Mosaic laser velocimeter for large facilities

Abstract

Detailed flow characterization and turbulence investigation in large facilities using laser velocimetry present several difficulties essentially due to the size of the optics to be installed, the limited optical access and the required window quality. The mosaic laser velocimeter is a new instrument: its compact and robust optical head is a probe introduced into the flow, but with a 467 mm working distance, so that negligible disturbances are induced at the probe volume level. The basic principle is the following: the image of the trajectory of a particle, crossing a 400 mu m probe volume illuminated by a focused argon laser beam, is made by a receiving optics on a mosaic of optical fibres, each fibre being linked to a photomultiplier; these fibres are arranged along two concentric circles: the external one is used for measurements, the internal one for validation purposes. Two velocity components, in the plane perpendicular to the receiving optics axis, are deduced from the recognition of the addresses of the lighted fibres and from the measured time interval between the pulses delivered by the corresponding detectors. The possibilities of this new instrument have been compared with those of a classical 3D laser velocimeter by making detailed measurements in the wake and in the boundary layer of an aerofoil in a 1.4 m by 1.8 m subsonic wind tunnel: the good agreement obtained with respect to the results of mean velocities and turbulence have brought a high degree of confidence for the future employment of the mosaic laser velocimeter in larger subsonic or transonic facilities.