The preservation of the excellent image quality of the 'Roque de los Muchachos' Observatory (ORM) is one of the most important design criteria for the enclosure of the future large telescope. Additionally, the cost of a large telescope and its instrumentation is so high that it would be regrettable if its own enclosure deteriorates the telescope performance for astronomical observations. For these reasons, the IAC and CEANI are conducting an aero- and thermo-dynamical study based on numerical simulations, attempting to minimize the seeing degradation due to factors within the control of the designers. This paper first describes the general concept of the enclosure we are planning for the future 'Gran Telescopio Canarias' (GTC) and second the set of three dimensional simulations we are conducting to evaluate the effect on seeing of four different enclosure topologies located at two possible sites.