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A Derivation of Ordinary Differential Equations and The
Numerical Calculations for Deep Water Time Domain Green
Functions
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Zhu Renchuan, Shen Liang, Miao Guoping
School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University
Shanghai, P. R. China (200030)
E-mail:gpmiao@sjtu.edu.cn
Abstract
How to evaluate time-domain Green function and its gradients efficiently is the key problem to analyze ship
hydrodynamics in time domain. Based on the Bessel function, an Ordinary Differential Equation (ODE) was
derived for time-domain Green function and its gradients in this paper. A new efficient calculation method
based on solving ODE is proposed. It has been demonstrated by the numerical calculation that this method
can improve the precision of the time-domain Green function.
Keywords: Time domain Green function, Series and Asymptotic expression, ODE.
1 Introduction
In recent years, ship hydrodynamics has achieved much progress and it shows a tendency that ship
hydrodynamics problems will be discussed and analyzed in time domain (Beck and Reed, 2001). While
solving a time domain hydrodynamic problem by use of panel method, the time domain Green function
involving multiple integrals need to be treated, and the boundary integral equations need to be solved and
reconstructed at every time step. Although the computer capacity and numerical technique have been
surprisingly improved in recent years, the calculation of the three dimensional Green function in time domain
is still a CPU intensive task. Therefore, how to calculate the time-domain Green function and its gradients
accurately and efficiently is the key problem to analyze ship hydrodynamics in time domain.
The three dimensional time domain Green function and its gradients are integrals with an oscillating kernel
over an infinity range. It is difficult to evaluate the integrals because its kernel function exhibits
high-frequency oscillatory behavior and its amplitude increases with wave number increases. In order to
evaluate the time domain Green function, the early strategy which are still commonly used, is a combination
of the series and asymptotic expansion as referenced by Newman (1985, 1992)
]
, Liapis (1986) and Beck and
Liapis (1987). This approach requires heavy computation consumption. Subsequently, a complementary trick
was introduced to decrease further the computation time of the Green function. Ferrant (1990), Magee (1991)
and Huang (1992) et al respectively developed the table interpolation method which can run on PC machine.
Taking advantage of the fact that the integrand has two parameters varying in bounded domains, one can
evaluate the Green function by a bivariate interpolation in a table which can be pre-computed once for all and
stored in a permanent file. Naturally the computing time decreases, but the precision of Green function is less
accurate than previous series expansion. As for the pre-computed table, it is commonly divided into two time
zones to calculate the oscillating part of integrals. In the small time zone series expansion is adopted, while in
the big time zone the asymptotic expression method is applied. However, it is very difficult to define the
boundary between the two time zones. Additionally due to the oscillating characters of Green function, the
precision can not be kept if the table nodal density is not high enough. Recently Clement (1998), Duan and
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Support by the Excellent Young Teachers Program of MOE,the Specialized Research Fund for the Doctoral
Program of Higher Education (Grant No.20030248014) and the National Natural Science Foundation of China
(NSFC) (Grant No. 50379026 and 50579034) All helps are greatly acknowledged by the authors…