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What is fluid flow?
Fluid flow is:
- breathing, drinking, digesting, washing, swimming, smoking;
- laundering clothes, and hanging them out to dry;
- heating or ventilating a room; extinguishing a fire with water;
- burning gasoline in an automobile engine to create power and (unfortunately) pollution;
- making soup, creating plastics from petroleum;
- flying an airplane, parachuting, surfing, sailing;
- soldering, making steel, electrolysing water;.... and so on ....
What is CFD?
CFD is predicting what will happen, quantitatively, when fluids flow, often with the complications of:
- simultaneous flow of heat,
- mass transfer
(eg perspiration, dissolution),- phase change
(eg melting, freezing, boiling), - chemical reaction (eg combustion, rusting),
- mechanical
movement (eg of pistons, fans, rudders),- stresses
in and displacement of immersed or surrounding solids.
Click here to see some examples of CFD predictions
How old is CFD?
Its early beginnings were in the 1960's.
- Its first successes came to prominence in the 1970's.
- The creation of the CFD-service industry started in the 1980's.
- The industry expanded significantly in the 1990's.
- Expansion continued in the Second Millennium as CFD packages devloped easier connexions with those for CAD and solid-stress analysis.
- A significant change of the near future is likely to involve the use of pay-as-you go remote computing, via Internet.
What use is CFD?
Knowing how fluids will flow, and what will be their quantitative effects on the solids with which they are in contact, assists:-
- building-services engineers and architects to provide comfortable and safe human environments;
- power-plant designers to attain maximum efficiency, and reduce release of pollutants;
- chemical engineers to maximize the yields from their reactors and processing equipment;
- land-, air- and marine-vehicle designers to achieve maximum performance, at least cost;
- risk-and-hazard analysts, and safety engineers, to predict how much damage to structures, equipment, human beings, animals and vegetation will be caused by fires, explosions and blast waves.
CFD-based flow simulations enable:-
- metropolitan authorities need to determine where pollutant-emitting industrial plant may be safely located, and under what conditions motor-vehicle access must be restricted so as to preserve air quality;
- meteorologists and oceanographers to foretell winds and water currents; - hydrologists and others concerned with ground-water to forecast the effects of changes to ground-surface cover, of the creation of dams and aquaducts on the quantity and quality of water supplies;
- petroleum engineers to design optimum oil-recovery strategies, and the equipment for putting them into practice;
- ... and so on.
Within a few years, it is to be expected, surgeons will conduct operations which may affect the flow of fluids within the human body (blood, urine, air, the fluid within the brain) only after their probable effects have been predicted by CFD methods.
How does CFD make predictions?
CFD uses a computer to solve the relevant science-based mathematical equations, using information about the circumstances in question.
Its components are therefore:
- the human being who states the problem,
- scientific knowledge expressed mathematically,
- the computer code (ie software) which embodies this knowledge and expresses the stated problem in scientific terms,
- the computer hardware which performs the calculations dictated by the software,
and
- the human being who inspects and interprets their results.
Can CFD be trusted?
Click here to read a more extended discussion of validation
CFD-based predictions are never 100%-reliable, because:
- the input data may involve too much guess-work or imprecision;
- the available computer power may be too small for high numerical accuracy (this is often the case);
- the scientific knowledge base may be inadequate (so is this).
The reliability is greater:
- for laminar flows rather than turbulent ones
- for single-phase flows rather than multi-phase flows;
- for chemically-inert rather than chemically-reactive materials;
- for single chemical reactions rather than multiple ones;
- for simple fluids rather than those of complex composition.
Therefore, coal-fired furnaces represent an extreme of uncertainty; but CFD is nevertheless used increasingly in their design because the uncertainties resulting from its non-use is even greater.
Click here to see some "validation" results
Click here for a more extended account of what CFD can and cannot do
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:-
- are based upon sufficiently precise formulations of the geometry and of the initial and boundary conditions;
- are performed with sufficiently fine grids;
- converge so as make the remaining imbalances in the equations sufficiently small;
- are based upon appropriate physical models;
- involve only phenomena of which the physical laws are well- described by the formulae embodied in the software.
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:
- low-Reynolds number effects;
- body forces (eg buoyancy, or strong swirl);
- kinetic-heating and compressibility effects;
- chemical reaction;
- free surfaces (eg between air and water);
- multi-phase phenomena, involving the intermingling of droplets, bubbles and solid particles.
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).