Sweep-Stick Mechanism of Heavy Particle Clustering in Fluid Turbulence

Abstract

It is proposed that the inertial range clustering of small heavy particles in fluid turbulence occurs as a result of the sweep-stick mechanism which causes inertial particles to cluster so as to mimic the clusters of points where the fluid acceleration is perpendicular to the direction of highest contraction between neighboring particles. Direct numerical simulations of inertial particles subjected to linear Stokes drag and suspended in homogeneous isotropic turbulence support the validity of the sweep and stick properties on which the sweep-stick mechanism is based, and also support the clustering consequences of this mechanism. It also explains the observed Stokes-number dependence of inertial particle clustering. DOI: 10.1103/PhysRevLett.100.054503 PACS numbers: 47.27.ÿi, 47.10.ÿg Turbulence spreads suspended impurities, but it can also bring them together to form clusters. For example, bubbles, which are much lighter than the surrounding fluid, may be used to visualize eddies (see, e.g., [1]), since they tend to accumulate along the center line of swirls. It is also well known [2 -4] that particles heavier than the surrounding fluid cluster in turbulence. This phenomenon, sometimes called preferential concentration, is crucial in various processes where the particle collision rate is a significant factor (such as combustion or chemical reaction in turbulence, raindrop growth in clouds, planet formation in the early solar system, etc.). Thus, it has been studied extensively in many areas of physics and mechanics.In turbulence at a relatively low Reynolds number, the heavy particle clustering is due to the action of the smallest-scale (i.e. the Kolmogorov length ) eddies. Specifically, heavy particles are centrifuged out of the coherent eddies, and accumulate in low-vorticity (highstrain-rate) regions. However, this picture is not valid for developed turbulence at a higher Reynolds number, and the clustering is not a single-scale phenomenon but has a multiscale nature [5][6][7]. This is because not only the smallest-scale but also multiscale coherent eddies in developed turbulence play a role in the preferential concentration, and therefore clusters differ significantly in space from the locations of low-vorticity regions.On the other hand, it has been pointed out [7][8][9][10][11] that the clusterings of heavy particles and fluid acceleration are strongly related. Especially, Refs. [7,9] claim that the spatial distribution of heavy particles reflects that of the stagnation points of the fluid acceleration in twodimensional inverse-energy cascading turbulence. Here, we extend this picture to three-dimensional turbulence by extending and refining the sweep-stick mechanism proposed (but not named) in [7,9] to three-dimensional turbulence.In what follows, we deal with the clustering of heavy particles suspended in statistically stationary homogeneous isotropic turbulence (HIT), u x; t . We assume that only the Stokes drag drives the motion of the particles, and the equation of motion for a particle (the veloc...

Anisotropy in pair dispersion of inertial particles in turbulent channel flow

“…This causes fluctuations of particle concentration that alter the structure of particle patterns. This phenomenology has been demonstrated and discussed in many experimental works, 2,20 as well as numerical studies [21][22][23][24][25][26][27][28] and theoretical analyses. 17,[29][30][31] From a modelling standpoint, reproducing accurately pair relative velocity and dispersion statistics is crucial to predict the effect of particle clustering on turbulent collisions and sedimentation.…”

Intermittency in the velocity distribution of heavy particles in turbulence

“…The first is preferential concentration, which is the development of strong inhomogeneities in their spatial distribution (see figure 1a) (Zhou, Wexler & Wang 1998, 2001Reade & Collins 2000;Goto & Vassilicos 2008). The second is the formation of fold caustics (a phenomenon also called the sling effect), which results in high probabilities that very close particles exhibit large relative velocities (see figure 1b) Wilkinson & Mehlig 2005;Wilkinson, Mehlig & Bezuglyy 2006;Falkovich & Pumir 2007;Olla 2008).…”

Preferential Concentration of Heavy Particles in Stably Stratified Turbulence

“…This leads to highly nonuniform particle distributions, where particles collect in regions of high strain rate and low vorticity [1]. A more detailed explanation is given by Goto and Vassilicos [2], who introduce the sweep-stick mechanism. The local high and low particle concentrations can have an enormous influence on mixing and clustering and can affect, for example, rain initiation in clouds [3].…”