Overview

To achieve a hypersonic airflow on the ground, a big pressure ratio of upstream to downstream is necessary. Therefore, in a hypersonic wind tunnel, a downstream pressure is withdrawn into a vacuum tank or is forcibly exhausted by an ejector, while a nozzle upstream pressure is kept high, to obtain the pressure ratio required for achievement of the hypersonic airflow. In general, the hypersonic airflow is generated by expansion of air from a nozzle. However, in a flow of Mach number > 5, air at room temperature liquefies due to drastically reducing of the airflow temperature caused by air expansion. To prevent this liquefaction, the hypersonic wind tunnel has an air heating facility such as a heat accumulator etc. upstream of the nozzle.