Computational simulation of starting dynamics in a ram accelerator

Computational fluid dynamics (CFD) solutions of the full Navier-Stokes equations along with finite-rate chemical kinetics, are used to numerically simulate the reacting in-bore flowfield for a 120mm ram accelerator projectile propulsion system. Various unsteady flow phenomena are investigated, including projectile starting, obturator discard, and high-velocity unstarting. These simulations illustrate the importance of obturator discard dynamics in achieving a successful starting of the ram acceleration process. However, once started, high projectile velocities can induce an unstart if the combustion wave precedes the projectile under certain conditions. In addition, a CFD code validation case is presented for low pressure, steady flow, hydrogen/oxygen combustion around a wedge as compared to Schlieren/PLIF data from Stanford. Introduction Experimental testing and gasdynamic modeling of the ram acceleration technique for in-bore projectile propulsion has been investigated at the U.S. Army Research Laboratory (ARL) under the Hybrid Inbore RAM (HIRAM) propulsion program. This research program seeks to provide a highly efficient method of achieving hypervelocity (> 3 km/s) projectile gun-launch for use in high speed impact testing applications. The ARL ram accelerator system used a 120-mm (bore diameter) tube which was modeled after the 38-mm system at the University of Washington. Numerical solutions of the Navier-Stokes equations have been obtained at the ARL via computational fluid dynamics (CFD) codes for non-reacting and reacting, two-and three-dimensional flows. These codes are being used to investigate the complex gas dynamic physics of ram accelerator projectile propulsion. A variety of CFD techniques have been brought to bear on this problem, including models for chemically frozen (non-reacting) gas, finite-rate global and multiple step chemical kinetics, and equilibrium chemical processes. Accurate numerical simulation of hydrocarbon-based reacting flow creates a very great demand on computational resources since the number of intermediate species and the number of kinetic steps for typical hydrocarbon fuels are prohibitively large. Global reaction mechanisms based on up to thirty-two steps have therefore been investigated for use in preliminary design studies. Within several hours on supercomputers, viscous and chemically reacting gas dynamic simulations are used to assess the influence of projectile velocity, tube fill pressure and mixture composition, and projectile geometry on species consumption, tube wall pressure and projectile thrust. These studies are being used to seek optimum performance for the 120-mm ram accelerator with minimal gun firings. In particular, significant advances in CFD * Aerospace Engineer, Ballistics &; Weapons Concepts Division. Associate Fellow AIAA. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. • modeling for ram acceleration have been made at the ARE." Recently, ARL CFD models have been modified to simulate the unsteady coupling between the launch gun and the ram accelerator. In the Army experimental gun, projectile transition from the conventional (solid propellant) launcher to the accelerator was made through a transition/vent section. This section decouples the launch gun movement from the accelerator and vents combustion gases from the conventional launcher. Redesign of the ram accelerator for Navy applications has required that the ram accelerator operate without the vent section. The removal of the vent section requires that the CFD code be linked to an interior ballistics (IB) simulation of the launch gun in order to supply the pressure behind the obturator. The IB code must be able to adjust to the pressure on the forward face of the obturator, supplied from the CFD code simulating ram combustion. Simulation of ventless ram acceleration has been accomplished. Reacting Flow Model The chemically reacting hypervelocity flow field around a body is numerically simulated using a computer code based on computational fluid dynamics. The ARL-NSRG2 code solves the 2D/axisymmetric, unsteady, real-gas Navier-Stokes equations including equations for chemical kinetics and diffusion. These partial differential equations are cast in conservation form and converted to algebraic equations using a finite-volume formulation. The conservation law form of the equations assures that the end states of regions of discontinuity (e.g., shocks, flames and deflagrations) are physically correct even when smeared over a few computational cells. Solution takes place on a mesh of nodes distributed in a zonal fashion around the projectile and throughout the flow field such that sharp geometric corners and other details are accurately represented. The Navier-Stokes equations for 2D/axisymmetric reacting and unsteady flow are written in the following conservation form with variables nondimensionalized in a conventional fashion. dU dF_ dG_ fat