NEW-TYPE OF EMBEDDED LASER-DOPPLER VELOCIMETER FOR MEASUREMENT OF ROTARY WINGS BOUNDARY

An experimental method has been developed to determine the velocity pr ofiles across the boundary layer of rotary wings. The measurement, bas ed on laser Doppler velocimetry, has been tested on a helicopter rotor blade in hover. The main components of the prototype are embedded in an untwisted blade. The beams converging at the measurement volume, wh ich can be moved along perpendicular to the surface, and the signals b ackscattered by flow particles, are collected through rotating fiber o ptic cables to a transmitter insuring the connection with fixed compon ents (laser source, photomultipliers, burst spectrum analyzer, compute r, etc.). Measurements, performed in a frame linked to the rotating bl ade for one radial distance from the rotation axis and at a chord absc issa x/c = 0.25, have involved the tangential velocity component (chor dwise) and the crossflow component (spanwise). The boundary layer has been explored at different rotating speeds of the blade. The accuracy of velocity components measurements has been evaluated in the region v ery close to the wall and far from the wall. The velocity profiles obt ained in different hovering test conditions have shown the efficiency of the present embedded laser Doppler velocimeter method, which can al so be applied to rotors in forward flight, wind turbines, etc. Undoubt ly, the new database obtained on the rotating boundary layer will cons titute an essential support for the physical models and computational works. (C) 1997 American Institute of Physics.