Characterizing virtual mass effects of a submersed body using pseudo-fluid elements

A recent hardware failure on a hydraulically-loaded experiment inspired an investigation into the method of determining natural frequencies of complex geometry immersed in a fluid. The failure indicated that the current methodology used to assess the hydrodynamic mass effect - namely, the estimation of the applicability of empirically-derived formulas based on a qualitative assessment of geometric similarity - may be significantly inaccurate. The methodology outlined in this paper demonstrates a potential solution to the problem faced. As opposed to utilizing a physics domain that includes the use of both computational fluid dynamics (CFD) and computational structural mechanics (CSM), a single physics domain was leveraged by modeling the fluid as a discrete 'pseudo-fluid' composed of solid finite elements with appropriatelydefined properties. Removal of the CFD component allows for the use of off-the-shelf tools, and reduces the uncertainty which exists between numerically coupling such a multiphysics problem. The current methodology allows for a computational vibrational response analysis of atypical shapes. This paper describes a study conducted on four systems. Two were idealized geometries with analytic solutions, one was an experiment with published results, and the fourth a prototypical system that is being developed for experimentation. The geometries in the first three systems represent submersed rigid body motion, and the fourth system is a submersed deformable body. The results show the methodology used to calculate the hydrodynamic mass shows significant promise, with close agreement achieved between calculated results and known solutions.