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The present paper describes control of wingtip vortices generated by vertical type wind turbine. The wind turbine consists of three circular cylinders. Each cylinder rotates on its own vertical axis and moves in orbit. It is known that wingtip vortices give rise to decrease of power generation performance as well as aerodynamic noise. Therefore, the goal of the study is to control wingtip vortices and to improve power generation performance. Numerical study was conducted for 14 models to find out control factors to suppress wingtip vortices. Numerical simulation visualized wingtips by streamlines as well as pressure distribution on the circular cylinder for evaluating Magnus effect. As a result, the following findings were obtained: 1) Installation of fully covered protection plates at both ends of the circular cylinder blades is greatly effective to suppress the wingtip vortices. 2) Curved wings attached to each cylinder are more effective to enhance power generation efficiency than flat ones, due to great increase in Magnus effect caused by large pressure difference on both sides of the curved wing. The power efficiency of the optimized model was improved up to 2.8%, which means 11 times that of the original model.

Wind-power generation is one of the most introduced energies regarding renewable energies across the world. However, wind power generation has not yet been widely used due to low power efficiency and poor strength [

To solve these problems, a new type of wind turbine equipped with circular cylinders is recently increasing in number. Takahashi et al. [

The wind turbine in interest consists of three circular cylinders. Each cylinder rotates on its own vertical axis and moves in the orbit. Compared with wind turbines with propellers, wind turbines with cylinder blades have sufficient higher rigidity. However, it is known that wingtip vortices give rise to decrease of power generation capacity as well as aerodynamic noise. Wingtip vortices shedding from a front cylinder interfere with a rear cylinder and the interference cause surface pressure distributions on the rear cylinder as shown in

Therefore, the paper aims to improve the power generation efficiency by control and suppression of wingtip vortices on vertical type wind turbine with circular cylinders.

Numerical study uses the software STAR-CCM+ with software V11.04.012. The numerical simulation was performed by unsteady airflow analysis. The study employs turbulent model of RANS (Reynolds Averaged Navier-Stokes) and LES (Large Eddy Simulation). The numerical simulation was conducted by DES (Detached Eddy Simulation) [

The prism layer mesh model [

For unsteady analysis, time step is 0.05 s and the number of an internal iteration is ten. The Magnus wind turbine was installed in the uniform flow whose speed is 10 m/s. This is due to the fact that although the speed is a little high when compared with the average wind speed in Japan, the wind turbine is expected to operate even in the circumstance of attack of typhoon.

Reynolds number is defined in Equation (1), where uniform velocity U = 10 m/s, diameter of a circular cylinder D = 89 × 10^{−3} m, and kinematic viscosity of air ν = 15.15 × 10^{−6} m^{2}/s and results in R_{e} = 5.9 × 10^{4}. This implies that the flow around the circular cylinder is laminar flow.

R e = U D ν (1)

The driving force of the wind turbine is caused by Magnus effect. When the rotating cylinder is immersed in the uniform, lift force is generated. The lift force is generated by the pressure difference between one side of the cylinder and the other side due to Bernoulli’s principle. To generate the Magnus effect, the three each cylinder has to be rotated. In the study the rotating speed is set as 200 rpm, based on the previous paper [^{4}. ^{2} < Re < 10^{5}, the flow still remains laminar and laminar separation occurs near 80˚ from the front stagnation as shown in

The test area was selected as reference size. The mesh size is 0.1 mm. When the reference mesh size is taken as 0.05 mm, the relative percentage becomes 200%. Other three areas, medium, small, and tiny area are 100%, 50%, 25%

respectively. The tiny test area has the finest mesh to simulate the shedding wing tip vortices accurately.

The study adopted the overset mesh region [

To find out control factors which can suppress wingtip vortices, fourteen numerical models from A to Q were produced.

• Output W

STAR CCM+ program can calculate the rotational energy as a function of time t, based on the rotational motion of the cylinder. Using this function, after calculating rotation energy for 10 seconds, average output (W) was obtained. In the simulation of power generation, both mechanical loss and electric generator loss are not taken into account.

