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.

With the growing importance of renewable energy, the interest in energy-harvesting technologies based on flow-induced vibrations of airfoils has been rekindled in the past few years. Compared to conventional turbines, these devices are centrifugal stress-free and hence they are structurally more robust. They are also environmentally friendly due to the relatively low speeds, thus reducing the impact on flying animals. In order to numerically investigate these types of devices, mesh motion techniques must be used so that the computational grid can adapt to the time-varying shape of the domain and boundaries, thus preserving its robustness and quality. During the last years, many different dynamic mesh approaches have been developed to better predict the behaviour of a flow interacting with solid moving bodies. The aim of the present work is to apply and validate the overset grid method offered by the OpenFOAM software. In this type of mesh, one or more grid blocks are allowed to overlap with other sets of cells to solve the fluid domain with moving bodies, thus showing many potential advantages compared to the other existing dynamic mesh approaches. In the first part of this work, a numerical investigation of the flow over a stationary SD 7003 airfoil at low Reynolds number has been performed using an unsteady RANS approach. The computed results have then been compared to experimental and high-fidelity computational data available from the literature in order to validate the presented model in terms of numerical domain configuration, mesh refinement, and turbulence modelling. Subsequently, unsteady RANS simulations have been performed on the SD 7003 and NACA 0012 airfoils for two different amplitudes of flapping motion, i.e., 30° and 45°. Finally, the results obtained with the OpenFOAM overset grid solver have been compared with the numerical and experimental benchmarks presented by Kurtulus, showing an excellent agreement with both computed and measured data.

The massive growth of the air traffic during the last years is leading to stricter limitations on the noise emission levels radiated from aircraft engines. To face this issue, the installation of acoustic liners on the intake duct and the exhaust nozzles is a common strategy adopted to properly abate noise emissions coming from the fan, the compressor, the turbine, and the jet. In this context, the aim of the present paper is to use high-fidelity LES simulations to validate a MDOF (multi-degree of freedom) extension of the single- and double-degree of freedom (SDOF and DDOF) analytical model provided by Hersh for impedance eduction of acoustic liners. Firstly, the results of the original Hersh model are compared with LES calculations performed with the OpenFOAM suite on a single-orifice and single-cavity layout (SDOF). Then the extension of the Hersh model to multi-cavity (MDOF) geometries by using a recursive formulation is presented. Finally, high fidelity simulations are carried out for single-orifice and multi-cavity (MDOF) configurations to validate the method extension and to understand how resonant coupling and acoustic impedance are affected by multi-cavity resonant elements. The excellent agreement between the high-fidelity results and the analytical predictions for the single-cavity pattern confirms that the Hersh model is a useful formulation for a preliminary design of a SDOF acoustic liner. The model extension to MDOF configurations enables the designers to broaden the design space, and thus a validated analytical method is strictly necessary to perform sensitivity studies to the multi-cavity geometrical parameters (i.e., face-sheet thickness, cavities depth, porosity). Basically, a multi-cavity configuration makes the liner element resonate at different frequencies leading to multiple absorption peaks in the audible range. In this way, the acoustic performance of the liner is extended to a wider frequency range, overcoming the limitations of a traditional SDOF configuration.

The massive growth of the air traffic during the last years is leading to stricter limitations on the noise emission levels radiated from aircraft engines. To face this issue, the installation of acoustic liners on the intake duct and the exhaust nozzles is a common strategy adopted to properly abate noise emissions. In this context, the aim of the present paper is to use high-fidelity LES simulations to validate a MDOF (multi-degree of freedom) extension of the SDOF (single-degree of freedom) analytical model provided by Hersh for impedance eduction of liners. Firstly, the results of the original Hersh model are compared with LES calculations performed with the OpenFOAM suite on a single-orifice and single-cavity layout. Then the extension of the Hersh model to MDOF geometries by using a recursive formulation is presented. Finally, high fidelity simulations are carried out for SDOF and MDOF configurations to validate the method extension. The excellent agreement between the high-fidelity results and the analytical predictions for the single-cavity pattern confirms the validity of the Hersh model for preliminary designs. The model extension to MDOF configurations enables the designers to broaden the design space, and to perform sensitivity studies to the multi-cavity geometrical parameters. A multi-cavity configuration makes the liner element resonate at different frequencies leading to multiple absorption peaks in the audible range. Doing so, the acoustic performance of the liner is extended to a wider frequency range, overcoming the limitations of a traditional SDOF configuration.