Features Of Sand-laden Flows And PM10 Concentration In The Atmospheric Surface Layer

Particle-laden flows are common in many engineering and environmental applications.The interaction between the turbulence and the dispersed particles in the particle-laden flows is one of the key issues in the fluid mechanics studies.Meanwhile,there have been increasing studies concerning the very large-scale motions?VLSMs?in the high Reynolds number wall turbulence.However,the modulation of dispersed particles on VLSMs and the spatial characteristics of the PM10 concentration in high Reynolds number wall turbulences are not revealed yet.Thus,field observations at the Qingtu Lake Observation Array?QLOA?site were performed in the atmospheric surface layer?ASL?during the dust weather event.The three velocity components,PM10 concentrations?particles with size less than 10?m?and sand saltation fluxes at different heights were measured synchronously.In the sand-laden flows,the average sand/dust particle diameters?d_p?ranged from 70-105?m,which is smaller than the Kolmogorov length scale(?=3.5×10^-4-7.2×10^-4 m);d_p/? ranged from 0.1 to 0.3,the Stokes number?St?ranged from 1.1-11 and the volume loading ratios ?_v<10^-6.Some new results were obtained from the investigation as following:1)By comparing with the results in clean-air flows,the influences of sand particles on the VLSMs were investigated.The present results indicate that the streamwise turbulent kinetic energy across all scales is enhanced in the sand-laden flows.However,the energy increment of the small-scale motions is larger than that of the VLSMs.As a result,the turbulent energy fraction associated with VLSMs is decreased in the sand-laden flows.The inclination angles of the VLSMs is found to increase with sand concentration,owing to the decreased velocity gradient.The streamwise length scale of VLSMs and the size of the most e nergetic turbulent structures were found to be unchanged in the sand-laden flows relative to those in the clean-air flows.Moreover,the presence of sand particles is found to strengthen the amplitude modulation of VLSMs on the small-scale motions.2)Two-point correlations of the fluctuating PM10 concentration confirm the existence of the large-scale structures of dust concentration.The structure inclination angle of dust concentration is larger than that of the flow structure.Moreover,the length scales of the dust concentration are larger than the flow structures in the near wall region,while they are equal to the flow structures in the higher region.3)To discriminate the gusty wind disturbances from large-scale turbulent structures in the ASL and to examine their contributions to the TKE and the momentum flux.This paper denote that the gusty wind disturbances contain only a portion of the energy contained in very large-scale motions and do not contain any of the information contained in large-scale motions.The amount of lost contributions to the streamwise turbulent kinetic energy and momentum flux increases linearly with the friction velocity,eventually becoming 53%and 67%,respectively.This indicates that large-scale turbulent structures?very large-scale motions and large-scale motions?better describe the coherent structures in the atmospheric surface layer when compared with the gusty wind disturbances.4)Systematic analysis of turbulence structure with the Monin–Obukhov similarity parameter was documented in this study.A parametric equation is fitted to the log-linear trend of unstable data to model the variation of the streamwise length scale of VLSMs and the energy fraction of the VLSMs with the stability parameter|z/L|;The variation of the length scales of the dust concentration with the stability parameter|z/L| follows a log-linear increase in the unstable regime.