Unsteady flow structures unrelated to rotating frequency in the turbine wheel space cavity has been observed and reported in a number of recent rim sealing investigations. These flow structures are relatively large in scale and have a significant influence on the sealing effectiveness prediction. As a result, it is important to capture these flow structures in numerical simulation. Small computation sectors, due to the circumferential symmetry assumption, have been proved to fail to capture these flow structures. This paper aims to find a minimum computation sector size that can capture these flow structures, at the same time save computation resources and shorten the convergence process for a simple axial rim seal. Four different sector model (10, 20, 30, 180-degree) are set into simulation using RANS and URANS method. The steady and unsteady simulation results are compared. By comparison, the 20-degree sector model is considered appropriate to conduct successive investigations. Then the 20-degree model is set into unsteady simulation under four different sealing flow rates c(w) = 0 (non-sealing flow case), c(w) = 2500, c(w) = 5000, c(w )= 10000). It was found that due to the large-scale flow structure, a staggering pressure distribution is found in the cavity. Increasing the sealing flow rate diminishes these structures and stabilizes the flow in the wheel space cavity. The staggering pressure distribution causes the sealing effectiveness to show an abnormal variation trend. Unsteady pressure oscillation waves at two different circumferential positions are subjected to cross-correlation analysis, by which the rotating speed and number of the flow structure could be calculated.