관성 입자 경로가 난류 소산 범위에서 입자 쌍 역학에 미치는 영향

Abstract

The relationship between inertial particle pair dynamics in the dissipation range and local turbulence characteristics is investigated using direct numerical simulations in homogeneous isotropic turbulence. Sub-Kolmogorov-size particles with the Stokes numbers ranging from 0.4 to 2.0 are tracked backward in time to investigate the formation of relative velocities of inertial particles in the dissipation range. The numerical experiment shows that particle pairs take different paths and sample different underlying flow depending on the intensity of the local turbulence activity at final locations where the distance between particles is much smaller than the Kolmogorov length scale. Taking different paths depending on the intensity of the local turbulence activity is described by sling time defined as the time when particle pairs are slung out from underlying flow. Particle pairs with shorter sling time values than characteristic time scales of particles are detached from the dissipation range of flow and approach each other in a ballistic fashion. In contrast, the large-scale flow guides particle pairs with longer sling time than the characteristic time scales. Especially, particle pairs forming caustics with the same sling time are detached from similar flow regardless of the magnitude of particle relative velocities. Samplings of similar flow at sling moments cause the scaling relationship between relative velocities of particles and sling time. These behaviors of particle pairs, especially for pairs with caustics in relative velocities, are also observed in the range of considered Stokes numbers. The relationship between the intensity of the local turbulence activity and the paths taken by particles highlights the importance of considering preferential sampling of flow in understanding inertial particle pair dynamics.