A Structural Priority Approach to Fluid-Structure Interaction Problems

In a large class of dynamic problems occurring in nuclear reactor safety analysis, the forcing function is derived from the fluid enclosed within the structure itself. Since the structural displacement depends on the fluid pressure, which in turn depends on the structural boundaries, a rigorous approach to this class of problems involves simultaneous solution of the coupled fluid mechanics and structural dynamics equations with the structural response and the fluid pressure as unknowns. Such an approach requires that the computer codes used to derive the forcing function and to analyze the structural response be radically modified or even abandoned. This paper offers an alternate approach to the foregoing problems. It is shown that for this kind of problem, the effect of fluid-structure interaction can be accounted for by a fluid mass matrix, which can be computed by the series expansion method. The advantage of this approach is that neither the computer code used to calculate the forcing function nor the ones for structural dynamics analysis need be modified. In fact, once the proper fluid mass matrix is computed, the problem can be solved by ordinary methods of structural dynamic analysis. Furthermore, since the fluid mass matrix is computed by a series expansion method, interfacing with a finite-element structural analysis computer code is relatively simple. On the other hand, the technique is restricted to small structural displacement with cylindrically symmetric fluid-structure boundaries. In addition, the fluid must be single phase.