DAA modal analysis for parametric investigations of fluid-structure interaction in underwater shock, revision 1

A program was initiated with the goal to develop a simulation of the underwater shock response of submarines. This capability was to be employed for systems analysis studies of a variety of tactical parameters, especially the orientation of the shock wave relative to the structure. Generic analytical models that avoided the details of specific structures, and the number of degrees of freedom was limited by the need for computational efficiency. It is shown how these objectives were met by using the modal expansion version of the doubly asymptotic approximation (DAA). Orthogonal assumed modes for the system are identified as the modes of free vibration for the isolated substructure. Based on these concepts three variants on a basic model are described; each modification provides the capability to evaluate different aspects of the system. The basic model, consists of a rigid cylinder capped by hemispheres which is subjected to a shock wave arriving from an arbitrary leading angle. The REFLECT model extends this fundamental model to treat multiple shock waves associated with reflections from the surface and bottom. The CAPS model was extended to assess the importance of an internal dead load. The new version includes a large internal mass suspended from the cylinder by four springs that give equivalent axial, transverse and rotational suspension stiffnesses. The center of mass of this dead load is located arbitrarily along the axis of symmetry of the cylinder.