An experimental study on a Natural Laminar Flow airfoil at off-design conditions

This study presents an experimental investigation on a natural laminar flow airfoil under operating conditions that deviate significantly from its design point. Aerodynamic force measurements revealed the section drag, lift and pitch moment coefficients converge for Reynolds number Re ≥ 462k. At lower Reynolds numbers (Re ≤ 308k), substantially reduced aerodynamic efficiencies and irregular variations of the aerodynamic forces with angle of attack were observed, which are attributed to flow separation without reattachment based on the particle image velocimetry (PIV) measurements. A discrepancy between the PIV and oil film interferometry (OFI) surface flow visualisation was observed on the suction side of the airfoil at Re=500k and α = 0°, where the PIV results indicate fully attached flow, while the OFI results reveal laminar boundary layer separation and turbulent reattachment. This discrepancy is attributed to the loss of PIV data in the near-wall region, coupled with the small vertical extent of the separation bubble, which can be on the order of 1% of the chord length.