One-Way Coupled Fluid Structure Simulations of Stores in Weapons Bays

Abstract

A one-way coupled fluid-structure interaction framework for the simulations of stores in weapons bays is described. The coupling as used here can be described as loose and one-way. The term loose coupling refers to the fact that the fluid and structural equation systems are only coupled through the boundary conditions and solved as separate systems. This is done primarily for efficiency and software maintainability reasons. The term one-way refers to the fact that the fluid loads are transferred to the structural solver, but the structural deflections are not coupled back to the fluid solver. The deflections of the structures are assumed to be small (even though the accelerations may be large), so that this assumption is valid. The framework is used to compute the flow field over a model rectangular weapons bay with a model store in it. examined the effects of these loads on the structural response of the stores carried in the weapons bays. Studies of the effects of the loads on stores have largely been restricted to separation trajectories. In these studies, experiments and simulations have been carried out to examine the effects of the unsteady flow field on the store trajectory once ejected. However, if the loads are large enough and contain energy in the right frequency ranges, it is possible that the store and its components can be dramatically affected. This effect can be an important consideration that must be included in the store design and qualification process. In this paper, we present a method to couple a fluid dynamics solver with a structural dynamics solver in order to be able to study the effects of the unsteady loads on the store structure and its components. The fluid dynamics solver used in this work is SIGMA CFD - an in-house block structured implicit code that solves the compressible flow governing equations. The structural solver used is Salinas - a massively parallel implementation of structural dynamics finite element analysis that uses a variant of the Newmark-Beta time integrator called the generalized alpha method for linear and nonlinear transient dynamics. As will be described in detail below, the coupling between the two codes is done through boundary conditions only - the two equation systems are still solved separately. In addition to this, the deflections computed by the structural dynamics solver are not passed back to the CFD code - the CFD code assumes rigid structures in its computations. These assumptions and their implications are discussed in greater detail below. This paper is organized as follows. Sections II and III describe the CFD and structural dynamics codes, respectively. The coupling approach is detailed in Section IV. Section V discusses the model problem the framework is applied to. Accuracy of the load transfers are evaluated in Section VI, mesh refinement results are presented in Section VII and finally, a model application is presented in Section VIII.