Embedded LDV measurement methods applied to unsteady flow investigation

Abstract

Embedded 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.