Body-Involved Fluid Dynamic Interactions and Performance Enhancement Mechanisms in Biological Propulsion

After millions of years of evolution, aerial and aquatic animals in nature have developed specialized morphological features and superior locomotion strategies to interact with the surrounding fluids to achieve fast and efficient flying/swimming. Pronounced fluid dynamic interactions between the body and the fins/wings are commonly observed in insects/birds and fishes, which can be beneficial for their propulsive performance.

This dissertation combines experimental and numerical approaches to examine the body-involved fluid dynamic interactions (BI-FDI) and the underlying flow physics in nature across major forms of body-propulsor configurations, morphological features, and locomotion modes. High-fidelity computational models with flying/swimming kinematics are built based on high-speed videos of live animal locomotion. Numerical simulations are then conducted using an immersed boundary based direct numerical flow solver to obtain the hydrodynamic performance and detailed flow field information for the analyses of performance enhancement and body-involved vortex dynamics.

It is found that aerodynamic/hydrodynamic performance enhancement owing to BI-FDI widely exists in the flying/swimming animals examined. Specifically, A 29% overall lift enhancement due to wing-body interaction is found in the “+”-type bilateral propulsion of hummingbird forward flight. Vortex dynamics results showed formations of unique body vortex pairs on the dorsal thorax of the hummingbird where low-pressure zones were created to generate more body lift. Significant interactions between body vortex and leading-edge vortex (LEV) were observed, resulting in strengthened LEVs near the wing root and enhanced wing lift generation during the downstroke of the wings.

For in-line propulsion of thunniform swimming in tuna, it is found that the independently mobile finlets help increase caudal fin thrust by 8% and reduce trunk drag by 7%. The effect of swimming with finlets is equivalent to adding a propulsor with nominal propulsive efficiency of 23.6%. Detailed flow analysis reveals that the presence of finlets at the dorsal and ventral margins is responsible for the trunk drag reduction and the interactions between the finlet-induced vortex pair and the caudal fin is responsible for the caudal thrust enhancement. The pitching kinematics of finlets help reduces the finlet drag, lateral force amplitude, and power consumption, resulting in a higher nominal propulsive efficiency of 23.6% than the 16.6% of body-fixed finlets.

For in-line propulsion of carangiform swimming in trout, the dorsoventrally asymmetric dorsal fin and anal fin use different mechanisms to reduce trunk drag and enhance caudal fin thrust. The dorsal fin induces lateral flow and dorsal fin vortex that strengthen the leading edge vortex and creates a larger pressure difference between the two sides of the caudal fin, resulting in an 11% thrust increase. The presence of the anal fin prevents both the local lateral flow across the ventral edge of the trunk and the formation of peduncle vortex that is destructive for caudal fin thrust production, resulting in a 6.9% trunk drag reduction and a 4.3% caudal fin thrust increase, respectively. In addition, the pelvic fins help reduce all the anal fin drag owing to beneficial interactions.

For in-line propulsion of anguilliform swimming in the leech, both the trunk and the posterior sucker produce thrust from pressure forces and suction forces acting on the body surface to balance the viscous drag. The trunk induces a counter-rotating edge vortex pair that then interacts with the lateral edges of the trunk in the following stroke. A strong correlation between the more intense vortex-trunk interaction and larger suction thrust production at the dorsal trunk surface is found when the aspect ratio of trunk cross section (AR) increases. The larger suction/pressure forces and the more dorsal-ventrally orientated normal vector of the trunk surface result in the larger thrust production at the trunk with higher AR. The vortex structure shows a less significant edge vortex at a lower Reynolds number. A similar pattern in thrust distribution along the body is found at different Reynolds numbers. The vortex structure shows severer edge vortex separations associated with more small-scaled vortex structures at higher Strouhal number and less strong edge vortex at lower Strouhal number. Thrust production of the anterior trunk is most sensitive to Strouhal number change.

Two major categories of the body-involved performance enhancement mechanisms—the deflection or prevention of transverse flow across body edges and wake capture or wake prevention at the propulsor—are found in the BI-FDIs examined in this dissertation work.

The overall findings of this dissertation advance the understanding of body-involved performance enhancement mechanisms in biological propulsion and provide novel physical insights into the design of aerial/underwater unmanned vehicles from a fluid dynamics perspective.