Encyclopaedia Index

1. An overview of computer-aided engineering


1.1 An overview of computer-simulation capabilities and difficulties 1.2 Summary of urgent needs 1.3 Outline of the present lecture

1.1 An overview of computer-simulation capabilities and difficulties

(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.

(b) CASA, ie computer-aided stress analysis

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.

(c) CFD, ie Computational Fluid Dynamics

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:-

  1. the phenomena to be simulated possess inherently greater variety involving, for example: turbulence, chemical reaction, more than one phase, and radiation; so more input data are required;
  2. the grid-fineness requirements are greater; and, since they can rarely be fully satisfied, more skill and experience are needed from the code users, if the simulations are still to be reliable;
  3. most practically-arising problems are three-dimensional in character and often time-dependent as well (turbo-machinery flows, properly considered, are of this kind); and finally
  4. science has not advanced sufficiently to enable all the relevant phenomena to be expressed reliably in tractable mathematical form.
As a consequence, very few CFD calculations deserve to be trusted fully; and the users of the relevant software packages should be constantly on their guard against believing that, because a particular model (eg of turbulence or multi-phase flow) is widely used, it must surely have been adequately validated.

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.

Press for notes on validation

(d) CAD-2; computer-aided Design

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.

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1.2 Summary of urgent needs

(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.

(b) The need for economy

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.

(c) The need for better physical modelling

(1) Turbulence

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.

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.

(b) Chemical reaction

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.)

(c) Radiation

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.

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1.3 Outline of the present lecture

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.] Back to top

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