Trapped Cylindrical Flow With Multiple Inlets for Savonius Vertical Axis Wind Turbines

2017 ◽  
Vol 140 (4) ◽  
Author(s):  
Aaron S. Alexander ◽  
Arvind Santhanakrishnan
Keyword(s):  
Flow Field ◽  
Wind Turbines ◽  
Swirling Flow ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Pressure Gradients ◽  
Freestream Velocity ◽  
Vertical Axis Wind Turbines ◽  
Low Efficiency ◽  
Induced Velocity

Savonius vertical axis wind turbines (VAWTs) typically suffer from low efficiency due to detrimental drag production during one half of the rotational cycle. The present study examines a stator assembly created with the objective of trapping cylindrical flow for application in a Savonius VAWT. While stator assemblies have been studied in situ around Savonius rotors in the past, they have never been isolated from the rotor to determine the physics of the flow field, raising the likelihood that a moving rotor could cover up deficiencies attributable to the stator design. The flow field created by a stator assembly, sans rotor, is studied computationally using three-dimensional (3D) numerical simulations in the commercial computational fluid dynamics (CFD) package Star-CCM+. Examination of the velocity and pressure contours at the central stator plane shows that the maximum induced velocity exceeded the freestream velocity by 65%. However, flow is not sufficiently trapped in the stator assembly, with excess leakage occurring between the stator blades due to adverse pressure gradients and momentum loss from induced vorticity. A parametric study was conducted on the effect of the number of stator blades with simulations conducted with 6, 12, and 24 blades. Reducing the blade number resulted in a reduction in the cohesiveness of the internal swirling flow structure and increased the leakage of flow through the stator. Two unique energy loss mechanisms have been identified with both caused by adverse pressure gradients induced by the stator.

FloVAWT: Progress on the Development of a Coupled Model of Dynamics for Floating Offshore Vertical Axis Wind Turbines

2013 ◽  
Author(s):  
Maurizio Collu ◽  
Michael Borg ◽  
Andrew Shires ◽  
Feargal P. Brennan
Keyword(s):  
Experimental Data ◽  
Wind Turbines ◽  
Three Dimensional ◽  
Coupled Model ◽  
Vertical Axis ◽  
Potential Method ◽  
Vertical Axis Wind Turbine ◽  
Vertical Axis Wind Turbines ◽  
Wind Profiles ◽  
Gyroscopic Effects

In the present article, progress on the development of an aero-hydro-servo-elastic coupled model of dynamics for floating Vertical Axis Wind Turbines (VAWTs) is presented, called FloVAWT (Floating Vertical Axis Wind Turbine). Aerodynamics is based on Paraschivoiu’s Double-Multiple Streamtube (DMST) model [1] [2], relying on blade element momentum (BEM) theory, but also taking into account three-dimensional effects, dynamic stall, and unsteady wind profiles and platform motions. Hydrodynamics is modelled with a time domain seakeeping model [3], based on hydrodynamic coefficients estimated with a frequency analysis potential method. In this first phase of the research program, the system is considered a rigid body. The mooring system is represented through a user defined force-displacement relationship. Due to the lack of experimental data on offshore floating VAWTs, the model has initially been validated by taking each module separately and comparing it against known experimental data, showing good agreement. The capabilities of the program are illustrated through a case study, giving an insight on the relative importance of aerodynamics loads and gyroscopic effects with respect to hydrodynamic load effects.

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.

scholarly journals Parked and Operating Loads Analysis in the Aerodynamic Design of Multi-megawatt-scale Floating Vertical Axis Wind Turbines

2021 ◽  
Author(s):  
Mohammad Sadman Sakib ◽  
D. Todd Griffith
Keyword(s):  
Aspect Ratio ◽  
Wind Turbines ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Lateral Force ◽  
Aerodynamic Design ◽  
Power Performance ◽  
Vertical Axis Wind Turbines ◽  
Design Variables ◽  
Loads Analysis

