Wingtip vortex dynamics at low Reynolds numbers under the influence of turbulence

Abstract The influence of turbulence on the aerodynamics of a NACA0012 wing and the behavior of the associated wingtip vortex is investigated at a chord-based Reynolds number of 5000. The study is conducted through water tunnel experiments using a half-wing setup with an aspect ratio of 6 and an angle of attack of 10 degrees. At this low Reynolds number, relevant for aerodynamics at low-speed, the flow over a stationary wing exhibits laminar separation. Longitudinal Particle Image Velocimetry (PIV) measurements show a laminar flow separation over the wing upper surface and its reduction with increased incoming turbulence. Four turbulence conditions are tested: three different passive grids and one baseline (no-grid) configuration, yielding turbulence intensities ranging from 1.4 to 8.2%. Transverse PIV measurements are conducted over three cross-sections up to 24 chord lengths downstream of the wing trailing edge, to characterize the vortex dynamics. At the wingtip, the flow rolls up into a single coherent tip vortex, which exhibits significant unsteadiness in the form of a dominant displacement mode, as Proper Orthogonal Decomposition (POD) shows. In the no-grid case, the vortex exhibits significantly lower-frequency and higher-amplitude motion than in the grid-generated turbulence cases. This indicates that, at low Reynolds numbers, vortex meandering can arise either from unsteady perturbations linked to large-scale flow separation on the wing under low turbulence intensity, or from the direct receptivity of the vortex to freestream turbulence at higher turbulence levels. These results highlight the coexistence and relative importance of two distinct sources of disturbance—wing-separated flow and inflow turbulence—in governing wingtip vortex dynamics. Graphic abstract