Editing Compressible flow (section)

==Applications==

===Supersonic wind tunnels===
[[Supersonic wind tunnel]]s are used for testing and research in supersonic flows, approximately over the Mach number range of 1.2 to 5. The operating principle behind the wind tunnel is that a large pressure difference is maintained upstream to downstream, driving the flow.

[[File:SWT alt.JPG|thumb|center|600px|Supersonic wind tunnel classification list]]

Wind tunnels can be divided into two categories: continuous-operating and intermittent-operating wind tunnels. Continuous operating supersonic wind tunnels require an independent electrical power source that drastically increases with the size of the test section. Intermittent supersonic wind tunnels are less expensive in that they store electrical energy over an extended period of time, then discharge the energy over a series of brief tests. The difference between these two is analogous to the comparison between a battery and a capacitor.

{|style="margin: 0 auto;"
|[[File:Supersonic-en.svg|thumb|center|Blowdown supersonic wind tunnel schematic]]
|[[File:Langley hypersonic wind tunnels.jpg|thumb|none|Part of the Langley hypersonic wind tunnel complex in 1969.]]
|}

Blowdown type supersonic wind tunnels offer high Reynolds number, a small storage tank, and readily available dry air. However, they cause a high pressure hazard, result in difficulty holding a constant stagnation pressure, and are noisy during operation.

Indraft supersonic wind tunnels are not associated with a pressure hazard, allow a constant stagnation pressure, and are relatively quiet. Unfortunately, they have a limited range for the Reynolds number of the flow and require a large vacuum tank.

There is no dispute that knowledge is gained through research and testing in supersonic wind tunnels; however, the facilities often require vast amounts of power to maintain the large pressure ratios needed for testing conditions. For example, [[Arnold Engineering Development Complex]] has the largest supersonic wind tunnel in the world and requires the power required to light a small city for operation. For this reason, large wind tunnels are becoming less common at universities.

===Supersonic aircraft inlets===
Perhaps the most common requirement for oblique shocks is in supersonic aircraft inlets for speeds greater than about Mach 2 (the F-16 has a maximum speed of Mach 2 but doesn't need an oblique shock intake). One purpose of the inlet is to minimize losses across the shocks as the incoming supersonic air slows down to subsonic before it enters the turbojet engine. This is accomplished with one or more oblique shocks followed by a very weak normal shock, with an upstream Mach number usually less than 1.4. The airflow through the intake has to be managed correctly over a wide speed range from zero to its maximum supersonic speed. This is done by varying the position of the intake surfaces.

Although variable geometry is required to achieve acceptable performance from take-off to speeds exceeding Mach 2 there is no one method to achieve it. For example, for a maximum speed of about Mach 3, the [[XB-70]] used rectangular inlets with adjustable ramps and the [[SR-71]] used circular inlets with adjustable [[inlet cone]].
{|style="margin: 0 auto;"
|[[File:North American XB-70 above runway ECN-792.jpg|thumb|left|XB-70 rectangular inlets with ramps (not visible)]]
|[[File:Lockheed SR-71 Blackbird.jpg|thumb|SR-71 round inlets with inlet cones]]
|}