(a) CAD-1; computer-aided drawing and display
In the last few decades, the use of software packages by engineers
and architects has become commonplace. The intense commercial
competition between package vendors, aided by great advances in
computer-graphics software has allowed even the poorest to acquire
easy-to-use packages for drawing and display.
The term CAD, which is used to describe these packages, is commonly regarded as an acronym for Computer-Aided Design. Yet the D is better regarded as standing for Drawing, or Display; for Design involves more than these, namely Decision-making, based on the systematic evaluation of alternatives.
This is why, in this review of current capabilities, it is useful to distinguish CAD-1 (drawing) from CAD-2 (design). Capabilities are more satisfactory in respect of the former than of the latter.
Many fully-sufficient packages also exist for computing the mechanical stresses in solid objects which come into (virtual) existence as a consequence of CAD-1 activities.
Some of the available packages combine both CAD-1 and CASA capabilities, to the great convenience of their users.
The CASA methods, it may be remarked, nearly all make use of the so-called finite-element techniques; and this sets them apart from the so-called finite-volume methods most commonly employed for fluid- flow simulations. This has led to some of the practical difficulties which will be referred to below.
Nevertheless, if calculations of the stresses in solids are all that are required, the availability of the relevant computer software must be regarded as rather satisfactory.
In many branches of engineering, calculations of fluid-flow phenomena and effects also require to be made. The subject of computational fluid dynamics has come into existence to meet this need.
Software packages have been created, and are in widespread use, which seek to satisfy this requirement. These fall into two categories, namely the general-purpose codes, of which PHOENICS was the first, and the special-purpose codes such as those dealing with the less general phenomena occurring in turbomachinery (eg VISIUN,) electronics cooling (eg FLOTHERM), power engineering, and environmental flows. Several of the general-purpose codes (PHOENICS is one) also have special- purpose manifestations (for example PHOENICS-HOTBOX for electronics- cooling simulations).
The satisfaction given by these codes to their users is lower than that given by CAD-1 and CASA packages to theirs, because:-
Even when some validating evidence can be provided, the question to be asked is: were the circumstances of the validation sufficiently close to those for which the model is now to be used?
Sceptics say CFD stands for Colourful Fluid Dynamics; stronger critics use the words: Cheats, Frauds and Deceivers.
If CAD-2 connotes computer-aided design in the extended sense mentioned in (a) above, it must be stated that it remains as an aspiration rather than a widely-practised activity.
Engineers proceed from design-object description (CAD-1) to analysis (CASA and/or CFD); they then make changes to the shapes, materials, loading, etc, of their to-be-designed objects; then THEY examine and assess the results of the analysis(es); then again THEY repeat the analysis once more.
This human-in-the-loop cycle is repeated until optimal results (ie design-object performance) are attained, or until time, money or patience run out.
CAD-2 happens when the human takes himself out of the loop, having instructed the computer, at the end of one cycle, to change shapes, materials, etc until the conditions for optimum performance have been established. This is the ultimate goal of CAE specialists.
(a) Ease of use of CFD software
In the foregoing overview, it is CFD software which has been pointed out as presenting the greatest ease-of-use difficulties.
Understandably, it is the more powerful CFD codes which present the greatest difficulties; for the user of a code possessing many turbulence models, for example, has more decisions to make than the user of a code possessing only one, or even none.
Adequate documentation, and usually-optimal default settings, can diminish the user's difficulty; but, once again, the more powerful the code, the harder it is to provide documents and defaults.
In all respects, the solution is to be found in "customization", which entails, in effect, making a powerful general-purpose code look, to the user, as though it possesses only those capabilities from which he or she needs to make selections.
A further ease-of-use requirement is the connexion between the widely-available CAD-1 packages and the software which computes the flows around and within the objects which they create.
As already indicated, the CASA packages tend to be better connected with CAD-1 than the CFD packages; but one of the few finite-volume CFD codes, CFDesign, has concentrated its attention on such connexions, with benefits to its users.
That such connexions are not more prevalent is in part due to the gap of understandinq between the finite-element and finite-volume communities. It is a gap which needs urgently to be either bridged or, by using finite-volume methods for CASA, eliminated.
