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We consider the rotation of neutrally buoyant axisymmetric particles
suspended in isotropic turbulence. Using laboratory experiments as well
as numerical and analytical calculations we explore how particle
rotation depends upon particle shape. We find that shape strongly
affects orientational trajectories but that it has negligible effect on
the variance of the particle angular velocity. Previous work has shown
that shape significantly affects the variance of the tumbling rate of
axisymmetric particles. It follows that shape affects the spinning rate
in a way that is, on average, complementary to the shape-dependence of
the tumbling rate. We confirm this relationship using direct numerical
simulations, showing how tumbling rate and spinning rate vary, on
average, in complementary ways for both rod-shaped and disk-shaped
particles. We also consider a random but non-turbulent flow. This allows
us to explore particle rotation in an unsteady flow in the absence of
turbulent vortical structures. As expected many of the features observed
for rotation in turbulent flow are due to the effects of particle
alignment in vortex tubes.
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