Abstract. A good understanding of aerodynamic loading is essential in the design of vertical axis wind turbines (VAWTs) to properly capture design loads and to estimate the power production. This paper presents a comprehensive aerodynamic design study for a 5 MW Darrieus offshore VAWT in the context of multi-megawatt floating VAWTs. This study systematically analyzes the effect of different, important design variables including the number of blades (N), aspect ratio (AR) and blade tapering in a comprehensive loads analysis of both the parked and operating aerodynamic loads including turbine power performance analysis. Number of blades (N) is studied for 2- and 3-bladed turbines, aspect ratio is defined as ratio of rotor height (H) and rotor diameter (D) and studied for values from 0.5 to 1.5, and blade tapering is applied by means of adding solidity to the blades towards blade root ends, which affects aerodynamic and structural performance. Analyses were carried out using a three-dimensional vortex model named CACTUS (Code for Axial and Crossflow TUrbine Simulation) to evaluate both instantaneous azimuthal parameters as well as integral parameters, such as loads (thrust force, lateral force, and torque loading) and power. Parked loading is a major concern for VAWTs, thus this work presents a broad evaluation of parked loads for the design variables noted above. This study also illustrates that during the operation of a turbine, lateral loads are on par with thrust loads, which will significantly affect the structural sizing of rotor and platform & mooring components.

scholarly journals Actuator cylinder theory for multiple vertical axis wind turbines

2016 ◽  
Vol 1 (2) ◽  
pp. 327-340 ◽  
Author(s):  
Andrew Ning
Keyword(s):  
Conceptual Design ◽  
Wind Turbines ◽  
Power Loss ◽  
Wind Farm ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Navier Stokes ◽  
Vertical Axis Wind Turbines ◽  
Wide Range ◽  
Wake Interactions

Abstract. Actuator cylinder theory is an effective approach for analyzing the aerodynamic performance of vertical axis wind turbines at a conceptual design level. Existing actuator cylinder theory can analyze single turbines, but analysis of multiple turbines is often desirable because turbines may operate in near proximity within a wind farm. For vertical axis wind turbines, which tend to operate in closer proximity than do horizontal axis turbines, aerodynamic interactions may not be strictly confined to wake interactions. We modified actuator cylinder theory to permit the simultaneous solution of aerodynamic loading for any number of turbines. We also extended the theory to handle thrust coefficients outside of the momentum region and explicitly defined the additional terms needed for curved or swept blades. While the focus of this paper is a derivation of an extended methodology, an application of this theory was explored involving two turbines operating in close proximity. Comparisons were made against two-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) simulations, across a full 360° of inflow, with excellent agreement. The counter-rotating turbines produced a 5–10 % increase in power across a wide range of inflow conditions. A second comparison was made to a three-dimensional RANS simulation with a different turbine under different conditions. While only one data point was available, the agreement was reasonable, with the computational fluid dynamics (CFD) predicting a 12 % power loss, as compared to a 15 % power loss for the actuator cylinder method. This extended theory appears promising for conceptual design studies of closely spaced vertical axis wind turbines (VAWTs), but further development and validation is needed.

A Robust Procedure to Implement Dynamic Stall Models Into Actuator Line Methods for the Simulation of Vertical-Axis Wind Turbines

2021 ◽  
Author(s):  
Pier Francesco Melani ◽  
Francesco Balduzzi ◽  
Alessandro Bianchini
Keyword(s):  
Flow Field ◽  
Wind Turbines ◽  
Signal To Noise Ratio ◽  
Angle Of Attack ◽  
Computational Cost ◽  
Vertical Axis ◽  
Dynamic Stall ◽  
Vertical Axis Wind Turbines ◽  
Lumped Parameter ◽  
Low Pass

Abstract The Actuator Line Method (ALM), combining a lumped-parameter representation of the rotating blades with the CFD resolution of the turbine flow field, stands out among the modern simulation methods for Vertical-Axis Wind Turbines (VAWTs) as probably the most interesting compromise between accuracy and computational cost. Being however a method relying on tabulated coefficients for modeling the blade-flow interaction, the correct implementation of the sub-models to account for higher order aerodynamic effects is pivotal. Inter alia, the introduction of a dynamic stall model is extremely challenging. As a matter of fact, two main issues arise: first, it is important to extrapolate a correct value of the angle of attack (AoA) from the CFD solved flow field; second, the AoA history required as an input to calculate the rate of dynamic variation of the angle itself is characterized by a low signal-to-noise ratio, leading to severe numerical oscillations of the solution. In the study, a robust procedure to improve the quality of the AoA signal extracted from an ALM simulation is introduced. The procedure combines a novel method for sampling of the inflow velocity from the numerical flow field with a low-pass filtering of the corresponding angle of attack signal based on Cubic Spline Smoothing (CSS). Such procedure has been implemented in the Actuator Line module developed by the authors for the commercial ANSYS® FLUENT® solver. In order to verify the reliability of the proposed methodology, two-dimensional unsteady RANS simulations of a test 2-blade Darrieus H-rotor, for which high-fidelity experimental and numerical blade loading data were available, have been eventually performed for a selected turbine unstable operation point.

