cfd: past, present and future by

Saint Petersburg, May 21-25, 2007

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CFD: Past, Present and Future

by

Brian Spalding

Lecture at the Sixteenth Leont’ev School-Seminar


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Introduction

An eightieth birthday is an occasion for looking back on the past and contemplating what the future may provide. That is why I have chosen the subject conveyed by my lecture’s title.

It is also an occasion for personal reminiscences; which I have therefore felt free to introduce.

Most of my career has been concerned with CFD (i.e. Computational Fluid Dynamics).


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Introduction

(contd)

What is the essence of CFD? It is the attempt to predict, quantitatively if possible, what will happen if, for example:• a kettle is placed on the hot-plate of an electric cooker;

• a wind-turbine is placed on a hill-top;

• an ignited explosive forces a bullet from a gun;• a swimmer dives into water; or

• a parachutist falls gradually to the ground.


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Introduction

(contd)

Who wishes to make these predictions? Of course,

• the designers of the kettles and cookers;

• the investors in wind-farms and the environment-protectors who oppose them;

• the manufacturers of armaments and of equipment which must withstand their effects;

• the whole sporting community (or it would if it knew that CFD could assist); and

• the designers of all aeronautical equipment, whether parachutes or jumbo-jets.


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Computational fluid dynamics can help all who have problems concerned with fluid flow, for example:

• surgeons who make incisions so as (perhaps) to improve the circulation of the blood;

Introduction (contd)

• cigarette smokers who hope that filters will protect their lungs (they won’t!)

• heating-and-ventilating engineers, who will not be paid unless the occupants of the buildings are comfortable;

• city authorities who must decide how large an area to evacuate when toxic gases are accidentally released;

• and so on.


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Introduction(end)

There is no end to the list. More than two thousand years ago, Heraclitus famously declared:

Were he alive today, he might pronounce:


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Prehistory: Boundary-

layer theory

When I started my scientific career, computational fluid dynamics (not yet so- called) was based on a combination of• ideal-fluid theory, which neglected the effects of viscosity and thermal conductivity, and• boundary-layer theory, which allowed for their effects close to solid surfaces.

The latter were particularly important in heat and mass transfer, which was my chosen field; and

the prediction method which I first adopted was that used by von Karman (1921) for aerodynamic friction

and by Kruzhilin (1936) for heat transfer.


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Prehistory

(contd)

This involved:• assuming a formula for the profile of velocity or temperature;• deducing its free parameters from the boundary conditions; and• calculating its thickness at any point of the surface by solving numerically an ordinary differential equation .

That was for laminar flows; but for turbulent ones it was necessary to introduce additional empirical information, augmented if possible by physical insight.

It was in this connexion that I first encountered the important work (1962) of Kutateladze and Leont’ev this week’s 80th birthday boy.


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Prehistory

(contd)

Indeed, I was so impressed by it that I translated their book into English (1964).

The effect of mass transfer through the surface was their particular study.


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Prehistory

(end)

Why did this interested me? Because my earliest researches had concerned the combustion of liquid fuels; and these exhibit just such a mass transfer.


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The finite-volume method:

The search for better profiles

and concentration in boundary layers, whether laminar or turbulent.

We also developed more-elaborate ‘weighting functions’ by which the equations were multiplied before integration.

My students and I made systematic efforts to invent more flexible formulae for the profiles of velocity, temperature


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The finite-volume method

(contd)

• that probably the most flexible formula of all would be a piece-wise linear one; • that if the widths of the ‘pieces’ were the limits of the integration, calculating their heights would be easy because • the weighting functions could then be extremely simple.

Then, by good fortune, I expressed during a lecture the following opinions :


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(contd)

Why was this fortunate?

Because my newest student, Suhas Patankar was in the audience; and listening!

The next day he told me that he had written, overnight, a small Fortran program which embodied my suggestions;

and they seemed to be correct!


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(end)

I mention this for three reasons:

1. This was how my colleagues and I came to adopt the finite-volume method, as it was later called, in all future work.

2. It led to the development of computer programs based on that method, which are now used throughout the world;

3. It shows that responsive students can play an important part in the development of science. They do not have to wait until they become professors themselves.


