On the mean characteristics of the juncture vortex in cross-flow planes

Abstract

Juncture flow of wings with leading edge sweep angles of 0°, ±15°, ±30°, and ±45° mounted on a flat plate for a turbulent approach boundary layer was investigated using laser Doppler velocimetry (LDV) in a subsonic wind tunnel. Time-averaged measurements in the cross-flow planes indicated incompletely formed vortical structures. Additionally, in the cross-flow plane at the airfoil maximum thickness location, measured velocity directions were inconsistent with vortex rotation. Complex motion of the juncture vortex system in the cross-flow direction was observed during flow visualization of the configuration in a water tunnel, suggesting masking of the mean vortex structure by the global motion of the structures. The mean vortex signature with correct rotational direction was ‘frozen’ by subtracting a constant cross-flow velocity component from the LDV mean measurements. Vorticity contours in the cross-flow planes were calculated using a finite differencing scheme on the measured data. Good comparison of vortex core estimated locations between the ‘frozen’ vortex and from vorticity contours was observed. Another artefact of the measurements was asymmetry in the vertical velocities, which is attributed to motion of the vortex core towards the flat plate in the maximum thickness plane and away from it in the 80 per cent chord plane. The horizontal location of the vortex core was observed to be influenced by wing sweep; however, the vertical location was insensitive to this parameter.