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The Water Channel Facility Scanning laser doppler velocimetry is used to take instantaneous measurements of the velocity of the flow. A Spectra-Physics Model 165 Argon Ion Laser and a Model 265 Exciter are connected to emit a green laser beam at a wavelength of 514.5 nm. The power setting for the laser is kept below 0.2 Watt, and is normally around 0.1-0.15 Watt to insure reasonable data rates in a range of 2000 to 3000 data points per second. The laser beam was first split into two beams, one of which was frequency shifted by 40 MHz with a Model 9182 Acousto-Optic modulator (Bragg cell). The two split beams diverge at 0.2 degrees from the Bragg cell, and then pass through two bi-convex lenses. The two lenses are used to focus the beams and reduce the diameter of the probe volume in the water channel. Next, the beams are steered with the prisms to a much wider angle, and reflected off from two stationary mirrors. These two mirrors were aligned so that the reflected beams would first strike the verical scanning mirror, and the strike the horizontal scanning mirrors. Because there are more beams at a higher order emitted from the Bragg cell, a pair of masks were put in front of the beams striking the vertical scanning mirror to block the unnecessary beams. Scanning was accomplished by triangular waveforms supplied by two Philips Model PM5132 function generators. One of these controls the vertical scanning and the other controls the horizontal scanning.
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