  Simultaneous two-phase particle image/tracking velocimetry (PIV/PTV)
measurement is conducted on particle-laden turbulent boundary layer (TBL) over
a horizontal smooth-flat-plate. The relatively high Reynolds number
(Re{\tau}=5500 based on friction velocity u{\tau} and boundary layer thickness
{\delta}) wind-sand TBL with large field-of-view (FOV) is experimentally
explored. With four high-resolution CCD cameras arranged along the flow
direction, this experiment can resolve a wide range of the spectrum ranging
from small-scale energetic eddies to large-scale motions (LSMs) in TBL. Dilute
desert sand grains with median diameter of 203{\mu}m and bulk volume fraction
of O(10-5) are used as discrete phase. Improved phase separation and discrete
particle matching method are developed for two-phase velocity measurement. The
results presented here provide new information concerning the effect of
high-inertia particles with dilute concentration on wall-bounded turbulence,
especially at high Re. The presence of sand grains attenuate turbulence
fluctuation by suppressing small-scale sweep-ejection cycle in the near-wall
region, and this suppression effect intensifies as the particle concentration
increases. A critical layer (y/{\delta}=0.12 or y+=670) is found to partition
the streamwise evolution of both the local concentration and the streamwise
mean velocity of the sand grains into a spatial developing near-wall region and
a quasi-parallel outer region. In addition, at this layer a balance of the
strength and probability between the sweep and ejection events of sand grains
is reached. Such a critical layer might be a good indicator of the upper bound
of the particle saltation process, in which LSMs are believed to play a
significant role.

Fluid Dynamics