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Improvement of vertical axis wind turbine performance using different J-shaped profiles
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A steady-state two-dimensional computational fluid dynamics analysis was performed using FLUENT 6.2 software to analyze the performance of a twisted three-bladed H-Darrieus rotor. The flow over the rotor was simulated by using unstructured-mesh finite volume method coupled with moving mesh technique to solve mass and momentum conservation equations. The standard k-&Vegr; turbulence model was chosen. Second-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The aerodynamic coefficients, such as lift coefficient, drag coefficient, and lift-to-drag coefficient, were evaluated with respect to angle of attack for two chord Reynolds numbers. The power coefficient of the rotor was also evaluated. The results were validated by using experimental values for the twisted three-bladed H-Darrieus rotor. The experiments were earlier conducted in a subsonic wind tunnel available in the department. The results showed good matching between the two approaches. The effect of twist angle at the chord ends on the performance of the rotor was also evaluated.
In this paper, a computational fluid dynamics analysis using the FLUENT package 6.2 was carried out to predict the performance characteristics such as power coefficient (Cp), torque coefficient (Ct), and tip speed ratio of a combined three-bucket Savonius and three-bladed Darrieus rotor for various overlap conditions, namely, 16.2%, 20%, 25%, 30%, and 35%. In the upper part of the rotor model, there was a three-bucket Savonius rotor of bucket diameter of 8 cm and height of 10 cm, whereas, in the lower part, there was a three-bladed Darrieus rotor of blade diameter of 8 cm and height of 10 cm. A two-dimensional unstructured computational grid was developed for the combined Savonius–Darrieus rotor model. A k-&Vegr; turbulence closure model with enhanced wall treatment function was chosen. A first-order upwind discretization scheme was adopted for pressure-velocity coupling of the flow. The values of Cp and Ct obtained computationally were then compared with those of the values of Cp and Ct obtained experimentally for all the overlap conditions. The experimental values of Cp and Ct for different overlap conditions were obtained from the tests conducted previously in an open-circuit subsonic wind tunnel available in the department. The comparison of experimental and computational studies is quite encouraging.
Increased concern for environment has led to the search for more environment friendly sources of energy. Wind energy can be a viable option in this regard. Vertical axis wind turbines offer promising solution for areas away from the integrated grid systems. However, they have certain drawbacks associated with different configurations. This paper reviews various configurations of VAWT along with their merits and demerits. Moreover, design techniques employed for VAWT design have also been reviewed along with their results. It was learned that coefficient of power (CP) for various configurations is different and can be optimized with reference to Tip Speed Ratio. Latest emerging design techniques can be helpful in this optimization. Furthermore, flow field around the blade can also be investigated with the help of these design techniques for safe operation.
A new correlation-based transition model has been developed, which is based strictly on local variables. As a result, the transition model is compatible with modern computa-tional fluid dynamics (CFD) approaches, such as unstructured grids and massive parallel execution. The model is based on two transport equations, one for intermittency and one for the transition onset criteria in terms of momentum thickness Reynolds number. The proposed transport equations do not attempt to model the physics of the transition pro-cess (unlike, e.g., turbulence models) but form a framework for the implementation of correlation-based models into general-purpose CFD methods. Part I (this part) of this paper gives a detailed description of the mathematical formulation of the model and some of the basic test cases used for model validation, including a two-dimensional turbine blade., pp. 423–434) of the paper details a significant number of test cases that have been used to validate the transition model for turbomachinery and aerodynamic applications. The authors believe that the current for-mulation is a significant step forward in engineering transition modeling, as it allows the combination of correlation-based transition models with general purpose CFD codes.
Selection of the airfoil is crucial for better aerodynamic performance and dimensions of a smaller-capacity fixed-pitch SB-VAWT. Most of the earlier research works with SB-VAWT mainly utilized symmetric airfoils as its blade shape, but several research works indicated that the performance of fixed pitch SB-VAWT with asymmetric blades have the potential to exhibit superior characteristics at low Reynolds numbers (RN). However, currently there is lack of comprehensive information in the public domain regarding the desirable aerodynamic and geometric features of prospective asymmetric airfoils for SB-VAWTs. Against this backdrop, this research has been undertaken with an objective to perform detail systematic investigative analysis with asymmetric airfoils appropriate for smaller-capacity fixed-pitch SB-VAWT with optimum design configuration.