The reason for promoting ease of use is already an economical one.
needless difficulties waste the time of intelligent humans; and
that, and they, are the most precious resources which we possess.
However, computer-time is also precious; and CFD-package users never
have enough of it. What is available should therefore be utilised in
a balanced manner, care being taken not to squander time by the use
(say) of excessively-fine computational grids when the models of the
physical processes are comparatively crude.
The opposite extreme is equally to be avoided. Some turbulence models
are rather elaborate and time-consuming; and these are sometimes (ill-
advisedly) employed in circumstances in which, because many small
solid objects are immersed within the fluid, the number of grid
nodes between two adjacent solids is far too small for (say) the
velocity gradients to be computed with adequate accuracy.
There is therefore a need for "balanced-accuracy" models, which, by
avoiding extremes, make optimal use of limited computer resources.
(1) Turbulence Most practically-occurring flows are turbulent. The methods for
simulating derive from ideas put forward by Kolmogorov (1942); but
these are inadequate in at least two here-relevant respects, namely:
Kolmogorov's idea (which others conceived later, but independently)
was that it might suffice to invent and solve equations for certain
statistical properties of the local turbulence. It was partly true.
Because it was partly true, Kolmogorov's followers (whether or not
they knew whom they were following) achieved success in predicting
the velocity (and sometimes temperature) distributions in:-
Unfortunately, the Kolmogorov concept, which is only one of several
possibilities, fails whenever the significant behaviour of a fluid
element depends on the differences of its properties, eg
temperature, or circumferential velocity, from the local time-mean.
Such circumstances are common; they include:-
Dopazo and O'Brien (1974) recognised that there was another
possibility; and Pope (1982) has explored it to same extent, but by
means of a computer-time-intensive (Monte-Carlo) method.
What is needed is needed is an economical method of exploration.
The scientific study of chemical kinetics is well advanced; and it
has revealed, in great detail, how engine fuel (for example)
combines with air to produce the desired products (carbon dioxide
and water vapour) and others that are undesired (oxides of
nitrogen, smoke, carbon monoxide, and unburned hydrocarbons).
The detailed knowledge is however TOO detailed, in the senses that
it involves more than designers want to know, and that its
computation necessitates enormous computer time. Therefore
simplified models have been devised, conveying the important
information well enough, while avoiding excessive detail.
That is however not the end of the computer-modeller's difficulties;
for chemical reaction rates depend not on time-mean gas properties,
which Kolmogorov-type turbulence models predict, but also upon the
instantaneous diferences therefrom.
Models of the Dopazo/O'Brien type are needed. (See MFM, below.)
Heat transfer by thermal radiation is, like chemical kinetics, one
of those phenomena for which it is easy to write down the relevant
mathematical equations, and indeed to devise general means of
solution. However, these solution means become computationally very
intensive, whenever the solid-surface geometry is complex.
Unfortunately, in many practically-important circumstances, the
resulting computational task is too great to be executed, at least
when the temperature and wave-length dependencies of the radiation
properties of gaseous and solid materials are taken into account.
What is commonly done is to neglect the latter dependencies, and
also, to at least some extent, the effect of the intervening gases
on the transfer from one solid surface to another.
What is therefore needed is the devising of a balanced method, which
may allow some geometrical inexactitude if wavelegth and temperature
dependences can be accommodated.
The lecture describes how the above-listed needs are being met.
Section 2 describes how the connexion between CAD-1 and CFD can be
effected by the use of the STL-file format.
Section 3 explains how the CASA and CFD worlds are being unified.
Section 4 describes some recent physical-modelling developments
directed towards:-
Finally, section 5 describes the tendency for remote computing to
replace the current practice of software-package purchase.
[Note: In the remainder of the lecture, the "-1" appendage to "CAD" will
be dropped, the point having been sufficiently made.]
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(b) The need for economy
(c) The need for better physical modelling
CASA specialists may have their own difficulties in repect of the
yield properties of plastically-deforming materials; but they are as
nothing (or so it seems to the present author) in comparison with
those of the CFD specialist in respect of turbulence.
(b) Chemical reaction
(c) Radiation
1.3 Outline of the present lecture