5.1 A typical scenario
5.2 The physical and mechanical processes considered
5.3 How PHOENICS takes these into account
5.4 A simple example of a blast wave
5.5 A simple two-phase flow
The effects of an explosion are felt in places which the burning
gases never reach, being carried by the pressure wave which these
gases have generated.
When it is strong enough to threaten damage to bodies in its path,
such a pressure wave is called a "blast".
An oil-platform explosion of the kind considered in section 4 can
cause a blast. A bomb can cause an even more violent one. It is
this which will now be considered.
Let it be imagined that a bomb explodes somewhere within an
oil-platform module. How can CFD predict what are likely to be the
consequences?
As far as the fluid mechanics is concerned, the process is somewhat
easier to simulate than that of explosion; for the complication of
flame acceleration is absent.
The CFD code has simply to solve the equations of motion, taking
care howver to ensure that the variations of gas density are
properly taken into account.
However, bombs are designed to cause structural damage; and this may
be such as to cause the creation and projection of missiles.
The simplest example is the shattering of a glass window: when the
pressure difference across it reaches a critical value, the glass
breaks into many fragments. These are then carried by the rush of
air past them; and they may cause secondary damage as they hit other
objects lying in their path.
The same is true, of course, of thin panels of metal, such as the
walls of oil-platform modules.
As already mentioned, PHOENICS is equipped to simulate multi-phase
flows, ie those in which gases, solids and liquids are all in
RELATIVE motion.
The earliest applications of this capability, indeed those which
motivated its creation, arose in the nuclear industry, which had
greatly to be concerned with predicting what happened when steam and
water were both in motion, within the same space but at differing
velocities.
Subsequently, the capability has been much used by the chemical and
petroleum industries, and for the simulation of environmental
phenomena such as sand-storms and avalanches.
For PHOENICS therefore, a shower of glass fragments carried by a
blast-wave wind is much the same as the spray of droplets borne by a
jet of steam which emerges from a ruptured pipe carrying hot
high-pressure water.
The differences lie solely in the properties of the two phases
(especially their densities) and in the quantitative laws governing
the momentum exchanges between them.
The latter exchanges, expressed as "drag coefficients" of the flying
fragments, depend om their shapes and sizes and on the Reynolds
number of their relative motion.
Of course, some guess-work is needed as to what shapes and sizes to
presume. It is best to make both upper- and lower-limit
presumptions, so that best- and worst-case conditions may both be
brought forward for examination.
This picture shows, on a plot of radius (vertical)
versus time (horizontal) the contours of radial
gas velocity consequent on the release of a sphere
of compressed gas, created for example by a bomb.
5.1 A typical scenario
5.2 The physical and mechanical processes considered
5.3 How PHOENICS takes these into account
(a) Related phenomena
(b) Distinguishing features
5.4 A simple example of a blast wave