â€œComputer simulation software - modern oracleâ€. A lecture at the Congress-Exhibition. by. Brian Spalding. 1. Outline of the argument. foresee , and so avoid, dangerous events ; select , from the options which are open, those which best promote happiness and well-being; and
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A lecture at the
«What will happen if .. » is the most important question which a conscious being can ask.
Answering it rightly more times than not is what keeps most of us safe, healthy and reasonably prosperous; for it allows us to:
In essence, all such techniques are the same: examine the past; and if elements of the present are seen there, suppose that what transpired before is likely to happen again.
“When last I pulled the tail of a cat, it scratched me; so, if I do it again, another scratch is what I must expect”.
It is a sound principle.
In ancient times, oracles were consulted on matters of importance, as being best fitted by age, experience or connections
to foresee, what
the past implied about the impending
That too was a sound principle, for those who could afford the oracle’s fees!
How are these principles applied in engineering?
If the task in hand involves little novelty, as
when one more engine is to be built for
an established and satisfactory production
line, simple repetition of past actions is
what the principle dictates.
But when the performance requirements have changed, exceeding what the old engine is capable of, novelty is needed;
and what is new has, by definition, no past to be examined.
What to do?
It is such laws,to which the engineer must turn,whenever actions without precedent are contemplated, in order to answer
“what will happen if ” questions
The reasons for both will be explained.
First, however a single example will be shown.
Though their causes are various (electrical faults, carelessness, arson, ‘spontaneous combustion’), undesired conflagrations are a fact of life,
for which building administrators must prepare by providing means for:
But how can they determine whether their preparations will be adequate?
Certainly they can check them against the requirements laid down in the municipal or state Building Regulations; and their compliance may save them from prosecution even if the preparations failed.
But that will be a poor comfort.
They can do better:
consult a ‘modern oracle’.
However they are started, fires spread in obedience to the general physical laws which were listed above,
as constrained by the particular circumstances in question.
Computational Fluid Dynamics,abbreviated toCFD,is the name which has been given to the body of knowledge and skill which forms the basis of such packages.
Those who participate in its various aspects include mathematicians, computer programmers, physicists and engineers.
Since last 80-ties it has grown into a multi-million dollar industry;
its potential being used particularly for aircraft.
Some pictures will now be displayed whicharise from the solution of a very simple fire-simulation problem.
It is the one which has been prepared for display in the Internet-café of this Exhibition.
Scene of the ‘virtual theatre’
Another man can be easily added to the ‘scene’ by clicking on the appropriate image; here it is the object ‘People’.
One can hide or remove objects
(here several objects disappeared from the scene).
The scene can be viewed from any point of view.
It is possible tomove and rotate objects, and
It is also possible to bring in a third man with a chair for him to sit upon.
Here are some results of calculation.
The next pictures show colour contours of temperature and arrows indicating air motion under normal conditions on
a horizontal and vertical planes.
Finally, an animation is shown of the spread of smoke and flame when combustible material under a chair is suddenly set alight.
The answer is three-fold.
First, computers are still not powerful enough; for CFD simulations proceed by representing what is in reality continuous with a discontinuous near-equivalent.
In this example the computational grid was regarded asan assembly of one hundred thousand imaginary boxes, in each of which conditions were treated as being uniform.
This is too coarse a grid to represent the solid objects or fluids.
If a hundred million boxes had been used, the boxes would still be of the order of one centimetre in length, width and height; and so hardly small enough to represent, for example, the leg of a burning chair.
Only the largest computer clusters in the world would have been able to handle so many; and the computation would take far too long for its outcome to remain of interest.
Secondly, although indeed chemists have accumulated immense amounts of information about how combustion occurs, the information is too immense to be useful.
Engineers like to think that fuel and oxygen combining form ‘combustion-product’ gases but
chemists have discovered thata great many of intermediate productsare formed in combustion, as the scheme shows.
Therefore, engineers create simplified combustion models and their accuracy always raises doubts.
Among intermediate products are those carbon-containing particles which we call ‘smoke’.
Its presence has a great influence on the intensity of radiative heat transfer,
which, in turn, has a great effect on the rate with which fire spreads.
As if this were not enough difficulty to contend with, it has also to be admitted that, chemists have mainly confined their researches to the behaviour of pure substances.
