1,721,068 research outputs found
Microeolic turbines in the built environment: influence of the installation site on the potential energy yield
No full textGeneric proposals for an effective integration of renewable energy sources in the urban environment are frequently carried out by project developers, local governments and media, although an in-depth knowledge of the technical and energetic limitations is often missing. In particular, the installation of small wind turbines on the rooftops of tall buildings is considered to represent an attractive solution thanks to the supposed possibility of exploiting local flow accelerations induced by the building façades. The real feasibility of this scenario has, however, yet to be proved, both in terms of real energy harvesting and of compatibility of the machines with a densely populated area.In this study, a critical examination of the flow conditions on the rooftop of a building in an urban environment has been carried out by means of CFD simulations. The main goal of the analysis was the assessment of some general criteria to evaluate the convenience of a microeolic turbine installation on the roof of a selected building as a function of both its geometrical features and those of its upwind building along the prevailing wind direction. In each configuration, the flow velocity and skew angle on the rooftop area of the installation building were calculated for different incoming wind profiles and compared to their levels in the undisturbed flow. Finally, an energy-based comparison between the flow potentials in the investigated environment is provided and some general tendencies are outlined
Dimensionless numbers for the assessment of mesh and timestep requirements in CFD simulations of Darrieus wind turbines
Full text linkComputational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of Vertical-Axis Wind Turbines. The unsteady flow past rotating blades is, however, a challenging application for a numerical simulation and some critical issues have not been settled yet. In particular, if some studies in the literature report detailed analyses on the assessment of the computational model, there is still no adequate convergence on the requirements in terms of spatial and temporal discretizations. In the present study, a multivariate sensitivity analysis was first carried out on a specific case study at different tip-speed ratios in order to define the optimal mesh and timestep sizes needed for an accurate simulation. Once full insensitivity had been reached, the spatial and temporal requirements needed to properly describe the flow phenomena were related to two dimensionless numbers, one for each domain, which can be used to assess the suitability of the selected settings for each specific simulation. The simulations revealed that the spatial requirements must be selected in order to ensure an accurate description of velocity gradients in the near-blade region. To this purpose, a Grid-Reduced form of vorticity is proposed as the best indicator for the quality of the mesh refinement. It is also shown that the temporal requirements are made stricter at low tip-speed ratios by the need of correctly describing the vortices detaching from the blades in the upwind region. To do so, proper thresholds for the Courant Number are highlighted in the study
Critical Analysis of Dynamic Stall Models in Low-Order Simulation Models For Vertical-Axis Wind Turbines
No full textThe efficiency of vertical-axis wind turbines (VAWTs) still lacks from those of horizontal-axis rotors (HAWTs). To improve on efficiency, more accurate and robust aerodynamic simulation tools are needed for VAWTs, for which low-order methods have not reached yet a maturity comparable to that of HAWTs' applications. In the present study, the VARDAR research code, based on the BEM theory, is used to critically compare the predictiveness of some dynamic stall models for Darrieus wind turbines. Dynamic stall, connected to the continuous variation of the angle of attack on the airfoils, has indeed a major impact on the performance of Darrieus rotors. Predicted lift and drag coefficients of the airfoils in motion are reconstructed with the different dynamic stall models and compared to unsteady CFD simulations, previously validated by means of experimental data. The results show that low-order models are unfortunately not able to capture all the complex phenomena taking place during a VAWT functioning. It is however shown that the selection of the adequate dynamic stall model can definitely lead to a much better modelling of the real airfoils' behavior and then notably enhance the predictiveness of low-order simulation methods
A computational procedure to define the incidence angle on airfoils rotating around an axis orthogonal to flow direction
No full textNumerical simulations provided in the last few years a significant contribution for a better understanding of many phenomena connected to the flow past rotating blades. In case of airfoils rotating around an axis orthogonal to flow direction, one of the most critical issues is represented by the definition of the incidence angle on the airfoil from the computed flow field. Incidence indeed changes continuously as a function of the azimuthal position of the blade and a distribution of peripheral speed is experienced along the airfoil's thickness due to radius variation. The possibility of reducing the flow to lumped parameters (relative speed modulus and direction), however, would be of capital relevance to transpose accurate CFD numerical results into effective inputs to low-order models that are often exploited for preliminary design analyses. If several techniques are available for this scope in the case of blades rotating around an axis parallel to flow direction (e.g., horizontal-axis wind turbines), the definition of a robust procedure in case the revolution axis is orthogonal to the flow is still missing. In the study, a novel technique has been developed using data from Darrieus-like rotating airfoils. The method makes use of the virtual camber theory to define a virtual airfoil whose pressure coefficient distributions in straight flow are used to match those of the real airfoil in curved flow. Even if developed originally for vertical-axis wind turbines, the method is of general validity and is thought to represent in the near future a valuable tool for researchers to get a new insight on many complex phenomena connected to flow past blades rotating around an axis orthogonal to flow direction, like for example dynamic stall
Critical issues in the CFD simulation of Darrieus wind turbines
Full text linkComputational Fluid Dynamics is thought to provide in the near future an essential contribution to the development of vertical-axis wind turbines, helping this technology to rise towards a more mature industrial diffusion. The unsteady flow past rotating blades is, however, one of the most challenging applications for a numerical simulation and some critical issues have not been settled yet.In this work, an extended analysis is presented which has been carried out with the final aim of identifying the most effective simulation settings to ensure a reliable fully-unsteady, two-dimensional simulation of an H-type Darrieus turbine.Moving from an extended literature survey, the main analysis parameters have been selected and their influence has been analyzed together with the mutual influences between them; the benefits and drawbacks of the proposed approach are also discussed.The selected settings were applied to simulate the geometry of a real rotor which was tested in the wind tunnel, obtaining notable agreement between numerical estimations and experimental data. Moreover, the proposed approach was further validated by means of two other sets of simulations, based on literature study-cases
