' and 'universal' threshold curves. Rates of aeolian entrainment over a range of values are found to be an inverse exponential function of time and wind speed. Different erosion rates were found to result from skewness of P(U) and degree of overlap between P(U) and P(U*T). Fluid sweeps in the turbulent bursting cycle are considered to be the primary entrainment mechanism. Medium/high-speed cine film gave insight into initiation processes. Grain oscillation due to vortex shedding and initial motion characterized by a progressive rolling and bouncing were observed. Bed permeability was not found to affect aeolian threshold values.
AuthorsWilliams, Jonathan Jowett
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