In search of random uncorrelated particle motion (RUM) in a simple random flow field

TL;DR

This study demonstrates the existence of random uncorrelated particle motion (RUM) in a simple flow field, showing it occurs only for particles with Stokes number greater than 0.25 and increases with particle inertia.

Contribution

It reveals RUM in a linear flow field and links its occurrence to particle inertia, providing insights beyond previous turbulence-based studies.

Findings

RUM occurs only when Stokes number > 0.25.

RUM increases monotonically with particle inertia.

Particle concentration does not reach equilibrium, increasing over time.

Abstract

DNS studies of dispersed particle motion in isotropic homogeneous turbulence [1] have revealed the existence of a component of random uncorrelated motion (RUM)dependent on the particle inertia {\tau}p(normalised particle response time or Stoke number). This paper reports the presence of RUM in a simple linear random smoothly varying flow field of counter rotating vortices where the two-particle velocity correlation was measured as a function of spatial separation. Values of the correlation less than one for zero separation indicated the presence of RUM. In terms of Stokes number, the motion of the particles in one direction corresponds to either a heavily damped ({\tau}p < 0.25) or lightly damped ({\tau}p > 0.25)harmonic oscillator. In the lightly damped case the particles overshoot the stagnation lines of the flow and are projected from one vortex to another (the so-called sling-shot effect). It is shown that RUM occurs only when {\tau}p > 0.25, increasing monotonically with increasing Stokes number. Calculations of the particle pair separation distribution function show that equilibrium of the particle concentration field is never reached, the concentration at zero separation increasing monotonically with time. This is consistent with the calculated negative values of the average Liapounov exponent (finite compressibility) of the particle velocity field.