Pressure Tunnels Completely sealed closed-circuit tunnels, which can be pressurized or partly evacuated, can be used to economize on power. If we consider two tunnels of the same size, one using atmospheric air and the other air at n times atmospheric pressure, we see that the power required by the latter to produce a given Reynolds number is n -2 times the power required by the former, since is proportional to absolute pressure. The first pressure tunnel was the NASA Langley Research Center's Variable Density Tunnel, now retired. Most other pressure tunnels are of conventional closed-circuit design, with the tunnel walls bearing the full pressure difference. Near the boiling point, gas properties, especially the ratio of specific heats, behave oddly.
Pressure increases the boiling point so a high-pressure cryo.
Modern pressure tunnels and cryo. Cooling or pressurizing takes a lot of power. Again the disadvantage is that the ratio of specific heats is no longer 1.4 so compressible-flow results may not be valid for air flows. Most companies or universities concerned with high-speed flows have supersonic tunnels, often of small size. The power required to bring the tunnel up to supersonic speed is greater than that required for continuous running at low supersonic Mach numbers, because when the flow through the throat is just subsonic it decelerates again downstream of the throat, the boundary layers separate from the diverging walls.