The modal dynamic characteristics of an underwater propeller are investigated using a coupling of the Finite Element Method (FEM) to model the propeller and the Boundary Element Method (BEM) to model the fluid. Results of this numerical model are presented for a fluid-loaded propeller and are compared with experimental holographic results.
The FEM is known to yield very reliable solutions in the analysis of the modal dynamic characteristics of solid structures such as a propeller and the BEM is very attractive in dealing with infinite domain problems and the radiation condition, such as the infinite fluid field. Combining the two methods exploits the best attributes of both. The fluid/structural coupling is achieved by discretizing Kirchhoff’s integral with boundary elements and isolating the effective mass of the fluid. This effective mass is in the form of a mass matrix which is coupled by the degrees of freedom of the propeller. The effective mass is then input into a finite element program in the form of user elements along with the propeller’s geometry, material properties, and boundary conditions to simulate an underwater propeller in a hub.
An experiment using time averaged holographic interferometry was performed to identify the resonant modes of the propeller, identical in geometry to that used in the FEM model. In order to simulate the boundary conditions of the model the propeller was rigidly clamped in a vise at it’s root and submerged in water. Excitation of the propeller was provided by means of a mechanical shaker mounted to the vise.
Both the resonant frequencies and their respective mode shapes agreed favorably with numerical predictions.