Mean and turbulence measurements in the near field of a wingtip vortex

The rollup of a wingtip vortex, at a Reynolds number based on chord of 4.6 x 10(6), was studied with an emphasis on suction side and very near-wake measurements (up to x/c = 0.678 downstream of the trailing edge). The research was conducted in a 32 x 48 in. (0.81 x 1.22 m), low-speed wind tunnel. The rectangular half-wing model had a semispan of 36 in. (0.91 m), a chord of 48 in. (1.22 m), and a rounded tip. Seven-hole pressure probe measurements of the velocity field surrounding the wingtip showed that a large axial velocity up to 1.77U(infinity) developed in the vortex core. This high a Level of core axial velocity has not been measured previously. Triple-wire probes were used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that meandering of the vortex was small and did not contribute appreciably to turbulence measurements. The flow was found to be turbulent in the near field (as high as 24% rms velocity), and the turbulence decayed quickly with streamwise distance because of the stabilizing effect of the nearly solid body rotation of the vortex-core mean flow. A streamwise variation of the location of peak levels of turbulence, relative to the core centerline, was also noted. Close to the trailing edge of the wing, peak shear stress levels were measured at the edge of the vortex core, whereas in the most downstream make plane, the peak levels were measured at a radius roughly equal to one-third of the vortex core radius. The Reynolds shear stresses (in Cartesian coordinates) were not aligned with the mean strain rate, indicating that an isotropic-eddy-viscosity-based prediction method cannot fully model the turbulence in the vortex. In cylindrical coordinates, with the origin at the vortex centerline, the radial normal stress was found to be larger than the circumferential component.