This paper presents a Navier-Stokes based numerical methodology for 3D unsteady viscous flow over a general mirror configuration. In this approach, the Reynolds averaged Navier-Stokes equations in finite volume form are solved unsteadily using Roe’s scheme on body fitted H-H type grid. A two-equation turbulence model was implemented. This method is three or five order accurate in space and first order accurate in time. The physical space modeled is a 120m×120m×120m region enclosing the primary mirror at the center. The inflow turbulence from the enclosure openings is represented by turbulence kinetic energy at the inflow boundary. The wind buffeting effect was studied by directly calculating flow field response to buffeting impinging wind. The unsteady pressure distribution on the mirror is extracted and analyzed for the amplitudes and frequencies of dynamic wind loading. The computational results are visualized to highlight the flow pattern, particularly on the mirror upper surface. Results are presented for a 30-meter aperture GSMT primary mirror. A 1:833 model of the GSMT primary mirror was tested in a water tunnel, in which the velocity distribution was measured using PIV technique. The preliminary experimental observation serves as qualitative validation of the simulation capability.