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.

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

2018 ◽  
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 (VAWTs) 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 180 k, and different incidence ranges: near stall, post stall and deep stall. Various bi-stable 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 is 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.

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.

scholarly journals Design and Testing of a LUT Airfoil for Straight-Bladed Vertical Axis Wind Turbines

2018 ◽  
Vol 8 (11) ◽  
pp. 2266 ◽  
Author(s):  
Shoutu Li ◽  
Ye Li ◽  
Congxin Yang ◽  
Xuyao Zhang ◽  
Qing Wang ◽  
...  
Keyword(s):  
Wind Turbines ◽  
Lift Coefficient ◽  
Pressure Coefficient ◽  
Vertical Axis ◽  
Reynolds Numbers ◽  
Optimization Method ◽  
Geometrical Parameters ◽  
Vertical Axis Wind Turbines ◽  
University Of Technology ◽  
Horizontal Axis Wind Turbines

The airfoil plays an important role in improving the performance of wind turbines. However, there is less research dedicated to the airfoils for Vertical Axis Wind Turbines (VAWTs) compared to the research on Horizontal Axis Wind Turbines (HAWTs). With the objective of maximizing the aerodynamic performance of the airfoil by optimizing its geometrical parameters and by considering the law of motion of VAWTs, a new airfoil, designated the LUT airfoil (Lanzhou University of Technology), was designed for lift-driven VAWTs by employing the sequential quadratic programming optimization method. Afterwards, the pressure on the surface of the airfoil and the flow velocity were measured in steady conditions by employing wind tunnel experiments and particle image velocimetry technology. Then, the distribution of the pressure coefficient and aerodynamic loads were analyzed for the LUT airfoil under free transition. The results show that the LUT airfoil has a moderate thickness (20.77%) and moderate camber (1.11%). Moreover, compared to the airfoils commonly used for VAWTs, the LUT airfoil, with a wide drag bucket and gentle stall performance, achieves a higher maximum lift coefficient and lift–drag ratios at the Reynolds numbers 3 × 105 and 5 × 105.

Surface Pressure Distribution on a Blade of a 10 m Diameter HAWT (Field Measurements versus Wind Tunnel Measurements)

2005 ◽  
Vol 127 (2) ◽  
pp. 185-191 ◽  
Author(s):  
T. Maeda ◽  
E. Ismaili ◽  
H. Kawabuchi ◽  
Y. Kamada
Keyword(s):  
Wind Tunnel ◽  
Surface Pressure ◽  
Field Measurements ◽  
Reynolds Numbers ◽  
Blade Surface ◽  
Pressure Data ◽  
Surface Pressure Distribution ◽  
Pressure Distributions ◽  
Wind Tunnel Data ◽  
Local Angle

This paper exploits blade surface pressure data acquired by testing a three-bladed upwind turbine operating in the field. Data were collected for a rotor blade at spanwise 0.7R with the rotor disc at zero yaw. Then, for the same blade, surface pressure data were acquired by testing in a wind tunnel. Analyses compared aerodynamic forces and surface pressure distributions under field conditions against analogous baseline data acquired from the wind tunnel data. The results show that aerodynamic performance of the section 70%, for local angle of attack below static stall, is similar for free stream and wind tunnel conditions and resemblances those commonly observed on two-dimensional aerofoils near stall. For post-stall flow, it is presumed that the exhibited differences are attributes of the differences on the Reynolds numbers at which the experiments were conducted.

Time-Resolved Experimental Characterization of the Wakes Shed by H-Shaped and Troposkien Vertical Axis Wind Turbines

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
Giacomo Persico ◽  
Vincenzo Dossena ◽  
Berardo Paradiso ◽  
Lorenzo Battisti ◽  
Alessandra Brighenti ◽  
...  
Keyword(s):  
Wind Turbines ◽  
Large Scale ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Speed Ratio ◽  
Local Dynamic ◽  
Performance Measurements ◽  
Time Resolved ◽  
Vertical Axis Wind Turbines ◽  
Blade Section

In this paper, the aerodynamics of two vertical axis wind turbines (VAWTs) are discussed, on the basis of a wide set of experiments performed at Politecnico di Milano, Milan, Italy. A H-shaped and a Troposkien Darrieus turbine for microgeneration, featuring the same swept area and blade section, are tested at full-scale. Performance measurements show that the Troposkien rotor outperforms the H-shaped turbine, thanks to the larger midspan section of the Troposkien rotor and to the nonaerodynamic struts of the H-shaped rotor. These features are consistent with the character of the wakes shed by the turbines, measured by means of hot wire anemometry on several surfaces downstream of the models. The H-shape and Troposkien turbine wakes exhibit relevant differences in the three-dimensional morphology and unsteady evolution. In particular, large-scale vortices dominate the tip region of the wake shed by the H-shape turbine; these vortices pulsate significantly during the period, due to the periodic fluctuation of the blade aerodynamic loading. Conversely, the highly tapered shape of the Troposkien rotor not only prevents the onset of tip vortices, but also induces a dramatic spanwise reduction of tip speed ratio (TSR), promoting the onset of local dynamic stall marked by high periodic and turbulent unsteadiness in the tip region of the wake. The way in which these mechanisms affect the wake evolution and mixing process for the two classes of turbines is investigated for different tip speed ratios, highlighting some relevant implications in the framework of wind energy exploitation.

