Overview of ballistic range program at Tohoku University

Abstract

The experimental activities at Shock Wave Research Center of Tohoku University using a ballistic range are reviewed. First, the shock stand-off distance for a sphere and the shock shape over a sharp cone are measured in the hypersonic speed regime. These measurements are made for 30 mm diameter models, flight speeds between 2.5 and 4.0 km/sec and ambient pressure 4.2 and 76 mmHg. The flow field is visualized by the schlieren method using a short-pulse Nd-YAG laser as the light source. Shock stand-off distance and shock shape are shown to vary with the ambient pressure, demonstrating the effect of the chemical nonequilibrium. Secondly, hypervelocity impact tests are conducted for simulation of space debris impacts against a bumper shield. The bumpers are made of metal and nonmetal materials. The projectile made of high density polyethylene has a cylindrical shape of 14 mm diameter and 3.7 g in weight. The impact velocity is about 4 km/sec. The debris cloud is visualized by shadowgraph. The nonmetal bumper shields used in this study are more effective than the metal bumper in regard to both the bumper mass and defensive capacity. Introduction The ballistic range is one of the readily available impulse facilities for the fundamental investigation of the hypersonic flow phenomena. The so-called two stage light gas gun can accelerate a model to hypersonic speeds. The facility is used mainly for the study of hypersonic aerothermodynamics and for the hypervelocity impact tests. In Shock Wave Research Center of the Institute of Fluid Science of Tohoku University, experiments are in progress in a ballistic range with a 14 mmand 30 mm-bore two stage light gas gun. First, the shock stand-off distance for sphere and the shock shape over sharp cone are measured in the thermochemical nonequilibrium regime. In the past, a large number of experimental investigations on high temperature real-gas effects have been carried out in the shock tunnel. However, there exist uncertainties in the characteristics of the flow produced in a shock * Graduate student f Professor, Institute of Fluid Science, Tohoku University; Member AIAA. Copyright ©1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. tunnel in real-gas regime, and therefore it is necessary to validate the facility.Theoretically, the real-gas effects are being studied using computational fluid dynamics (CFD). However, the CFD calculation of nonequilibrium flow encounters many difficulties because of the complex processes of chemical reactions in the nonequilibrium state, particularly in the case of air. Therefore reliable experimental data are needed for validating shock tunnel results and CFD. One of the most appropriate reference quantity is the shock stand-off distance for a sphere and shock shapes for a sharp cone, because the heat absorption by the real-gas phenomena behind shock wave sensitively affects the shock stand-off distance and shock shapes.'' Ballistic ranges are the only impulse facility capable of producing a reliable data, because, in a ballistic range, a model flies in a quiescent atmosphere. Measurement of shock stand-off distance for sphere has been made by Lobb in the ballistic range (see Figure I). His experiment misses the range of the binary scaling parameter pR above 2X10 kg/m and the flight velocity below 4 km/sec. The accuracy of Lobb's experiment is unclear because of the relatively long pulse duration of the light source used for flow visualization and small size of the models. Therefore, we conduct an experiment using a Nd-YAG laser of very short pulse duration for the light source and use 30 mm-diameter models. The shock shapes have been also measured in real gas regime. However, these measurements are conducted in high enthalpy shock tunnels and expansion tubes.' As mentioned above, the data