Improved Formulation for Underwater Sound Radiation Problem and It's Application

Abstract

This report is concerned with a numerical calculation method for the underwater sound radiation by the hull vibration.Iwasaki reported a numerical calcuation method for coupled structure-fluid interaction problem. As for the method that Iwasaki suggested, the computer model would necessarily have large number of degree freedom and would require an excessive computer cost. And also, there was a problem of Irregular Frequency that are the eigen frequencies of the corresponding interior problem. In this study, we examine the following improved numerical calaulation method.(1) We employ a modal approach whereby the structure displacement is approximated by a linear combination of the dominant in-vacuo, undamped normal modes of the structure. By this procedure we can shorten calculation time largely.(2) We combine discretized surface and interior Helmholtz integral equations to obtain an overdeter-mined system. If the interior points do not all happen to fall on a nodal point of an interior normal mode, then the method uniquely solves the Helmholtz integral equation for all wave numbers. We combine the procedures (1) and (2), then we can estimate the characteristics of the under water sound radiation by mechanical vibration of hull appropriately.We examine the effect of anti-noise and vibration system of engine by using the improved numerical calculation method. The relation between the property of anti-vibration rubber and the radiated under water sound are investigated numerically.