^{1}

^{1}

^{1}

^{1}

^{1}

^{*}

This paper describes a new actively controlled multi-fan wind tunnel that generates natural wind as a type of turbulence wind tunnel at a reduced cost. The driving section of the wind tunnel has 100 PC cooling fans that are controlled by an original embedded system. The fluctuating velocity wind is successfully generated with a mean velocity of 7 m/s and two turbulent intensities of 2% and 3% based on Karman’s power spectrum density function. The case of 2% has the integral scales of 5 m, 10m and 20 m, and the case of 3% has the integral scales of 3 m, 6 m and 15 m with a turbulence grid. In particular, the wind with the turbulent intensity of 2% satisfies the Kolmogorov’s -5/3 multiplication rule of inertial subrange with the frequency range from 0.01 Hz to 2.0 Hz. Consequently, the new wind tunnel can be used for studying engineering technology and research regarding conditions with natural wind.

Turbulent flow is a significant concern within engineering technology and fluid dynamics research. Wind turbine design and environmental assessments of natural wind and buildings in urban development are typical real-world applications of such related research. For these applications, it is important that we can produce the turbulent flow experimentally and clarify the influence of turbulence on the research objects. Therefore, various types of turbulence wind tunnels have been developed to generate the turbulent flow for the research purposes. Makita [

This paper shows that the possibility of the low-cost multi-fan actively controlled turbulence wind tunnel which consists of driving PC cooling fans and an original embedded system. One purpose of using the low-cost wind tunnel is to easily clarify the influence of natural wind on the performance of a wind turbine through a laboratory-based experiment. Therefore, we used the multi-fan type wind tunnel as the generator of natural wind to investigate the effect of turbulent flow on the wind turbine performance. Specifically, this paper shows the composition of the new wind tunnel and the characteristics of the generated natural wind. Furthermore, we clarify the generating conditions and reproducibility of the real natural wind with our original procedure method.

The horizontal and vertical contraction ratios in the convergent section are 1 and 2 respectively.

Firstly, we clarify the steady wind velocity distribution as the basic characteristic of the wind tunnel. The velocity distribution on the cross-section is measured as shown points along x, y, xy+ and xy− directions in

of the velocity distribution is the same or more uniform than one with the turbulence grid.

in the range of −15 cm to 15 cm on the x and y axes at B, respectively. Since we plan the research using a wind turbine with a rotor of 20 cm diameter (−10 cm to 10 cm), the uniformity is sufficient enough to study. In addition, the wind tunnel can generate the mean steady wind velocity from 3 m/s to 8.6 m/s.

We verified that the steady wind can be generated as almost uniform velocity distribution on the cross section A and B in Section 3.1. This is required as the fundamental character of the wind tunnel for testing wind turbine performance. Next, we will discuss about generating natural wind with fluctuating velocity.

The prescribed wind tunnel generates arbitrary artificial natural wind that is based on Kármán’s power spectral density function (PSD) [

S u ( f ) = 4 I 2 L U 1 [ 1 + 70.8 ( f L U ) 2 ] 5 6 (1)

I = σ U (2)

Here, S_{u}, f, U, I and L denote the power spectral density function [m^{2}/s], frequency [Hz], mean wind velocity [m/s], turbulent intensity and integral scale, respectively. The turbulent intensity I is defined by Equation (2) with standard deviation σ of fluctuating wind velocity. Here, we use the integral scale means as a statistical parameter representing the mean turbulent scale under Taylor's hypothesis of “frozen turbulence”. Hence, L cannot be directly connected to the wind turbine size.

through FFT analysis of the time series velocity as shown in the two left green graphs. On the other hand, the theoretical control PSDs are made from the time series velocity data with changing I and L to control the wind tunnel, which are shown in the right blue symbol graphs in

Figures 10-15 show PSDs of the generated natural wind with mean velocity of about 7 m/s as No.1 to 6 in

In the cases without the turbulence grid (Figures 10-12), the theoretical and experimental wind PSDs are in good agreement at the turbulent intensity, I = 2%. In particular, the PSDs satisfy the Kolmogorov’s −5/3 multiplication rule of inertial subrange with frequency from 0.01 Hz to 2.0 Hz.

While the turbulence grid increases turbulent intensity by about 1% (i.e. I = 3%) as shown in Figures 13-15. The theoretical and experimental wind PSDs are in good agreement in the range of the frequency from 0.01 Hz to 1.0 Hz with I = 3%. The turbulence grid raises PSD in the range of over frequency of 1.0 Hz, and experimental PSD is larger than theoretical PSD. This result is commonly known as the turbulence grid effect. The actual natural wind of the offshore wind at Fukushima, Japan has this character [

Consequently, the natural winds can be generated as the conditions of

No. | Control conditions | Generated wind conditions | ||||
---|---|---|---|---|---|---|

U [m/s] | I [%] | L [m] | U [m/s] | I [%] | L [m] | |

Without the turbulence grid | ||||||

1 | 7.0 | 4 | 5 | 6.8 | 2 | 5 |

2 | 7.0 | 4 | 10 | 6.7 | 2 | 10 |

3 | 7.0 | 4 | 20 | 7.0 | 2 | 20 |

With the turbulence grid | ||||||

4 | 6.9 | 4 | 5 | 7.0 | 3 | 3 |

5 | 6.9 | 4 | 10 | 7.0 | 3 | 6 |

6 | 7.0 | 4 | 20 | 7.0 | 3 | 15 |

on Karman’s PSD. Hence, these natural winds are useful to investigate the influence of natural wind on the performance of the wind turbines through a laboratory-based experiment.

This paper describes a low-cost actively controlled multi-fan wind tunnel with an original embedded system and 100 PC fans of the driving section to generate artificial natural wind through Kármán’s power spectrum density function (PSD). This wind tunnel can successfully generate two kinds of natural winds with turbulent intensities of 2% and 3% and with a mean velocity of about 7 m/s. To this wind, we apply inverse Fast Fourier Transformation assuming a random phase to the Kármán’s PSD. Winds with a turbulence intensity of 2% can be reproduced with the varying integral scale 5 m, 10 m and 20 m without the turbulence grid. Winds with a turbulence intensity of 3% can be reproduced with the varying integral scale 3 m, 6 m and 15 m with the turbulence grid. The winds with a turbulence intensity of 2% satisfy the Kolmogorov’s −5/3 multiplication rule of inertial subrange with the frequency range from 0.01 Hz to 2.0Hz.

As a feature of this wind tunnel, the costs of the driving section are about one-tenth or less than one consisting of servo motors. Hence, it has the advantage that one can construct the driving section at a very low cost, and it is possible to construct a multi-fan turbulence wind tunnel at a low cost. Consequently, this wind tunnel is useful for laboratory-based experimental research using natural wind.

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

Kikuchi, H., Matsubara, H., Pornthisarn, P. and Toshimitsu, K. (2019) A Low-Cost Active Control Multi-Fan Turbulence Wind Tunnel with an Embedded System to Generate Natural Wind. Open Journal of Fluid Dynamics, 9, 158-167. https://doi.org/10.4236/ojfd.2019.92011