Ludwieg tube
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A Ludwieg tube is a cheap and efficient way of producing supersonic flow. Mach numbers up to 4 are easily obtained without any additional heating of the flow.
[edit] Principle
A Ludwieg tube is a wind tunnel that produces supersonic flow for short periods of time. A large evacuated dump tank is separated from the downstream end of a convergent-divergent nozzle by a diaphragm. The upstream end of the nozzle connects to a long cylindrical tube, whose cross-sectional area is significantly larger than the throat area of the nozzle. Initially, the pressure in the nozzle and tube is high. To start the tunnel, the diaphragm is ruptured, e.g., by piercing it with a suitable cutting device. As always when a diaphragm ruptures, a shock wave propagates into the low-pressure region (here dump tank) and an expansion wave propagates into the high-pressure region (here the nozzle and long tube). As this unsteady expansion propagates through the long tube, it sets up a steady subsonic flow toward the nozzle, which is accelerated by the convergent-divergent nozzle to a supersonic condition. The flow is steady until the expansion, having been reflected from the far end of the tube, arrives at the nozzle again. For practical reasons, flow times are on the order of 100 milliseconds. For many purposes this is a flow duration that is quite sufficient.
[edit] History
The Ludwieg Tube was invented by Hubert Ludwieg (1912-2000) in 1955 in response to a competition for a transonic or supersonic wind tunnel design that would be capable of producing high Reynolds number at low operating cost. Professor Ludwieg was also responsible for the experimental demonstration and explanation of the large effect of sweep on the drag of transonic wings (his dissertation in 1937).
