Computational flow modelling of the T4 free-piston driven reflected shock tunnel

Computational flow modelling of the T4 free-piston
driven reflected shock tunnel

Free-piston driven shock tunnels, such as the T4 tunnel that is the focus of this study, are a class of impulse facilities able to cover a wide range of flight Mach numbers, which have been used for decades to study hypervelocity flows at true total enthalpies and flow speeds. Their test gas is generated in a succession of flow processes involving complex and unsteady wave processes which are modified by viscous and high-temperature gas effects. A complete transient and spatial characterisation of the test gas properties is crucial for accurate interpretation of the experimental data obtained. However, experimental characterisation of these highly energetic shock tunnel flows is arduous and typically only limited measurements are performed. Routine experimental data analysis is generally based on quasi-steady state calculations coupled with basic pressure measurements to deduce test gas properties. This simplified methodology introduces unquantified errors between these estimated and actual test gas properties, and thus propagated errors in experimental data analysis. The present paper outlines the use of transient one-dimensional and axisymmetric numerical models for the full facility flow reconstruction to obtain more accurate test gas properties. The quasi-one-dimensional model is used to calculate the transient flow history in the driver tube, and serves as an inflow to the axisymmetric calculations of the downstream region. These simulations are validated through comparison with experimental data obtained from the T4 facility. The comparison shows that the shock speeds for two out of three locations were simulated with sufficient accuracy leading to the conclusion that the numerical modelling is adequately capturing relevant fluid processes for most of the shock tube length. The present work is the initial part of a planned investigation of a full facility flow reconstruction where instead of the one-dimensional modelling of the driver section, a higher fidelity moving mesh approach will be used to capture the piston motion.