Keri M. Collins

Variable stroke timing of rubber fins' duty cycle improves force

Swimming animals can tune their kinematics to achieve increased propulsive performance. To engineer effective propulsive mechanisms, a better correlation between kinematics and dynamics is required in artificial designs. Two rubber fins: one with a NACA aerofoil shape, the other with a biomimetic shape, were used in two asymmetric oscillations in a static water tank. The force generation patterns within the parameter space and the response to the change in stroke timing, were dependant on the fin. The biomimetic fin produced peak force at a similar frequency and amplitude regardless of its kinematics and duty cycle. The response of the NACA fin, however, was dependent on the duty cycle. For the NACA fin, the fast-to-centreline kinematics caused larger resultant force over a narrow range of frequencies. For the fast-to-maximum-amplitude stroke, a lower resultant force was achieved, but over a larger range of frequencies. Digital Particle Image Velocimetry (DPIV) analysis showed the wake pattern of shed vortices. We present experiments and qualitative flow analysis that relate kinematic parameters, particularly the trailing-edge angle to resultant forces.

The interaction between vortices and a biomimetic flexible fin

The fluid-structure interaction of flexible bodies in steady and unsteady flow is a key area of interest for the development of underwater vehicles. In the design of marine vehicles the flow can often be seen as an obstacle to overcome, whilst in nature a fish interacts with the flow and is capable of achieving a high level of efficiency. Therefore by understanding how fish – or flexible bodies – interact with the flow we may be able to achieve a better level of co-operation between our vehicles and their environment, potentially attaining a better efficiency in design.