Importance of fluid inertia for the orientation of spheroids settling in turbulent flow

Abstract

How non-spherical particles orient as they settle in a flow has important practical implications in a number of scientific and engineering problems. In a quiescent fluid, a slowly settling particle orients so that it settles with its broad side first. This is an effect of the torque due to convective inertia of the fluid set in motion by the settling particle, which maximises the drag experienced by the particle. Turbulent flows tend to randomise the particle orientation. Recently the settling of non-spherical particles in turbulence was analysed neglecting the effect of convective fluid inertia, but taking into account the effect of the turbulent fluid-velocity gradients on the particle orientation. These studies reached the opposite conclusion, namely that a rod settles preferentially with its tip first, wheras a disk settles with its edge first, therefore minimizing the drag on the particle. Here, we consider both effects, the convective inertial torque as well as the torque due to fluctuating velocity gradients, and ask under which circumstances either one or the other dominate. To this end we estimate the ratio of the magnitudes of the two torques. Our estimates suggest that the fluid-inertia torque prevails in high-Reynolds number flows. In this case non-spherical particles are expected to settle with a maximal drag. But when the Reynolds number is small then the torque due to fluid-velocity gradients may dominate, causing the particle to settle with its broad side first.

References in corpus (14)

• On the drag of freely falling non-spherical particles
• Rotation Rate of Rods in Turbulent Fluid Flow
• Finite-size effects in the dynamics of neutrally buoyant particles in turbulent flow
• Gravity-driven enhancement of heavy particle clustering in turbulent flow
• Shape-dependence of particle rotation in isotropic turbulence
• Tumbling of small axisymmetric particles in random and turbulent flows
• Accumulation of motile elongated micro-organisms in turbulence
• Preferential sampling and small-scale clustering of gyrotactic microswimmers in turbulence
• Orientation dynamics of small, triaxial-ellipsoidal particles in isotropic turbulence
• Statistical model for the orientation of non-spherical particles settling in turbulence
• Theory for the effect of fluid inertia on the orientation of a small particle settling in turbulence
• Time-dependent lift and drag on a rigid body in a viscous steady linear flow
• Angular dynamics of a small particle in turbulence
• Intrinsic viscosity of a suspension of weakly Brownian ellipsoids in shear

Cited by in corpus (8)

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• Inertial torque on a small spheroid in a stationary uniform flow
• Effect of particle inertia on the alignment of small ice crystals in turbulent clouds
• Fluid inertial torque is an effective gyrotactic mechanism for settling elongated micro-swimmers
• Experimental validation of fluid inertia models for a cylinder settling in a quiescent flow
• Velocity and acceleration statistics of heavy spheroidal particles in turbulence
• Pair statistics of oblate spheroids settling in a turbulent flow
• Palabos Turret: A Particle-Resolved Numerical Framework for Settling Dynamics of Arbitrary-Shaped Particles