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

Abstract

For blunt bodies the reduced bow shock wave h stand-off distance and the shock layer density rise in I the stagnation region as compared to ideal gas flow are K phenomena caused by dissociative effects. In this work L experiments with a RBODY = 45 mm radius cylinder and an Ma aspect ratio of L/RBoDY ~ 11 are described. The tests were M carried out in the High Enthalpy Shock Tunnel in G6ttingen m (HEG), a free piston driven shock tunnel. Two different test N conditions at reservoir enthalpies of around 21 MJ/kg and n Mach numbers of around 9 in air and nitrogen were available, pOptical measurements with a holographic phase step inter-RBODY ferometer to obtain complete flow field density gradients have 9t been carried out. By increasing the signal-to-noise ratio with t the technique of phase stepping over the original recording u 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 7 density fields for the two free-stream conditions are presented. 3The stagnation line densities are quantified. The measured e. results are slhown and compared with Navier-Stokes calcuOa lations which account for chemical reactions in the flow. The fi numerical code underpredicts the stand-off distance of the # bow shock wave. It is shown that the flow behind the bow p shock wave is in non-equilibrium and that it reaches equilib-4) rium before the body for one condition, d q0 f2 List of symbols F grating frequency [lines/mm] f focal length [mm] , whose support is gratefully acknowledged. This study of stagnating high enthalpy flows has exceedingly benefited from discussions with S. Briick and V. Hannemann. The operation of the large wind tunnel HEG is the result of a team effort. The authors wish to thank the whole team, represented by Dr. W.H. Beck, for keeping the tunnel going. specific enthalpy [Ml/kg] irradiance [W/m 2 ] Gladstone-Dale constant [m3/kg] length of the cylinder [m] Mach number molecular weight of the gas mixture [kg/mol] fringe contrast number of fringe shifts refractivity index pressure [MPal body radius [mm] universal gas constant [J/mol K] time [s ] velocity [m/s] Greek symbols degree of dissociation angle between object and reference beam [°] ratio of specific heats shock wave stand-off distance [mm] reference beams angle shift [°] characteristic dissociation temperature [K] laser wavelength [nm] free-stream energy parameter density [kg/m 3 ] optical pathlength [mm] phase change reaction rate parameter Indices oo free-stream 0 reservoir sh at the shock wave equ equilibrium fro frozen 1

Optical imaging techniques for hypersonic impulse facilities

“…This allows the removal of noise and the purported increase in sensitivity is up to one twentieth of a fringe, a factor of two improvement. Such a system has been developed for the high enthalpy facility in Göttingen, Germany (HEG) and has been used to study a variety of flows (40) . The path length of the gas that the light passes through in the test section limits the sensitivity of interferometry systems.…”