Ballistic range experiments on superbooms generated by refraction

Experiments are reported which pertain to the phenomenon of enhanced sonic booms or superbooms generated as a result of atmospheric refraction in threshold-Mach-number flights. Such ’’refraction superbooms’’ are created in a ballistic range by firing projectiles at low supersonic speeds into stratified medium of varying sound speed consisting of an inhomogeneous mixture of CO2 and air. The pressure signature recorded by a fast-response transducer inserted in the flow field is studied in conjunction with sets of dual-schlieren pictures obtained simultaneously during each run. The flow field observations indicate that the generated shocks are reflected near the sonic cutoff elevation, where the local sound speed equals the projectile speed, provided that such an elevation exists. The incident shock and its reflected portion join to form a Y-shaped configuration near the cutoff region. Enhanced shock overpressures are detected near the ’’critical altitude’’ where the flow speed behind the shock becomes sonic. A maximum focus factor of 1.7 has been measured for the leading shock which, with some theoretical assumptions, leads to an estimated value of 1.3 for the constant which appears in the Guiraud–Thery scaling law. The same value is also deduced from field data taken during aircraft threshold Mach-number flight by Haglund and Kane. The substantial agreement between the two experimentally derived values of the constant and the theoretical estimate of 1.4 given by Thery, Lecomte, and Reggiani seems to suggest the validity of the above-mentioned scaling law. However, the discrepancy of these numbers with the value of 2.6 derived by Gill and Seebass indicates that further investigation is still needed. Subject Classification: [43]25.20, [43]25.30; [43]28.55; [43]50.55.