Unsteady propulsion and the acoustic signature of undulatory swimmers in and out of ground effect

The propulsive performance and acoustic emission of undulatory swimmers are investigated using an integrated unsteady potential flow and acoustic boundary element solver. Anguilliform and carangiform swimming gaits are modeled by a deforming NACA 0012 airfoil section for various reduced frequencies and dimensionless wave numbers based on the body length. The most efficient swimming motions are achieved when the reduced frequency and dimensionless wave number are approximately equal, a condition which also minimizes the thrust, required power, and radiated acoustic pressure. A vertically oriented dipole dominates the transient acoustic response in the near and far fields for both classes of swimming gait. The effect of a ground plane on undulatory swimming and its associated noise generation is examined as a function of reduced frequency and altitude above the plane using the method of images. Ground effect becomes pronounced when swimming within half of a chord length from the ground plane, where the thrust, power, propulsive efficiency, and peak acoustic pressure all increase for both gaits. All swimming configurations simulated in this study experience positive period-averaged lift at proximal distances where increased thrust and other hydrodynamic performance benefits associated with ground effect occur.