New type of embedded laser Doppler velocimeter for measurement of rotary wings boundary layer

Abstract

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.

A miniature three-component LDV probe

“…Measurements over models in large wind tunnels require either large focal length LDV probes to access the flow field from outside the wind tunnel (Butefisch 1989) or unique probes that can be inserted into the model. Examples of unique miniature LDV probes exist in the literature, such as the probes by Chesnakas and Simpson (1997), and by Byun et al (2004) for measurements on stationary models, and by Favier et al (1997), for measurements on rotating frames. Additionally, measurement in unique applications such as the measurements within the cylinder of working cars requires special probes to be built.…”

The Role of Wind Tunnel Experiments inCFDValidation

“…The ELDV measurement principle basically consists in using a specific arrangement of the optical fibers, installed either inside or outside the wing model and always linked to the moving frame, and thus with the model motion in translation/rotation. Such a method has been developed at Laboratory of Aerodynamics and Biomechanics of Movement (LABM), which has now become Institute of Movement Sciences (ISM) over the past 15 years (Favier et al, 1997;Berton et al, 2001Berton et al, , 2003Pascazio et al, 1996) by means of 2C and 3C systems and across different Reynolds number regimes in the range 10 5 < Re c < 10 6 (Berton et al, 2003;Barla et al, 2005). Figure 1 shows a typical example of a boundary layer flow measured using ELDV by Berton et al (2003) abscissa from the leading edge, c the airfoil chord) on a NACA0012 airfoil at rest in the 2D steady flow.…”

“…When the airfoil is forced to oscillate in 2D/3D flows, specific ELDV unsteady data acquisition and reduction procedures, as well as ensemble averages for periodic flows, have been specifically developed at LABM (Favier et al, 1997;Berton et al, 2001;Pascazio et al, 1996). A data file containing around 20 000 samples (of velocity and time values) is created for each altitude y above the wall to provide the (< U >, < V >, < W >) phase-averaged velocity components and the phase-averaged turbulent quantities as turbulence intensities (< σ u >, < σ v >, < σ w >) and Reynolds stresses (< σ uv >, < σ uw >, < σ vw >).…”

“…At the same time, such CFD advances have stimulated the development of new efficient measurement methods such as, for instance, non-intrusive Laser Doppler Anemometry (LDA), Particle Image Velocimetry (PIV) and embedded laser Doppler velocimetry (ELDV) with two and three components (Raffel et al, 2007;Favier et al, 1997;Berton et al, 2001). Such experiments have then played a major role in a better understanding of flow physics and allowed the development of new numerical approaches.…”