Part I of the thesis deals with the implementation of a computational uid dynamic analysis using open source tools. The presented study aims to highlight the accuracy and the stability aspects of the open source codes, speci cally OpenFOAM, compared to the commercial software.
Firstly, a theoretical background of the Computational Fluid Dynamic (CFD) is provided in Chap-ter 1. The well known Navier-Stokes equations are derived with full attention to the principles which underlie the physics involved. They are presented in their conservative form, which represents the most suitable formulation for the implementation in a numerical analysis. The discretization of the equations on a computational grid and the dierences between explicit and implicit methods are also illustrated. In addition, the theoretical formulation of a problem that involves rotating zone of uids is also mentioned. The Moving Reference Frame technique allows to simplify the numerical analysis of the Horizontal Axis Wind Turbine, the topic addressed in Chapter 7.
Chapter 2 provides a general overview on the open source codes. Firstly, the concept of copyleft is explained and the most important types of open source license (GPL and LGPL) are presented. The open source codes, used in the thesis, are therefore brie y introduced. The panel codes XFOIL and RFOIL represent a high-accuracy alternative to the CFD analysis: they allow to evaluate the aerody-namic polars of an airfoil in a fast way. For this reason, they are suitable for the implementation in an optimization loop. A powerful collection of open source tools for the optimization is DAKOTA, a software developed by Sandia and released, for a few years, under a LGPL license. OpenFOAM rep-resents the most important open source CFD code and its community of users continuously develops it. In order to generate a geometry, the 2D and 3D computational grids, the CAE software SALOME can be used. SALOME consists of several open source libraries and algorithms with a simple graphical interface. Finally, ParaView represents a great tool that helps the user to post-process the numerical results from FEM or CFD analysis, or simply to visualize a geometry.
The thesis aims to develop an open source environment for the wind turbines multi-disciplinary opti-mization. In the last decade, several tools for the design of both Horizontal and Vertical Axis Wind
under the GPL license) for the wind turbine design and calculation. It integrates the XFOIL/XFLR5 functionality to compute the polars and it includes them into the design process. It also contains several modules for the VAWT design calculation, the Viterna extrapolation, the structural Euler-Bernoulli beam module and the integration with the aeroelastic code FAST. The latter one is developed by NREL; FAST [2] is an aeroelastic CAE tool (distributed under the Apache License, Version 2.0) for simulating the coupled dynamic response of wind turbines. The tool allows the analysis of a wide range of wind turbine con gurations, considering a dierent number of blades, pitch or stall regula-tion, rigid or teetering hub, upwind or downwind rotor, and lattice or tubular tower. It implements aerodynamic, hydrodynamic, control and electrical system and structural-dynamics models that can interact and be coupled. AeroDyn [3] is also developed by NREL and it represents a time-domain wind turbine aerodynamics module that has been coupled into FAST tool to enable aero-elastic simulation of horizontal-axis wind turbines. The cited tools are examples of multidisciplinary analysis of the wind turbine design and they take into account several aspects that should be considered in the design process. However, an optimization module is still not considered and they have not the exibility to also include high-accuracy calculations evaluated with methods as the FEM and CFD codes.
Being the aero-dynamical study one the most important in the considered machine design, an extensive application of a CFD analysis (on a dierent topic, a tiltrotor external aerodynamic analysis), with open source codes, is presented in Chapter 3. The purpose is to test the capabilities of an open source CFD tool and compare the results with both numerical and experimental data. The work is part of the European research program DREAm-Tilt [4], signed by the HIT09-UNIPD-RUAG consortium and the European community organism Clean Sky JU in the framework of the call for proposal, issued by the helicopter manufacturer AgustaWestland (AW). Speci cally, the analysis carried in Chapter 3 are complementary to the work accomplished in the framework of the DREAM-Tilt WP2, task 2.2:
Blind test assessment via CFD simulation of both the baseline and optimized tiltrotor geometry in wind tunnel ow conditions, [4]. The analysis are based on the previous European research program Clean Sky Cfp (CODE-Tilt, [5]). In CODE-Tilt, the numerical model of the ERICA 1/8 scaled baseline geometry has been validated using experimental data available from a previous wind tunnel campaign, carried out at the Politecnico di Milano in the framework of the European project NICETRIP WP 4.5, [6]. Furthermore, the numerical prediction capabilities, at near-stall conditions, have been improved thorough numerical analysis carried in [7] for medium and high angles of attack. Finally, the numerical results have been compared against experimental data coming from a further dedicated experimental campaign, performed in June 2011 at the Politecnico di Milano wind tunnel, [8]. In addition, a
numerical validation with the open source OpenFOAM code has also been assessed in [9]. In the presented work, both the commercial software ANSYS Fluent and the open source code OpenFOAM have been tested in order to validate a CFD model of the tiltrotor baseline con guration, in wind tunnel conditions. The validation of the numerical models has been carried out on the 1/8 scaled tiltrotor geometry and it is described in detail in Chapter 3: the settings are reported in terms of boundary conditions, numerical schemes and simulation strategies. Finally, the obtained numerical results are compared against the experimental data. The global aerodynamic coe cients, ow distortion, total pressure losses and ow separation phenomena are analysed. In addition, a PIV analysis has also been carried in the framework of the DREAm-Tilt project [10]: the numerical results of ANSYS Fluent and OpenFOAM are compared with the PIV data in terms of velocity and vorticity eld distributions.