An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines: Part 2 — Post-Stall Data Extrapolation Methods

2015 ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
John M. Rainbird ◽  
Joaquim Peiro ◽  
J. Michael R. Graham ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Full Range ◽  
Vertical Axis ◽  
Wind Tunnel Tests ◽  
Numerical Assessment ◽  
Wide Range ◽  
Naca0018 Airfoil ◽  
Lift And Drag ◽  
Data Extrapolation

Accurate post-stall airfoil data extending to a full range of incidences between −180° to +180° is important to the analysis of Darrieus vertical-axis wind turbines (VAWTs) since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios encountered during startup. Due to the scarcity of existing data extending much past stall, and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, BEM codes). Most of these models assume post-stall performance to be virtually independent of profile shape. In this study, wind tunnel tests were carried out on a standard NACA0018 airfoil and a NACA 0018 conformally transformed to mimic the “virtual camber” effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25. Unsteady CFD results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades. Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna-Corrigan, Montgomerie and Kirke, was analyzed and discussed.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part II: Post-stall Data Extrapolation Methods

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
John M. Rainbird ◽  
Joaquim Peiro ◽  
J. Michael R. Graham ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Full Range ◽  
Vertical Axis ◽  
Wind Tunnel Tests ◽  
Numerical Assessment ◽  
Wide Range ◽  
Computational Fluid Dynamics Cfd ◽  
Lift And Drag ◽  
Data Extrapolation

Accurate post-stall airfoil data extending to a full range of incidences between −180 deg and +180 deg are important to the analysis of Darrieus vertical-axis wind turbines (VAWTs), since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios (TSRs) encountered during startup. Due to the scarcity of existing data extending much past stall and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, blade element momentum (BEM) codes). Most of these models assume post-stall performance to be virtually independent of profile shape. In this study, wind tunnel tests were carried out on a standard NACA 0018 airfoil and a NACA 0018 conformally transformed to mimic the “virtual camber” effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25. Unsteady computational fluid dynamics (CFD) results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades. Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna–Corrigan, Montgomerie, and Kirke, was analyzed and discussed.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines: Part 1 — Flow Curvature Effects

2015 ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
John M. Rainbird ◽  
Joaquim Peiro ◽  
J. Michael R. Graham ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Incidence Angle ◽  
Turbine Blades ◽  
Performance Data ◽  
Power Coefficient ◽  
Wind Tunnel Tests ◽  
Curvature Effects ◽  
Lift And Drag ◽  
The Impact

A better comprehension of the aerodynamic behavior of rotating airfoils in Darrieus Vertical-axis wind turbines (VAWTs) is crucial both for the further development of these machines and for improvement of conventional design tools based on zero or one-dimensional models (e.g. BEM models). When smaller rotors are designed with high chord-to-radius (c/R) ratios so as not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be heavily impacted by a virtual camber effect imparted on the blades by the curvilinear flow they experience. To assess the impact of this virtual camber effect on blade and turbine performance, a standard NACA0018 airfoil and a NACA0018 conformally transformed such that the airfoil’s chord line follows the arc of a circle, where the ratio of the airfoil’s chord to the circle’s radius is 0.25 were considered. For both airfoils, wind tunnel tests were carried out to assess their aerodynamic lift and drag coefficients for Reynolds numbers of interest for Darrieus VAWTs. Unsteady CFD calculations have been then carried out to obtain curvilinear flow performance data for the same airfoils mounted on a Darrieus rotor with a c/R of 0.25. The blade incidence and lift and drag forces were extracted from the CFD output using a novel incidence angle deduction technique. According to virtual camber theory, the transformed airfoil in this curvilinear flow should be equivalent to the NACA0018 in rectilinear flow, while the NACA0018 should be equivalent to the inverted transformed airfoil in rectilinear flow. Comparisons were made between these airfoil pairings using the CFD output and the rectilinear performance data obtained from the wind tunnel tests and XFoil output in the form of pressure distributions and lift and drag polars. Blade torque coefficients and turbine power coefficient are also presented for the CFD VAWT using both blade profiles.

