PHOENICS can be used successfully by persons of varied experience and background; however, the probability of success is greatest if they are already familiar with the general features of computer simulation of flowing continua; in particular, they should be aware of the following:

- that the continua of Nature must be represented in a digital computer by 'discontinua',
*i.e.*by sets of discrete volumes of space retaining their properties for discrete intervals of time; - that the predicted behaviour of the hypothetical discontinua will be close to the actual behaviour of Nature's continua only if the volumes and intervals are numerous enough to reflect the steepness of variation in space and time of the characterising properties (eg temperature, or velocity);
- that, because of the non-linearity of the physical phenomena which are being simulated (in most cases), or because of the large numbers of equations requiring simultaneous solution (in other cases), iterative procedures of solution must ordinarily be employed;
- that iterative procedures do not invariably converge (
*i.e.*arrive in the end at the solution), and may require user-intervention before they do so; - that, either because of the round-off limits of the computer, or because the iteration process is prematurely terminated, the solution yielded by a computer program will differ somewhat from the true solution of the equations governing the discontinua;
- that to compute solutions to the discontinua equations which are very close to the solutions of the continuum equations therefore entails greater costs than providing approximate solutions, because both accuracy and cost increase with the number of volumes, with the number of time intervals and with the number of iterations;
- that providing computed solutions which are close to those for the continuum equations
is still not the same as simulating physical reality, the success of which depends
additionally on: the validity (
*i.e.*truth to Nature) of the continuum equations them- selves; the appropriateness of the information concerning material properties; and the correctness of the formulation of the initial and boundary conditions; - that, as a consequence, the use of computer codes for the simul- ation of flow processes is an art as well as a science; for it concerns the creation of significant abstracts from reality, which have most merit when they can economically and rationally persuade experienced persons to base their decisions and actions upon them;
- and finally that credibility, economy and relevance to important practical concerns are all qualities that the simulation spec- ialist values highly.

The ideal user of PHOENICS has been identified in this way in order that, should less experienced persons use it, they should have received due warning that computer simulations of fluid flow differ appreciably from calculations of the kind which are customary in, say, structural engineering or in rigid-body dynamics.

In the those fields, non-linearities are often of secondary importance, if indeed they are present at all; the computer code can therefore be left to produce the solution automatically.

Fluid-flow phenomena, by contrast, are essentially **non-linear**; so
the user of a
flow-simulating code must reconcile himself to having, on occasion, to help
the code
along, using his intuition and experience in the selection of the 'switches'
and 'tuning
knobs' of the solving procedure.

The final solution itself, *i.e.* the result of the solving procedure,
of course must **not**
depend upon the settings so chosen, any more than the scenery at the port
of destination
can depend upon the course which the ship's navigator selected.

It is WHETHER the solution is arrived at, and HOW SWIFTLY, that these steering adjustments may beneficially influence.

PHOENICS does already possess a primitive EXPERT system for making optimal adjustments of the numerical parameters as the calculation proceeds; and this auto-pilot feature is being further developed by CHAM. However what has been stated above remains true: the user of PHOENICS, or of any other CFD code, should never trust their predictions blindly, or expect them to proceed efficiently no matter how injudicious are the user's settings.