Laser-generation of controlled localized perturbations for boundary-layer transition research

Controlled localized perturbations are generated in the freestream of a quiet supersonic wind tunnel, in order to investigate receptivity and laminar-turbulent transition under controlled conditions. The local 'hot-spot' disturbances are generated in air using a tightly focused Nd:YAG laser, via the photo-ionization effect. The injection-seeded laser operates at 532 nm and creates 7 ns pulses with energies to 240 mJ; the shot-to-shot energy repeatability is within about 3%. Measurements of the localized 'hot-spot' in stagnant air at 760 torr show an edge-spreading that is repeatable in both transverse and axial directions, to within 12%, as long as the 'hot-spot' is less than about 8 mm in diameter. Injection-seeding is essential to obtaining this repeatability. The repeatable shock-induced flow emanating from the focus suggests a new method of calibrating high-frequency flow sensors. Successful generation of the perturbations at Mach 4 is also demonstrated, although details of the high-frequency convective disturbances remain to be measured.