Modelling of the operation of the Oxford High Density Tunnel (HDT) was performed to establish the level and duration of test gas heating required in the facility and the thermal structural implications of this heating. 1-D gas dynamic simulations of the transient flow conditions in the facility were performed for a range of starting pressures and tunnel configurations demonstrating potential test times of 80ms at M6 and unit Re = 1.45e8m -1 . Gas-state analysis was used to establish the liquefaction limits for the oxygen and nitrogen components of the air test gas at the outlet of the nozzle to quantify the heater requirements (power and duration) for the facility. The HDT is required to contain high-temperature and high-pressure test gas safely for extended periods to ensure uniform heating and transient FEM simulations showed that the structural and thermal-structural loads on the facility during its heat up and operation were well within the static limits of the facility. FEM was also used to perform transient simulations of the heat loss from the test gas during the experimental runs. CFD simulations of the buoyant mixing were performed to establish the duration of preheating of the test gas required in the facility to ensure gas temperature uniformity prior to operation of the tunnel. This was found to be of the order of minutes.