Modeling fluid/structural interaction in a pulsed power accelerator

Abstract

A key component of pulse power technology is the engineering of these complicated structures. The goal of this work is to develop a finite element model that captures the complex physical interactions of all components within the Z machine. A large driver of physical motion in this machine is not found at the target but rather upstream at the water switches. Where this high current passes through the water a strong acoustic wave is generated. The pressure in an individual wave is fairly weak, only approximately 3.45 MPa peak pressure. However, due to the number of these switches (108 total), their axial symmetry, and switch timing, the impulse generated by these waves has the potential to create significant damage within the structure. Modeling this type of fluid/structure interaction (FSI) on this scale pushes many finite element codes to the limit of their capabilities. In order to get the correct input to the model great care must be taken in the selection of a sensor deployed to capture this pressure time history. A number of technologies have been investigated via shock tube testing. In a shock tube a square wave can be transmitted through air or water to the sensor mounted in the tube end cap. From this test the sensor rise time, response frequency, and decay can be evaluated for suitability in this application. Sensor technologies studied include off the shelf quartz pressure sensors, polyvinylidene fluoride (PVDF), and phase Doppler interferometry (PDI) with thin film TPX. A number of modeling approaches have been investigated during the course of this work including structural/acoustic elements, arbitrary Lagrangian Eulerian (ALE), and coupled Eulerian-Lagrangian (CEL). A coupled Eulerian Lagrangian approach was ultimately selected for this effort. This technique allows the structural mesh (Lagrangian) to occupy the same volume in space as the fluid mesh (Eulerian). A third component, the Eulerian material (water in this case) flows through the Eulerian mesh and interacts with the structure. Techniques for initiating such acoustic waves in the Eulerian domain, as well as techniques for getting the correct reflection/transmission response at the fluid structure interface have been studied.