Aero Optics of Subsonic Boundary Layers over Backward Steps

An experimental study of the two-dimensional transition from turbulent boundary layer over small backward-facing steps is presented in order to investigate the effect of the separation bubble behind the back-step on the aero-optical environment, as well as to demonstrate the capacity for characterizing complex flow fields using non-intrusive optical measurement techniques such as the Malley Probe. The flow field surrounding backward facing step is characterized using hot-wire anemometry, wall pressure measurements, and the Malley Probe. These measurements are presented for two step heights, and the results are discussed in details. The development of the sub-shear layer is observed behind the step, along with increased levels of OPDrms within the separated region. After re-attachment, an eventual recovery of the boundary layer is observed, although aero-optical levels recover to higher than pre-step levels. A simple model to account for step effects on boundary layers downstream of the back-step is proposed. Also, a relative contribution from the near-wall region of the boundary layer to overall levels of the aero-optical distortions was estimated.