Simultaneous Shack-Hartmann and digital holography measurements of the optical effect of a subsonic compressible shear layer

The compressible shear layer is an important fundamental flow for the determination of aero-optical effects on airborne beam directors. Past research has shown disagreement in regards to the magnitude of thermodynamic gradients between the predictions of computational fluid dynamics (CFD) simulations and experimental data for the aero-optical effect of the shear layer. One reason these density-related discontinuities were not observed experimentally is that Shack Hartmann wavefront sensors (SHWFS), used to study shear layers optically, cannot resolve the sharp wavefront gradients in the flow using standard algorithms. One way to resolve this issue is to employ a digital holographic (DH) measurement approach which directly measures the phase of the optical aberration. In this paper, a comparison is presented of simultaneous SHWFS and DH measurements of the optical wavefront on a laser beam propagating in the cross-stream direction through a subsonic shear layer. In addition to processed wavefronts, a comparison of the analysis methods employed, as well as pros and cons for each technique is presented. The findings have general application to the measurement of optical wavefronts containing steep OPD gradients.