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Title: Physical Mechanisms of Force Production for Swimming in Fishes
Abstract: Fishes must swim effectively to perform many behaviors that are important for evolutionary fitness. So, understanding how a fish’s body morphology, mechanical properties, and movements combine to enable swimming can provide insight into the evolution of their present body forms. Here, I use physical models and experimental fluid dynamics to study the relationships between fish body properties and swimming performance.
The first section of this dissertation centers on the contributions the body’s flexural stiffness makes to overall swimming performance. In my previous work, I found that many swimming and flying animals, including fishes, bend their bodies and appendages in a similar way during steady locomotion in part due to non-uniform flexural stiffness in their propulsive structures. In Chapter 1, I use a simple physical modeling system to isolate the effects of flexural stiffness from the complexity of a fish body. Using versions of the simple model with different flexural stiffnesses, I show that having non-uniform stiffness along the body length leads to faster, more efficient swimming. I extend these results in Appendix I, where I compare several tuna-tail-shaped models of different stiffnesses and find that certain combinations of stiffness and fish-like movements improves swimming ability. The findings indicate that the non-uniform physical properties of different components of a fish’s body are paramount for maintaining high overall swimming performance, and I hypothesize that fish may be controlling their body stiffness and movements to achieve performance benefits.
In the second section of my dissertation, I focus on how the fish’s body interacts with the surrounding water to create swimming forces. In the past, our understanding of these interactions has been limited by the difficulty of measuring forces in a fluid medium. Therefore, in Chapter 2, I develop and validate new tools that allow me to estimate these forces at high spatial and temporal resolution using a pressure-based approach. I then apply these tools in Chapter 3 to describe how carangiform swimmers – fishes swimming using only their bodies and caudal fins and undulate only the posterior portion of their bodies at large amplitudes – generate swimming forces. I find that both positive and negative pressure contribute to thrust and drag. Further, in contrast to previous hypotheses, I find that carangiform fishes generate both thrust and drag along much of the length of their bodies in complex temporal patterns. I conclude by describing how subtle differences between different carangiform swimmers’ movements and body shape lead to significant differences in force production.
Taken together, my results suggest that fishes may simultaneously use a variety of mechanisms to produce swimming forces, and identifies specific body features and movements which dictate swimming abilities. Future work will continue to explore the relationships between body morphology, material properties, and swimming abilities, and to apply this knowledge to critically evaluate existing frameworks of hypotheses linking fish morphology to specific habitats and ecological roles.
Committee: Peter R. Girguis (Advisor), Dr. Eric D. Tytell (Tufts)