Copyright © A. Filippone (1999-2003). All Rights Reserved.

Aerodynamic Flow Devices


Flow devices are systems used to produce internal flows of specified characteristics. They are a common component of several types of engines and laboratory instrumentation. Here are some of them.

The Shock Tube

The shock tube is a device used to produce very high pressure ratios for a short time at very high Mach numbers. Basically the tube consists in a pipe section with two areas (pressurized gas and vacuum) separated by a diaphragm.

The diaphragm is designed to withstand a certain pressure ratio. When this ratio is exceeded, the high pressure gas ruptures the diaphragm and expands into the low-pressure gas.

The pressure discontinuity creates a strong shock wave. To achieve strong shocks, pressure ratios of may be required. The shock Mach number can be derived from the pressure ratio and the ratio of specific heat.

One problem with the shock tube is that the temperature can be maintained uniform across the shock for only a very short time, which limits the use of the shock tube as a useful high speed device.

Further limitations arise from the length of the tube, since the boundary layer growth behind the shock will tend to interfere with flow uniformity.

Shock Tube

Figure 1: Shock Tube

The Supersonic Nozzle

In its most elementary configuration a nozzle is a tube having a convergent section followed by a divergent section. The discharge flow can be either subsonic or supersonic (Laval nozzle).

For flows effectively accelerated above the speed of sound (choked flow), the sonic speed is reached at the throat (section of minimum area). Elements of nozzle design include size of the convergent section, size of the throat, angle and length of the divergent section.


Figure 2: Convergent-divergent nozzle

Flow in the nozzle can be maintained as long as the discharge pressure is lower than the reservoir pressure. Given this ratio, it is possible to compute both the throat and the discharge speeds. Sonic speed is reached for a critical value of the pressure ratio.

For larger pressure ratios a few phenomena may be observed: the internal flow in the subsonic section is insensitive to pressure changes at discharge; separation and condensation in the divergent section may occur. There is a number of complex flow phenomena.

Selected References

  • Thompson PA. Compressible Fluid Dynamics, McGraw-Hill, 1971.

  • Crocco L. One Dimensional Gas Dynamics, Part B in Fundamentals of Gas Dynamics, Vol III of High Speed Aerodynamics and Jet Propulsion, Princeton Univ. Press, 1958.

  • Campbel AB, Jennings BH. Gasdynamics, McGraw-Hill, 1958.

  • Kuethe AM, Chow CY. Foundations of Aerodynamics,
    John Wiley, New York, 1997 (5th edition).
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Copyright © A. Filippone (1999-2003). All Rights Reserved.