The effect of freestream turbulence on wing-tip vortex meandering and deformation

The effect of freestream turbulence (FST) on a wing-tip vortex was investigated at a chord-based Reynolds number of $10^5$. Experiments were conducted at four FST intensities, generated by an active grid, 0.30%, 1.84%, 7.70%, and 13.25%, with integral scales ranging from 0.5 to 2.6 times the wing chord length. Stereoscopic particle image velocimetry measurements document the effects of FST on the meandering motion of the wing-tip vortex in the near field and middle field of a NACA0012 wing, i.e., planes $x/c=[2,5,12]$ downstream from the trailing edge of the wing. Conditional averaging based on recentring the coordinate system on the vortex center has been used to eliminate the influence of vortex motion. When the analysis was conditionally averaged on the vortex’s core position, a reduction in vortex strength with increasing FST was observed, along with a slight increase in diffusion for the highest FST cases (FST levels $>6\text{\%}$). Snapshot proper orthogonal decomposition (POD) analysis on the coherent component of the streamwise vorticity field revealed two dominant modes associated with meandering displacement for all FST cases. POD analysis further reveals turbulence accelerates the onset of spatial modes associated with vortex deformation, which typically emerge in the far wake ($x/c\ge 36$) under non-turbulent conditions but appear in the near wake ($x/c=5$) when FST is present. The vortex deformation modes contribute more significantly to the total enstrophy in turbulent cases in the near wake than in the far wake under non-turbulent conditions.