Hydroxyl Tagging Velocimetry in a Supersonic Flow Over a Ramped-Wall Cavity Flameholder with an Upstream Strut

Abstract

Supersonic air flow over a wall cavity with a rear ramp both with and without an upstream strut was studied with Hydroxyl Tagging Velocimetry (HTV). HTV is a non-intrusive means of measuring gas velocity by producing a hydroxyl tag from water vapor via a two step process: 1) the “write” phase where water molecules are dissociated into OH+H, 2) the “read” phase where after a fixed time delay the OH field is interrogated with planar laser-induced fluorescence. An 11x11 OH grid was used to obtain velocities at ~120 grid points. Mean and RMS fluctuation velocity profiles both with and without the strut were analyzed and compared to each other and to a rectangular cavity studied by other authors. In the ramped-cavity, increased recirculation and turbulence was observed over the ramp compared to the rectangular cavity, and the shear layer both grew slower and was lower into the cavity. Once the strut was installed, it was observed that cavity recirculation along the centerline increased relative to the no-strut configuration and extended above the step. Furthermore, expansion waves off the top of the strut and weak compression waves off the back of the strut were observed. To best analyze vorticity, two separate interpolation schemes were interrogated by comparing velocity, and vorticity data. In general, the two methods provided similar results for irrotational flows. However for rotational flows, it was found that a Powell-Sabin interpolation more accurately determined the location of velocity gradients compared to a cubic interpolation scheme.