Attribution of low-frequency leading-edge suction to complete dynamic-stall-vortex formation

Determine whether the leading-edge suction observed during the lowest-reduced-frequency ramp-up motions of a NACA0021 airfoil at chord Reynolds number Re_c = 6 × 10^6 is caused by the complete formation of a dynamic stall vortex.

Background

The study examines the unsteady aerodynamic response of a NACA0021 airfoil pitched through its static stall angle at reduced frequencies between k = 0.001 and k = 0.1 and at Re_c approximately 6 × 106. The measurements show stall delay and elevated leading-edge suction even at the lowest reduced frequencies.

For the lowest-frequency motions, the leading-edge suction signature is weaker and is not accompanied by the downstream-convection signature that would normally help identify a fully formed dynamic stall vortex. Consequently, the paper leaves unresolved whether this suction reflects complete dynamic stall-vortex formation or another boundary-layer-related unsteady mechanism. Resolving this attribution is important for accurately modeling unsteady loading on wind-turbine airfoil sections at very low reduced frequencies.

References

For the lowest $k$ motions studied herein, it is inconclusive to determine whether the suction at the leading edge may be attributed to the complete formation of a dynamic stall vortex due to its weaker peak suction signature and lack of a corresponding downstream convection signature.

— Modern aerodynamics models do not capture important unsteady forces--the failure of the quasi-steady approximation  (2608.13424 - Malarczyk et al., 13 Aug 2026) in Conclusion