MEASUREMENTS IN THE TURBULENT WAKE OF A LIFTING WING

Experiments have been performed to study the spiral wake that surrounds the vortex core is shed from the tip of a rectangular NACA 0012 half wing. The measurements were performed 10 chordlengths downstream of the wing in the 3 X 2 Subsonic Wind Tunnel using 3-component hot wire probes singly and in pairs. Grids of measurements were made to reveal the overal flow structure, single point profiles were measured through the core to reveal its form, and two-point profiles were measured in the two and three-dimensional parts of the wake and through the core to reveal correlation length and timescales.

The structure of this flow (see above figure which shows the cross-section turbulence field) 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. 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 the spiral wake. The maximum turbulence levels exist in the region of the wake that begins to spiral about the core. 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. Stronger correlations and coherence in the 3-D region than in the 2-D region reveal that the three-dimensionality is organizing the spanwise turbulent structures. Two point measurements were transformed into upwash wavenumber frequency such as would be used to predict the noise radiated by an airfoil passing through this flow.

Detailed description of the results and their implications is given in the papers

• J Miranda and W J Devenport, "Two-point measurements in trailing vortices", AIAA 34th Aerospace Sciences Meeting, Reno, Nevada, January 15-18, 1996.
• W J Devenport, C W Wenger, S A L Glegg and J A Miranda, "Wavenumber frequency spectra of turbulence in a lifting wake for broadband noise prediction", 3rd AIAA/CEAS Aeroacoustics Conference, May 12-14, 1997, Atlanta, GA, AIAA-97-1699.
• J A Miranda and W J Devenport, "Turbulence structure in the spiral wake shed by a lifting wing", AIAA Journal in press.
• W J Devenport, C W Wenger, S A L Glegg and J A Miranda, "Wavenumber frequency spectra in a lifting wake for broadband noise prediction", in press AIAA Journal.