Editing Wind tunnel (section)

====High Reynolds number tunnels====
[[Reynolds number]] is one of the governing similarity parameters for the simulation of flow in a wind tunnel. For [[mach number]] less than 0.3, it is the primary parameter that governs the flow characteristics. There are three main ways to simulate high Reynolds number, since it is not practical to obtain full scale Reynolds number by use of a full scale vehicle.{{citation needed|date=April 2022}}
* Pressurised tunnels: Test gases are pressurised to increase the Reynolds number.
* Heavy gas tunnels: Heavier gases like [[freon]] and [[R-134a]] are used as test gases. The transonic dynamics tunnel at [[NASA]] Langley is an example of such a tunnel.
* Cryogenic tunnels: Test gas is cooled down to increase the Reynolds number. The [[European transonic wind tunnel]] uses this technique.
* High-altitude tunnels: These are designed to test the effects of shock waves against various aircraft shapes in near vacuum.  In 1952 the University of California constructed the first two high-altitude wind tunnels: one for testing objects at {{convert|50|to|70|mi|km}} above the earth and the second for tests at {{convert|80|to|200|mi|km}} above the earth.<ref>{{Citation|url=https://books.google.com/books?id=8dwDAAAAMBAJ&pg=PA105|title= "Windless Wind Tunnels for High Altitude Tests."|date= February 1952|publisher= Hearst Magazines}}</ref>