Embedded LDV measurement methods applied to unsteady flow investigation

Abstract

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

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.…”

“…From phase-averaged ELDV velocity profiles, measured on airfoil surfaces of different geometries and thicknesses (NACA0012, OA209) set at either different steady flow conditions on Reynolds number (10 5 < Re s < 3 × 10 6 ) or different unsteady flow conditions generated by the oscillation in translation, pitching and combined translation-pitching, a transition onset criterion has been proposed (Berton et al, 2001(Berton et al, , 2003 respectively based on the integral term δ 3 and on the length s along the curved airfoil surface. The above formulation thus allows to delineate the instantaneous border between the laminar flow state and the onset of transition, as a function of the phase ωt of the oscillation.…”

“…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.…”

Estimating Horizontal Displacement between DEMs by Means of Particle Image Velocimetry Techniques

“…PIV comes from a distant research field, more commonly used in studies involving aerodynamics [13], transonic flows [14], wind tunnels [15], or aeroacoustic predictions [16]. PIV can be used to determine the velocity field generated by a flow, tracking the particles inside that flow by repeated photography of the same area; an algorithm then computes the displacement of a particle (or a set of particles) from one photograph to the next.…”