High-Fidelity Fluid Structure Coupled Simulations for Underwater Propulsion Using Flexible Biomimetic Fins

Abstract

The ability of fish to maneuver in tight places, perform stable high acceleration maneuvers, and hover efficiently has inspired the development of underwater robots propelled by flexible fins mimicking those of fish. In general, fin propulsion is a challenging fluid-structure interaction (FSI) problem characterized by large structural deformation and strong added-mass effect. It was recently reported that a simplified computational model using the vortex panel method for the fluid flow is not able to accurately predict thrust generation. In this work, a high-fidelity, fluid-structure coupled computational framework is applied to predict the propulsive performance of a series of biomimetic fins of various dimensions, shapes, and stiffness. This computational framework couples a three-dimensional finite-volume Navier-Stokes computational fluid dynamics (CFD) solver and a nonlinear, finite-element computational structural dynamics (CSD) solver in a partitioned procedure. The large motion and deformation of the fluid-structure interface is handled using a validated, state-of-the-art embedded boundary method. The notorious numerical added-mass effect, that is, a numerical instability issue commonly encountered in FSI simulations involving incompressible fluid flows and light (compared to fluid) structures, is suppressed by accounting for water compressibility in the CFD model and applying a low-Mach preconditioner in the CFD solver. Both one-way and two-way coupled simulations are performed for a series of flexible fins with different thickness. Satisfactory agreement between the simulation prediction and the corresponding experimental data is achieved.