This study focuses on the detection and characterization of vortices in low Reynolds number separated flow over the elliptical leading edge of a low aspect ratio, flat plate wing. Velocity fields were obtained using the time-resolved particle image velocimetry. Experiments were performed on a wing with aspect ratio of 0.5 for velocities of 1.1 m/s, 2.0 m/s, and 5.0 m/s corresponding to chord length Reynolds numbers of 1.47×104, 2.67×104, and 6.67×104, respectively, and angles of attack of 14 deg, 16 deg, 18 deg, and 20 deg. A local swirl calculation was used on proper orthogonal decomposition filtered data for vortex identification and corresponding vortex centers were tracked to determine convective velocities. The swirl function was also analyzed for its temporal frequency response at several discrete points in both the shear layer and in the separated recirculation region. A peak frequency was detected in the shear layer with a corresponding Strouhal number of approximately 3.4 based on the flow direction projected length scale. The Strouhal number increases with both angle of attack and Reynolds number. The shear layer convective length scale, based on the vortex convection velocity, is found to be consistent with the mean separation distance between vortices within the shear layer. This length scale decreases with increasing Rec.
This study examines the generation of large scale vortices caused by flow separation from a flat wing at various angles of attack. Time-resolved particle image velocimetry is used to determine the evolution and convective characteristics of the large scale structures. A rectangular airfoil with aspect ratio of 0.5 is used and data are collected at a Reynolds number of 23,500, for angles of attack from 0° to 20°. Data consists of two dimensional velocity fields obtained at 500 Hz located at the airfoil centerline. The region of interest is near the separation point but fields of view extend over approximately one half of the chord length from the leading edge to document the downstream progression of the large scale vortical flow elements. The velocity data were processed to identify the vorticity field dynamics in terms of the Kelvin-Helmholtz instability occurring near the leading edge. The vortical structures are identified using vortex detection based on local circulation. The convective nature of the vortex elements are shown to consist of merging, stalling and convecting, with convective velocities on the order of 20% of the freestream velocity with an associated Stouhal number based on chord length and freestream velocity of approximately 1.0.
This study focuses on the detection and characterization of vortices in low Reynolds number separation flow over the elliptical leading edge of a flat plate airfoil. Velocity fields were obtained using Time Resolved Particle Image Velocimetry (TRPIV). The Reynolds number based on chord length ranged from 14,700 to 66,700. Experiments were performed for velocities of 1.1, 2.0 and 5.0 m/s and angles of attack of 14°, 16°, 18° and 20°. These velocities correspond to chord length Reynolds numbers of 1.47×104, 2.68×104, and 6.70×104, respectively. A local swirl calculation was used to determine regions of high circulation, and the convection of the centers of these regions was used to determine convective velocities of these vortical structures. The streamwise convective velocity normalized by the freestream velocity is observed to range from approximately 0.4 to 0.65 over the range of angles of attack, with slightly increasing values as the angle of attack increases.
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