Analysis of the Boundary Layer and Near-Field Flow Around Swimming Fish

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The object of this research is to measure fluid velocities to within 0.1 mm of the body surface of a freely swimming fish, and use these measurements to determine drag and other characteristics of the unsteady flow field. Up until now, the drag of a swimming fish has not been definitively measured, yet its value is important for determining how much power a fish uses for locomotion. In the past, researchers have used a "rigid-body" concept, in which the drag of an equivalently shaped object stretched straight in the flow is used as a proxy for the swimming case. Such estimates have led to a number of contradictory conclusions about fish swimming ability. There is no doubt that the lateral body motion used by fish for propulsion will alter the fluid velocities near the fish body. How these changes to the velocity field affect the drag remains unknown. The major source of drag on any streamline body, rigid or swimming, is confined to a very thin layer of fluid along the body surface. Within the very thin layer, called the "boundary layer", fluid velocity increases from zero at the body surface to approximately the speed of the current in only a few millimeters. In order to measure the drag of a swimming fish, high-resolution measurements of the fluid velocities must be made within this microscopic region and also just outside the boundary layer in the "near-field." For this research, non-invasive techniques based on digital particle image velocimetry are used to measure flow velocity in the boundary layer and near field of swimming fish. Pressures and shear stresses over the body surface are determined from these measurements and combined to form estimates of total viscous and pressure drag. The nature of the flow is analyzed to determine if the boundary layer is laminar or turbulent and if the boundary layer has separated over any part of the fish body. These questions have been debated for years and are important for examining proposed drag-reducing capabilities in swimming fish. Five local species of fish will be used in a comparative study. These include scup, Atlantic mackerel, American eel, and two local dogfish species. These species encompass a range of swimming motions from anguilliform to carangiform modes. Scup are also included because the observed drag can be compared to existing measurements of in vivo muscle power. The results of this study will further the understanding of how fish swim and how fish power swimming with their muscles. The results will also improve understanding of energetic behavior as it applies, for example, to migration, schooling behavior, and flow sensing by the fish lateral line.