Non-isotropy of slip velocity in large negatively buoyant non-spherical particles in turbulence

For inertial particles in turbulent flows, the difference between the particle’s velocity and that of the surrounding fluid can be substantial. This “slip velocity,” though difficult to measure experimentally, is important in particle-laden flow, particularly for studies of turbulence modulation or sedimentation. We present experimental measurements of the slip velocity from simultaneous stereo-PIV of both the solid particle and the surrounding fluid. Our particles are cylindrical (aspect ratios 0.5-4), negatively buoyant (specific gravities of 1.003 and 1.006), and larger than the Kolmogorov scale (by 10-40 times). We find that the slip velocity is anisotropic, with significantly less slip in the vertical (gravity-coupled) direction. Slip velocity increases with particle mass density, even in the non-gravity-coupled directions. We find that aspect ratio barely affects mean particle rotation rate, but higher densities correspond to lower rotation rates. Lastly, we compare particle rotation rate to the surrounding fluid vorticity, hypothesize mechanisms for angular momentum transfer between the fluid and solid phases, and outline future studies to further distinguish between gravitational and finite-size effects on the motion of inertial particles in flow.