Hydrodynamic derivative determination based on CFD and motion simulation for a tow-fish

Abstract

A new methodology to determine hydrodynamic derivatives of a tow-fish underwater vehicle using computational fluid dynamics (CFD) was presented. Hydrodynamic derivatives were derived from a second-order modulus expansion and consist of added mass, linear damping, and nonlinear damping coefficients. Added mass coefficients were analytically obtained using strip theory. All components of linear and nonlinear damping coefficients were determined by CFD simulations without any pre-elimination. The determination procedure for damping coefficients is as follows. First, a series of simulations was prepared for various situations reflecting the actual operating conditions of the tow-fish. Then, actual CFD simulations were performed for each of the cases, and 6 degree-of-freedom (6-DOF) forces and moments acting on the tow-fish were obtained. Finally, all linear and nonlinear damping coefficients were determined by optimized fitting of the CFD results using the least-squares procedure. The effectiveness of the determined hydrodynamic derivatives was demonstrated by reproducing the 6-DOF forces and moments and comparing them with those obtained from the CFD simulations. The relative significance of each coefficient was estimated by a global comparative view of normalized coefficients. To demonstrate the applicability of the current approach, 6-DOF simulations of the tow-fish for three different scenarios (L -, U -, and S -turn scenarios) at various towing speeds were conducted. The efficacy of the current methodology was strengthened by graphical and physical analyses of the virtual simulation results.