Bending Moment Distribution along Swimming Fish

A dynamic analysts of body undulation was carried out for the saithe and an assumed eel-like swimmer. It was found that the three-dimensional unsteady waving plate theory results in a better momentum balance than the slender body theory. For subcarangiform swimmers the recoil effect requires that the envelope of the waving motion increases non-linearly close to an exponential function starting from a non-zero value at the head. The local bending moments necessary to produce such a movement pattern appear almost simultaneously (standing wave) within the middle segment. Within the following caudal segment the bending moment travels backward with a phase-velocity close to that of the undulatory wave. For both the eel and the saithe bending moments display a rostrally running wave within the rostral body segment. The phase coupling between moment and bending moments roughly agrees with the one observed between movement and muscle activity. Owing to the influence of hydrodynamic forces a phase shift between muscle activity and movement is necessary for both subcarangiform and anguilliform swimmers.