Vertical Axis Wind Turbine Simulation Research Articles

Enabling Floating Offshore VAWT Design by Coupling OWENS and OpenFAST

Vertical-axis wind turbines (VAWTs) have a long history, with a wide variety of turbine archetypes that have been designed and tested since the 1970s. While few utility-scale VAWTs currently exist, the placement of the generator near the turbine base could make VAWTs advantageous over tradition horizontal-axis wind turbines for floating offshore wind applications via reduced platform costs and improved scaling potential. However, there are currently few numerical design and analysis tools available for VAWTs. One existing engineering toolset for aero-hydro-servo-elastic simulation of VAWTs is the Offshore Wind ENergy Simulator (OWENS), but its current modeling capability for floating systems is non-standard and not ideal. This article describes how OWENS has been coupled to several OpenFAST modules to update and improve modeling of floating offshore VAWTs and discusses the verification of these new capabilities and features. The results of the coupled OWENS verification test agree well with a parallel OpenFAST simulation, validating the new modeling and simulation capabilities in OWENS for floating VAWT applications. These developments will enable the design and optimization of floating offshore VAWTs in the future.

Tailoring the actuator line theory to the simulation of Vertical-Axis Wind Turbines

Analysis tools with a fidelity higher than the ubiquitous Blade Element Momentum (BEM) method are needed by now in wind energy; in particular, different research groups have recently proposed the application of the Actuator Line Method (ALM) to wind turbines, to exploit the benefits of an accurate discretization of the wake through Computational Fluid Dynamics and the computational cost saving associated to the lumped parameter modeling of the blade. When applied to Vertical-Axis Darrieus rotors, however, several shortcomings of present models are known to the scientific community, especially regarding the spreading of aerodynamic forces in the domain and the implementation of robust aerodynamic polars and dynamic stall models. Moving from this background, an ALM method numerical model for the simulation of VAWTs has been here developed within the commercial solver ANSYS® FLUENT®. Then, in the effort of tailoring the ALM to this type of machines, different features have been implemented and discussed in the present study, including a novel strategy for the sampling of the angle of attack from the resolved flow field, a sensitivity analysis on the force spreading within the domain and numerous sub-models to account for secondary aerodynamics effects. Attention has been given at ensuring robustness to the implementation of the pivotal modeling of dynamic stall. To prove the effectiveness of proposed solutions, an extensive validation has been carried out on selected test cases, for which both high-fidelity CFD and experimental data were available: a real 2-blade H-Darrieus rotor and a fictitious 1-blade machine. The developed solutions have increased the accuracy of the predicted torque up to 16% with respect to the ALM standard formulation.

Influences of trailing edge split flap on the aerodynamic performance of vertical axis wind turbine

AbstractTo improve the performance of a 3.5 kW small H‐type vertical axis wind turbine (VAWT) with the airfoil of NACA0015, a design method for installing split flaps at the trailing edge of a blade is proposed. By solving the two‐dimensional URANS equations, the simulation of VAWTs is carried out. The influences of the length (10% c‐30% c), arrangement positions (90% c‐70% c), and deflection angles (10°‐30°) of the split flaps on the performance of VAWT are determined by orthogonal design when Reynolds number is 2 × 105 and 1 < TSR < 3. In accordance with the influence degrees of these parameters and adopting the maximum wind energy utilization ratio CPmax and the maximum efficient operating range Δλmax as research objectives, the optimum flap length is determined as 22% c, the optimum deflection angle is 10°, and the optimum arrangement position is 92% c. Further studies show that the CPmax is increased by 5.8% and its Δλmax is increased by 25.9% after installing split flaps. Meanwhile, split flaps can change the Kutta‐Joukowsky trailing edge boundary condition, which increases the flow circulation around the airfoil. Also, split flaps can increase the positive pressure difference in the upwind area and reduce the negative pressure difference in the downwind area, which significantly improves the aerodynamic performance of VAWTs.

