Research

In nature, birds, insects, and aquatic animals evolved wings and fins to control their movements in complex fluid environments, maintaining precise postures and obtaining efficient propulsion. Inspired by evolution, as part of research on active flow control(AFC), we are conducting a study on improving the propulsion of hydrofoils by applying flow jets. We use deep reinforcement learning(DRL) models for the real-time fine-tuning flow jets on hydrofoils under various flow conditions. This research will be devoted to improving propulsion in underwater vehicles, aircraft wings, and turbine blades, similar to the foil shape. 

Plumed seeds, such as dandelion or milkweed seeds, are wind-dispersal seeds that passively fly by the wind. While other types of wind-dispersal seeds disperse a few hundred meters at maximum, plumed seeds can fly a distance of kilometers. Augmenting counter-gravity drag force is essential for the long-distance dispersion, and the drag is significantly affected by the configuration of a flier. The plumed seed consists of many microscopically thin hairs, which makes the structure porous and elastic, i.e. poroelastic. We adopt a simplified poroelastic cluster to investigate fluid-dynamic mechanisms underlying the long-distance flight of poroelastic seeds. In opposite to common fixed plants that change their shapes to reduce the frontal area encountering the flow, the poroelastic cluster rearranges its constituents to enlarge the frontal area, thereby effectively augmenting the drag force. Moreover, the identification of an optimal porosity that maximizes the drag force has the potential to inform the design of micro-sized drones or wind-dispersal sensors.

Regarding the flapping motion of drag-based propulsion, the deforming profile of the propulsion unit act as a critical factor in determining the thrust performance. Proposing a structure composed of rigid segments connected via elastic hinges as a propulsion unit, the passive deflection of the structure by its fluid-structure interaction during periodic flapping motion and the generated hydrodynamic forces are investigated experimentally. Appropriate expressions of the stiffness and deflection of the structure are developed that well characterizes the behavior of the multi-segmented structure. Furthermore, an analytical model is proposed that estimate the unsteady deformation and propulsive forces of the structure.

For the development of a bio-inspired underwater swimmer with a flexible body, analytical modeling of its dynamics enables comprehensive understanding of the system of interest and can be further utilized to better design propulsion strategies and optimize dynamical behaviors. A feather-like elastic leg structure motivated by feather stars (marine animal) is fabricated, which consists of an elastic shaft and rigid barbs elastically connected to the shaft. Shape reconfiguration of the elastically hinged leg structure under translating motion is experimentally investigated, and a simplified analytical model is developed to estimate the deformation and propulsive force of the structure.