March 25-28,

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Computer simulation software -modern oracle

• A lecture at the
• Congress-Exhibition

by
Brian Spalding
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1. Outline of the argument
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

• foresee, and so avoid, dangerous events
• select, from the options which are open, those
  which best promote happiness and well-being and
• create opportunities which never before existed.

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1. Outline of the argument

• Society seeks, through education,
• to inculcate in the young the habit of asking
  this question
• and to convey how the right answers can be
  arrived at,
• which means to teach to all techniques of
  prediction.

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.
Thus 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.
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1. Outline of the argument
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 future.
That too was a sound principle, for those who
could afford the oracles fees!
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1. Outline of the argument
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?
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1. Outline of the argument

• There is however a more general encapsulation of
  the past, which we call science
• and for engineers it takes the form of
• laws of conservation of mass, momentum and
  energy

Lomonosov Newton
Joule
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1. Outline of the argument

• laws of transport of those same entities by
• diffusion, viscous action and heat conduction
  

Fick Newton
Fourier Hooke

• laws of deformation of solids in response to
  mechanical and thermal stresses (Hooke)

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1. Outline of the argument

• laws governing rates of chemical
  transformation, and electrical and magnetic
  interactions.

Arrhenius
Faraday
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
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1. Outline of the argument

• The present lecture explains
• how simulation-by-computer has become the
  engineers favoured prediction technique, and
• how specialised software packages have become
  the oracles which they consult.

• Two things the modern oracles share with the
  ancient ones
• they cost money (or sheep, oxen or other
  currency) and
• their pronouncements are never 100 reliable.

The reasons for both will be explained. First,
however a single example will be shown.
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2. Example What happens if a fire starts in a
building?
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

• extinguishment (e.g. sprinklers, foam
  canisters)

• prevention of spread (e.g. fire doors) and
• escape for personnel and livestock.

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2. Example What happens if a fire starts in a
building?
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. Why? 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.
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2. Example What happens if a fire starts in a
building?

• The relevant circumstances include
• the locations and thickness of the walls which
  impede the flow of air
• the locations and sizes of the apertures
  (doors, windows, air vents) which allow air (and
  smoke and flames) to pass through
• the amount and location of combustible
  material, not only that in the source of the
  fire, but also that in furniture and furnishings
  (e.g. chairs, tables, curtains)
• the positions of water sprinklers, and the
  flow rates of liquid through them and
• external conditions such as the strength and
  direction of the wind and, if it arrives in time,
  the water injected by the firemens hoses.

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2. Example What happens if a fire starts in a
building?

• During the last thirty years computer-software
  packages have been created which embody both the
  general physical laws and (templates of) the
  particular circumstances.
• Their users can, in effect
• open a store cupboard containing
• fires,
• walls,
• windows,
• tables,
• sprinklers, etc.
• place them on a stage, in appropriate
  relative positions
• declare which physical laws are to be obeyed
  and then
• observe, as though in a theatre, how the
  play develops.
• The users have supplied the if then the
  package pronounces upon
• the what will happen , just as did the
  ancient oracles.

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2. Example What happens if a fire starts in a
building?
Computational Fluid Dynamics, abbreviated to CFD,
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.
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2. Example What happens if a fire starts in a
building?
Some pictures will now be displayed which arise
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.

• The pictures show the scenario which is to be
  studied, namely
• an office containing a standing man, some
  desks and some computers.
• The store-cupboard from which chairs, desks
  and other objects can be extracted is visible
  near the top of the picture .

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2. Example What happens if a fire starts in a
building?
Store-cupboard
Scene of the virtual theatre
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2. Example What happens if a fire starts in a
building?
Another man can be easily added to the scene by
clicking on the appropriate image here it is
the object People.
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2. Example What happens if a fire starts in a
building?
One can hide or remove objects (here several
objects disappeared from the scene).
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2. Example What happens if a fire starts in a
building?
The scene can be viewed from any point of view.
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2. Example What happens if a fire starts in a
building?
It is possible to move and rotate objects, and
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2. Example What happens if a fire starts in a
building?
It is also possible to bring in a third man with
a chair for him to sit upon.
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2. Example What happens if a fire starts in a
building?
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.
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2. Example What happens if a fire starts in a
building?
Finally, an animation is shown of the spread of
smoke and flame when combustible material under a
chair is suddenly set alight.
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2. Example What happens if a fire starts in a
building?

