Validating PIV Measurements in Supersonic Jets Using Shadowgraph Optical Imaging

Abstract

Particle Image Velocimetry (PIV) is a planar fluid velocity measurement technique, wellestablished worldwide in research and in industry to acquire two or three component planar velocity data in a wide variety of fluid flows. Specifically in this work, 2D PIV was applied to characterize the supersonic flow field of a screech-reducing jet nozzle design on the Nozzle Acoustic Test Rig (NATR) in the Aero-Acoustic Propulsion Lab (AAPL) at NASA Glenn Research Center (GRC). The PIV data were acquired in the stream-wise configuration allowing for mapping the jet plume flow field at numerous locations downstream. Based on a growing experience level and understanding of jet flows, more confidence is being given to Computational Fluid Dynamic (CFD) prediction codes. Typically, PIV as well as other diagnostic measurement tools are used to validate the CFD predictions. Recently, a large difference between the PIV measurements and the CFD predictions of a supersonic jet flow was observed, leading to concerns about accuracy and overall quality of the PIV measurement campaign. In order to alleviate these apprehensions, the shadowgraph optical imaging method was applied to the same flows to provide supplementary supersonic flow field data. Shadowgraph images are completely independent of PIV measurements which provide a robust qualitative PIV validation solution in supersonic flows.