THE DIODE-ARRAY VELOCIMETER

Diode-array velocimetry is an optical technique for measuring turbulen t flows. It involves timing the passage of seed particles through a sm all section of a light beam by imaging the light they scatter onto one or more photodiode arrays. The arrays have a few carefully shaped ele ments, the shapes and positions of which are used to control the measu rement-volume geometry and thus select the measurement made. Measureme nt volumes sensitive to velocity, position and acceleration may be des igned. Measurements in highly turbulent and reversing flows are possib le. A diode-array velocimeter (DAV) for one-component velocity measure ments has been developed to demonstrate this concept. This uses a sing le laser beam to illuminate particles and a photodiode array with two rectangular elements to sense their motion. The sensitivity of this DA V to electrical noise in the photodiode circuitry decreases with reduc tion in measurement-volume size. The angle response is closely cosinus oidal to about 60-degrees. Changes to the photodiode-array design coul d substantially increase this limit. Measurements of mean velocity, no rmal 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 separ ated shear layer and recirculation including the sub-boundary layer fo rmed beneath the backflow. Histograms measured in the reversing part o f this flow show a hole near zero velocity that is a consequence of th e imperfections in the DAV angle response and limitations on the maxim um 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.