International audienceEmbedded laser Doppler velocimetry (ELDV) is shown to be an efficient tool for investigating nonsteady flows near moving surfaces. The present study gives some examples of this methodology applied to velocity measurements in the boundary layer of rotary wings, and of oscillating or rotating models as flat plate, airfoil or half wing, considered in 2D and in 3D flow configurations. The paper presents some typical steady and unsteady data sets (model at rest or oscillating and helicopter rotor blade in hover) that illustrate different aspects of the ELDV capability in determining the instantaneous velocity held within the boundary layer close to the moving surface. In each how configuration (2D, 3D and rotating 3D), the same measurement principle has been developed and based on an optical fibres option. The optical head used is either embedded inside the moving model (ELDV method 1 and 3) or installed outside the model and linked with the oscillating frame and thus with the motion of the model (ELDV method 2). In all cases, velocity measurements are directly realized in a reference frame linked to the moving wall. This paper presents a detailed description of the three ELDV measurement methods and gives some examples of typical features concerning the boundary layer response of oscillating and rotating models to unsteadiness
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.
The Flow Visualization Gun (FVG), a novel time line visualization technique, has been used to investigate the flow field of a helicopter rotor with swept back tip shape in hover flight condition. After introducing the FVG-technique, the paper presents some visualization photographs of the rotor blade tip vortices and the rotor downwash. Using orthogonal sets of flow photographs and digital image analysis, the 3-dimensional time line displacement within the flow and the tip vortex structure are determined. The data of 16 time line experiments are interpolated in space and time to obtain velocity data on an evenly spaced 3-dimensional grid. Vorticity contour plots of the flow field show the complex arrangement of the tip vortices of the blade itself and of the preceding blade and some additional vorticity in the direct wake which may form a secondary vortex. Understanding and control of this vorticity distribution is important for the design of new efficient tip shapes. While the FVG technique has been used so far for qualitative investigation of complex flow patterns at local velocities of up to 20 m/s, a good comparison to laser velocimetry data validates the technique as an interesting tool for both qualitative and quantitative investigation.
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