Relative importance of initial conditions on outflows from multiple fans

Generation of homogeneous isotropic turbulence was attempted using an innovative "multifan wind tunnel" with 99 fans installed. The driving method used is based on a principle that the shear layers generated between outflows from the adjacent ducts lead to turbulent flow downstream. First, a signal composed of two frequency components is set, and then it is fed to all the fans for three kinds of arrangements of phases. Here, parameter N is introduced as the number of phases used for the 99 fans, which represents a variety of emanated shear layers. Furthermore, S is introduced as a measure of shear magnitude at the inlet of the test section. Relative importance of the initial conditions (N and S) in the development of turbulence was investigated. To estimate the contribution from naturally induced turbulence, we numerically decomposed the resulting velocity fluctuations into the periodic and nonperiodic component. Energy spectra for three values of N were calculated using nonperiodic data. The inertial subrange of a gradient of -5/3 widens with increasing N. The value S is the largest for N=2, but the turbulence intensity of the nonperiodic component is the largest for N=99. Hence, it might be suggested that the shear magnitude at the inlet of the test section is not as important as the variety of shear layers for effective generation of high-Reynolds-number turbulence.