Aeolian entrainment thresholds in a developing boundary layer.
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.