Shock-shadowgraphy as aerodynamic in-flight diagnostics tool

Although computational fluid dynamics (CFD) has made considerable progress in the simulation of compressible airflows, discrepancies remain between numerical predictions and in-flight data. As part of the EFT project with Airbus, the 2D synthetic study involving the tracing of light rays through shock waves has shown that it is possible to generate simulated shock-shadow images from CFD data. The study has further shown that there is, for a given flow condition, a unique relationship between the recorded shock-shadow intensities and the distance between the shock and shadow. In practice this 2D approach will however be too prone to various inaccuracies rendering the method infeasible. However, the study does suggest that ray tracing can be used to verity CFD calculations and provide indications of the presence of unexpected flow features. Moreover, by avoiding the restrictions in degrees of freedom in 2D simulations but extending the methodology to 3D and incorporating controlled parameter variations, it may be possible to make in-flight shock-shadowgraphy a viable measurement tool.
In this project the investigation into the ability to use common day-to-day optical effects to characterise and quantify compressible flows will be continued, focussing on practicality and ability to produce quantitative aerodynamic data such as shock strength and position. The ultimate goal of the envisaged research is therefore to be able to infer shock properties from observed in-flight shadows. The proposed project addresses the required extension of the 2D ray tracing technique into 3D, including multiple camera arrangements. Subsequently, with the validated tool at hand, more fundamental research is conducted to assess the feasibility to extract shock properties from formed shadows. Based on these findings a preliminary yet encompassing tool will be developed to extract such data from in-flight observations on aircraft wings.
The project is divided into three separate work packages (subject to change though as the project evolves). First, the 2D shock-shadow simulator will be extended to enable 3D simulations. This this extent, the light scattering model in 2D Ray tracing will be improved, ray tracing will be extended to accept 3D velocity fields, varying observation angles and cameras will be incorporated as well as photogrammetry and a preliminary validation can be performed based on comparisons with CFD-based simulations of shock-shadows with in-flight observations. The second work package determines the feasibility of shock characterisation with shock-shadowgraphy. A parameterised shock-shape will be used to investigate the relationship between parametrised shock and shadow formation using the 2D ray tracing model before investigating the 3D tracing relationship between parametrised shock and observation angle. Finally, a tool/methodology will be established to characterize shock and/or flow from observable in-flight shock-shadows.