Aerodynamic Measurements on a Vertical Axis Wind Turbine in a Large Scale Wind Tunnel

2010 ◽  
Author(s):  
L. Battisti ◽  
L. Zanne ◽  
S. Dell’Anna ◽  
V. Dossena ◽  
B. Paradiso ◽  
...  
Keyword(s):  
Wind Tunnel ◽  
Flow Field ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Large Scale ◽  
Fluid Dynamic ◽  
Vertical Axis ◽  
Vertical Axis Wind Turbine ◽  
Aerodynamic Optimization ◽  
Vertical Axis Wind Turbines

This paper presents the first results of a wide experimental investigation on the aerodynamics of a vertical axis wind turbine. Vertical axis wind turbines have recently received particular attention, as interesting alternative for small and micro generation applications. However, the complex fluid dynamic mechanisms occurring in these machines make the aerodynamic optimization of the rotors still an open issue and detailed experimental analyses are now highly recommended to convert improved flow field comprehensions into novel design techniques. The experiments were performed in the large-scale wind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for micro generation can be tested in full similarity conditions. Open and closed wind tunnel configurations are considered in such a way to quantify the influence of model blockage for several operational conditions. Integral torque and thrust measurements, as well as detailed aerodynamic measurements were applied to characterize the 3D flow field downstream of the turbine. The local unsteady flow field and the streamwise turbulent component, both resolved in phase with the rotor position, were derived by hot wire measurements. The paper critically analyses the models and the correlations usually applied to correct the wind tunnel blockage effects. Results evidence that the presently available theoretical correction models does not provide accurate estimates of the blockage effect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in particular, seem to play a key role for the prediction of the turbine performance; large-scale unsteadiness is observed in that region and a simple flow model is used to explain the different flow features with respect to horizontal axis wind turbines.

scholarly journals Formulation, Validation, and Application of a Novel 3D BEM Tool for Vertical Axis Wind Turbines of General Shape and Size

2021 ◽  
Vol 11 (13) ◽  
pp. 5874
Author(s):  
Andrea G. Sanvito ◽  
Vincenzo Dossena ◽  
Giacomo Persico
Keyword(s):  
Wind Tunnel ◽  
Wind Turbines ◽  
Three Dimensional ◽  
Vertical Axis ◽  
Small Scale ◽  
Engineering Practice ◽  
Blade Profile ◽  
General Shape ◽  
Vertical Axis Wind Turbines ◽  
Shape And Size

Low order models based on the Blade Element Momentum (BEM) theory exhibit modeling issues in the performance prediction of Vertical Axis Wind Turbines (VAWT) compared to Computational Fluid Dynamics, despite the widespread engineering practice of such methods. The present study shows that the capability of BEM codes applied to VAWTs can be greatly improved by implementing a novel three-dimensional set of high-order corrections and demonstrates this by comparing the BEM predictions against wind-tunnel experiments conducted on three small-scale VAWT models featuring different rotor design (H-shaped and Troposkein), blade profile (NACA0021 and DU-06-W200), and Reynolds number (from 0.8×105 to 2.5×105). Though based on the conventional Double Multiple Stream Tube (DMST) model, the here-presented in-house BEM code incorporates several two-dimensional and three-dimensional corrections including: accurate extended polar data, flow curvature, dynamic stall, a spanwise-distributed formulation of the tip losses, a fully 3D approach in the modeling of rotors featuring general shape (such as but not only, the Troposkein one), and accounting for the passive effects of supporting struts and pole. The detailed comparison with experimental data of the same models, tested in the large-scale wind tunnel of the Politecnico di Milano, suggests the very good predictive capability of the code in terms of power exchange, torque coefficient, and loads, on both time-mean and time-resolved basis. The peculiar formulation of the code allows including in a straightforward way the usual spanwise non-uniformity of the incoming wind and the effects of skew, thus allowing predicting the turbine operation in a realistic open-field in presence of the environmental boundary layer. A systematic study on the operation of VAWTs in multiple environments, such as in coastal regions or off-shore, and highlighting the sensitivity of VAWT performance to blade profile selection, rotor shape and size, wind shear, and rotor tilt concludes the paper.

