Clustering and preferential concentration of finite-size particles in forced homogeneous-isotropic turbulence

摘要

We have performed interface-resolved direct numerical simulations of forcedhomogeneous-isotropic turbulence in a dilute suspension of spherical particlesin the Reynolds number range Re-lambda=115-140. The solid-fluid density ratiowas set to 1.5, gravity was set to zero, and two particle diameters wereinvestigated corresponding to approximately 5 and 11 Kolmogorov lengths. Notethat these particle sizes are clearly outside the range of validity of thepoint-particle approximation, as has been shown by Homann & Bec (2010). At thepresent parameter points the global effect of the particles upon the fluid flowis weak. We observe that the dispersed phase exhibits clustering with moderateintensity. The tendency to cluster, which was quantified in terms of thestandard deviation of Voronoi cell volumes, decreases with the particlediameter. We have analyzed the relation between particle locations and thelocation of intense vortical flow structures. The results do not reveal anysignificant statistical correlation. Contrarily, we have detected a small butstatistically significant preferential location of particles with respect tothe `sticky points' proposed by Goto & Vassilicos (2008), i.e. points where thefluid acceleration field is acting such as to increase the local particleconcentration in one-way coupled point-particle models under Stokes drag. Thepresently found statistical correlation between the `sticky points' and theparticle locations further increases when focusing on regions with high localconcentration. Our results suggest that small finite-size particles can bebrought together along the expansive directions of the fluid accelerationfield, as previously observed only for the simplest model for sub-Kolmogorovparticles. We further discuss the effect of density ratio and collectiveparticle motion upon the basic Eulerian and Lagrangian statistics.