Abstract: In order to accurately evaluate the mixing phenomenon between tight lattice subchannels, the open-source computational fluid dynamics (CFD) software OpenFOAM2.0 and an explicit geometric Reynolds stress turbulence model based on k-ω were used to simulate periodic large scale vortices. The variation of the periodic large-scale vortex wavelength and the peak frequency among the closely spaced channels are studied. The calculation results showed that the averaged peak frequency of the coherent fluctuations is increased linearly with the increasing of Re number. However, the wavelength can approximately be considered as a constant at different Re, and is only dependent on the geometry. The quasic-periodic large scale vortices cause a significant flow pulsation between tight lattice subchannels, which significantly enhances the turbulent mixing between adjacent subchannels.