Realization of both high-intensity and large-scale turbulence using a multi-fan wind tunnel

When experiments are conducted on the effects of freestream turbulence on the flow around bluff structures, generation of both high-intensity and large-scale turbulence in a wind tunnel is of major importance. We attempt this by developing a new type of wind tunnel named 'multi-fan wind tunnel' (MFWT), wherein airflow is driven by multiple fans. We focus on the use of shear layers generated between the outflows from adjacent ducts. Herein, we propose an efficient driving method in which an input signal composed of numerous sinusoidal waves is fed to each fan with quasi-random phases ('random-phase mode'). It was confirmed that this mode generates approximately homogeneous turbulence within a short downstream distance. We found that pulsatile components were eliminated, which prevented the development of turbulence in our preceding experiments using the MFWT. The initial rectangular spectrum provided by forcing rapidly changes its shape as turbulence convects downstream, and asymptotes to a typical broad spectrum including a few decades of $-5/3$ power law. The intensity and the integral scale can be controlled by the amplitude of the input signal, and for an approximately isotropic region at large downstream stations, the intensity reaches 12–16% and the integral scale 0.6–0.7 m. Graphical abstract:

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