Large ellipsoids rising in quiescent fluids and turbulent flows*

Solid particles in nature or industrial applications are almost never perfectly spherical; there is always some measure of anisotropy in their topology. Understanding the effect of geometry on the rise trajectory and dynamics of such particles is therefore highly relevant, as is their interaction with free-stream turbulence. Many particles are well described by assuming them to be ellipsoidal with aspect ratio χ = h/d. We experimentally investigate light (buoyant) particles of density ratio Γ  = 0.52, all with a volume equivalent sphere-diameter of d = 0.02m and a Galileo number of 6300. The aspect ratio is varied from χ = 0.25, oblate, to χ = 4 prolate. Using translational and rotational particle tracking, we investigate the particle behaviour when rising through a quiescent fluid and a turbulent flow of Re_λ ≈ 300. A strong dependence of drag coefficient on aspect ratio is found. Generally the value of C_d is reduced for turbulent flow compared to a quiescent liquid. Further, we characterise the particle kinematics and dynamics in terms of a Strouhal number and orientation and acceleration statistics.