Encyclopaedia Index

5. CFD applied to explosion or blast damage and missile projection

    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

5.1 A typical scenario

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?

5.2 The physical and mechanical processes considered

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.

5.3 How PHOENICS takes these into account

(a) Related phenomena

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.

(b) Distinguishing features

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.

5.4 A simple example of a blast wave

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.

The high outward velocities at the start (on the left) are followed by almost equally high inward velocities (the blue region)

5.5 A simple two-phase flow

PHOENICS in fact possesses two methods for handling multi-phase flows: namely the two-continua method (called IPSA, which stands for Inter-Phase Slip Algorithm) and a particle-tracking method (called GENTRA, for GENeral TRacking Algorithm).

IPSA having already been demonstrated, in connexion with the transition to detonation, the opportunity is here taken to demonstrate GENTRA.

The case shown is not related to blast waves but to an industrial spray-drying process. This has been chosen so as to exemplify how EXPLOITS, through its attachment to PHOENICS, can draw on experiences from (what may seem like) remote fields of engineering.

Gas flowfield

Particle trajectories