Consequently, even if one could compute with accuracy the intensity of the radiation reaching the curtains, the chemical literature contains nothing from which one could compute the speed with which the fabric would burst into flames.
Thirdly, chemistry and radiation apart, even the fluid-flow aspects of the simulation are, in most circumstances, subject to doubt; and the reason is: turbulence.
The smoke from a chimney, blown by the wind, although it certainly moves mainly in the wind direction, also exhibits seemingly random motions at right angles to it.
Such randomness, which is called ‘turbulence’, pervades all flows, whether of gases or of liquids when their velocity is such as to make inertia forces exceed viscous ones. (This is the so-called Reynolds-number criterion.)
Although turbulence has been much studied, and is represented to some extent in the CFD packages, none of those representations are known to correspond with reality in all circumstances.
This regrettable fact seems likely to remain until some Newton reduces chaos to order.
Until then, all CFD predictions of turbulent flows must be regarded as no more than probable forecasts of
“what will happen if.. ”.
When chemical reaction and two-phase effects are present (as they are when water from sprinklers interact with burning gases), the margin for error widens.
What is to be done?
The optimists and those who make their living by selling CFD packages and services based on them, find it easy to be impressed by the plausible-seemingattractively-coloured images which the packages produce.
They look realistic; and often packages from different vendors give results which are qualitatively and even quantitatively similar.
The second fact especially easily dismisses doubts in the accuracy of the results obtained.
The pessimistsargue that the agreement between the packages from different vendors means nothing; for all use the same dubious models of turbulence, etc, and all are compelled to use far-too-coarse grids.
They are mainly influenced by the above arguments on the inaccuracy of models.
Aristotle’s advice is here appropriate:
The new oracles demand their sacrifices.
What are they?
1. The software
In the first two decades of the CFD
industry, licences to use the software
packages could be sold for tens or even
hundreds of thousands of dollars.
Nowadays their price is hundreds times less.
There isno obstacle to purchase them by industrial organizations.
A minor impediment to academic ones still remains.
Students can usually acquire low- or zero-cost versions … and not only by ‘pirating’.
2. The hardware
Nowadays hardware costs
have decreased dramatically;
so much so that, were
‘parallel-computing’ more widely
promoted by the CFD vendors and
its merits appreciated by
customers, many of the latter
would acquire clusters of
computers, using thus more adequate computational grids.
Another possibility is to use remote clusters via Internet paying for actual services just as we pay for piped gas or water.
In this case it is only a laptop that a user needs to solve great multifactor problems.
3. The personnel
Nowadays, therefore, it is the cost of hiring
CFD-literate personnel which is the most
serious impediment to the extension of computer
However, although suitably competent personnel are indeed in short supply, that is in part because what that competence comprises has not been adequately defined.
Consider the situation of a Building-Licensing authority which, having been impressed, by the demonstration of section 2 of this lecture, decides:
“Yes, it’s good. We must have it. Let’s hire some staff.”
What kind of staff should it hire?
Not, I would suggest, a person who has just
completed a PhD study concerned with turbulence or
radiative heat transfer modelling.
Such specialists are expensive; but their prolonged specialisation is likely to cause them to over-estimate the importance of one small aspect of the subject, and to lack well-balanced common-sense.
Intense enthusiasts for a particular software package are also to be employed only with caution.
Such a person seeing the animation of the fire in a computer room, would say:
He would be right and his doubts would impel him to search for, and find
the human error which causes the computer to produce unrealistic simulations.
Once the error was corrected, the simulations became more reasonable, thus:
But human error must always be suspected and guarded against.
There are some persons for whom every novelty is bad per se.
It is not necessary to consider their opinions.
Others may adduce the expense of introducing computer simulation as a teaching tool; and it is not negligible because first it is the teachers who will have to be taught.
However, the most-frequently-propagated negative view is that computer simulation produces such attractive and plausible-seeming output that students will be persuaded to believe that they correspond with reality.
That is perhaps possible; but only if the teachers are themselves naive, having not been well-taught.
Its use seems certain to continue to grow.
It is therefore obvious that with the framework of the «Global Education» project the educational systems of the world should prepare their students to participate and to contribute to it.