A combined holographic interferometer and laser-schlieren system applied to high temperature, high velocity flows

The High Enthalpy tunnel in Gottingen (HEG) is operated by the German Aerospace Research Establishment (DLR). Nominal test conditions of a free stream Mach number of 10, reservoir enthalpies of up to 23 MJ/kg and reservoir pressures of up to 100 MPa make the HEG ideal to study the "hot" entry phase of space vehicles. For the flow visualisation in the HEG a holographic interferometer with a 300 mm field of view is set-up. It uses the phase step technique for the evaluation of the holograms. This method enables computerized evaluation of the holograms to create high quality interferograms by increasing the signal to noise ratio. Complete flow field density gradients around two dimensional models can be calculated from the interferograms. Nevertheless for three dimensional models, where the integration length of the light path is too short to obtain a large fringe shift, the exact localisation of the shocks is difficult. To overcome this lack of the holographic interferometric system it has been extended by a laser-schlieren system to obtain simultaneous interferometric and schlieren information. Results that are presented for the hyperboloid flare model demonstrate the feasibility of the combined system. A comparison of laser-schlieren and hologram-schlieren images made for the axisymmetric Electre model show that the resolution of a direct recorded schlieren image is much higher than the latter.