Laser Velocimeter for Wind Tunnel Measurements

Laser velocimetry is an optical technique used to make local measurements of mean velocity and turbulence in fluid flows without disturbing them. Laser velocimetry principles are briefly recalled; the optical and mechanical elements making up a velocimeter are described. Their purpose is to create two laser beams of equal intensity which are focused and crossed in a probe volume V where a fringe pattern is formed; the light scattered by submicron particles passing through V is collected by an optics rigidly connected to the emitting optics and focused on a photomultiplier. Among the various signal processing techniques, counters are now widely used, in connection with minicomputers. The different means used to determine the velocity sign and to measure simultaneously two components of the velocity are described; the simultaneous measurement of the three velocity components is more a matter of technology and cost than a theoretical problem. The modular operational velocimeter developed at ONERA is described; as examples of application, some typical results obtained in different aerodynamic flows are reported.