Biologically-inspired bodies under surface waves - Part 1: Load measurements

Measurements have been carried out in a tow tank on cylindrical bodies subm erged in proximity of traveling surface waves. Two bodies are considered: a reference plain cylinder and another cylinder containing a pair of wings ( or hydrofoils) below the cylinder, not above. The latter body owes its orig in to certain species of fish which has small wings for maneuverability. Th e wavelength of the surface waves (lambda) is of the order of the cylinder length (L) or higher (1 < lambda/L < 10). Temporal measurements of axial an d vertical forces and pitching moments, phase matched to the surface elevat ion of traveling waves, have been carried out. The time periods of the wave s and depth of water pertain to deep water and intermediate depth waves. Th e forces and moments exhibit characteristic phase relationship with water e levation. Towing affects only vertical forces in the speed range of a to 1 m/s. The effect of towing and surface waves an vertical forces is roughly a dditive. Within the low speed range of rowing evaluated, the effects of sur face waves dominate those of towing. The presence of the hydrofoil and inte rmediate depth waves bring in some additional effects which are not well un derstood. In intermediate depth waves, a small plain cylinder may encounter a resonance with traveling waves which can be averted by attaching a pair of small wings to dampen pitching moment and make it speed invariant, altho ugh at a cost of increased vertical forces.