Near-wall fluid jet velocity profile measurements using laser speckle velocimetry

Cavitation damage has long been known to be the result of the impulsive pressure forces that occur with the collapse of bubbles. It has been hypothesized that a major contributor to cavitation damage is the jet of water resulting from the bubble's collapse impinging on a hairline crack or other imperfection in an otherwise solid surface. If the velocity profile near such cracks could be measured accurately, a better understanding of the forces that are active in this region would be obtainable. Velocities at distances of tens to hundreds of microns from the surface must be measured. The intention of this study is to measure near-wall velocity profiles immediately following bubble collapse. The technique used in these measurements is called laser speckle velocimetry (LSV). In LSV a thin sheet of pulsed laser light is passed through a transparent fluid medium. A suitably chosen (neutrally buoyant) seed material reflects the laser light, usually at 90 deg, and the scattered light is captured on film. A second pulse is sent through a short time later, and the reflected light is captured on the same piece of film. The end result of these two pulses is a doubly exposed film, which is then analyzed by a variety of methods to correlate the images into pairs of points. From this, the fluid's velocity profile is determined