title: The role of the lateral line in active drag reduction by clupeoid fishes.
abstract: The lateral-line canals, confined in clupeoid fishes to the two sides of the head, are centred on the two lateral recesses, where thin membranes separate the sea water in the lateral-line system from another fluid (perilymph) in that subcerebral canal which passes through the head between the two lateral recesses. Any pressure difference between the recesses can accelerate fluid in the subcerebral canal, but it is only the effective acceleration of that fluid (i.e. relative to the lateral acceleration of the head) which can tend to generate motions--sensed by neuromasts--in lateral-line canals near the lateral recesses. Furthermore, it is the same effective lateral acceleration (relative to that of the head) that is experienced by water in the thin boundary layer on the surface of the head, where it tends to generate 'crossflows' that may act to increase hydrodynamic resistance (i.e. drag) to the fish's normal swimming movements. These regular swimming movements produce oscillatory sideslip of the fish's head which, by itself, would give such substantial values to the effective acceleration that lateral-line sensors near the lateral recesses would be saturated during normal swimming movements. Any such permanent state of saturation seems rather unlikely. An alternative hypothesis is that the fish actively produces an oscillatory turning of the head, controlled by the sensory output of those same neuromasts in such a way that this output is kept to a minimum. Then that effective pressure difference, which is responsible for the effective lateral acceleration both of perilymph in the subcerebral canal and of sea water in the boundary layer on the head, would be minimised--with advantageous drag-reduction consequences. In order to test this hypothesis, a detailed hydrodynamic analysis was carried out. It suggested that, in order to minimise the effective pressure difference, the yaw angle (in radians) of the fish's head would need to be kept in phase with the sideslip velocity, their magnitudes being in a ratio of about 0.87 U-1 (where U is the swimming speed). Experiments on a swimming clupeoid fish confirmed these conclusions, both about phases and about magnitudes. By contrast, a purely passive response of the head to oscillatory sideforce on the caudal fin would be expected to give the yaw angle a substantial lag behind sideslip, along with a ratio of magnitudes much smaller than 0.87 U-1. Thus, the experiments seems to support the hypothesis regarding active control of drag reduction.
authors.affiliation: Department of Mathematics, University College London.
authors.forename: J
authors.initials: J
authors.lastname: Lighthill
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D005399:Fishes / Q000502:physiology*; D008954:Models, Biological; D012679:Sense Organs / Q000502:physiology*; D013314:Stress, Mechanical; D013550:Swimming / Q000502:physiology*