Experiments were conducted in a wind tunnel in which a turbulent boundary layer
was naturally grown over flat beds of three types of nearly mono-disperse spherical
particles with different diameters, densities and coefficient of restitution (r) (snow,
0.48 mm, 910 kg m−3; mustard seeds, 1.82 mm, 1670 kg m−3,
r = 0.7; ice particles, 2.80 mm, 910 kg m−3, r = 0.8–0.9).
The surface wind speeds (defined by the friction
velocity u∗) were varied between 1.0 and 1.9 times the threshold surface wind speed
(defined by u∗t). The trajectories, and ejection and impact velocities of the particles
were recorded and analysed, even those that were raised only about one diameter into the flow.
Measurements of the average horizontal flux of saltating particles per unit area, f(z),
at each level z above the surface showed that, for
u∗/u∗t [les ] 1.5, f(z) is approximately
independent of the particle density and decreases exponentially over a vertical scale
length lf, that is about 3 to 4 times the estimated mean height of the particle
trajectories 〈h〉. Numerical simulations of saltating grains were computed using the
measured probabilities of ejection velocities and the mean velocity profile of the air
flow, but neglecting the direct effect of the turbulence. The calculated mean values
of the impact velocities and the trajectory dimensions were found to agree with the
measurements in the saltation range, where
u∗/u∗t < 1.5. Similarly, in this range
the simulations of the horizontal flux profile and integral are also consistent with the
measurements and with Bagnold's u∗3 formula, respectively.
When u∗/u∗t [ges ] 1.5, and
u∗/VT [ges ] 1/10, where VT
is the settling velocity, a transition
from saltation to suspension occurs. This is indicated by the change in the mean mass
flux profile which effectively becomes uniform with height (z) up to the top of
the boundary layer. An explanation is provided for this low value of turbulence
at transition relative to the settling velocity in terms of the random motion of
the particles under the action of the turbulence when they reach the tops of their
parabolic trajectories. The experiments show that, as u∗/u∗t
increases from 1.0 to 1.9 the normalized mean vertical impact velocity
〈V3I〉/u∗ decreases by nearly 60% to
about 0.6, which is less than 50% of the value for fluid particles. There is also a
decrease in the vertical and horizontal component of the ejection velocity to values
of 0.8 and 2.3, which are much less than their values in the saltation regime. We
hypothesize that at the transition from saltation to suspension the ejection process
changes quite sharply from being determined by impact collisions to being the result
of aerodynamic lift forces and upward eddy motions.