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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Nobuhito Oka Kyushu University, Japan
Co-authors:
Nobuhito Oka (1) F P Masato Furukawa (1) Kazutoyo Yamada (1) Kenta Kawamitsu (1) Kota Kido (1) Akihiro Oka (1)
(1) Kyushu University, Fukuoka city,Fukuoka, Japan

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Nobuhito Oka is a doctral course student in mechanical engineering at the Kyushu university. He is studying and developing an optimum aerodynamic design method for turbo-machinery like a wind turbine, an axial fan, a centrifugal compressor and so on. His optimum aerodynamic design method utilizes a genetic algorithm and a computational fluid dynamics. He can be contacted at: [email protected]

Abstract

Aerodynamic design optimization of wind-lens turbine

Introduction

In this research, the optimization technique has been applied to the “wind-lens turbine”, which is one of the new types of wind turbine system shown in Figure 1[1], design method. Figure 2 shows a schematic illustration of the "wind-lens", which has the wind concentration effect on the turbine rotor. The wind-lens consists of bell-mouth, diffuser and brim. Because of its shape, the non-uniform meridional flow distributions are generated and wind velocity at the rotor is increased. In the previous study, a quasi-three-dimensional aerodynamic design method has been developed for the wind-lens turbine rotor which can take into account the non-uniform meridional flow distributions[2].

Approach

The quasi-three dimensional aerodynamic design method mainly consists of the three parts: decision of wind-lens meridional shape and blade loading distributions, meridional viscous flow calculation and two-dimensional blade element design. The wind-lens turbine meridional shape and blade loading distributions are decided as design specifications. The meridional viscous flow calculation is introduced to obtain the flow rate in the wind-lens and the velocity distribution of the turbine rotor inlet[3]. Using the blade loading distributions and the velocity diagrams obtained by the meridional viscous flow calculation, the 3-D blade shape is designed by the two dimensional blade element design method. Taking into account the blade force evaluated by the 3-D blade shape and the blade loading distributions, the meridional viscous flow calculation is performed again. By repeating the meridional viscous flow calculation and the two dimensional blade element design, the blade shape and the flow field are converged [2].
In the previous study, it is found that the wind-lens turbine meridional shape and the blade loading distribution affect the flow field and aerodynamic performances. In the present study, one of the optimization techniques called genetic algorithm (GA) has been applied to the wind-lens turbine design method. The optimization objects are the wind-lens turbine meridional shape and the blade loading distribution. The same design specifications are adopted except for the wind-lens shape and blade loading distribution. A flow chart of the present optimal design method by GA is shown in Figure 3. The evaluation and selection model is the Non-dominated Sorting Genetic Algorithm II (NSGA-II) which is well-known optimization method by its performance[4]. The crossover model is Real-coded Ensemble Crossover (REX)[5]. In the optimization procedure, the aerodynamic performances of each individual are obtained from the meridional viscous flow calculation.
Reynolds averaged Navier-Stokes simulations have been applied to conventional and optimal design cases of the wind-lens turbine in order to verify the validity of the present optimization technique and to elucidate the relation between aerodynamic performances, three-dimensional flow fields and design specifications.


Main body of abstract

Figure 4 shows the aerodynamic performance of the individuals designed by the present optimization procedure. The abscissa shows the wind collection coefficient K and the ordinate shows the output power coefficient evaluated by the meridional viscous flow calculation CWM*. The wind collection coefficient K corresponds to the amount of the wind flow rate inside of the wind-lens, namely the wind concentration effect by the wind-lens. The output power coefficient, CWM*, describes the efficiency of the wind-lens turbine. In this research, the three-dimensional flow field of the optimized individual indicated as a red symbol in Figure 4 is analyzed by a three-dimensional RANS simulation. This individual named WLT-opt has the highest performance.
Figure 5 shows the wind-lens shape and its position relative to the rotor in the conventional and WLT-opt cases. In the figure, the abscissa and ordinate denote the axial and radial positions of the wind-lens, respectively. The diffuser shape of the wind-lens shows no large difference between the optimum and conventional designs. However, the brim height in the WLT-opt case is lower than that in the conventional case. Figure 6 shows the blade loading distributions in the conventional and WLT-opt cases. Although a qualitative coincidence of the blade loading distributions is observed between the optimum and conventional designs, the blade loading coefficients from mid-span section to tip section are significantly different.
Total performances and three-dimensional flow fields in the optimum and conventional design cases have been investigated by Reynolds averaged Navier-Stokes simulations (RANS), in order to verify the present design method. The Reynolds averaged Navier-Stokes equations were solved by an unfactored implicit upwind relaxation scheme with the k-omega two-equation turbulence model of Wilcox. Vortical flow structures were visualized by a semi-analytic method for identifying vortex cores based on the critical point theory. The RANS simulations and the flow visualization have been applied to the conventional and optimum design cases, in order to elucidate the relation between their aerodynamic performances and the flow fields around them.
Table 1 shows the aerodynamic performances calculated from the three dimensional RANS simulations at the design condition. As a result of the optimization, the wind-lens turbine of WLT-opt has the output power coefficient superior to the conventional one. It is found that the output power coefficient CW* in the WLT-opt case exceeds the Betz' limit (CW* =0.593). However, the wind collection coefficient in the conventional case is higher than that in the WLT-opt case. As seen in Figure 5, the brim height, which strongly affects the wind-collection coefficient, in the WLT-opt case is lower than that in the conventional case.
Figure 7 shows vortex cores identified by the critical-point concept and limiting streamlines on the blade suction surface in the conventional and the WLT-opt cases. As seen in Figure 7, the conventional case forms the separation vortex located away from the brim. However, the WLT-opt case forms the smaller vortex attached to the brim. It is clearly seen that the wind-lens turbine that achieves higher aerodynamic performances forms the smaller separation vortex attached to the brim. This fact agrees with the result of the previous study[6]. In the previous study, it was concluded that the diffuser shape of the wind-lens affected the aerodynamic performances[6]. In the present study, it is concluded that the blade loading distribution and the brim height also affect the aerodynamic performance. Therefore, it is important that the optimum design of the rotor blade coupled with the wind-lens is performed in order to achieve higher aerodynamic performance of the wind-lens turbine. These results indicate that the present aerodynamic optimization for the wind-lens turbine design works well using the quasi-three dimensional design method.


