Design and Characterization of a Hypervelocity Expansion Tube Facility

We report on the design and characterization of a 152-mm-diam expansion tube capable of accessing a range of high-enthalpy test conditions with Mach numbers up to 7.1. Expansion tubes have the potential to offer a wide range of test flow conditions as gas acceleration is achieved through interaction with an unsteady expansion wave rather than expansion through a fixed-area-ratio nozzle. However, the range of test flow conditions is limited in practice by a number of considerations such as a short test time and large-amplitude flow disturbances. We present a generalized design strategy for small-scale expansion tubes. As a starting point, ideal gasdynamic calculations for optimal facility design to maximize test time at a given Mach number test condition are presented, together with a correction for the expansion-head reflection through a nonsimple region. A compilation of practical limitations that have been identified for expansion tube facilities, such as diaphragm rupture and flow-disturbance minimization, is then used to map out a functional design parameter space. Experimentally, a range of test conditions are verified through pitot pressure measurements and analysis of schlieren images of flow over simple geometries. To date, there has been good agreement between theoretical and experimental results