Aerodynamic characterization of transitionally-operating airfoils under a set of flow conditions going from ideal to real configurations

Abstract

Applications in which airfoils operate under transitional flow conditions are widespread, comprising microand nano-aerial-vehicle wings, portions of mid- and large-scale wind-turbine blades or turbomachinery vanes. The paradigm of the renewables, among which wind energy stands as one of the major sources, or the ever-growing use of MAVs for new transportation and delivery modes, field monitoring operations or rescue missions alike, are indicative of the relevant role that transitional flows are acquiring within aerodynamics. Beyond the interest that such applications may have, the scientific contributions of this work are focused on performing an experimental-numerical characterization of airfoils under different flow conditions. Thus, the goals of the study are headed in two directions: on experimental grounds, it aims at complementing the understanding about how transitional conditions influence the characterization and design process of an application-oriented airfoil. This is done by undertaking wind tunnel tests in configurations that attempt to reproduce an ever-more realistic set of flow conditions. The starting point corresponds to the clean-flow paradigm, in which the aerodynamic characterization is performed under a uniform, steady and two-dimensional flow. Then, real effects are introduced either in the flowfield or in the geometry of the airfoil. These two constitute the turbulent- and rough-flow paradigms respectively, and lead to pseudo-real conditions that contrast significantly with the clean configuration. The real-flow paradigm is obtained by combining those two pseudo-real effects, and the behaviour of the airfoil is shown to differ substantially from the previous flows. The characterization process is complemented by an analysis of a discrete-roughnessbased flow-control technique that aims at improving either the lift or the efficiency of the airfoil. As occurs for the characterization itself, the flow paradigm is shown to affect the suitedness of the roughness elements for enhancing the aerodynamic behaviour. In fact, it is proven that certain discrete roughness distributions downgrade the aerodynamic behaviour under clean or pseudo-real configurations, but improve it when imposing real-flow conditions. On numerical grounds, two-dimensional RANS simulations and potential-flow-based panel-method calculations are performed under clean- and turbulent-flow configurations. The comparison with their experimental counterparts shows that further advancements are required in the turbulence modelling of the transitional regime for obtaining acceptable experimental-numerical matches.