Abstract. Simulations of a complete reflected shock tunnel facility have been performed with the aim of providing a better understanding of the flow through these facilities. In particular, the analysis is focused on the premature contamination of the test flow with the driver gas. The axisymmetric simulations model the full geometry of the shock tunnel and incorporate an iris-based model of the primary diaphragm rupture mechanics, an ideal secondary diaphragm and account for turbulence in the shock tube boundary layer with the Baldwin-Lomax eddy viscosity model. Two operating conditions were examined: one resulting in an over-tailored mode of operation and the other resulting in approximately tailored operation. The accuracy of the simulations is assessed through comparison with experimental measurements of static pressure, pitot pressure and stagnation temperature. It is shown that the widely-accepted driver gas contamination mechanism in which driver gas 'jets' along the walls through action of the bifurcated foot of the reflected shock, does not directly transport the driver gas to the nozzle at these conditions. Instead, driver gas laden vortices are generated by the bifurcated reflected shock. These vortices prevent jetting of the driver gas along the walls and convect driver gas away from the shock tube wall and downstream into the nozzle. Additional vorticity generated by the interaction of the reflected shock and the contact surface enhances the process in the over-tailored case. However, the basic mechanism appears to operate in a similar way for both the over-tailored and the approximately tailored conditions. 2 R.
Measurements from a rake of heat flux probes installed in a 62.2 mm diameter shock tunnel have been used to deduce the thickness of the shock tube interface and its distribution across the tube for a primary shock Mach number of 2.3. The axial thickness of the interface was between about 0.32 and 0.42 m for locations from about 2.0 to 2.5 m downstream of the diaphragm station. Axisymmetric simulations using a compressible Navier Stokes solver to model the entire shock tunnel operating at this condition show a simulated interface distributed over an axial length of about 0.20 m at 2.0 m downstream of the diaphragm, thus underestimating the measured interface length by about 37 %. The simulations indicate that the diaphragm opening process has a strong influence on the interface development within the nominally inviscid core flow region of the tube. The shape of the interface in these axisymmetric simulations differs from the experimental results and this is probably because the turbulent mixing within the interface is not adequately modelled. A review of previous data on the shock tube interface development indicates that the present results (both the experimental data and numerical simulations) are consistent with interface axial lengths obtained in previous shock tube studies.
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