Schlieren visualization for high-speed flows based on laser-induced fluorescence

The use of a short-duration laser as a light source for Schlieren flow visualization is described. The coherent nature of this light source has been removed using the laser-induced fluorescence principle. The quality of the speckle-free pictures obtained competes with the one achieved using conventional illumination techniques. Flow features are frozen in space thanks to the short duration of the laser flashes. The applicability of the technique is demonstrated with the characterization of hypersonic conical boundary layers at Mach 11. Pairs of images have been obtained using successive laser flashes with a separation time down to 2 μs. Instantaneous convection velocities and spectral properties of the dominant boundary layer disturbances have been determined. These characteristics compare favorably with the values determined from synchronized time-resolved wall pressure measurements.