Wake-induced dynamics of buoyancy-driven and anisotropic particles

The dynamics and kinematics of particles rising and falling in a fluid is a fundamental problem in fluid mechanics that has been studied for centuries. Despite the apparent simplicity the behaviour of particles due to gravity and buoyancy is still not fully understood and remains hard to predict a priori. The reason for this is the complex interaction between the particle motion and the fluid structures in the wake of the body. This coupling can result in unpredictable and often chaotic behaviour. In this work we contribute to this field by decomposing the parameter space and separately examining effects of centre of mass (CoM) offset, rotational moment of inertia (MoI), and geometrical anisotropy.

We show that CoM offset, an often overlooked parameter, can be used to shape the dynamics of spherical particles through a coupling of rotational and translational motion. The CoM offset introduces an intrinsic rotational timescale to the problem akin to that of a pendulum. The torque induces by the offset interacts with the natural, vortex-shedding induced, particle rotation, and thus modifying the vortex-shedding dynamics themselves, the oscillation frequency and amplitude, and most importantly the drag. Similarly we investigate the role of MoI. We find that this parameter only has a marginal effect in governing the dynamics of rising spheres. However, we do observe that the kinematics and dynamics are affected by the ratio of particle density to fluid density, confirming the existence of a critical density ratio for rising spheres. The previous two properties are related to the particle internal mass distribution, the final parameter we examine is an external one: the particle geometry. We find, by gradually varying the particle aspect ratio of positively buoyant spheroids (from disc-shaped to needle-shaped), that the particle motion can be classified into 6 distinct regimes, each exhibiting its own characteristic dynamics, alignment, amplitude, and drag. Finally, we numerically investigate the alignment and distribution of slender fibers (needle-shaped spheroids) in Taylor-Couette turbulence and find these to be strongly related to the background flow state.