Static and dynamic analysis of a NACA 0021 airfoil section at low reynolds numbers based on experiments and CFD

The wind industry needs airfoil data for ranges of Angle ofAttack (AoA) much wider than those of aviation applications,since large portions of the blades may operate in stalledconditions for a significant part of their lives. Vertical axiswind turbines (VAWTs) are even more affected by this need,since data sets across the full incidence range of 180 degreeare necessary for a correct performance prediction at differenttip-speed ratios. However, the relevant technical literaturelacks data in deep and post stall regions for nearly everyairfoil. Within this context, the present study showsexperimental and numerical results for the well-knownNACA 0021 airfoil, which is often used for Darrieus VAWTdesign.Experimental data were obtained through dedicated windtunnel measurements of a NACA 0021 airfoil with surfacepressure taps, which provided further insight into the pressurecoefficient distribution across a wide range of AoAs. Themeasurements were conducted at two different Reynoldsnumbers (Re=140k and Re=180k): each experiment wasperformed multiple times to ensure repeatability. Dynamic AoAchanges were also investigated at multiple reduced frequencies.Moreover, dedicated unsteady numerical simulations werecarried out on the same airfoil shape to reproduce both thestatic polars of the airfoil and some relevant dynamic AoAvariation cycles tested in the experiments. The solved flow fieldwas then exploited both to get further insight into the flowmechanisms highlighted by the wind tunnel tests and to providecorrection factors to discard the influence of the experimentalapparatus, making experiments representative of open-fieldbehaviour. The present study is then thought to provide thescientific community with high quality, low-Reynolds airfoildata, which may enable in the near future a more effectivedesign of Darrieus VAWTs