COMPUTATION OF UNSTEADY BOW THRUSTER HYDRODYNAMICS USING A LAGRANGIAN VORTICITY METHOD

摘要

A novel method to compute unsteady hydrodynamics with application to Unmanned Undersea Vehicle bow thrusters is presented. This approach solves the vorticity equation which is derived from the momentum equation of the Navier-Stokes equations. For incompressible flow, the entire flow can be described in terms of the vorticity. Velocity is an integral quantity of the instantaneous vorticity field. For this problem, a UUV body is represented using surface source and vortex panels whose strength is prescribed to satisfy the no-slip and no-flux boundary conditions. The thruster is modeled using these panels with the inlet and outlet normal velocity component specified. Vorticity is diffused from the vortex sheets onto the body surface to maintain a vorticity balance. Vorticity in the flow is specified at points and the vorticity at any other point in the field is obtained via linear interpolation. Interpolation is performed by constructing tetrahedra using Delaunay traingularization. Tetrahedra provide the control volume to integrate over to obtain the velocity and the connectivity of the control points provides a basis to construct derivatives. Application of this method illustrates the unsteady flows produced by the application of the bow thruster. Vortex structures are produced which give rise to unsteady loading over the UUV. Plots of the instantaneous vorticity field, flow streamlines, surface pressure and integrated thruster normal and drag force are compared with experimental data.