The Effect of Aspect Ratio and Sweep Angle on 3D Stall Cell Formation on Wings with NACA0015 Airfoil

The effects of wing’s aspect ratio and sweep angle are investigated on the characteristics of stall cell phenomenon at different Reynolds numbers and beyond angle of attack (AOA) at which trailing edge flow separation initiates (A_s). To study the effect of aspect ratio on post-stall flow, three different models with NACA0015 cross-section and AR=3.5, 4.5 and 5.5 were examined experimentally. For the sweep angle effect, four wing models with NACA 0015 airfoils, AR = 4.5 and sweep angles of ?? = 0°, 3°, 6°, and 10° were utilized.  The experiments were conducted at different Reynolds numbers varying in the range of Re = 3 × 10⁵ - 6 × 10⁵ based on the wing mid-chord length and free stream velocity (Re = ????₀??_m/μ).

The flow visualization results indicated that stall cells can be observed in a relatively narrow range of angles of attack. By increasing the angle of attack, the observed flow pattern changed from a separated region without any vortical structure at A_s ≤ AOA ⪅ A_s + 1°, to a single stall cell which contained at least two counter-rotating structures at A_s +1° ⪅ AOA ⪅ A_s + 8°. Further increasing angle of attack led to the distortion of stall cell structure. While changing the Reynolds number affected the post-stall flow patterns significantly, present visualization results showed approximately similar trends for wings with different aspect ratios.

The results from the sweep angle comparison indicated that in contrast with the effect of aspect ratio, patterns on the different swept wings are not following the same trend. For instance, present results showed that increasing sweep angle up to 10°, led to the formation of a single vortex on the wing surface instead of a stall cell with two counter-rotating vortices. For ?? = 3° and 6°, by increasing the swept wings’ angle of attack, the size of the vortex, which is closer to the wings’ root increased significantly while the vortex closer to the swept wings tip remained approximately unchanged.