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The diode-array velocimeter

Diode-array velocimetry is an optical technique for measuring turbulent flows. It involves timing the passage of seed particles through a small section of a light beam by imaging the light they scatter onto one or more photodiode arrays. The arrays have a few carefully shaped elements, the shapes and positions of which are used to control the measurement-volume geometry and thus select the measurement made. Measurement volumes sensitive to velocity, position and acceleration may be designed. Measurements in highly turbulent and reversing flows are possible. A diode-array velocimeter (DAV) for one-component velocity measurements has been developed to demonstrate this concept. This uses a single laser beam to illuminate particles and a photodiode array with two rectangular elements to sense their motion. The sensitivity of this DAV to electrical noise in the photodiode circuitry decreases with reduction in measurement-volume size. The angle response is closely cosinusoidal to about 60°. Changes to the photodiode-array design could substantially increase this limit. Measurements of mean velocity, normal turbulence stress and velocity skewness made with this DAV in two attached boundary-layer flows compare well with hot-wire measurements. Useful DAV measurements were made as close as 0.2 mm from the wall. DAV measurements made in a separated flow formed downstream of a fence are also presented. These show all the expected features of the separated shear layer and recirculation including the sub-boundary layer formed beneath the backflow. Histograms measured in the reversing part of this flow show a hole near zero velocity that is a consequence of the imperfections in the DAV angle response and limitations on the maximum transit time. These are not fundamental problems, however, and the hole could be minimized or eliminated by using a different photodiode array design and/or measurement strategy.