An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines—Part I: Flow Curvature Effects

2015 ◽  
Vol 138 (3) ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
John M. Rainbird ◽  
Joaquim Peiro ◽  
J. Michael R. Graham ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Incidence Angle ◽  
Turbine Blades ◽  
Performance Data ◽  
Power Coefficient ◽  
Wind Tunnel Tests ◽  
Curvature Effects ◽  
Lift And Drag ◽  
The Impact

A better comprehension of the aerodynamic behavior of rotating airfoils in Darrieus vertical-axis wind turbines (VAWTs) is crucial both for the further development of these machines and for improvement of conventional design tools based on zero- or one-dimensional models (e.g., blade element momentum (BEM) models). When smaller rotors are designed with high chord-to-radius (c/R) ratios so as not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be heavily impacted by a virtual camber effect imparted on the blades by the curvilinear flow they experience. To assess the impact of this virtual camber effect on blade and turbine performance, a standard NACA 0018 airfoil and a NACA 0018 conformally transformed such that the airfoil's chord line follows the arc of a circle, where the ratio of the airfoil's chord to the circle's radius is 0.25 were considered. For both airfoils, wind tunnel tests were carried out to assess their aerodynamic lift and drag coefficients for Reynolds numbers of interest for Darrieus VAWTs. Unsteady computational fluid dynamics (CFD) calculations have been then carried out to obtain curvilinear flow performance data for the same airfoils mounted on a Darrieus rotor with a c/R of 0.25. The blade incidence and lift and drag forces were extracted from the CFD output using a novel incidence angle deduction technique. According to virtual camber theory, the transformed airfoil in this curvilinear flow should be equivalent to the NACA 0018 in rectilinear flow, while the NACA0018 should be equivalent to the inverted transformed airfoil in rectilinear flow. Comparisons were made between these airfoil pairings using the CFD output and the rectilinear performance data obtained from the wind tunnel tests and xfoil output in the form of pressure distributions and lift and drag polars. Blade torque coefficients and turbine power coefficient are also presented for the CFD VAWT using both blade profiles.

Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers

2018 ◽  
Vol 141 (3) ◽  
Author(s):  
D. Holst ◽  
B. Church ◽  
F. Wegner ◽  
G. Pechlivanoglou ◽  
C. N. Nayeri ◽  
...  
Keyword(s):  
Design Process ◽  
Surface Pressure ◽  
Wind Turbines ◽  
Full Range ◽  
Vertical Axis ◽  
Reynolds Numbers ◽  
Time Resolved ◽  
Pressure Distributions ◽  
Pressure Development ◽  
Flow Phenomena

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if, e.g., dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine's lifetime. In addition, the design process of vertical axis wind turbines needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag, and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180k, and different incidence ranges: near stall, poststall, and deep stall. Various bistable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper are dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

scholarly journals Wind Tunnel testing of small Vertical-Axis Wind Turbines in Turbulent Flows

2017 ◽  
Vol 199 ◽  
pp. 3176-3181 ◽  
Author(s):  
Andreu Carbó Molina ◽  
Gianni Bartoli ◽  
Tim de Troyer
Keyword(s):  
Wind Tunnel ◽  
Turbulent Flows ◽  
Wind Turbines ◽  
Vertical Axis ◽  
Wind Tunnel Testing ◽  
Vertical Axis Wind Turbines ◽  
Tunnel Testing

scholarly journals Characterization of a new open jet wind tunnel to optimize and test vertical axis wind turbines

2017 ◽  
Vol 9 (3) ◽  
pp. 033302 ◽  
Author(s):  
Silvana Tourn ◽  
Jordi Pallarès ◽  
Ildefonso Cuesta ◽  
Uwe Schmidt Paulsen
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Vertical Axis ◽  
Vertical Axis Wind Turbines

Predicting Wind Loads on the Topside of a Self-Propelled Wind Turbine Installation Jack-Up Using CFD

2021 ◽  
Author(s):  
Zana Sulaiman
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Wind Turbine ◽  
Full Range ◽  
Design Tool ◽  
Wind Loads ◽  
Computational Domain ◽  
Wind Tunnel Tests ◽  
Jack Up ◽  
Turbine Installation

Abstract This paper presents the results of wind load computational fluid dynamics (CFD) calculations performed on the topside structures of a self-propelled wind turbine installation jack-up. The CFD calculations were performed for the jack-up topside structures with and without the deck load. An atmospheric boundary layer profile was applied for the model-scale calculations. The full range of heading angles was considered. The CFD results were validated through comparison with the wind tunnel tests which were carried out at the German-Dutch wind tunnels (DNW) in Marknesse, The Netherlands. Moreover, a comparison is presented between the applied boundary layer profiles throughout the CFD computational domain with those profiles measured in the wind tunnel. The CFD results were found to be in good agreement with the wind tunnel tests for the considered cases, verifying the feasibility of the CFD method as an important design tool for the prediction of wind loads during the design processes of these types of jack-ups.