Conclusion

One of the optimization methods, GA, has been applied to a quasi-three-dimensional aerodynamic design of the wind-lens turbine. In order to evaluate the wind concentration in the design procedure, the quasi-three dimensional aerodynamic design method has successfully developed. The quasi-three dimensional aerodynamic design method consists of a meridional viscous flow calculation and two-dimensional blade element designs. The meridional viscous flow calculation is introduced to obtain the flow rate in the wind-lens and the velocity distribution of the turbine rotor inlet. Using the quasi-three dimensional design method, the optimization method has been applied to the wind-lens turbine design. This optimization is based on the Non-dominated Sorting Genetic Algorithm II (NSGA-II) and Real-coded Ensemble Crossover (REX). The optimum design result shows a wind-lens diffuser shape similar to the conventional design and a lower brim height than the conventional one. However, the optimum design achieves much higher aerodynamic performance than the conventional one, which exceeds the Betz limit. It is found that the blade loading distribution and the brim height also affect the aerodynamic performance of the wind-lens turbine. Therefore, it is important that the optimum design of the rotor blade coupled with the wind-lens is performed to achieve higher aerodynamic performance of the wind-lens turbine. Also, it is concluded that the present optimization method for the aerodynamic design of wind-lens turbine works well.
The Reynolds Averaged Navier-Stokes (RANS) simulations and the flow visualization have been applied to the conventional and optimum design cases of the wind-lens turbine, in order to elucidate the relation between their aerodynamic performances and the vortical flow fields around them. It is found that the wind-lens turbine which achieves higher aerodynamic performance forms the smaller separation vortex attached to the brim.



Learning objectives
The objective of the research is the development of the wind-lens turbine which achieves high aerodynamic performances and the investigation of three-dimensional flow fields around the wind-lens turbines and their aerodynamic performances. In order to achieve the objective, an aerodynamic optimization using a genetic algorithm, a quasi-three dimensional aerodynamic design method and Reynolds Averaged Navier-Stokes (RANS) simulations have been applied.


References
1. Yuji, O., et al., “Development of a shrouded wind turbine with a flanged diffuser”, 2008, Journal of Wind Engineering, Vol.96, pp.524-539.
2. Oka, N., et al., “Aerodynamic Design for Wind-Lens Turbine Using Optimization Technique”, 2013, Proceedings of the ASME 2013 Fluids Engineering Summer Meeting, Paper No. FEDSM2013-16569.
3. S., Tabata, F., Hiratani, and M., Furukawa, 2007, “Axisymmetric Viscous Flow Modeling for Meridional Flow Calculation in Aerodynamic Design,” Memories of the Faculty of Engineering, Kyushu Univ. Vol.67, No.4, pp. 199-208.
4. K., Deb, Associate Member, IEEE, A., Pratap, S., Agarwal, and T. Meyarivan, 2002, “A Fast and Elitist Multiobjective Genetic Algorithm:NSGA-II”, IEEE Transactions on evolutional computation, Vol. 6, No. 2, pp.182-197.
5. S., Kobayashi, “The frontiers of real-coded genetic algorithms”, 2009, Journal of the Japanese Society for Artificial Intelligence, Vol24, No.1, pp.147-162.
6. Oka, N., Kawamitsu, K., Tabata, S., Furukawa, M., Yamada, K., Kido, K., 2013, “Numerical Analysis of Vortical Flow Field Around Wind-lens Turbines”, 4th International Conference on Jets, Wakes and Separated Flows,