A numerical simulation of the Navier-Stokes equations is conducted to investigate the intense 1T mode oscillation with a mean flow, and the characteristic features of the acoustic structure are revealed. The compressible perfect-gas Navier-Stokes equations are numerically solved using a finite volume solver on unstructured grids to capture a nonlinear acoustic structure. The viscosity and thermal conductivity are evaluated by the Sutherland law. When the amplitude of the pressure oscillation exceeded approximately 10-20% of the chamber plenum pressure, the nonlinear acoustic structure appeared, as in the case without a mean flow. The highest acoustic-pressure region was located away from the chamber wall, and the positive half-wave of the wall pressure had a twin-peak structure. On the other hand, the effect of a mean flow appeared as an axial movement of the global acoustic structure, which would induce a phase difference in the wall-pressure signal along the chamber axis.