37.5 Saturday, Jan. 5 Pressure across the head of a freely-swimming rainbow trout (Onchorynchus mykiss) in uniform flow LIAO, J.C.*; CHAMBERS, L. M.; AKANYETI, O.; The Whitney Lab for Marine Bioscience, University of Florida Gainesville; Robotics Laboratory, University of Bristol, UK; The Whitney Lab for Marine Bioscience, University of Florida Gainesville firstname.lastname@example.org
The mathematician Sir James Lighthill indicated that in order to reduce drag swimming fish should move in a manner that reduces the pressure difference across the head. According to his theory, this pressure difference is equal to C1A - C2UΩ , where the coefficients C1 and C2 are defined by the head morphology, U is the swimming speed of the animal and A and Ω are the lateral acceleration and the angular velocity of the head, respectively. The maximum drag reduction is predicted to occur when A and UΩ are in-phase and their ratio is C2/C1. Passive undulating body does not naturally achieve this indicating that active head control is likely required. In this study, we provide the first direct pressure measurements on a free swimming trout and use these measurements to experimentally validate Lighthill’s equation. We swam four rainbow trout of total body length (L) 18.5±1.0 cm (mean ± standard deviation) at 3, 4 and 5 L/s. We simultaneously measured the swimming kinematics and pressure along the head using a high speed camera and miniature pressure catheters. Pressure measurements from all speeds closely matched values estimated by Lighthill; the Pearson’s linear correlation coefficient was 0.89±0.04 (p<0.05). In contrast, the ratio (0.64±0.09) and the phase difference (43.8±6.4°) between A and UΩ differed significantly from the theoretical optimums (0.95±0.08 and 0°, respectively), resulting in an average pressure difference 17.0±7.2 Pa. However, this value is still 49% less than the expected pressure difference for a head rotating passively, indicating that active head control is correlated to the reduced pressure difference.