USE OF HEAVY GASES IN EXPLOSIVE-DRIVEN SHOCK TUBES
Aerodynamic model tests with C4F8 in explosive-driven shock tubes revealed irregular pressure-time histories at random temporal and spatial locations on the tube wall and on the model surface. Shock-front photographs and analytical studies suggested that the irregularities were caused primarily by the decomposition of the gas behind the shock for shock-front pressure ratios > 15. A nitrogen buffer placed between the explosive and the test gas to eliminate conduction of heat to the dFs only partly alleviated the problem. Therefore, a second heavy gas SFe was selected for investigation. Photographs of the shock fronts in SFe indicated that there was no decomposition in the flow, although some irregular pressure-time histories were observed. These irregularities were neither spatially random nor were they attributable to oscillating, nonplanar shock fronts. Numerical computations of the flow-field and experimental studies confirmed that the irregular pressures were produced by the nonplanar shock fronts formed by an asymmetrical distribution of the explosive charge and by perturbations caused by an impedance mismatch at the contact surface. When these conditions were remedied, reasonably planar shocks and well-defined flow conditions were obtained in the shock tube.