Design of a wind tunnel set up to measure airfoil aeroelastic gust response

Abstract

Introduction Gusts of moderate and large magnitude induce flow separation and other complexities when they interact with the lifting surfaces of air vehicles or other aerodynamic surfaces, such as wind turbine blades. Separated flow poses a significant challenge for the classic potential flow-based methodologies used in the past. To improve our understanding of the underlying physics and develop new and effective mitigation strategies, state-of-the-art tools and new ways of analysis are required. The present paper presents the design of a wind tunnel set up to measure airfoil aeroelastic gust response at Reynolds numbers . The set up consists of a vane Gust Generator (GG) and an aeroelastic support for extruded airfoil models, see Figure 1. Figure 1. . Methodology Wind tunnel The set-up was developed for the small test section of the subsonic wind tunnel at the National Technical University of Athens (NTUA). The test section is 1.4 m high and 1.8 m wide with a length of 3.3 m. The maximum velocity is 60 m/s and the free stream turbulence is 0.2%. Gust Generator The GG consisted of four independently controlled vanes, see Figure 2. The vanes have a NACA 0015 profile and span the test section vertically. The GG design is based on the work by (Balatti et al., 2022), but modified for the larger wind tunnel cross section of the wind tunnel at NTUA. Based on the existing literature and the relevant applications (Wind Turbines, Aircraft, UAV) the GG specifications are defined as given in Table 1. Figure 2. A 3D render of the Gust Generator CAD (left) and a photo of the Gust generator vanes in the wind tunnel (right) Aeroelastic Model Support A simplified aeroelastic system, which consists of an airfoil mounted through torsional and bending springs of stiffness and , respectively, is shown in Figure 1. The wing model support in the test section is designed to reflect this, with the model being supported only on the lower end on a carriage that can perform a heaving motion. The carriage located below the test section is restricted by a pair of compression springs. The carriage also hosts a bearing, which allows for the model pitching motion, and a pair of springs connected to the model axis, which exert restoring forces to when the model is displaced from its initial equilibrium position. Spring stiffness can be selected to reflect the desired elastic properties. The set up is explained in detail in (Gkiolas, 2022). Table 1. Gust Generator Specifications Geometry Nr of Vanes # 4 Vane chord m 0.2 Vane Profile c NACA 0015 Vane Span m 1.4 Centre of Rotation c/4 Material 3D printed Operation Max Frequency Hz 20 Max Angle range deg +/-10 Actuation One motor per Vane Gearbox type low backlash Gearbox Ratio 10:01 CFD approach The GG and vane motion protocol design was assisted by CFD simulations (see Figure 3, left) using the inhouse computational fluid dynamics solver MaPFlow (Papadakis and Voutsinas, 2019). MaPFlow is a cell centred CFD Solver capable of using both structured and unstructured grids and is parallelized using the MPI library in a multiblock fashion. In the present case, Unsteady Reynolds Averaged Navier Stokes (URANS), with the Spalart – Allmaras turbulence model, and Improved Delayed Detached Eddy Simulation closure models were employed. For the vane motion, deformable grids are employed, which are treated using a radial basis function approach that allows for large motions without degrading the grid quality. Results At the time of writing only preliminary results are available and presented below, in Figure 3. A more detailed discussion and presentation will follow at the conference. Figure 3. (Top) 2D RANS investigation on a suitable vane motion. Comparison with ‘1-cos’ and the motion presented by Balatti et al. 2022; (Bottom) Comparison between CFD and Experiments. Flow angle for a ‘1-cos’ vane movement. , , . The negative peak prior to the main gust is visible, typical of the ‘1-cos’ motion (Balatti et al., 2022) Acknowledgements The authors are grateful to NTUA technician Sotirios Mavrakis for his help setting up the experiments. This project is carried out within the framework of the National Recovery and Resilience Plan Greece 2.0, funded by the European Union – NextGenerationEU (H.F.R.I. Project Number: 016749) References Balatti, D., Haddad Khodaparast, H., Friswell, M. I., and Manolesos, M.: Improving Wind Tunnel “1-cos” Gust Profiles, J. Aircr., 1–15, https://doi.org/10.2514/1.C036772, 2022. Gkiolas, D.: Experimental flow study of an oscillating airfoil, National Technical University of Athens, https://doi.org/http://dx.doi.org/10.26240/heal.ntua.22396, 2022. Papadakis, G. and Voutsinas, S. G.: A strongly coupled Eulerian Lagrangian method verified in 2D external compressible flows, Comput. Fluids, 195, 104325, https://doi.org/10.1016/j.compfluid.2019.104325, 2019.