Abstract

The vortex structure and hydrodynamic performance of a tadpole undulating in the wake of a blunt body are studied by solving the Navier-Stokes equations for the unsteady incompressible viscous flow. A dynamic mesh fitting the tadpole's deforming body surface is used in the simulation. It is found that three main factors can contribute to thrust of the tadpole behind a blunt body. At different undulating frequency, a tadpole may break or dodge vortices from the D-cylinder. However, vortices from a sphere are broken by the tadpole's snout.

Introduction

Aquatic animals are subject to the complex flows caused by abiotic and biotic sources. They are able to reduce locomotory cost by interacting with vortices that arise from fluid flow past stationary objects. Understanding how aquatic animals extract energy from environmental vortices is a topic of considerable interest. In particular, the flow around a circular cylinder or a sphere has been well studied and is one of the classical problems of fluid dynamics. Studies on the interaction between fish and vortices shedding from a cylinder have provided a focused way to begin to understand how fish swim in complex flows.

Experimental studies on the fish swimming in the vortex wakes have been carried out in recent years. Beal et al (2006) and Fish et al (2006) found the vortex wakes may lead to a passive propulsion to the fish in the wake. Liao et al (2003a, 2003b, 2007) compared fish swimming in the wake of a D-cylinder to those swimming in free stream. It was shown that fish behind a D-cylinder adopts novel body kinematics behind a cylinder, termed the Karman gait. The fish changes its undulating frequency and wavelength to synchronize the vortices shedding from the D-cylinder, slaloming between vortices shedding from the D-cylinder rather than swimming through them. On the other hand, many numerical simulations on fish interacting with the environmental vortices have also been carried out.

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