Encyclopaedia Index


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1.How reliable are predictions made by a CFD code?

Computer software such as PHOENICS is commonly used for the making of predictions about what flow phenomena, temperature distributions, reactor yields, etc, will actually occur in prescribed circumstances of interest to its user, who usually wishes to be assured that the predictions will turn out to be correct.

Such assurances, when given, are usually based upon arguments of one one or both of two kinds, namely:-

(a) inherent probability; and

(b) validation.

2. Conditions conducive to reliability

High inherent probability of correctness attends predictions which:-

The sufficiency implied in the first three conditions is of course for the user to judge; and often there is no way to be certain except by performing further calculations with, for example: - more precisely detailed geometric input, - 2, 4 or 8 (according to dimensionality) times as many cells, - RESREF (ie reference residual) values which are 0.1 times those used before, each of these changes being introduced SEPARATELY.

The appropriateness of the physical models is also sometimes a matter of judgement. Thus PHOENICS may be given the task of solving the velocity-potential equations for flow around a stream- lined object, on the grounds that viscous effects are probably small.

However, whether the neglect of the viscous effects is truly justified also requires checking, for example by performing a further calculation in which the Navier-Stokes equations are solved and allowance is made for turbulence.

If turbulence IS to be activated, the last of the above five considerations assumes importance; for the question to be decided is: which turbulence model should be used?

Science has not advanced far enough for definitive answers to be made to questions of this type.

Uncertainty about which model is best for given circumstances increases when any of the following phenomena make their appearance:

3. Validation

Uncertainty about the reliability of the predictions made by PHOENICS, in so far as it derives from doubts about the physical models which have been invoked, can be allayed by making comparisons with reliable experimental data.

Usually no data can be found which fit exactly the circumstances in which the user is interested; for, if they did, he would not be seeking to make computer predictions at all. However, the nearer are the conditions of the experiment to those which concern the user, and the more closely the PHOENICS predictions agree with those data, the greater will be the reliance which can be prudently placed on the predictions.

PHOENICS predictions have been subjected to many such validation tests; and the results have been reported in many places, for example in the PHOENICS Journal and other publications. (See the Documentation section of POLIS).

4. Has PHOENICS been sufficiently validated?

The above question is often asked, for understandable reasons, as it is also about other CFD codes; but it cannot be answered by a simple "yes" or "no". The greater is the range of flow simulations which a code is capable of making, the less is it possible for comparisons with experiment to be regarded as adequate.

PHOENICS can simulate a VERY great range of flows; so it can be regarded as LESS well validated than some codes of much more restricted capability. On the other hand, beacuse it has been used and tested by more persons than any other, and over a longer period, the claim can also reasonably be made that it is the BEST-validated CFD code.

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