Experimental Methods for Studying Post Shock Relaxation

Experimental studies of post-shock relaxation are typically performed in one of two ways. The first method is to drive a shock down a wind tunnel and take data at a series of points as the shock moves down the facility. The second method is to stagnate a hypersonic test-flow against a scaled model in order to produce a shock stand-off region for optical probing. This study proposes the production of Mach disks and Mach stems as an additional method for investigation of the post-shock region. A wedge-ring called the Mach Disk Generator has been used to produce a disk shaped shock which is stationary in the lab frame, allowing for unobstructed optical probing in a single spatial location. The flow behind this shock is unobstructed by a scaled model allowing for unstagnated flow dissociation downstream. The Mach disk is cylindrically axisymmetric and requires Abel inversion to produce a slice of calibrated data along the flow centreline. To complement the cylindrically symmetric model, a dual wedge configuration called the Mach Stem Generator was used to produce a standing shock which is quasi two-dimensional. Mach disk diameter and Mach stem height were both approximately 10 mm, and the depth of the radiating gas from The Mach stem was 120 mm. A more pronounced shear layer was seen from the Mach stem, and the increased luminosity in that region can allow for better investigation into the influence of turbulent viscosity in a region not constrained within a boundary layer. The study used filtered imaging techniques to isolate three features of interest; Nitrogen dissociation around 740 nm and 820 nm, and Oxygen dissociation around 780 nm. Two test conditions were used, an established condition used for previous Mach disk studies, and a new condition designed to produce a longer non-equilibrium region. While the shock positions were known for the Mach Disk Generator, a computational fluid dynamics modelwas used to predict the location of the Mach stem as thiswas the first use of this model. The computational model showed good agreement with experimental data. For this work, test time is categorised in terms of the steadiness of free-stream pressure, luminosity, and shock position. For the Mach disk and Mach stem, the importance of this third consideration has been identified since the shocks are able to move more freely in the flow direction due to the absence of a stagnating body.