Laser-induced gas breakdown as a light source for schlieren and shadowgraph particle image velocimetry

The "schlieren PIV" technique combines schlieren or shadowgraph optics with particle image velocimetry (PIV) equipment to measure the velocities of turbulent eddies in flows with sufficiently strong changes of the refractive index without actual particle seeding. Prior work on this technique used direct laser illumination that produced inadequate schlieren image quality due to coherent artifact noise and other problems. By way of a simple equipment modification, we show the white-light emission of a laser-induced air or argon breakdown to be an improved light source for schlieren and shadowgraph PIV. The Nd:YAG illumination used in standard PIV is converted to a white-light pulse by this means. High-quality schlieren images are obtained and measurements in a helium jet using this new approach compare well with previous data. The method is especially applicable to high-speed flows requiring time delays of 5 µs or less between the images of a PIV pair.