Trial production of a new wind tunnel with directly controlled fans to simulate fluctuations in velocity and direction of natural wind

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The present project has been conducted to develop a new wind tunnel to simulate fluctuations in velocity and direction of natural wind. We made for trial a wind tunnel of which fans are driven by an AC servomotor. The servo motor is controlled by computer-generated voltage histories. The maximum wind velocity is 13 m/s. The generated wind velocity is measured by a hot wire anemometer.The measured velocity histories under step-wise voltage changes show that the wind velocity can change abruptly within a infinitesimal time less than a quarter second. However, in the cases that the target velocity is zero, it takes about one second because the wind is not able to stop due to its inertia even after the fan has stopped rotating. The measured velocity histories under sinusoidal voltage changes show that the wind velocity varies sinusoidally, except for the cases that the lowest velocity is zero.We have next conducted a series of experiments where the target velocity fluctuations obey the Karman spectrum. The measured velocity histories and their power spectra show that the new wind tunnel can reproduce not only details of the target velocity fluctuation but also its statistics. Especially, velocity fluctuation with quite large turbulence length scale can be achieved easily. There is a limitation in generating high frequency fluctuation beyond several cycles per second.These results show that the present new wind tunnel possesses sufficient ability to simulate velocity fluctuation of natural wind as far as the longitudinal fluctuations are concerned.As a preliminary investigation to control wind direction, we investigated the effect of inhalation by two wind tunnels in tandem alignment, and found that it is not effective. It is now planned to control wind direction by combining air flows from two wind tunnels aligned with a horizontal angle.