The structure of a trailing vortex wake
Experiments have been performed in the spiral wake that surrounds the vortex shed from the tip of a rectangular NACA 0012 half wing. Both single-point and simultaneous two-point hot-wire measurements were made ten chord lengths downstream of the wing at a Reynolds number of 320,000.
The structure of this flow consists of a small concentrated vortex core, surrounded by a circulating velocity field that winds the wake into a spiral about the core. Small amplitude wandering motions of the core were detected and quantitative estimates of the wandering amplitude and its contributions to the Reynolds stress fields were found. Outside of the core region the turbulence structure is dominated by this spiral wake. The maximum turbulence levels exist in the part of of the wake that begins to spiral about the core. The 3-D region is the region where the wake is wound about the vortex and the 2-D region is the nearly two-dimensional portion of the wake inboard of the wing-tip. The stresses in the 3-D region, when scaled on local length and velocity scales, increase significantly over those in the 2-D region. This increase is presumably a consequence of the additional rates of strain imposed on the wake by the vortex. Stretching intensifies turbulent structures aligned with the stretching direction.
Two-point measurements were made in the 2-D and 3-D regions of the wake to reveal the spanwise extent of the instantaneous turbulent structures in the wake. A high correlation was found at the smallest probe separations. There are stronger correlations and coherence in the 3-D region than in the 2-D region. Correlation length scales and coherence were found to increase significantly from the 2-D region to the peak turbulence region. This reveals that the stretching and additional rates of strain imposed on the wake by the vortex are organizing the spanwise turbulent structures.