Acoustic impedance of resonators or perforated plates with a backing cavity is affected by sound pressure level and flow velocity. To understand why they change the acoustic impedance, we investigated the flow field around a resonator by used of numerical fluid analysis. We used a quasi-three-dimensional model of a resonator in the flow duct. The sound and the grazing flow enter from an inlet of the flow duct. The frequency was set to the resonance frequency of the resonator and the sound incident pressure was varied at 3 levels. The grazing flow velocity was set to various values. The non-reflection boundary conditions for pressure fluctuation were applied at both ends of the flow duct. We used pressure fluctuation at two points to calculate the acoustic impedance: One is at the end of the resonator and the other is at the point in a flow duct near the mouth of the resonator. When the grazing flow is slow, the resistance, real part of the acoustic impedance, depends on incident sound pressure. As the incident sound pressure increases, the resistance increases. When the grazing flow is high, the resistance depends on incident grazing flow. As the grazing flow increases, the resistance increases. These results are qualitatively consistent with the experimental results.