Encyclopaedia Index

What CFD can and cannot do

Contents

  1. General considerations about the capabilities of CFD
    1. Why the subject is important
    2. What affects the degree of difficulty
    3. How to get value from CFD despite the difficulty
  2. What is easy
  3. What is difficult
    1. Transition from laminar to turbulent flow
    2. Chemical reaction in turbulent fluids
    3. Fluid-structure interactions
    4. Erosion
    5. Atomisation
  4. What is impossible

1. General considerations about the capabilities of CFD

1.1 Why the subject is important

All who are interested in using CFD need to understand that, although all fluid-flow phenomena are in principle amenable to simulation by CFD techniques , it is only the minority of those which arise in practice which are easy to simulate; most are rather difficult; and many will have to be regarded for many years as impossible.

The sources and natures of the difficulties need to be recognised; and it is useful to distinguish those difficulties which can be surmounted by spending more money (for example on more-powerful software or hardware) and those which derive from inadequacies of scientific knowledge.

Most important of all is to recognise that, even in cases of great difficulty, much value can be obtained from CFD simulations, if the problem statement is judiciously simplified, and the demand for accuracy is reduced.


1.2 What affects the degree of difficulty

The features which affect the relative ease or difficulty are:

In summary, the degree of difficulty of a flow-simulation problem is strongly dependent on its magnitude, ie on the number of:

The greater the magnitude of the problem, the greater must the size of the computer, and the time for which it must run; and computers possessing the necessary power and speed for even moderately large problems are simply not available.

However, there is another equally insuperable source of difficulty: scientific knowledge is also lacking or inadequate for many of the processes and materials to which one might wish to apply CFD.

Thus the fire in the waste-paper basket cannot truly be simulatd by CFD, even if simplified in respect of geometry, because the chemistry and physics of the combustion of paper have not yet been reduced to quantitative scientific order.


1.3 How to get value from CFD despite the difficulty

Faced with the difficulties just enumerated, some would-be CFD users may decide to proceed no farther, preferring either to obtain the information which they require by making physical experiments, or to do without it.

Most, however, wisely decide to lower their expectations, and to learn what they can from the simulation of a simpler situation which still, they may believe, contains the essence of the more complex one.

Thus, they may reduce the size of the problem by:-

In addition to these quantitative reductions, the prudent user of CFD will also look for qualitative ones, determining to represent some physical phenomena in an approximate manner, even when science knows better.

For example, in a combustion simulation, he or she may choose to use the "mixed-is-burned" presumption, thus dispensing with the necessity to simulate in detail the many chemical reactions which are known to take place.

Such simplifications abound in CFD and are necessarily much employed. So important is this that the selector of a CFD software package should ask not only what complex problems it can solve exactly but how many and which affordable approximations it possesses.

As will be seen, PHOENICS possesses quite a large number of these, for example: LVEL, IMMERSOL and the multi-fluid model of turbulence .


To be continued in the near future

2. What is easy


3. What is difficult

Transition from laminar to turbulent flow

The special features of transition

What PHOENICS can do now

What PHOENICS could do

Chemical reaction in turbulent fluids

The special features of turbulent chemical reaction

The special difficulty about predicting correctly the rates of chemical reaction which take place in turbulent fluids is discussed in several places within the PHOENICS documentation, for example:

What PHOENICS can do now

What PHOENICS could do

Fluid-structure interactions

The special features of Fluid-structure interactions

What PHOENICS can do now

What PHOENICS could do

Erosion

The special features of erosion

What PHOENICS can do now

What PHOENICS could do

Atomisation

The special features of Atomisation

What PHOENICS can do now

What PHOENICS could do


4. What is impossible