Propulsive performance of unsteady tandem hydrofoils in a side-by-side configuration

Abstract

Experimental and analytical results are presented on two identical bio-inspired hydrofoils oscillating in a side-by-side configuration. The time-averaged thrust production and power input to the fluid are found to depend on both the oscillation phase differential and the transverse spacing between the foils. For in-phase oscillations, the foils exhibit an enhanced propulsive efficiency at the cost of a reduction in thrust. For out-of-phase oscillations, the foils exhibit enhanced thrust with no observable change in the propulsive efficiency. For oscillations at intermediate phase differentials, one of the foils experiences a thrust and efficiency enhancement while the other experiences a reduction in thrust and efficiency. Flow visualizations reveal how the wake interactions lead to the variations in propulsive performance. Vortices shed into the wake from the tandem foils form vortex pairs rather than vortex streets. For in-phase oscillation, the vortex pairs induce a momentum jet that angles towards the centerplane between the foils, while out-of-phase oscillations produce vortex pairs that angle away from the centerplane between the foils.

Learning to school in the presence of hydrodynamic interactions

“…9) to weaken anti-correlation effects, allowing for better stability and reduced effort, consistent with the above observations. We conclude that our findings, within the limits of our modeling approach, qualitatively capture the salient features associated with two-body swimming interactions [48,49].…”

“…The results reported in this section may be put in context in light of recent experiments that systematically investigated the thrust, power and efficiency performance of two side-by-side [48] and in-line pitching airfoils [49]. In the side-by-side case [48], it is found that the performance of individual airfoils are always anti-correlated except for perfectly in-phase or out-of-phase actuation (which may not be realistic or robust in a schooling system). This arrangement entails an overall constant system efficiency and the generation of a net torque, which needs to be compensated for in stable schooling arrays.…”

Locomotion of a bioinspired flyer powered by one pair of pitching foils

“…Although the complicated three-dimensional effects and the wing flexion are excluded in the model, this study can still shed light on the aerodynamic characteristics of the flying machine reported in [13]. It should be noted that some investigations have been conducted before on the dual pitching-foil system under either tethered or free-swimming condition [20,21]; however, the performance of such system in flying against gravity has received little attention.…”

Unsteady Performance of Finite-Span Pitching Propulsors in Side-by-Side Arrangements

“…The case of foils in side-by-side arrangements has also received some limited attention [1,15,17,18]. In the numerical study of Dong and Lu [18], two wavy foils traveling in side-by-side arrangement were considered and a reduction in the power consumption and an enhancement in the fluid mediated forces was reported for in-phase and out-of-phase cases, respectively.…”

“…No other synchronies were considered. In the experimental study of Dewey et al [1], a larger synchrony and foil spacing parameter space was investigated, and flow field analysis was provided in connection with force measurements. It was found that in-phase oscillations lead to increases in both thrust generation and power consumption while propulsive efficiency stays at the same level of two foils in isolation whereas for out-of-phase oscillations a decrease in the collective thrust and power and a concurrent increase in collective efficiency occurs.…”