The Wet Mode Analysis of Rudder System

Abstract

To calculate a rudder system's vibration characteristic, MSC Nastran was carried out to simulate an underwater model's wet modes based on finite element method (FEM) and virtual mass method. The model was simplified from the rudder system using a torsion spring instead of the transmission parts. The frequencies and mode shapes of the system's first 4 order wet modes were obtained and showed differences in comparison with the dry modes. It could be concluded that the water's effect plays a significant role on rudder system's structural dynamics which should not be neglected in its normal modes analysis. Keywords-rudder; wet mode; MSC Nastran, FEM, virtual mass method. I INTRODUCTION For a rudder system, flutter will be induced by incorrect design as the wings of aircraft (1). Since the strong flutter which leads to violent vibration and structure damage only happens when the speed is higher than 100 knots which is out of marine's speed limit, the flutter problem of the rudder system is often neglected in the early research. But the weak flutter may exist in low speed and it will produce noise in the water which goes against marine's stealth. Thus, it is necessary to study on rudder system's flutter problem. Prior to the flutter analysis, an accurate structural dynamics model should be built (2). Finite element method (FEM) is widely used because of its ability to obtain high fidelity and reliable solutions for complex geometries with various boundary conditions (3). Thus, the structural dynamics modelling of the rudder system are performed by the FEM. It should be noticed that the rudder is under the water, therefore, the wet mode analysis of the structure considering fluid's effect is different with the dry mode. The vibrating structure in the fluid is subjected to the effect of fluid's resistance, and a part of the fluid vibrates in coupling with the structure which presents as an added mass in system's mass matrix. Consequently, it is necessary to compute the virtual fluid mass matrix for the underwater system. The method is available in MSC Nastran. Through the normal modes analysis on the finite element model, rudder system's wet modes can be obtained using MSC Nastran.