Microwave Shock-Speed Diagnostic Development and Analysis of Expansion Tunnel Viscous Effects

The development of a nonintrusive microwave shock-speed measurement for expansion tunnels is presented, based on microwave standing-wave reflection. Testing is performed in a large-scale expansion-tunnel facility, with the goal of characterizing viscous effects. Expansion tunnels generate high-enthalpy test conditions for short test times, and the test gas is never heated such that excessive freestream dissociation or ionization occurs. The microwave system measures primary and secondary shock speeds accurately and with high spatial resolution along the length of the facility, yielding more accurate freestream conditions. It is shown to be both practical and low cost. The high spatial resolution along the tunnel is used to assess shock-speed attenuation. Negligible shock attenuation is found over a wide range of test conditions and gases, attributed to the large diameter of the facility’s driven and expansion tubes. Shock-tube boundary-layer growth solutions based on Mirels’s theory (“Shock Tube Test Time Limitation Due to Turbulent-Wall Boundary Layer,” AIAA Journal, Vol. 2, No. 1, 1964, pp. 84–93) and a Pitot-probe survey confirm that, in the current facility, the test conditions should not be adversely affected by viscous effects. Mirels’s theory is also used to determine the displacement thicknesses for quasi-one-dimensional analyses, showing how viscous effects become significant in long, smaller-diameter facilities. The high spatial resolution allows for local shock-speed information to evaluate nonideal secondary-diaphragm ruptures. Additionally, when postshock electron-density levels are near the cutoff required for standing-wave reflection, test-gas velocities are measured rather than shock velocities.