Seedless Velocimetry Measurements by Schlieren Image Velocimetry

Schlieren optical systems have been used to perform velocity measurements in refractive turbulent flows using particle image velocimetry algorithms. This schlieren image velocimetry (schlieren "particle image velocimetry") technique makes use of naturally occurring refractive turbulent eddies in a flow as virtual "seed particles" upon which velocimetry is performed. Current experiments are performed in a supersonic wind tunnel to measure the Mach 3 turbulent boundary-layer mean velocity profile. Results from schlieren, shadowgraph, and focusing schlieren image velocimetry are compared with the boundary-layer velocity profile derived from a pitot-pressure survey. Focusing schlieren optics allow the visualization of refractive disturbances within a limited depth of focus, resulting in seedless velocimetry within a narrower depth of field. The natural intermittency of the turbulent boundary layer complicates schlieren image velocimetry, but useful measurements are still possible. The velocity profile in a subsonic turbulent boundary layer is also measured using this technique through thermal seeding of the boundary layer to provide refractive turbulent structures for velocimetry. An important improvement in schlieren image velocimetry, the use of a pulsed light-emitting-diode light source in place of the twin pulsed lasers required for traditional particle image velocimetry measurements, is introduced. This comparatively inexpensive white-light source eliminates traditional problems of coherent laser illumination in schlieren imaging and improves the overall results.