Physics-Informed Background-Oriented Schlieren of Turbulent Underexpanded Jets

Background-oriented schlieren (BOS) is an imaging technique that can be used to characterize the density field in a compressible flow. This information is often employed to assess shock formations and predict aerodynamic performance in high-speed ground tests. However, measurements in ground test facilities are subject to large uncertainties due to limitations on optical access, short exposure times/high signal-to-noise ratios, and intense vibrations that lead to calibration drift. Further, it can be difficult to interpret scalar measurements in a complex flow. Data assimilation (DA) can ameliorate these issues by optimally combining measurement information with the relevant governing equations. Doing so enhances the accuracy of parameter estimates and provides access to latent (i.e., not directly measured) flow fields. We previously developed a DA algorithm for BOS to recover the density, velocity, and total energy fields of compressible inviscid flows from noisy experimental images. Here, we refine and deploy our “physics-informed BOS” technique using the compressible RANS equations, testing the method on a suite of turbulent underexpanded jets. The resulting mean fields agree with simulations and measurements reported in the literature.