Focusing-Schlieren PIV Measurements of a Supersonic Turbulent Boundary Layer

A focusing-schlieren optical system has been developed for performing velocity measurements in refractive turbulent flows using commercial particle image velocimetry (PIV) algorithms. Focusing-schlieren optics allows the visualization of refractive disturbances within a limited depth-of-focus, resulting in quasi-planar schlieren images. The schlieren “PIV” technique makes use of naturally-occurring refractive-turbulent eddies in a flow as PIV “particles” upon which velocimetry is performed. Current experiments are performed in a small supersonic wind tunnel to measure the Mach 3 turbulent boundary layer mean-velocity profile. Results from both focusing-schlieren PIV and shadowgraph PIV are compared to the velocity profile from a standard pitot-pressure survey. The natural intermittency of the outer part of the turbulent boundary layer plays a role in the schlieren PIV results, but useful measurements of the velocity profile can still be made. We also introduce an important improvement in schlieren “PIV”, the use of a pulsed LED light source in place of the twin pulsed lasers typically required for traditional PIV measurements. This comparatively-inexpensive white-light source eliminates the traditional problems of laser illumination in schlieren optical systems and improves the overall results.