Tight lattice fuel bundle is widely adopted in the small reactor design. Large scale, quasi-periodical flow pulsation exists in the gap region between fuel rods, which strongly increases the heat transfer between fuel rods and coolant. This phenomenon is tightly attributed to the Reynold Number (Re) and pitch-to-diameter ratio (P/D). However, it is still not possible to predict the occurrence of flow pulsation under given conditions. In this work, the Unsteady Reynolds Averaged Navier-Stokes (URANS) methodology is used to establish guidelines to predict the occurrence of large scale vortex structure. The simulation results are in satisfactory agreement with experimental data. A Strouhal number (Sr) threshold is found, below which no actual flow pulsation phenomenon occurs. One model for estimating the threshold value via Re and P/D is established, from which one can predict the occurrence of flow pulsation.