New type of embedded laser Doppler velocimeter for measurement of rotary wings boundary
Résumé
An experimental method has been developed to determine the velocity profiles across the boundary layer of rotary wings. The measurement, based 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, which can be moved along perpendicular to the surface, and the signals backscattered by flow particles, are collected through rotating fiber optic cables to a transmitter insuring the connection with fixed components (laser source, photomultipliers, burst spectrum analyzer, computer, etc.). Measurements, performed in a frame linked to the rotating blade for one radial distance from the rotation axis and at a chord abscissa x/c = 0.25, have involved the tangential velocity component (chordwise) 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 very close to the wall and far from the wall. The velocity profiles obtained in different hovering test conditions have shown the efficiency of the present embedded laser Doppler velocimeter method, which can also be applied to rotors in forward flight, wind turbines, etc. Undoubtly, the new database obtained on the rotating boundary layer will constitute an essential support for the physical models and computational works. (C) 1997 American Institute of Physics.