Wind loading on elongated structures characterized by bluff sections: comparison between 2D and 3D simulations

The flow around bluff-bodies is characterized by specific flow features (e.g. flow separations, flow re-attachments and vortex shedding) which clearly differentiate it from flows over aerodynamic bodies, such as aerofoils or turbine blades. Due to the lack of analytical solutions, bluff-body aerodynamics relies on experimental measurements and, increasingly, on Computational Fluid Dynamics (CFD), which involves the solution of fluid flow equations by means of numerical methods. However, since the computational cost for Direct Numerical Simulations (DNS) is often prohibitive, the use of turbulence models is essential for the application of CFD to engineering problems. Such models can be developed in various ways. In particular, Reynolds Averaged Navier-Stokes (RANS) models are derived by Favre-averaging Navier-Stokes equations, whereas Large Eddy Simulations (LES) rely on their spatial filtering. The major drawback in the use of such models is the loss of flow details, which may however have a strong influence on the flow field development. The resulting flow can, indeed, be remarkably different from DNS and experimental measurements. This is particularly true for RANS models, especially when used in a two-dimensional (2D) framework. In fact, in cases for which the time-averaged flow is approximately two-dimensional, the use of 2D RANS models appears to be an extremely convenient and widely adopted solution. However, such simplification can lead to strong inaccuracies. The aim of this thesis is to explore and assess the accuracy of 2D RANS models for the prediction of the aerodynamic coefficients for the wind loading of elongated structures characterized by bluff sections. Results show an overall tendency to overestimate drag due to the impossibility to generate three-dimensional structures in the wake zone. Additional LES simulations confirm the obtained results.