Yunxing Su, PhD

We present the design, manufacture, and validation of the Pleobot—a unique krill-inspired robotic swimming appendage constituting the first platform to study metachronal propulsion comprehensively. We combine a multi-link 3D printed mechanism with active and passive actuation of the joints to generate natural kinematics. Using force and fluid flow measurements in parallel with biological data, we show the link between the flow around the appendage and thrust. 

We focus on the effects of swimming directions on the swimming speed and the induced flow fields by an individual swimmer as well as the related mixing effects by quantifying the Darwinian drift volume of a swimming organism. Experimental observation data and numerical modeling (squirmer) are presented and compared to further understand the three-dimensional flow fields and the related drift volume of individual swimmers at different swimming directions.

Hemolysis persists as a common and serious problem for neonatal patients on extracorporeal membrane oxygenation (ECMO). While the cannula within the ECMO circuit is associated with hemolysis-inducing shear stresses, a complete understanding of this mechanism is currently needed. This fluid dynamics study generates what is believed to be the first in vitro data for shear stresses inside clinical dual-lumen cannulas (DLCs), and the analysis is extended to quantify hemolysis risk over a typical ECMO treatment duration.

We examine the process of an isolated vortex ring crossing the interface between two miscible liquids of different densities (two-layer stratification). Experiments were compared for vortex rings crossing the interface from below and above the interface. Using experimental measurement techniques of Planar Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), we visualize the vortex ring crossing the density interface, track the vortex ring size and position, and examine the vorticity conservation of the vortex ring.

The effects of viscoelasticity have been shown to manifest themselves via symmetry breaking. In this investigation, we show a novel phenomenon that arises from this idea. We observe that when a dense sphere is rotated near a wall (the rotation being aligned with the wall-normal direction and gravity), it levitates to a fixed distance away from the wall. Since the shear is larger in the gap (between the sphere and the wall) than in the open side of the sphere, the shear-induced elastic stresses are thus asymmetric, resulting in a net elastic vertical force that balances the weight of the sphere.

We experimentally investigate the interaction between a pair of bubbles ascending in a stagnant viscoelastic shear-thinning fluid. In particular, we focus on the effect of bubble size, across the velocity discontinuity, on the bubble-bubble interaction. In the dancing phase, the bubble pair repeatedly interchange their relative leading and trailing positions as they rise to the free surface. The flow fields around the bubble pair interaction are obtained using particle image velocimetry (PIV).

The aeroelastic response and energy harvesting performance of an elastically mounted hydrofoil subject to a prescribed pitching motion are experimentally studied using a cyber-physical force-feedback control system in a uniform flow. By taking advantage of this cyber-physical system, we systematically sweep through the parameter space of the elastic support (stiffness, damping, and mass) for various frequencies of the prescribed pitching motion. It is found that the flow-induced heave amplitude and the energy harvesting performance are both strongly affected by the frequency ratio between the prescribed pitching frequency and the natural frequency of the system and the damping coefficient.