The T6 Stalker Tunnel is a multi-mode, high-enthalpy, transient ground test facility. It is the first of its type in the UK. The facility combines the original free-piston driver from the T3 Shock Tunnel with modified barrels from the Oxford Gun Tunnel. Depending on test requirements, it can operate as a shock tube, reflected shock tunnel or expansion tube. Commissioning tests of the free-piston driver are discussed, including the development of four baseline driver conditions using piston masses of either 36 kg or 89 kg. Experimental data are presented for each operating mode, with comparison made to numerical simulations. In general, high-quality test flows are observed. The calculated enthalpy range of the experimental conditions achieved varies from $$2.7\hbox { MJ kg}^{-1}$$2.7MJkg-1 to $$115.0\hbox { MJ kg}^{-1}$$115.0MJkg-1 .Graphical

Shock tubes are a crucial source of experimental data for the aerothermodynamic modelling of atmospheric entry vehicles. Notably, many chemical-kinetic and radiative models are validated directly against optical measurements from these facilities. Typically, the incident shock speed at the location of the experimental measurement is taken to be representative of the test slug; however, the shock velocity can vary substantially upstream of this location. These variations have been long posited as a source of disagreement with computational predictions, although a definitive link has proved elusive. This work describes a series of experiments which aim to isolate and confirm the importance of the shock deceleration effect. This is achieved by generating different shock trajectories and comparing the post-shock trends in atomic oxygen emission and electron density. These trends are shown to be directly linked to the upstream shock speed variations using a recently developed numerical tool (LASTA). The close agreement of the comparisons confirms the importance of shock speed variation for shock tube experiments; these findings have direct and potentially critical relevance for all such studies, both past and present.

Complex processes related to nonequilibrium thermochemistry and radiation are a fundamental aspect of atmospheric entry flowfields. Shock tubes provide a means of generating test gas conditions analogous to those found on the stagnation line of flight shock layers, which allows extraction of thermochemical rates and radiative intensities. Currently, the NASA Electric Arc Shock Tube (EAST) is the best source of such data. Although simple in principle, nuances of these experimental facilities can affect the observed results. Notably, electron densities and radiance levels in excess of equilibrium predictions have been observed at EAST for many years. The deceleration of the shock as it passes along the tube has been posited as a source of these discrepancies. In this work, a recently developed numerical methodology (LASTA) is applied to these results from the literature. Using the experimental shock speed profile as an input, trends in postshock electron density are computed. Radiance throughout the shock layer is also predicted by coupling the simulation to the NASA NEQAIR code. It is shown that the predictions of LASTA provide a good match to the magnitudes and trends of the experimental differences, confirming shock speed deceleration as their cause.

View Video Presentation: https://doi.org/10.2514/6.2022-0267.vid A fundamental aspect of atmospheric entry flow-fields is the complex interaction between non-equilibrium thermochemistry and radiation. Shock tubes provide a means of generating flight-representative shock-layers, which allows extraction of thermochemical rates and radiative intensities. Currently, the NASA Electric Arc Shock Tube (EAST) is the best source of such data. Although simple in principle, nuances of these experimental facilities can affect the observed results. Notably, electron densities and radiance levels in excess of equilibrium predictions have been observed at EAST for many years. The deceleration of the shock as it passes along the tube has been posited as a source of these discrepancies. In this work, a recently developed numerical methodology (LASTA) is applied to these results from the literature. Using the experimental shock speed profile as an input, trends in post-shock electron density are computed. Radiance throughout the shock-layer is also predicted by coupling the simulation to the NASA NEQAIR code. It is shown that the predictions of LASTA provide a good match to the magnitudes and trends of the experimental differences, confirming shock speed variation as their cause.

Shock tube experiments are essential in understanding the environment encountered by hypersonic vehicles. Such experiments provide information used to determine rate constants of chemical, relaxation and radiative processes taking place in non-equilibrium plasmas. These constants are significant drivers of uncertainty in surface heat flux predictions. Recent work has shown that flow non-uniformities in real shock tube experiments can be misinterpreted as a need to alter these parameters; however, no comprehensive model exists to decouple the effects. We show that there is a rigorous method to achieve this by using experimental measurements as boundary conditions and including their effects via reverse time integration. This method improves over previous implementations by rigorously enforcing conservation laws, incorporating two-temperature, non-equilibrium thermochemistry and explicitly modelling both forward- and backward-running sound waves in the shock tube test slug through a method of characteristics formulation. This approach allowed the effect of shock speed variation in highly non-equilibrium tests, specifically those relevant to Titan entry, to be studied for the first time. A validation study showed that properties predicted by the method were found to agree with results from a viscous, two-dimensional axisymmetric Navier–Stokes solver within 1.5 %. When applied to shock tube test cases from the EAST and T6 facilities for simulation of lunar return and Titan entry representative conditions, the method offered improved agreement with experimentally measured oxygen 777 nm and 240–440 nm radiance, respectively, when compared with previous implementations, particularly towards the rear of the test slug where forward-running sound waves from the driver become influential.