Contemporary design of large telescopes requires optimization of the telescope environment in order to fully realize the capabilities of advanced mirror technology, telescope control, and instrumentation. Telescope enclosure design is a critical element in providing this optimized environment. Enclosures must protect the telescope while minimizing the local degradation of image quality, and the large cost of such structures requires that a successful design be in place before construction begins. In order to test various enclosure designs, three-dimensional nonlinear hydrodynamic calculations have been carried out to determine the flow of air within and around proposed enclosure configurations. Such calculations can test the effectiveness of dome venting, evaluate the dynamic pressures that cause possible deformation of primary mirror surfaces and structural windshake, and isolate sources of turbulent flow that may cause image degradation. Results are presented from a series of calculations that investigated characteristic flows in the Gemini 8-meter enclosure and around its associated primary mirror cell. In general the enclosure design is found to meet its overall design goals. Good dome venting is achieved under a variety of conditions, yet the telescope structure is kept in a low wind environment.