Design of a shock-free expansion tunnel nozzle in HYPULSE

A shock-free expansion tunnel nozzle has been designed for the NASA HYPULSE facility at GASL. The main purpose of the nozzle is to increase the available test core size to accommodate a large-scale model, with the initial design intended for aeroheating studies at re-entry flow conditions. The nozzle was designed to expand the hypervelocity flow at the acceleration tube exit to a larger test core size of 30 cm, while maintaining a unit Reynolds number of at least 9.8 x 105/m. The two main challenges encountered in the design were the high entrance Mach number and the thick incoming tube-wall boundary layer. The paper discusses the design strategies of using a contour obtained using the method-of-characteristics as opposed to the conical geometry, and the skimmer versus the full-capture approaches, to achieve shock-free uniform exit flow with acceptable Reynolds numbers. To fully characterize the final design, computational fluid dynamics analyses were carried out using the nonuniform viscous flow profile at the acceleration tube exit as a start-line condition. The pitot pressure distribution at nozzle exit, as well as the heat flux on a hemisphere placed at nozzle exit, were calculated and compared with experiments. Besides discussing the fluid dynamic aspects of the design, a short description of a cost-effective manufacturing technique of fabricating the nozzle from fiberglass is also included.