Particle-turbulence interaction in High-Reynolds-number Sand-laden Turbulent Boundary Layer

Abstract

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 (Reτ=5500 based on friction velocity uτ and boundary layer thickness δ) 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 203μm 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/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 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.