Dynamic Correlation on a Schlieren Image in a Transonic Airflow

Photographic data produced by schlieren and other opticalsystems often appear to have a regular structure. This is particularly true in the transonic flow region which is sensitive to periodic and quasi-periodic fluctuations. The objective of this method is to provide a simple signal averaging technique which can quantify this structure in optical images by temporal and spatial correlation. As an example of the technique measurements have been made in a two-dimensional linear transonic turbine cascade. The auto-and cross-correlations obtained over a Mach number range 0.5 to 1.25 show a strong coherent vortex structure in the trailing edge region of the blades. Using the method of dynamic correlation it has also been possible to show the persistence of this vortex structure despite the presence of a trailing edge shock for the supersonic Mach numbers. In further more recent work the technique has been combined with holographic interferometry to visualize the vortex street in the trailing edge region on an isolated aerofoil with a blunt trailing edge. The combination of these methods gives both the density distribution of the vortex and its periodicity in the flow.