scholarly journals Application of Richardson extrapolation method to the CFD simulation of vertical-axis wind turbines and analysis of the flow field

2019 ◽  
Vol 13 (1) ◽  
pp. 359-376 ◽  
Author(s):  
Andrés Meana-Fernández ◽  
Jesús Manuel Fernández Oro ◽  
Katia María Argüelles Díaz ◽  
Mónica Galdo-Vega ◽  
Sandra Velarde-Suárez
Keyword(s):  
Flow Field ◽  
Wind Turbines ◽  
Cfd Simulation ◽  
Vertical Axis ◽  
Richardson Extrapolation ◽  
Extrapolation Method ◽  
Vertical Axis Wind Turbines ◽  
Richardson Extrapolation Method

scholarly journals Actuator Cylinder Theory for Multiple Vertical Axis Wind Turbines

2016 ◽  
Author(s):  
Andrew Ning
Keyword(s):  
Wind Turbines ◽  
Wind Farm ◽  
Vertical Axis ◽  
Total Power ◽  
Velocity Models ◽  
Wake Interference ◽  
Vertical Axis Wind Turbines ◽  
Wake Interactions ◽  
Wake Zone ◽  
Induced Velocity

Abstract. Actuator cylinder theory is an effective approach for analyzing the aerodynamic performance of vertical axis wind turbines at a conceptual design level. Existing actuator cylinder theory can analyze single turbines, but analysis of multiple turbines is often desirable because turbines operate in near proximity within a wind farm. For vertical axis wind turbines, which tend to operate in closer proximity than do horizontal axis turbines, aerodynamic interactions may not be strictly confined to wake interactions. We modified actuator cylinder theory to permit the simultaneous solution of aerodynamic loading for any number of turbines. We also extended the theory to handle thrust coefficients outside of the momentum region, and explicitly defined the additional terms needed for curved or swept blades. It is found that even out of the wake zone, aerodynamic interactions are not negligible at typical separation distances (i.e., 3–6 rotor diameters). If turbines are co-rotating then for the two turbine cases examined in this paper the sum of the total power was effectively constant except within the wake zone. However, if turbines counter-rotate then both beneficial and detrimental changes in power production were observed depending on the relative positions. However, these benefits are on the order of a few percent and unlikely to be advantageous in practice because of wake interference, except for within highly directional wind sites. Limitations of these analyses identified the need for integration with viscous wake models, and potentially with higher-fidelity induced velocity models.

The Effect of Number of Blades on the Performance of Helical-Savonius Vertical-Axis Wind Turbines

2012 ◽  
Author(s):  
Majid Rashidi ◽  
Jaikrishnan R. Kadambi ◽  
Asuquo Ebiana ◽  
Ali Ameri ◽  
James Reeher
Keyword(s):  
Wind Turbines ◽  
Three Dimensional ◽  
Vertical Axis ◽  
3D Models ◽  
Test Results ◽  
Rotor Design ◽  
Vertical Axis Wind Turbines ◽  
3D Geometry ◽  
Bladed Rotor ◽  
Series Of Experiments

This work presents the results of a series of experiments conducted on three different scaled-down Helical-Savonius vertical axis wind turbines (VAWT) systems. The work was aimed at investigating how the number of blades may affect the performance of the Helical-Savonius VAWTs. The first turbine consisted of two helical blades, the second turbine had three blades, and the third turbines had four blades. The work included a design phase in which the three dimensional (3D) geometry of each of three VAWTs were developed using a 3D drawing software. The 3D models were then uploaded to a rapid-prototyping machine to fabricate the VAWTs. The projected areas of each of the VAWTs were that of a rectangle of 4″ × 6″. A test setup was designed and developed to examine the performance of the scaled-down turbines. A 1.1 KW floor fan was used to simulate wind flow in the laboratory for testing of the turbines. A flow straightener was also designed and developed in order to minimize the turbulent flow of the air at the discharge opening of the floor fan. The test results show that the 3-bladed rotor design performs better than the two and four bladed turbines. Under the same wind speed conditions the 3-bladed turbine produced 18% more power compared to the 2-bladed turbine, whereas the 3-bladed turbine produced 30% more power compared to the 4-bladed turbine.

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