Development of Advanced Techniques for Aerodynamic Assessment of Blunt Bodies in Hypersonic Flows

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Abstract

The breakup of large rocket or satellite structures during destructive re-entry is not well understood and depends heavily on numerical simulations to predict the entry-trajectories. Experimental studies in high-speed wind tunnels provide validation data for such simulations with non-intrusive optical methods extracting quantified field data and true free-flight capabilities of object motion in free-flight. Optical schlieren have become the de facto method to visualise the flow structures around objects in high-speed flow. It affords high spatiotemporal resolution from precision optics and both still and high-speed cameras. It is not possible to simultaneously resolve multiple-strength density gradients in a single schlieren image, which requires either multiple costly tunnel runs to tune knife edge orientation and cut-off which cannot precisely duplicate transient flow features. The background-oriented schlieren (BOS) method allows a single image to be interrogated for multiple knife edge orientations and different dynamic range values. Two sets of experiments were performed in this study; steady-state and free-flight. Steady-state tests were performed to evaluate the spatial resolution, sensitivity and dynamic range of schlieren and (BOS) applied to hypersonic flows with multiple density gradients. Metrics were devised for optimisation of the physical BOS arrangement to minimise the shock and model blur and maximise the shock displacement. A processing study developed metrics to achieve minimal processing blur and highest signal to noise ratio. The BOS results have an overall lower spatial resolution and sharpness than the schlieren images, but had lower shock blur than schlieren for the optimised arrangement used. A synthetic knife edge and nonlinear colour were applied to the BOS studies to resolve an increased number of weaker flow features in a single image than multiple orientations of optical schlieren knife edge and cut off settings. These methods provided increased spatial resolution of the density gradients in a BOS image than a standard linear colourmap for the full range of displacement vectors. For thefree-flight testing both BOS and optical schlieren captured the six distinct Edney shock interaction types. Application of nonlinear colourmaps and synthetic knife edge orientations improved resolution of weaker shear-layer and boundary layer separation flow features behind the sphere than captured with the schlieren. Sphere free-flight drag coefficient was reconstructed using moving linear fit to derive acceleration. Uncertainty with this approach was less than 3%. A lower drag coefficient was seen for the sphere in free-flight but this was comparable to studies with high-sphere surface roughness. Spheres experiencing an Edney shock interaction produced an increased drag coefficient with a maximum value for the Edney IV case. Expanding the applicability range of non-intrusive optical field measurement techniques for high-speed flows is vital to support experimental testing and validation of numerical research results. Significant developmental work was performed on the BOS technique to evaluate its performance, sensitivity, blur and expand its dynamic rage to support future experimental tests in conjunction with the established schlieren methods.

Keyword(s)

Aerodynamics; Background-Oriented Schlieren; Free-Flight Testing; Hypersonic; Image Processing; Optical Flow Diagnostics; Re-entry; Schlieren; Shock Interactions; Wind Tunnels

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