The dorsal and anal fins of fish interact with the tail fins to produce higher thrust and efficiency. In particular, we focused on thin elongated dorsal fins, like those of jackfish. We discovered that the fins act like wing strakes on fighter jets, promoting flow attachment on a main lifting surface (wing/tail) by inducing spanwise flow and reducing the effective angle of attack. We found that beyond a critical sharpness, this effect is more costly than beneficial, meaning dorsal fins may be optimal when they are slightly blunted rather than razor sharp. (This work was done in collaboration with the Flow Simulation Research Group at the University of Virginia.)
Fish flap their tails asymmetrically to maneuver around obstacles. In contrast, classic fish tail models assume symmetric motions in a uniform flow. We tested how well these classic models work for maneuvering tails. In some cases, the models work well: even 2D wakeless models were able to predict the phase of high frequency lateral displacements. As for predicting overshoot and settling time, only a semi-empirical model was accurate to within 10%.
Fish and birds experience different forces when they swim/fly near a flat surface (e.g. seabed, solid ground, still lake). We discovered that the vertical forces they feel switch from negative (downward) to positive (upward) at a particular distance from the surface. In other words, there’s a stable equilibrium altitude where they are neither pushed down nor up. Animals and bio-inspired robots should factor this altitude into their control schemes; ignoring it could lead to high energy costs when swimming/flying near a flat surface. (This work was done in collaboration with the Biofluids Research Lab at Lehigh University.)
Pilots need complex instruments and training to safely fly through gusts when their vision is deprived. In contrast, birds fly reliably over open water and at night, despite being more susceptible to gusts due to their much lower flight speeds. We found that even inexperienced lovebirds can navigate through strong opposing gusts in the dark. Their ability is surprising, because it was previously thought that diurnal animals needed a visual horizon and image features moving over their retina to maneuver. (This work was the culmination of a multi-year project that started when PI Quinn was in the Lentink Lab at Stanford University.)
Building on our previous work on 2-D pitching airfoils, we explored how forces and torques scale for 3-D pitching airfoils. The terms we added to existing theories were inspired by the 3-D elliptical ring shapes of wake vortices. We validated the new terms by comparing our predictions with water channel experiments over a range of frequencies, amplitudes, and aspect ratios. The modified scalings offer guidance for theories about fish morphology and design strategies for bio-inspired robots. (This work was done in collaboration with the Biofluids Research Lab at Lehigh University.)