Ludwieg-Rohr
A Ludwieg pipe is a cost-effective and simple test setup to produce supersonic and hypersonic flows . Mach numbers up to 4 can be reliably achieved without additional heating; Mach numbers of around 11 can be achieved by preheating the expansion gases.
Working method
A Ludwieg pipe is a wind tunnel that enables short-term measurements in supersonic or hypersonic flows. A large evacuated pressure vessel, separated by a fast-switching valve or a diaphragm, is connected behind a convergent-divergent nozzle (arrangement in front of the nozzle is also possible). On the other side of the nozzle there is a long cylindrical tube, the cross section of which is significantly larger than the cross section of the nozzle. To start up, the pressure in the expansion pipe is increased and - if necessary - the gas is heated. At the start of the measurement, the valve is opened suddenly or the diaphragm (mechanically by a pointed body) is destroyed. This creates a shock wave in the vacuum tank and a dilution wave runs into the expansion tube. While a subsonic flow develops in the pressure accumulator (expansion pipe) in the direction of the nozzle, the gas there is accelerated into the supersonic. The resulting flow is stationary until the expansion fan is reflected from the rear end of the expansion tube and reaches the nozzle. Typical measurement times are on the order of 100 milliseconds, which is sufficient for most measurements.
history
The Ludwieg tube was invented by Hubert Ludwieg (1912–2000) in 1955. He developed it as a contribution to a competition to develop the design of a transonic or supersonic channel that could achieve high Reynolds numbers at low operating costs. Professor Ludwieg demonstrated and also explained the strong influence of the sweep of wings in the transonic range on the flow resistance (dissertation from 1937).