Abstract
The very important engineering problem of modeling the fluid-structure interaction occurring during the shedding of vortices has defied, and will probably continue to defy, a closed form exact solution for the foreseeable future. Therefore, an attempt must be made to extract relevant information about the process in order to be able to have a basic understanding of it for the purpose of analysis. A useful method involves the flow-oscillator concepts of Hartlen and Currie [1] redefined here for stochastic processes. The fluid-structure system is assumed to be governed by the cross-coupled equations
x
̈
(t)+2ξω
n
x
̇
(t)+ω
2
n=C
e(t)pV
2
0(t)DL/2m (i)
C
̈
e(t)+{α − βC
2
e(t)+γC
4
e(t)}
C
̇
e(t)+ω
2
0C
e(t)=b
x
̇
(t), (ii)
where these equations govern the structure and fluid oscillators, respectively. The fluid damping is non-linear. These equations are taken as stochastic differential equations because of the many unpredictable, random effects that determine the loading and response. The lift coefficient C
l
(t) is assumed to be a zero mean, narrow band process and the velocity
V
0, composed of a uniform, constant velocity current plus oscillating wave, a broad band process. The analysis is based on solving equation (i) for
x(
t) by using Duhamel's integral and substituting its derivative
x
̇
(t)
into equation (ii). This equation is then used to derive the Fokker-Planck equation for the process
C
l
(t). To obtain the Fokker-Planck equation, slowly varying variables are replaced by their long-time averages [2] and then the method of stochastic averaging is employed [3, 4]. The moment equation for the lift-oscillator process is derived from the Fokker-Planck equation and, as equation (ii) is non-linear, one finds the moment equation to be in terms of higher order moments. A truncation scheme [5] is used to derive the moment generating function. It is possible then to generate the first and second order statistics of the lift coefficient and the structure response in terms of the empirical parameters of fluid damping. This work was carried out in conjunction with an analysis of ocean wave-current forces with application to offshore fixed structures [6].