Phase step holographic interferometry applied to hypervelocity,non-equilibrium cylinder flow

For blunt bodies the reduced bow shock wave stand-off distance and the shock layer density rise in the stagnation region as compared to ideal gas flow are phenomena caused by dissociative effects. In this work experiments with aR_BODY=45 mm radius cylinder and an aspect ratio ofL/R_BODY 11 are described. The tests were carried out in the High Enthalpy Shock Tunnel in Göttingen (HEG), a free piston driven shock tunnel. Two different test conditions at reservoir enthalpies of around 21 MJ/kg and Mach numbers of around 9 in air and nitrogen were available.Optical measurements with a holographic phase step interferometer to obtain complete flow field density gradients have been carried out. By increasing the signal-to-noise ratio with the technique of phase stepping over the original recording quality, high quality interferograms are obtained. The high spatial resolution of the holograms results in the creation of hologram-schlieren images which are compared to directly recorded laser-schlieren images.Infinite and finite fringe interferograms and the complete density fields for the two free-stream conditions are presented. The stagnation line densities are quantified. The measured results are shown and compared with Navier-Stokes calculations which account for chemical reactions in the flow. The numerical code underpredicts the stand-off distance of the bow shock wave. It is shown that the flow behind the bow shock wave is in non-equilibrium and that it reaches equilibrium before the body for one condition.This work was an offshoot of an ESA research contract monitored by Dr. D. Vennemann, whose support is gratefully acknowledged. This study of stagnating high enthalpy flows has exceedingly benefited from discussions with S. Brück and V. Hannemann. The operation of the large wind tunnel HEG is the result of a team effort. The with to thank the whole team, represented by Dr. W.H. Beck, for keeping the tunnel going.