Development of Simpler Coarse-Grain Model for Analyzing Behavior of Particles in Fluid Flow

A new simpler coarse-grain model (SCG) for analyzing particle behaviors under fluid flow in a dilute system, by using a discrete element method (DEM), was developed to reduce calculation load. In the SCG model, coarse-grained (CG) particles were enlarged from original particles in the same way as the existing coarse-grain model; however, the modeling concept differed from the other models. The SCG model focused on the acceleration by the fluid drag force, and the CG particles’ acceleration coincided with that of the original particles. Consequently, the model imposed only the following simple rule: the product of particle density and squared particle diameter is constant. Thus, the model had features that can be easily implemented in the DEM simulation to comprehend the modeled physical phenomenon. The model was validated by comparing the behaviors of the CG particles with the original particles in the uniform and the vortex flow fields. Moreover, the usability of the SCG model on simulating real dilute systems was confirmed by representing the particle behavior in a classifier. Therefore, the particle behavior in dilute particle-concentration systems would be analyzed more simply with the SCG model.

Mechanisms of size segregation in granular flows with different ambient fluids

Particle size segregation is ubiquitous in granular systems with differently sized constituents but is found to diminish in the presence of viscous ambient fluids. We study this inhibiting effect through coupled fluid-particle numerical simulations. It is found that size segregation is indeed slower in the presence of fluid and this effect becomes more significant as fluid viscosity is increased. Direct calculation of segregation forcing terms reveal that the ambient fluids affect segregation in two major ways: buoyant forces reduce contact pressures, while viscous dissipation diminish particle-fluctuation driven kinetic pressures, both of which are necessary in driving large particles up. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not directly affect segregation.

Fluid Drag With and Without Vortex-Induced Vibrations

Fluid drag is an integrated force that depends on the velocity of the fluid flow relative to the motion of a structure. In previous OMAE papers, we used nonlinear physics-based time-domain simulations to show how fluid drag evolves geometric changes in slender (long and thin) structures. We then showed how these changes physically determine the specific dynamic nature of the vibrations that the fluid can induce in the structure. Induced vibrations are four-dimensional oscillations in a marine riser, suspended pipe or other slender structure, whereby the maximum amplitude of deflection is generally perpendicular to the sustained action. The sustained action is often fluid drag. In this paper, we study the physical relationship between fluid drag and induced vibrations. By focusing on the nonlinear interaction between fluid and structure, we revisit a longstanding belief that vortex-induced vibrations amplify fluid drag. Using nonlinear physics-based simulations of a slender structure interacting with flowing fluid, we show how amplification depends on the type of vibration (imposed or free). In other words, drag amplification can occur when we impose a vibration on the structure, but does not occur when we allow sufficient geometric freedom so that the fluid merely induces the structure to vibrate. Using simple visual experiments, we confirm that Vortex-Induced Vibrations (VIV) do not amplify fluid drag. This result is consistent with basic energy conservation principles.

Flapping, swirling and flipping: Non-linear dynamics of pre-stressed active filaments

Initially straight slender elastic filaments and rods with geometrically constrained ends buckle and form stable two-dimensional shapes when compressed by bringing the ends together. It is known that beyond a critical value of this pre-stress, clamped rods transition to bent, twisted three-dimensional equilibrium shapes. Here, we analyze the three-dimensional instabilities and dynamics of such pre-stressed, initially twisted filaments subject to active follower forces and dissipative fluid drag. We find that degree of boundary constraint and the directionality of active forces determines if oscillatory instabilities can arise. When filaments are clamped at one end and pinned at the other with follower forces directed towards the clamped end, stable planar flapping oscillations result; reversing the directionality of the active forces quenches the instability. When both ends are clamped however, computations reveal a novel secondary instability wherein planar oscillations are destabilized by off-planar perturbations resulting in three-dimensional swirling patterns with periodic flips. These swirl-flip transitions are characterized by two distinct and time-scales. The first corresponds to unidirectional swirling rotation around the end-to-end axis. The second captures the time between flipping events when the direction of swirling reverses. We find that this spatiotemporal dance resembles relaxation oscillations with each cycle initiated by a sudden jump in torsional deformation and then followed by a period of gradual decrease in net torsion until the next cycle of variations. Our work reveals the rich tapestry of spatiotemporal patterns when weakly inertial strongly damped rods are deformed by non-conservative active forces. Practically, our results suggest avenues by which pre-stress, elasticity and activity may be used to design synthetic fluidic elements to pump or mix fluid at macroscopic length scales.