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Boundary Layers

Supersonic boundary layer measurements

LDV beams

Credit: Holger Babinsky

A supersonic turbulent boundary layer is being measured by Laser Doppler Velocimetry (LDV). A pair of laser beams are crossed in the middle of the tunnel, which creates an interference pattern of bands of light and dark with a known separation. Tiny droplets of oil are introduced into the wind tunnel and carried along by the air. As they pass through the pattern, they flash at each band. By timing the difference between flashes, the speed of the droplets can be calculated.