Study on benefit of guide vane for vertical axis wind

Guide vane was introduced to overcome the problem of high turbulence intensity due to the high rise building in the urban area. The guide vane is a fixed aerofoil that directs air into the moving wind turbine. Guide vane borders the Vertical Axis Wind Turbine (VAWT) and improves the VAWT performance by increasing the wind velocity, decreasing turbulence intensity and increasing torque because it acts as wind concentrator and wind shielding to an optimum angle before it interacts with turbines. This project is based on the VAWT on the College of Engineering (COE) building in Universiti Tenaga Nasional (UNITEN). This VAWT had been fabricated in CREO software to study the benefit of guide vane onto it and the design of guide vane also was constructed by using similar software. SOLIDWORKS Flow simulation was used to analyze the performance of the VAWT. The simulation had been done by simulation of VAWT without and with guide vane to study the benefit and analyze the flow characteristic of the guide vane towards wind turbine. The simulation result shows that the presence of guide vane increase wind speed by maximum 0.0051 m/s and torque increment is about seven times from VAWT without guide vane. Other than that, decreasing of turbulence intensity value also detected in flow simulation result. To sum up, guide vane had the potential to be installed in the urban area because it improves the power output of the wind turbine.

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

There is still discrepancy regarding the verification of CFD U-RANS simulations of vertical-axis wind turbines (VAWTs). In this work, the applicability of the Richardson extrapolation method to assess mesh convergence is studied for several points in the power curve of a VAWT. A 2D domain of the rotor is simulated with three different meshes, monitoring the turbine power coefficient as the convergence parameter. This method proves to be a straightforward procedure to assess convergence of VAWT simulations. Guidelines regarding the required mesh and temporal discretization levels are provided. Once the simulations are validated, the flow field at three characteristic tip-speed ratio values (2.5 - low, 4 - nominal and 5 - high) is analyzed, studying pressure, velocity, turbulent kinetic energy and vorticity fields. The results have revealed two main vortex shedding mechanisms, blade- and rotor-related. Vortex convection develops differently depending on the rotor zone (upwind, downwind, windward or leeward). Finally, insight into the loss of performance at off-design conditions is provided. Vortex shedding phenomena at the low tip-speed ratio explains the loss of performance of the turbine, whereas at the high tip-speed ratio, this performance loss may be ascribed to viscous effects and the rapid interaction between successive blade passings.

The strategy of simulation effects of wind speed, variation of turbine blades and it’s interaction to power generated by vertical axis wind turbine using NACA 2412

The objective of this paper is to simulate the effects upon the wind speed, variation of turbine blades and interaction of wind speed and variation of turbine blades to the power capacity generated by Vertical Axis Wind Turbine (VAWT) using NACA 2412 and to stratify the power capacity generated by the VAWT simulation. The research backgrounds are the wind-energy potential in Indonesia is about 9.290 MW and has already elaborated by Ministry of Mining, and Energy Resources is about 50 MW. This wind energy is environmentally (clean energy), economically (cheapest), easy to operate and easy to maintain, also renewable energy. The method of analysis is quantitative approach using two way classification (analysis of variance or design of experiments). The research variables are wind speed, variation of turbine blades and this interaction among independent variables and the power capacity as dependent variables. Data wind speed simulation vary from 3 m/s till 6 m/s. The quantity of turbine blades vary from 3 till 8 units. The finding from this research is accepted the null hypothesis or not differ significantly at 5% from each independent variable. The scenario and the parameters during the strategy development use turbine blades, wind speed and power generated by VAWT and the maximum power generated is 16.38 watt. The wind speed is 6 m/s and the number of turbine blade is 4 units. However, the minimum power generated by VAWT is 0.45 watt, the wind speed is 6 m/s and the number of turbine blade is 3 units.

Numerical simulation of VAWT on the effects of rotation cylinder

Based on Finite Element Analysis Method, studying on Vertical Axis Wind Turbine (VAWT) which is added rotating cylinder in front of its air foils, especially focusing on the analysis of NACA6 series air foils about variation of lift to drag ratio. Choosing the most suitable blades with rotary cylinder added on leading edge. Analysis indicates that the front rotating cylinders on the VAWT is benefit to lift rise and drag fall. The most suitable air foil whose design lift coefficient is 0.8, the blades relative thickness is 20%, and the optimistic tip speed ratio is about 7.

