Turbulent air flow through subchannels is simulated to obtain characteristic frequency on large scale flow pulsation in square arrayed 6-rod bundles. The turbulent mixing between the subchannels is dominated by the large scale flow pulsation phenomenon. Therefore, it is crucial to understand flow patterns in the subchannels for the design of reliable nuclear reactor elements. Numerical simulation using a CFD code is performed by solving the unsteady equations with large eddy simulation (LES) model. For the numerical simulation, two kinds of grid including two subchannels are created. Models with two different P/D ratios (P/D=1.08, 1.35) are simulated. P/D=1.08 case is our main concern to investigate flows in tight lattice rod buddle. The shortened channel length with periodic boundary condition is employed to reduce the computing time. The geometry of the simulated model is based on the 3-D experimental equipment. The velocity fluctuation data in the stream-wise and cross directions are obtained from the simulated model to find the characteristic frequency. The large scale flow pulsations are explained by time history of velocity, Fast Fourier Transform analysis and cross-correlation analysis. The fluctuation frequency at the center of gap and 20 mm away from that point are obtained. The characteristic frequency is 87.5 Hz through the entire monitoring points. Results demonstrate that the periodic flow pulsations occur with small pitch-to-diameter ratio (P/D=1.08) and the vortices trains are highly synchronized. There is no large scale pulsation in P/D=1.35 case.