Detailed Analysis of the Wake Structure of a Straight-Blade H-Darrieus Wind Turbine by Means of Wind Tunnel Experiments and Computational Fluid Dynamics Simulations

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Alessandro Bianchini ◽  
Francesco Balduzzi ◽  
Giovanni Ferrara ◽  
Lorenzo Ferrari ◽  
Giacomo Persico ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Experimental Data ◽  
Computational Fluid Dynamics ◽  
Wind Tunnel ◽  
Wind Turbines ◽  
Large Scale ◽  
Vertical Axis ◽  
Performance Estimation ◽  
Small Scale ◽  
Dynamics Simulations

Darrieus vertical axis wind turbines (VAWTs) have been recently identified as the most promising solution for new types of applications, such as small-scale installations in complex terrains or offshore large floating platforms. To improve their efficiencies further and make them competitive with those of conventional horizontal axis wind turbines, a more in depth understanding of the physical phenomena that govern the aerodynamics past a rotating Darrieus turbine is needed. Within this context, computational fluid dynamics (CFD) can play a fundamental role, since it represents the only model able to provide a detailed and comprehensive representation of the flow. Due to the complexity of similar simulations, however, the possibility of having reliable and detailed experimental data to be used as validation test cases is pivotal to tune the numerical tools. In this study, a two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (U-RANS) computational model was applied to analyze the wake characteristics on the midplane of a small-size H-shaped Darrieus VAWT. The turbine was tested in a large-scale, open-jet wind tunnel, including both performance and wake measurements. Thanks to the availability of such a unique set of experimental data, systematic comparisons between simulations and experiments were carried out for analyzing the structure of the wake and correlating the main macrostructures of the flow to the local aerodynamic features of the airfoils in cycloidal motion. In general, good agreement on the turbine performance estimation was constantly appreciated.

Static and Dynamic Analysis of a NACA 0021 Airfoil Section at Low Reynolds Numbers: Drag and Moment Coefficients

2019 ◽  
Author(s):  
David Holst ◽  
Francesco Balduzzi ◽  
Alessandro Bianchini ◽  
Christian Navid Nayeri ◽  
Christian Oliver Paschereit ◽  
...  
Keyword(s):  
Experimental Data ◽  
Wind Tunnel ◽  
Wind Turbines ◽  
Reynolds Numbers ◽  
Correction Method ◽  
Hysteresis Cycle ◽  
Low Reynolds Numbers ◽  
Static And Dynamic Analysis ◽  
Wide Range ◽  
Low Reynolds

Abstract Wind industry needs high quality airfoil data for a range of the angle of attack (AoA) much wider than that often provided by the technical literature, which often lacks data i.e. in deep- and post-stall region. Especially in case of vertical axis wind turbines (VAWTs), the blades operate at very large AoAs, which exceed the range of typical aviation application. In a previous study, some of the authors analyzed the trend of the lift coefficient of a NACA 0021 airfoil, using the suggestions provided by detailed CFD analyses to correct experimental data at low Reynolds numbers collected in an open-jet tunnel. In the present study, the correction method is extended in order to analyze even the drag and moment coefficients over a wide range of AoAs for two different Reynolds numbers (Re = 140k and Re = 180k) of particular interest for small wind turbines. The utility of these data is again specifically high in case of VAWTs, in which both the drag and the moment coefficient largely contribute to the torque. The investigation involves tunnel data regarding both static polars and dynamic sinusoidal pitching movements at multiple reduced frequencies. Concerning the numerical simulations, two different computational domains were considered, i.e. the full wind tunnel and the open field. Once experimental data have been purged by the influence of the wind tunnel by means of the proposed correction method, they were compared to existing data for similar Reynolds both for the NACA0021 and for similar airfoils. By doing so, some differences in the static stall angle and the extent of the hysteresis cycle are discussed. Overall, the present paper provides the scientific community with detailed analysis of low-Reynolds NACA 0021 data in multiple variations, which may enable, inter alia, a more effective VAWT design in the near future.

Sign in / Sign up

Export Citation Format

Share Document