Sand-turbulence interaction in a high-Reynolds-number turbulent boundary layer under net sedimentation conditions

Simultaneous two-phase particle image/tracking velocimetry (PIV/PTV) measurements are conducted on a particle-laden turbulent boundary layer (TBL) over a horizontal smooth-flat plate under net sedimentation conditions. Dilute desert sand grains with a median diameter of 203 μm and bulk volume fraction of O(10⁻⁵) are used as the discrete phase, and the characteristic Reynolds number of the TBL is relatively high (Re_τ = 5500 based on the friction velocity u_τ and boundary layer thickness δ). The two-dimensional velocity filed in the streamwise/wall-normal plane is measured by four CCD cameras aligned along the streamwise direction, which resolves a wide spectrum of the scales, ranging from small-scale energetic eddies to large-scale motions (LSMs) in the TBL. An improved phase separation algorithm and discrete particle matching method are developed for two-phase measurements. The results presented here provide new information concerning the effect of high-inertia particles with a dilute concentration on wall-bounded turbulence, especially at high Re. Analysis of the velocity spectrum of the gas phase reveals, for the first time, that the presence of sand grains with a size on the order of the Kolmogorov length scale attenuates turbulence fluctuation by suppressing small-scale sweep-ejection cycles in the near-wall region. A critical layer (y/δ = 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 non-equilibrium near-wall region and a quasi-parallel outer region. In addition, a balance between the strength and probability between the sweep and ejection events of sand grains is reached at this layer. 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.