Experimental analysis of vertical axis wind turbine active pitch control system with Permanent Magnet Synchronous Motor using MATLAB Simulink tools

Efficient vertical axis wind turbine (VAWT) technology is a key topic for the future wind energy market. At the moment, VAWTs are seldom used for electricity production. The development of new technologies for building a new generation of VAWTs, which will be more efficient, user-friendly, and with very low noise pollution levels is the target for many researchers. The goal of this research was to analyse an active pitch control system in an experimental setting through construction of an active pitch control system test bench using a Permanent Magnet Synchronous Motor (PMSM) and to develop new testing programmes for analytical system performance tests. The current commercial turbines do not carry an active pitch system for a VAWT; however, the concept of an active turbine blade pitching opens new opportunities for boosting the efficiency, safety, and user-friendliness of VAWTs. The research consists of the mathematical model and control system operating in a simulation environment in a closed loop with the test bench setup consisting of an active pitch control system. By applying a specially developed VAWT simulation model implemented in MATLAB Simulink, an active pitch system was tested and analysed under various conditions, which were as close as possible to the real-world operating conditions. The results of the testing and analysis show that an active pitch system using the PMSM can be very efficient and fast-operating. An active pitch system is able to work on the needed conditions by using the PMSM. Analysis shows that while using the PMSM for a turbine active pitch system, certain conditions should be taken into account in order to achieve the best results and to reduce costs. Full and effective use of active pitch system components can improve VAWT performance.

Wake modelling of a VAWT in dynamic stall: impact on the prediction of flow and induction fields

AbstractDynamic stall is a relevant phenomenon in the design and operation of a vertical axis wind turbine (VAWT) as it impacts loading, control and wake dynamics. Although streamtube models and single‐wake vortex models are commonly used for VAWT simulation, they either do not explicitly simulate the distribution of vorticity in the wake (streamtube models) or simplify it into a single‐wake release point (single‐wake vortex models). This can lead to inaccurate predictions of the vorticity distribution and wake dynamics, and therefore of the induction field, rotor loading and wake development, including wake mixing and re‐energizing. In this work, we use a double‐wake panel model developed for the simulation of dynamic stall in a VAWT to analyse (i) what is the flow field in dynamic stall, including the induction field, (ii) what is the error due to assuming a simplified wake, in both vorticity distribution and induction and (iii) how an incorrect simulation of the vorticity distribution can affect the prediction of the dynamics of the near and far wake. The results demonstrate that for mild separation (tip speed ratio λ≥3), single‐wake models can produce acceptable results. However, for lower tip speed ratios (λ < 3), the inaccuracy in the prediction of loads, induction field and vorticity distribution becomes significant because of an inadequate representation of the wake dynamics. These results imply that using lower order models can lead to inaccurate estimations of loads, performance and power control requirements at low tip speed ratios. Copyright © 2014 John Wiley & Sons, Ltd.

Large Eddy Simulation of Vertical Axis Wind Turbine Wakes

In this study, large eddy simulation (LES) is combined with a turbine model to investigate the wake behind a vertical-axis wind turbine (VAWT) in a three-dimensional turbulent flow. Two methods are used to model the subgrid-scale (SGS) stresses: (a) the Smagorinsky model; and (b) the modulated gradient model. To parameterize the effects of the VAWT on the flow, two VAWT models are developed: (a) the actuator swept-surface model (ASSM), in which the time-averaged turbine-induced forces are distributed on a surface swept by the turbine blades, i.e., the actuator swept surface; and (b) the actuator line model (ALM), in which the instantaneous blade forces are only spatially distributed on lines representing the blades, i.e., the actuator lines. This is the first time that LES has been applied and validated for the simulation of VAWT wakes by using either the ASSM or the ALM techniques. In both models, blade-element theory is used to calculate the lift and drag forces on the blades. The results are compared with flow measurements in the wake of a model straight-bladed VAWT, carried out in the Institute de Méchanique et Statistique de la Turbulence (IMST) water channel. Different combinations of SGS models with VAWT models are studied, and a fairly good overall agreement between simulation results and measurement data is observed. In general, the ALM is found to better capture the unsteady-periodic nature of the wake and shows a better agreement with the experimental data compared with the ASSM. The modulated gradient model is also found to be a more reliable SGS stress modeling technique, compared with the Smagorinsky model, and it yields reasonable predictions of the mean flow and turbulence characteristics of a VAWT wake using its theoretically-determined model coefficient.

