  Through laboratory measurements, we compare the rotation of spherical and
ellipsoidal particles in homogeneous, isotropic turbulence. We find that the
particles' angular velocity statistics are well described by an
Ornstein-Uhlenbeck (OU) process. This theoretical model predicts that the
Lagrangian autocovariance of particles' angular velocity will decay
exponentially. We measure the autocovariance using stereoscopic particle image
velocimetry (SPIV) applied to particles whose size is within the inertial
subrange of the ambient turbulence. The SPIV resolves the motion of points
interior to the particles, from which we calculate the solid body rotation of
the particles. This provides us with the angular velocity time series for
individual particles. Through ensemble statistics, we determine the
autocovariance of angular velocity and confirm that it matches the form
predicted by an OU process. We can further use the autocovariance curve to
quantify the turbulent rotational diffusivity.
