Fluid Dynamic Efficiency of Unsteady Propulsion
Propulsion systems in biology differ from most engineering systems in their use of ambient flow as the working fluid and in their unsteadiness. These effects cannot be unambiguously accounted for in traditional definitions of propulsive efficiency, which are typically based on steady-flow rocket and jet technologies. The objective of this project is to develop general metrics for fluid dynamic efficiency that are sufficiently robust to facilitate quantitative comparison between engineering and biological propulsion systems. Within this framework, the design of “bio-inspired” propulsion devices can proceed using an objective metric to evaluate their performance. Efficiency tests are conducted in the 40-meter water tunnel at Caltech. The length of the facility allows for the measurement of mobile systems operating over long time scales.
Fluid Dynamic Energy Conversion
The extraction of energy from a moving fluid is a fundamental challenge in the field of fluid dynamics. Success requires a quantitative understanding of the physical processes governing the transfer of energy between the fluid and solid bodies in the flow. The goal of this project is to understand the mechanisms of fluid dynamic energy conversion by studying biological systems that are known to regularly accomplish this feat. The problem is posed in terms of the coupled fluid-structure interactions that govern the energy extraction process. These interactions are reproduced in controlled laboratory experiments and are observed in situ using laser velocimetry techniques. Models of the energy conversion process are being developed to guide the design of engineering systems based on the discovered physical principles.
Lagrangian Analysis of Fluid Transport in Vortex Flows
Unsteady flows such as those that are commonly observed in biological systems are not easily described using metrics based on the Eulerian velocity field. New methods of analysis based on a Lagrangian description of fluid motion have the potential to facilitate more accurate descriptions of fluid transport in vortex flows that exhibit transient, aperiodic behavior. This project is concerned with the application of theoretical methods of Lagrangian analysis to empirically measured flows. Canonical fluid systems being studied include vortex rings and bluff-body wakes, providing a range of both simple and more complex flows in which to apply the analytical methods. Experiments are conducted using mechanical flow generators and live animals such as jellyfish to explore the robustness of the developed methods in practice.
Jellyfish Biomechanics
Medusae in the phylum Cnidaria are an integral component of ocean ecology and also represent one of the oldest forms of biological propulsion still in existence. Their simple morphology and body kinematics make them a promising candidate for studies aimed at uncovering design principles that have led to the success of these and more complex biological propulsion systems. Empirical observations and theoretical modelling of normal and artificially-stimulated swimming and feeding behavior are used to aid in building a system-level understanding of jellyfish function, which can be mimicked and improved upon in engineering systems.
Optimal Vortex Formation
Animal phyla that require macro-scale fluid transport for functioning have repeatedly and often independently converged on the use of jet flows. During flow initiation these jets form fluid vortex rings, which facilitate mass transfer by stationary pumps (e.g. cardiac chambers) and momentum transfer by mobile systems (e.g. jet-propelled swimmers). Previous research has shown that vortex rings generated in the laboratory can be optimized for efficiency or thrust, based on the jet length-to-diameter ratio (L/D), with peak performance occurring at 3.5 < L/D < 4.5. This research is aimed at developing a generic, quantitative framework in which to observe and characterize optimal vortex formation in biological fluid transport systems. The implemented approach identifes simple rules for effective fluid transport that can be exploited in engineered fluid transport systems.
Biological Flow Velocimetry/Dynamometry
Effective flow visualization is essential to the analysis of biological propulsion systems. Currently, standard laser-based tools are utilized in the study of biological fluid transport. The applicability of these methods is often limited when studying biological systems due to sensitivity of the target animal to environmental factors (e.g., laser intensity, flow seeding, etc.). Rather than adapt existing engineering tools to biological studies, this project aims to develop new tools for non-intrusive measurement of the generated flow fields and resultant fluid dynamics. The flow patterns generated by jellyfish are used as a canonical application of the newly developed experimental methods.
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