Abstract
The working conditions of airfoils along modern wind turbine blades are putting new focus on the importance of properly characterizing the aerodynamic performance of different airfoil families also at high angles of attack (AoAs) beyond stall and at Reynolds numbers much lower (from few thousands to one million) than those commonly analyzed before. Several test cases are showing that even higher-order computational methods (like RANS/URANS CFD) are unable to properly capture the complex flow physics taking place past the blades, when deep stall occurs or when the AoA changes so rapidly to provoke the onset of dynamic stall. To fill this gap, the use of high-fidelity methods, like the Large Eddy Simulation (LES) is proposed, even though it implies a massive increase of the calculation cost. In order to analyze the prospects of using LES in comparison to RANS for low Reynolds, high AoAs, this work presents an in-depth study of the NACA 0021 aerodynamics at the Reynolds number of 80,000, by means of both traditional RANS approaches and high-fidelity (LES) simulations using the OpenFOAM suite. The selected airfoil has been showing in fact several issues in the correct characterization of its performance in similar conditions in many recent wind energy applications. The LES approach showed the ability to overcome the limitations of traditional RANS simulations, improving the accuracy of the results and reducing their dispersion thanks to the fact that the flow structures in the separated-flow regions are properly captured. Overall, this work underlines that accurate investigations of the aerodynamic performance of the NACA 0021 at low Reynolds require multiple sensitivity studies when RANS approaches are used, and suggests the use of LES simulations in order to increase the accuracy of estimations, especially when studying the stalledflow operating conditions of the airfoil.
Highlights
The aerodynamic performance of airfoils has been studied extensively since the beginning of the 20th century, with both dedicated experiments in the wind tunnel and numerical approaches with different levels of fidelity
This work underlines that accurate investigations of the aerodynamic performance of the NACA 0021 at low Reynolds require multiple sensitivity studies when RANS approaches are used, and suggests the use of Large Eddy Simulation (LES) simulations in order to increase the accuracy of estimations, especially when studying the stalledflow operating conditions of the airfoil
The aerodynamic performance of the NACA 0021 airfoil available from the open literature have been found to be affected by a huge dispersion
Summary
The aerodynamic performance of airfoils has been studied extensively since the beginning of the 20th century, with both dedicated experiments in the wind tunnel and numerical approaches with different levels of fidelity Among these latter, the ubiquitous panel method (e.g. applied through the XFoil software [1]) is often used by researchers to obtain a first estimation of the airfoil aerodynamic coefficients in the attached flow region up to the region in proximity of the static stall limit. Several test cases are showing that even these methods are unable to properly capture the complex flow physics taking place past the blades when deep stall occurs or when the AoA changes so rapidly to generate dynamic stall To fill this gap, the use of high-fidelity methods, like the Large Eddy Simulation (LES) is on the spotlight, even though it implies a massive increase of the calculation cost
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