Application of Self-Aligned Focusing Schlieren for Flowfield Characterization in a Transonic Diffuser Flow Path

A commercially-available, self-aligned focusing schlieren (SAFS) system from RH Technologies LLC (RHT) has been used to take flowfield measurements within the Air Force Research Laboratory (AFRL) Research Cell 21 (RC-21) diffuser. The RC-21 facility is a transonic diffuser flow path that enables the study of complex internal flows relevant to aerospace propulsion systems. The unsteady lambda-shock structure in the diffuser was measured and characterized at multiple SAFS planes of focus across the span of the facility, and the results are compared to shadowgraph data collected under the same conditions. The SAFS measurements were able to successfully identify three-dimensional features of the shock structure at local spanwise locations that the comparative shadowgraph could not due to its inherent line-of-sight path integration. The features extracted from the SAFS measurements that were not clearly observed in the shadowgraph measurements include: (1) precise shock location, thickness, and angle, (2) freestream acoustic noise, (3) separated shear-layer structures from the lambda-shock-foot/boundary-layer separation, and (4) clean slip lines emanating from the two triple points. In addition, a shock-tracking algorithm was developed with moderate success for identifying the location of the shock structure for this highly unsteady flow. In general, the SAFS system developed shows promising capability for use in three-dimensional flowfields due to its ability to: (1) provide quasi-planar measurements, (2) resist vibration imaging issues, (3) provide relatively large fields of view with limited optical access, and (4) be setup precisely with ease.