Application of Variable Blade Pitch Control on Improving the Performance of Vertical Axis Wind Turbine

A three blades vertical axis wind turbine simulation is performed to study the unsteady aerodynamic characteristics with blade pitch control. Several fixed and variable blade pitch models under different tip speed ratio are adopted to improve performance of the wind turbine. Results show that an appropriate pitch control model can effectively decrease the range of negative torque regime to reduce the vibration of the wind turbine. Besides, the average torque coefficient as well as the energy capture efficiency can be also improved, especially for the lower tip speed ratio. The overall efficiency of the wind turbines in power generation will be enhanced. For the cases under the tip speed ratio between 1 and 3, the efficiency can be enhanced 243% and 486% for fixed and variable pitch control models respectively as comparing with non-pitch control cases.

Operator- and template-based modeling of solid geometry for Isogeometric Analysis with application to Vertical Axis Wind Turbine simulation

This article describes a novel approach to modeling and storage of NURBS-based solid objects for Isogeometric Analysis. The proposed method is based on a procedural description of the modeling process. Creation of geometric objects as well as the steps of the modeling process are formulated as a list of simple commands. This provides an abstraction from the often times tedious manual specification of control point locations to create a given geometric object. This operator-based approach, in conjunction with the existing template-based geometry modeling methods, allows one to create complex and multi-level adaptive models. To illustrate our method, we construct the geometry of a Vertical Axis Wind Turbine (VAWT) that is suitable for isogeometric fluid and fluid–structure interaction analysis. A new template is proposed for modeling VAWTs together with a novel algorithm for constructing wind turbine airfoil profile B-Spline curves from point data. The resultant model has a compact representation that makes use of a small number of parameters. A preliminary aerodynamics simulation of a newly constructed VAWT model in 3D under realistic wind conditions and rotation speed is presented.

Investigation of Self-Starting Capability of Vertical Axis Wind Turbines Using a Computational Fluid Dynamics Approach

Vertical axis wind turbines have always been a controversial technology; claims regarding their benefits and drawbacks have been debated since the initial patent in 1931. Despite this contention, very little systematic vertical axis wind turbine research has been accomplished. Experimental assessments remain prohibitively expensive, while analytical analyses are limited by the complexity of the system. Numerical methods can address both concerns, but inadequate computing power hampered this field. Instead, approximating models were developed which provided some basis for study; but all these exhibited high error margins when compared with actual turbine performance data and were only useful in some operating regimes. Modern computers are capable of more accurate computational fluid dynamics analysis, but most research has focused on horizontal axis configurations or modeling of single blades rather than full geometries. In order to address this research gap, a systematic review of vertical axis wind-power turbine (VAWT) was undertaken, starting with establishment of a methodology for vertical axis wind turbine simulation that is presented in this paper. Replicating the experimental prototype, both 2D and 3D models of a three-bladed vertical axis wind turbine were generated. Full transient computational fluid dynamics (CFD) simulations using mesh deformation capability available in ansys-CFX were run from turbine start-up to operating speed and compared with the experimental data in order to validate the technique. A circular inner domain, containing the blades and the rotor, was allowed to undergo mesh deformation with a rotational velocity that varied with torque generated by the incoming wind. Results have demonstrated that a transient CFD simulation using a two-dimensional computational model can accurately predict vertical axis wind turbine operating speed within 12% error, with the caveat that intermediate turbine performance is not accurately captured.