• This rather trivial simulation was created in
  such a way that it could be executed in a few
  minutes on a lap-top computer.
• In engineering practice, of course, situations of
  much greater magnitude are in question and CFD
  simulations may take many hours of computation on
  powerful computer clusters working in parallel.
• Such simulations are routinely
• conducted for
• multi-storey car parks

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2. Example What happens if a fire starts in a
building?

• super-markets

• concert-halls

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2. Example What happens if a fire starts in a
building?

• under-ground railway stations
• tunnels
• airplanes and
• ocean liners.

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2. Example What happens if a fire starts in a
building?

• In many countries such computer simulations are
  already the necessary intermediate stage for
  every application for a licence to build.
• However, the licensing authorities need to be
  appropriately educated.
• They must know that
• such simulations can be made
• the expense of making them, although not
  negligible, is tiny in comparison with the cost
  of allowing an unsafe building to be erected and
• that, although the simulations cannot be
  relied upon to predict
• with certainty what will happen if.. ,
  enough tests have been made to persuade even
  sceptics they are good indicators of what is
  probable.

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3. Why the Oracles are not completely reliable

• Since
• the quantitative laws of Newton, Joule, Hooke
  and Fourier have stood the test of time,
• computers are becoming increasingly more
  powerful, and
• physicists and chemists have accumulated
  extensive knowledge of the properties of
  materials,
• it is natural to ask
• why it should be necessary to issue the warning
  that CFD can predict only what is probable, not
  what is certain?

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3. Why the Oracles are not completely reliable
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 as an assembly of one hundred thousand
imaginary boxes, in each of which conditions were
treated as being uniform.
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3. Why the Oracles are not completely reliable
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.
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3. Why the Oracles are not completely reliable
Secondly, although indeed chemists have
accumulated immense amounts of information about
how combustion occurs, the information is too
immense to be useful .
Why?
Engineers like to think that fuel and oxygen
combining form combustion-product gases but
chemists have discovered that a great many of
intermediate products are formed in combustion,
as the scheme shows.
Therefore, engineers create simplified
combustion models and their accuracy always
raises doubts.
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3. Why the Oracles are not completely reliable
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.
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3. Why the Oracles are not completely reliable
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.)
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3. Why the Oracles are not completely reliable
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.
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3. Why the Oracles are not completely reliable
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-seeming
attractively-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.
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3. Why the Oracles are not completely reliable
The pessimists argue 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.
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3. Why the Oracles are not completely reliable
Aristotles advice is here appropriate

• The best lies between the extremes.
• It entails recognising that
• the CFD-based predictions are no more than
  indicators of probability but
• they are immensely better than the mere
  guess-work which is mankinds only alternative.

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4. The cost of CFD
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 is no
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.
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4. The cost of CFD
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.
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4. The cost of CFD
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 simulation. 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, its good. We must
have it. Lets hire some staff.
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4. The cost of CFD
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.
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4. The cost of CFD

• Better are those,
• who have experience of several packages,
• who recognise that most of their claims to
  superiority are ill-founded, and
• who understand the limitations from which they
  all suffer.
• Best are those who are pragmatically sceptical
• who can extract value from order-of magnitude
  predictions,
• who have so strong a grasp of physical reality
  that, when the occasion arises, they can conclude
  that
• A particular computer simulation simply must be
  wrong.

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4. The cost of CFD

• Such a person seeing the animation of the fire in
  a computer room, would say
• Why does the hot air not rise vertically from
  the burning chair?
• Surely the arrows appearing above the man are
  too big!
• I can not believe that simulation.
• 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.

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4. The cost of CFD

• Once the error was corrected, the simulations
  became more reasonable, thus

But human error must always be suspected and
guarded against.
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5. Should computer simulation be used in
education?

• The above question is the most important one for
  our Congress.
• Arguments in favour
• computer-simulation packages do embody
  physical principles which all students should
  study
• they are cheap to obtain and to run
• they convey a sense of immediacy akin to that
  of hand-on experimentation
• their use allows some burdensome topics to be
  omitted from the teaching syllabus.

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5. Should computer simulation be used in
education?
Arguments 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.
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6. Concluding remarks

• The use of CFD has been increasing steadily in
  the last three decades for the design of
• vehicles for land, sea and air travel
• their engines, and those of stationary power
  plants
• chemical and nuclear reactors
• gas and oil pipelines and the pumps and
  compressors connected with them
• kinds of electrical equipment including
  computers themselves,
• and in many other directions.

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.