Slip velocity of large low-aspect-ratio cylinders in homogeneous isotropic turbulence

Abstract

Using laboratory experiments, we investigate the turbulent slip velocity of large (Taylor-scale), slightly negatively buoyant cylinders at two mass densities and four aspect ratios. Using refractive index matching and stereoscopic Particle Image Velocimetry, we simultaneously record the velocities of both the cylinders and the surrounding homogeneous isotropic turbulence. We find that despite the large size and concomitant inertia of the cylinders, the cylinder velocity is well predicted by the fluid velocity immediately surrounding it: the particle velocity is close to the fluid velocity across a large range of fluid velocities, barring a small slip velocity. We apply a decomposition to this slip velocity and find that its average is nonzero in the vertical direction, indicating gravitational settling, but is approximately zero in the two lateral directions, reflective of the isotropy in the turbulence. The average slip velocity is subtracted from instantaneous slip velocity to find the “true slip”, with the anisotropic effects of gravitational settling ostensibly removed. However, this true slip also shows the effects of gravity, indicating that large particles with even a small departure from neutral buoyancy may sample the flow anisotropically. In other words, gravity creates anisotropy in the slip velocity even without considering the effects of settling/sedimentation. We discuss potential mechanisms for this effect, including preferential concentration and resuspension, and conclude that gravity should be considered in studies of large particles even when the particles are very close to neutrally buoyant.