  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.
