We attempted to determine the number of frequency components required for efficient turbulence generation using a multi-fan type wind tunnel where 99 fans were driven to generate turbulence. In a previous study, a random-phase mode was applied, where an input signal composed of forty frequency components was fed to each fan with quasi-random phases. Using this driving mode, we achieved high-Reynolds-number homogeneous turbulence of Re_λ ～ 750 in a relatively short distance. In the present study, in order to understand the elementary process of the evolution, one single frequency or two frequencies were used, instead of forty. When using the single frequency, initial dominant spectral peaks remain at lower frequencies over the tunnel length. In the case of two frequencies, f_1 and f_2 (f_1 = n_1f_0 and f_2 = n_2f_0; n_1 ＜ n_2), where n_1 and n_2 are integers, and f_0 is defined as the reciprocal of a basic input data period, the turbulence characteristics depend on the relation between n_1 and n_2. When n_1 and n_2 are not coprime (i.e., n_2 can be divided by n_1), dominant spectral peaks remain over the tunnel length as in the case of using a single frequency, but when coprime (i.e., n_2 cannot be divided by n_1), the spectral shape becomes relatively smooth with the initial dominant peaks disappearing. However, it was found that the development of turbulence is much slower for the two-frequency case than for the forty-frequency case.
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