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The flow simulation is effective and useful method for fan design and analysis. Though it is difficult to use it for the sound source analysis. It needs a lot of simulation time and data to analyze the fluctuating characteristics of whole fan flow. We proposed the one pitch unsteady simulation for the turbulent noise characteristics of the multi-blades fan. It is used the circumferential variation to inlet boundary condition that is obtained from the results of steady state analysis of fan. In this report, it was investigated the influence of the inlet boundary condition of one pitch blade simulation on the unsteady flow characteristics that the three types of inlet boundary conditions are examined, (1) constant inflow velocity with time, (2) variation of inflow velocity with time but the constant velocity with circumference and (3) variation of inflow velocity with time and circumference. It makes clear that condition (1) is difficult to simulate the unsteady flow around blade, but the condition (2) and (3) is almost same and simulate the unsteady flow well. The flow separation at the leading edge is occurred in all conditions, but the flow after the separation is different in condition (1) compared to (2) and (3). The pressure fluctuations on the blade surface in condition (2) and (3) is well indicated the characteristics of the sound sources that the main location of the broad band noise is located to the end of scroll., 長崎大学大学院工学研究科研究報告, 50(95), pp.1-6; 2020