A computational method has been developed in the present study after identifying and considering the main aerodynamic challenges of smaller-capacity SB-VAWT using theoretical coefficients rather than using rarely available expensive experimental results. After conducting literature survey and detail performance analyses with available asymmetric airfoils, it has been found that there is a need for designing special-purpose airfoils for smaller-capacity SB-VAWT. Under this circumstance, a new airfoil “MI-VAWT1” has been designed and it has been found that its performance is much superior to other prospective asymmetric airfoils and conventionally used symmetric NACA 0015 at low RN and low tip speed ratio ranges. Another airfoil, named as “MI-STRUT1”, has been designed for blade supporting struts to reduce the detrimental parasitic drag losses. After considering the design parameters and detailed sensitivity analyses with selected important parameters, a new class of 3kW SB-VAWT (named as “MI-VAWT 3000”) has been proposed.
Many attempts have been carried out in the past few years to build a general understanding of the straight blade vertical axis wind turbine aerodynamics. For this purpose many models have been developed. These models may be classified into four main categories: momentum, vortex, cascade and computational fluid dynamic based modelling. The computational fluid dynamics modelling has become more favourable due to the modelling aerodynamical complexity involved in the other modelling approaches. This approach has become more feasible in the last few years as the computational power has been significantly improved by the development of high performance computers. However, there has not been enough investigations on computational fluid dynamics simulations in the application of straight blade vertical axis wind turbines in the literature. This paper focuses on the computational fluid dynamics modelling aspects and their effects on the prediction accuracy of vertical axis wind turbines performance.
This thesis reports on a numerical and experimental investigation of the unsteady loading of high solidity vertical axis wind turbines (VAWTs). Two-dimensional, unsteady Reynolds averaged Navier-Stokes simulations of a small scale, high solidity, H-type Darrieus vertical axis wind turbine revealed the dominant effect of dynamic stall on the power production and vibration excitation of the turbine. Operation of the turbine at low blade speed ratios resulted in complex flow-blade interaction mechanisms. These include; dynamic stall resulting in large scale vortex production, vortex impingement on the source blade, and significant flow momentum extraction.
To validate the numerical model, a series of full-scale experimental wind tunnel tests were performed to determine the aerodynamic loading on the turbine airfoils, vibration response behaviour, and wake velocity. In order to accomplish this, a complex force measurement and wireless telemetry system was developed. During the course of this investigation, high vibration response of the turbine was observed. This resulted in conditions that made it difficult or impossible to measure the underlying aerodynamic loading. A vibration mitigation methodology was developed to remove the effect of vibration from the measured aerodynamic forces. In doing so, an accurate and complete measurement of the aerodynamic loading on the turbine blades was obtained.
Comparison of the two-dimensional numerical model results to the experimental measurements revealed a considerable over-prediction of the turbine aerodynamic force and power coefficients, and wake velocity. From this research, it was determined that the three-dimensional flow effects due to the finite aspect ratio of the turbine and blades, as well as parasitic losses, could be accounted for through the application of inlet velocity and turbine height correction factors. In doing so, the two-dimensional numerical model results could be properly scaled to represent the three-dimensional flow behaviour of the turbine prototype. Ultimately, a validated VAWT design tool was developed.
This paper presents developments in numerical simulations of a cross-flow vertical-axis marine current turbine (straight-bladed Darrieus type) with particular emphasis on rotor-performance prediction and hydrodynamic loads for structural design calculations. This study initially used theoretical double-multiple-streamtube models, followed by physical testing on a scaled-down model turbine and primarily numerical simulations. Numerical investigations of a proposed full-scale turbine (power coefficient, blade loads and flow behaviour) were undertaken using the developed computational models. The turbine design was studied using a time-accurate Reynolds-averaged Navier-Stokes (RANS) commercial solver. A transient-rotor-stator model with a moving mesh technique was used to capture the change in flow field at a particular time step. A shear stress-transport k-ω turbulence model was used to model turbulent features of the flow. The numerical results show good agreement with experimental measurements and the theoretical double-multiple-streamtube model. Turbine sensitivity to parametric variations was also demonstrated in the full-scale numerical study. This work concludes that the developed model can effectively predict hydrodynamic performance and structural design blade loads of a vertical-axis marine current turbine.
A method based on aerofoil data is proposed which enables the
self-starting capabilities of an aerodynamic vertical-axis turbine to be
determined. Aerofoil and rotor performance are evaluated at various
blade angles and the effects of free-wind acting upon the operating
rotor as well as the various parameters affecting the initial start are
examined. All results are discussed in relation to operations of the
Maximill turbine (Watson 1977) and the Canadian turbine at Magdalen
Island. Consideration is also given to the possible benefits of
low-speed operation for different rotor machines.
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