• Number of revolution n

STAR CCM+ can also calculate the rotational angle θ (deg.) just like rotational energy. Based on Equation (2), the number of averaged revolution was obtained.

n = 1 t θ 360 × 60 [ rpm ] (2)

• Torque T

Torque T can be obtained as the function of W and n in the following Equation (3).

T = 1 2π 1000 60 × W n [ N ⋅ m ] (3)

• Power efficiency η

Power efficiency η is given as Equation (4) by dividing output W with motion energy Q of the wind.

Q = 1 2 ρ S U 3 , η = W Q (4)

Model Q installs fully covered protection plates at both ends of the circular cylinders as well as curved wings along the circular cylinders. The large protection plate contributes greatly to flow control of wingtip vortices. Wingtip vortices

shedding from the front cylinder has no interference with the rear cylinder. This leads to the maintenance of high Magnus force.

Further the curved wing can produce the wind resistance. Compared with the flat plate wing, the curved wing can generate large pressure difference on both sides of the curved plate wing. This large pressure difference causes the great rotating force along with Magnus force.

output [W] | Rotation speed [rpm] | torque [Nm] | power efficiency [%] | |
---|---|---|---|---|

A | 0.33 | 11.27 | 0.28 | 0.19 |

B | 0.37 | 29.96 | 0.12 | 0.21 |

C | 2.67 | 169.65 | 0.15 | 1.50 |

D | 2.86 | 134.39 | 0.20 | 1.60 |

E | 0.00 | 0.46 | 0.02 | 0.00 |

G | 0.57 | −10.93 | −0.50 | 0.32 |

H | 2.89 | 150.98 | 0.18 | 1.62 |

I | 2.88 | 181.77 | 0.15 | 1.61 |

J | 0.66 | −11.05 | −0.57 | 0.37 |

K | 0.42 | −8.55 | −0.46 | 0.23 |

L | 0.58 | 39.84 | 0.14 | 0.33 |

O | 1.34 | 37.19 | 0.34 | 0.75 |

P | 3.68 | 199.32 | 0.18 | 2.06 |

Q | 4.06 | 197.12 | 0.20 | 2.28 |

for model Q it is recognized that the fully covered protection plates successfully prevent wingtip vortices from interfering with the end of the circular cylinder. Since pressure loss on the wing surface was not induced by wingtip vortices, sufficient lift forces due to Magnus effect can be obtained.

shown in the figure. The other effect is to enhance Magnus force. Airflows impinging the curved wing cause the increasing positive pressure which leads to greater pressure gap on the cylinder surface. Solution time indicates the time when the most representative situation was obtained. Solution time of the figures is different since it depends on the simulation results.

The paper aims to control wingtip vortices to improve power generation capacity. As a result, following findings were obtained.

Numerical study was conducted for 14 models to find out control factors to suppress wingtip vortices. Numerical simulation visualized wingtips by streamlines as well as pressure distribution on the circular cylinder for evaluating Magnus effect. As a result, the following findings were obtained.

1) Installation of fully covered protection plates at both ends of the circular cylinder blades is greatly effective to suppress the wingtip vortices.

2) Curved wings attached to each cylinder are more effective to enhance power generation efficiency than flat ones, due to great increase in Magnus effect caused by large pressure difference on both sides of the curved wing.

The power efficiency of the optimized model was improved up to 2.8%, which means 11 times that of the original model.

This work has been supported by Japan Grant-in-Aid for Scientific Research (C) under contract No. 17K06174.

The authors declare no conflicts of interest regarding the publication of this paper.

Ogawa, S. and Kimura, Y. (2018) Performance Improvement by Control of Wingtip Vortices for Vertical Axis Type Wind Turbine. Open Journal of Fluid Dynamics, 8, 331-342. https://doi.org/10.4236/ojfd.2018.83021