Steady swimming in the pufferfish Diodon holocanthus : propulsive momentum enhancement is an adaptation for thrust production in undulatory median and/or paired fin swimmers

Lighthill and Blake (1990) developed a form of elongated body theory appropriate for the analysis of undulatory fins attached to a rigid body and proposed a possible benefit due to momentum enhancement relative to the fins “on their own.” In this study, the magnitude of this momentum enhancement was determined by particle image velocimetry (PIV). This momentum enhancement depends on the ratio between half body depth for the major body axis s and the distance from the body midline to the fin tip l (s/l) and the half body depth for the minor axis i and s (i/s). The theoretical momentum enhancement factor for Diodon holocanthus were of the order of 2.2 and 2.7 for the median and pectoral fins respectively and compared well with inferred values based on thrust determined from PIV wake measurements with factors ranging from 2.2 - 2.4 and 2.7 - 2.9. Mean theoretical thrust for the caudal fin with no provision for momentum enhancement was compared to those measured from PIV and no significant difference was found (P > 0.05). There was no significant difference between the average sum of the total fin thrust and fish drag (P > 0.05). Pectoral fin thrust was half that of the median and caudal fins due to high jet angles, low circulation and momentum. Pectoral fin lift was insufficient to overcome the excess weight over buoyancy of the fish, implying weight support from the swim bladder and/or the body. Vortex rings generated by the fins were elliptical and depended on fin chord and amplitude. Hydrodynamic advantages are likely universal among rigid bodied organisms propelled by undulatory fins with the extent of momentum enhancement dependent on fin number, position, s/l and i/s. However, a trade-off between momentum enhancement and thrust generation sets a practical limit to the former. When s/l and i/s are large (small fins), whilst momentum enhancement is high, low absolute thrust is produced to propel a body of large surface area with consequent high drag. Limits to enhancement are also set by reductions in propulsive force associated with progressive reductions in fin wavelength.Science, Faculty ofZoology, Department ofGraduat