An experimental study on the interaction between free-stream turbulence and a wing-tip vortex in the near-field

The effect of external free-stream turbulence was investigated experimentally in the near-field of a wing-tip vortex. A free-stream turbulent flow with 4.6% turbulence intensity was created with the aid of a grid turbulence generator, while the turbulence intensity in the base flow case with no grid was approximately 0.5%. The experiments were conducted in a closed-circuit wind tunnel while a dynamic force transducer was used to measure the aerodynamic forces on a NACA 0015 wing section in the range of 0° to 25° angle of attack. The wing section with a semi-span aspect ratio of 5.4 spanned approximately half height of the wind tunnel section and fixed vertically to the load cell at the base. Hot-wire anemometry was used to measure the velocity field downstream of the wing at crossflow planes of x/c=0.1,0.42,0.77 and 1.03 while the Reynolds number and angle of attack were set at 1.6×105 and 10°, respectively. The results of aerodynamic force measurement for the no-grid case agree fairly with other experimental and analytical works. In addition, circumferential velocity and circulation profiles inside the vortex flow-field fit well with analytical expressions proposed by others. The crossflow velocity contours show that the free-stream turbulence tends to increase the vortex diffusion at x/c=0.77 and 1.03. The inboard and upward movement of the tip vortex downstream of the wing was not significantly affected by the introduction of free-stream turbulence, while external free-stream turbulence increases the turbulence intensity peak. External turbulence also increased the circumferential velocity and vortex circulation with respect to the streamwise distance at radial locations far from the vortex center.