Effectiveness of two-dimensional CFD simulations for Darrieus VAWTs: a combined numerical and experimental assessment
Full text linkThanks to the continuous improvement of calculation resources, computational fluid dynamics (CFD) is expected to provide in the next few years a cost-effective and accurate tool to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines. This rotor type is in fact increasingly welcome by the wind energy community, especially in case of small size applications and/or non-conventional installation sites. In the present study, unique tow tank experimental data on the performance curve and the near-wake structure of a Darrieus rotor were used as a benchmark to validate the effectiveness of different CFD approaches. In particular, a dedicated analysis is provided to assess the suitability, the effectiveness and the future prospects of simplified two-dimensional (2D) simulations. The correct definition of the computational domain, the selection of the turbulence models and the correction of simulated data for the parasitic torque components are discussed in this study. Results clearly show that, (only) if properly set, two-dimensional CFD simulations are able to provide - with a reasonable computational cost - an accurate estimation of the turbine performance and also quite reliably describe the attended flow-field around the rotor and its wake
Virtual incidence effect on rotating airfoils in Darrieus wind turbines
Full text linkSmall Darrieus wind turbines are one of the most interesting emerging technologies in the renewable energies scenario, even if they still are characterized by lower efficiencies than those of conventional horizontal-axis wind turbines due to the more complex aerodynamics involved in their functioning. In case of small rotors, in which the chord-to-radius ratios are generally high not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be moreover influenced by the so-called "flow curvature effects". Recent works have indeed shown that the curved flowpath encountered by the blades makes them work like virtually cambered airfoils in a rectilinear flow. In the present study, focus is instead given to a further effect that is generated in reason of the curved streamline incoming on the blades, i.e. an extra-incidence seen by the airfoil, generally referred to as "virtual incidence". In detail, a novel computational method to define the incidence angle has been applied to unsteady CFD simulations of three airfoils in a Darrieus-like motion and their effective angles of attack have been compared to theoretical expectations. The analysis confirmed the presence of an additional virtual incidence on the airfoils and quantified it for different airfoils, chord-to-radius ratios and tip-speed ratios. A comparative discussion on BEM prediction capabilities is finally reported in the study
Aerodynamics of Darrieus Wind Turbines Airfoils: The Impact of Pitching Moment
No full textRecent studies have demonstrated that, when rotating around an axis orthogonal to the flow direction, airfoils are virtually transformed into equivalent airfoils with a camber line defined by their arc of rotation. In these conditions, the symmetric airfoils commonly used for Darrieus blades actually behave like virtually cambered ones or, equivalently, rotors have to be manufactured with countercambered blades to ensure the attended performance. To complete these analyses, the present study first focuses the attention on the airfoils' aerodynamics during the startup of the rotors. It is shown that, contrary to conventional theories based on one-dimensional aerodynamic coefficients, symmetric airfoils exhibit a counterintuitive nonsymmetric starting torque over the revolution. Conversely, airfoils compensated for the virtual camber effect show a more symmetric distribution over the revolution. This behavior is due to the effect of the pitching moment, which is usually neglected in lumped parameters models. At very low revolution speeds, its contribution becomes significant due to the very high incidence angles experienced by the blades; the pitching moment is also nonsymmetric between the upwind and the downwind zone. For upwind azimuthal positions, the pitching moment reduces the overall torque output, while it changes sign in the downwind section, increasing the torque. The importance of accounting for the pitching moment contribution in the entire power curve is also discussed in relationship to the selection of the best blade-spoke connection (BSC) point, in order to maximize the performance and minimize the alternate stresses on the connection due to the pitching moment itself
Aerodynamics of darrieus wind turbines airfoils during start-up
No full textRecent studies have demonstrated that, when rotating around an axis orthogonal to the flow direction, airfoils are virtually transformed into equivalent airfoils with a camber line defined by their arc of rotation. In these conditions, the symmetric airfoils commonly used for Darrieus blades actually behaves like virtually cambered ones or, equivalently, rotors have to be manufactured with counter-cambered blades in order to have the performance of a symmetric airfoil. To complete these analyses, the present study focuses the attention on the airfoils' aerodynamics during the start-up of the rotors. This phase of turbines' functioning is indeed of particular interest since it actually defines the cut-in speed of the rotors and then notably impacts on the annual energy production, especially in case of small-size machines. In the work, unsteady CFD simulations have been carried out in start-up like conditions on three airfoils, i.e. a NACA 0018 and two modified profiles based on the same airfoil. The modified profiles have been conformally transformed to fit their camber lines to the arc of a circle, such that the ratio of the airfoil chord to the circle's radius is 0.114 or 0.25. The study demonstrates that all the conventional theories based on one-dimensional aerodynamic coefficients (e.g. blade element momentum models) are affected by an intrinsic error in evaluating the starting torque profiles. Symmetric airfoils in fact exhibit a counter-intuitive non symmetric starting torque over the revolution. Conversely, airfoils compensated for the virtual camber effect show a substantially different starting torque profile, with a more symmetric distribution between the upwind and the downwind halves. This behavior is due to the effect of the pitching moment, which is usually neglected in lumped parameters models. At very low revolution speeds, its contribution becomes significant due to the very high angles of attack experienced by the blade. In particular, the pitching moment is non symmetric between the upwind and the downwind halves of the revolution. For upwind azimuthal positions the pitching moment reduces the overall torque output, while it changes sign in the downwind section, increasing the torque. The importance of accounting for the pitching moment contribution in low-order models (e.g. a blade element momentum model) is finally discussed by comparing the predicted torque profiles with those obtained by CFD
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