Analytical Solutions for a Family of Gaussian Impinging Jets

2008 ◽  
Vol 75 (2) ◽  
Author(s):  
Zhuyun Xu ◽  
Horia Hangan ◽  
Pei Yu
Keyword(s):  
Boundary Layer ◽  
Excellent Agreement ◽  
Surface Pressure ◽  
Analytical Solutions ◽  
Reynolds Numbers ◽  
Impinging Jets ◽  
Experimental Results ◽  
Jet Flows ◽  
Pressure Distributions ◽  
Circular Plane

Various types of impinging jet flows are analytically modeled using inviscid free Gaussian jet solutions superimposed with experimentally fitted boundary layer models. Improved (more robust) and simplified solutions to existing models are defined. Velocity profiles, surface pressure distributions, and streamline plots are calculated for circular, plane, and annular impinging jets. The models show excellent agreement with existing experimental results in both laminar and turbulent conditions and for different Reynolds numbers.

Circulation Controlled Airfoil Analysis Through 360 Degrees Angle of Attack

2009 ◽  
Author(s):  
Henry Z. Graham ◽  
Meagan Hubbell ◽  
Chad Panther ◽  
Jay Wilhelm ◽  
Gerald M. Angle ◽  
...  
Keyword(s):  
Power Generation ◽  
Wind Turbines ◽  
Electrical Power ◽  
Vertical Axis ◽  
Reynolds Numbers ◽  
Experimental Investigations ◽  
Circulation Control ◽  
Wind Speeds ◽  
Wide Range ◽  
Turbine Shaft

Wind turbines are a source of renewable energy with an endless supply. The most efficient types of wind turbines operate by utilizing the lift force of its blades to create a rotational force. The power capabilities of a wind turbine are tied to the blades’ ability to convert the aerodynamic forces into rotational energy. Vertical axis wind turbines (VAWT), unlike the more common horizontal axis (HAWT) type, do not need to be directed into the wind and can place the transmission and electrical power generation components at the bottom of the turbine shaft, near the ground. Currently VAWTs cannot feather or pitch the blades, in the same fashion as a HAWT, for a lift change to control power generation and/or rotational speed at different or changing wind speeds. A method of increasing the lift of a blade without physically moving the blade is to use circulation control (CC), via a blowing slot over a rounded trailing edge. The CC air flow entrains the air around the blade to create more lift. Adding an actuated valve for the blowing slot allows a CC-VAWT to control the amount of lift generated, as well as the location of the augmentation relative to the wind direction, resulting in augmented power generation. In order to study the performance capabilities of a CC-VAWT, a NACA0018 blade was modified to incorporate circulation control. This modified shape was analyzed using computational fluid dynamics at two Reynolds numbers and a wide range of angles of attack. The lift to drag ratio of the CC-VAWT blade shows benefits at low Reynolds numbers over a NACA0018 blade for post stall angles of attack, but there is a decrease in the lift to drag before stall due to a significant increase in drag of the circulation control models. Further CFD refinement and experimental investigations are recommended to validate the predicted effects circulation control will have on the performance of a VAWT.

The Influence of Reynolds Number and Row Position on Surface Pressure Distributions in Staggered Pin Fin Arrays

2006 ◽  
Author(s):  
F. E. Ames ◽  
L. A. Dvorak
Keyword(s):  
Reynolds Number ◽  
Surface Pressure ◽  
Measurement Techniques ◽  
Reynolds Numbers ◽  
Secondary Flows ◽  
Pressure Gradients ◽  
Stagnation Region ◽  
Pin Fin ◽  
Pressure Distributions ◽  
Pin Fin Array

Full surface pressure distributions over the endwall and pin in a staggered pin fin array have been acquired over a ten to one range in Reynolds numbers. These pressure distributions allow us to visualize the strong inertial pressure gradients that are responsible for driving secondary flows in pin fin passages. These strong pressure gradients include endwall regions near the pin stagnation region and near the pin at 90° from the stagnation region. Pressure distributions have been acquired on pin and endwall surfaces at eight consecutive rows using conventional static pressure measurement techniques. Pressures have been taken at 380 locations per row and, assuming symmetry, provide a well resolved visualization of surface pressure. Generally, surface and pin pressure distributions vary significantly from row to row in the entrance of the array at a given Reynolds number but stay relatively consistent after row four. Dimensionless pressure distributions are quite similar for row one for all Reynolds numbers but vary significantly at a given row downstream with Reynolds number. These data are expected to enhance our understanding of pin array fluid dynamics and to compliment full surface heat transfer data presented in a future paper.

Responses of vertical axis wind turbines to gusty winds

2020 ◽  
pp. 095765092091059
Author(s):  
Zhenlong Wu ◽  
Qiang Wang ◽  
Galih Bangga ◽  
Hao Huang
Keyword(s):  
Wind Turbines ◽  
Atmospheric Condition ◽  
Vertical Axis ◽  
Vertical Axis Wind Turbine ◽  
Dynamics Model ◽  
Velocity Amplitude ◽  
Vertical Axis Wind Turbines ◽  
Flow Phenomena ◽  
German Aerospace ◽  
Mesh Technique

Gust is a common atmospheric condition encountered by wind turbines. Despite the presence of a vast amount of literature on this topic, few of them involve lateral gust influence. Due to this motivation, this paper presents an investigation on the effects of sinusoidal gusts on a three-bladed vertical axis wind turbine under varying gust parameters including gust direction, velocity amplitude, and frequency. The chimera mesh technique is used to model the real rotation of the rotor, as well as the resolved gust approach model in the DLR (German Aerospace Center) TAU code for gust simulation. Both the general aerodynamic computational fluid dynamics model and the gust model are validated before the following simulations. Numerous new flow phenomena and physics are revealed. The influences of gust on the rotor power output and flowfield characteristics are discussed and analyzed in detail. The findings in this study may be helpful for some practical wind engineering applications, such as atmospheric influence evaluation and field site selection.

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