Aerodynamic Flow Devices
Summary
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 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
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.
Figure 1: Shock Tube
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.
The Shock Tube
may be required. The shock Mach number
can be derived from the pressure ratio and the ratio of specific heat.
The Supersonic Nozzle
Selected References
John Wiley, New York, 1997 (5th edition).