A Ludwieg tube is a cheap and efficient way of producing
supersonic
Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
flow. Mach numbers up to 4 in air are easily obtained without any additional heating of the flow. With heating, Mach numbers of up to 11 can be reached.
Principle
A Ludwieg tube is a
wind tunnel
Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...
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
A de Laval nozzle (or convergent-divergent nozzle, CD nozzle or con-di nozzle) is a tube which is pinched in the middle, making a carefully balanced, asymmetric hourglass shape. It is used to accelerate a compressible fluid to supersonic speeds ...
by a diaphragm or fast acting valve. 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, or opening the valve respectively. As always when a diaphragm ruptures, a
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
propagates into the low-pressure region (here the dump tank) and an
expansion wave
Expansion may refer to:
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* ''L'Expansion'', a French monthly business magazine
* ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004
* ''Expansions'' (McCoy Tyner album), 1970
* ''Expansio ...
propagates into the high-pressure region (here the nozzle and the 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 about 100 milliseconds for most Ludwieg tubes. For many purposes, this flow duration is sufficient. However, by taking advantage of multiple quasi-static flows between expansion wave reflections, experimentation times of up to 6 seconds can be achieved.
Facilities and Instrumentation for Hypersonic Measurements of Transition Mechanisms at Purdue University
{{webarchive, url=https://web.archive.org/web/20070117151651/http://cobweb.ecn.purdue.edu/~aae519/BAM6QT-Mach-6-tunnel/summary-2006.pdf , date=2007-01-17
History
The Ludwieg tube was invented by Hubert Ludwieg (1912-2000) in 1955 in response to a competition for a transonic
Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach number, but transonic ...
or supersonic wind tunnel design that would be capable of producing high Reynolds number
In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...
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).
See also
*Shock tube
: ''For the pyrotechnic initiator, see Shock tube detonator''
The shock tube is an instrument used to replicate and direct blast waves at a sensor or a model in order to simulate actual explosions and their effects, usually on a smaller scale. ...
* Supersonic wind tunnel
*Hypersonic wind tunnel
A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section, thus simulating the typical flow features of this flow regime - including compression shocks and pronounced boundary layer effects, entropy layer an ...
References
External links
Ludwieg Tube Laboratory at the California Institute of Technology
Operation of a transonic Ludwieg tunnel(Video)
Fluid dynamics
Aerodynamics
Wind tunnels