It is very difficult to achieve a very high speed in the wind tunnel to match the Reynolds number of model and prototype. Even if we are able to achieve the speed the Mach number would be too high. If we use a pressurized wind tunnel the density of air would be very high there. At the same Reynolds number larger density leads to a lower air speed requirement.

Therefore, a pressurized wind tunnel can achieve higher Reynolds number for the same scale model.

If the air pressure inside the wind tunnel increases by a factor of 1.8 the air density also increases by a factor of 1.8.