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Two-dimensional

recirculating flows

Two other talented and industrious students joined me during that time: Akshai Runchal and Micha Wolfshtein.

Therefore, so as to give them a distinct field of study, I asked them to extend the finite-volume method to flows such as arise in chemical reactors and furnaces.


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Two-dimensional

recirculating flows (contd)

Unlike boundary layers, in which the flow has a single predominant direction, large-scale vortices are present. Therefore we called them recirculating flows.


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Two-dimensional

recirculating flows (contd)

We used a rectangular grid of computational cells, which we thought of as ‘tanks’ connected by ‘tubes’.

Strictly-speaking, Akshai and Micha did this indirectly, by employing the vorticity and stream function as dependent variables; these were suitable for the two-dimensional flows to which we then confined attention.

The task was to calculate what distribution of pressures in the ‘tanks’ would cause the fluid to flow through the ‘tubes’ at rates which would satisfy the laws of physics.


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Two-dimensional

recirculating flows (end)

During the course of this work, we discovered the crucial importance of upwind-differencing.

Also, I had chanced upon Prandtl’s little-known (1945) paper on the production, dissipation and transport of turbulence energy.

We were therefore able, for the first time, to incorporate a turbulence model into a flow-simulation program.

Both that program, and the one based upon Patankar’s work, were simple enough to be published in book form (1967, 1969).


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Three-dimensional flows (steady and unsteady)

We did this by inventing the one-cell-at-a-time SIVA (ie Simultaneous Variable Adjustment) method, which worked well (1973).

Although two-dimensional flows do have some practical importance, three-dimensional ones have much more.

We tried, with varied success, to generalise the vorticity-based approach to three-dimensions.

Finally we decided to solve for pressure and velocity components directly.

Larry Caretto from the University of California made an important contribution.


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Three-dimensional flows

(contd)

I must explain that it was only ignorance which led us to believe that we were the first in the field:

Frank Harlow, of Los Alamos, was already computing unsteady flows numerically by means of the finite-volume method.

The only ‘first’ which we can now claim is to have been simulating steady flows rather than transient ones.


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(contd)

Then Suhas Patankar returned for a second period of study with me; and that resulted in the many-cells-at-once SIMPLE (ie Semi-Implicit Pressure-Linked-Equation) method (1973), which worked even better.

It has been followed by minor variants (SIMPLER, SIMPLEST, SIMPLEC and some others).

Nowadays most of the “CFD world” uses one or other of them.


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(end)

At that time, the whole range of fluid-flow phenomena seemed open to us. Heat exchangers (1974),combustion chambers (1974) and reciprocating engines (1969) were shown to be capable of simulation.Soon we were able to help the nuclear industry to overcome the seriousextending the equation-solving algorithms to two-phase (1980).

The potential of Computational Fluid Dynamics (which had a name at last) appeared then to be limitless.

difficulties which it was having with its steam generators by


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Commercial CFD;

The start

What happened then? Commercialization.

It was I, it appears, who began it, and it seemed to be a good idea at the time.

My argument was this:

• Many people need the benefits of CFD;• but few can become skilled computer programmers.• Let us therefore provide software packages with a central core which users cannot change, and an outer shell to which they can supply all that is relevant.• Experts will maintain and develop the sealed-off core;• Users will benefit and pay accordingly.


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Commercial CFD;

companies and packages

The first such program was called PHOENICS (1981), created by my own company, CHAM. Here you see the sealed-off core, ‘EARTH’, and the user-accessible, special-purpose ‘SATELLITEs’.

PHOENICS was followed by FLUENT, FIDAP, STAR-CD, FLOW3D and others, created by other companies, in several of which my former students were active.

At the present time, the largest of these companies have all been acquired by one US corporation, ANSYS Inc; the independents are few, and much smaller.

‘Big Business’ has taken over.


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Commercial CFD;a personal opinion

The benefits:• the basic idea did work; CFD is now widely used; and

education, engineering, environment and society all benefit;

• some companies have become very prosperous.

The disadvantages:

• Onlookers suppose

1. that the whole of CFD is what the commercial packages provide; and

2. that these packages are reliable predictors of “what will happen if…”.

Both these suppositions are totally false!


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Commercial CFD;

stagnation: an example

• To leave CFD development to the package vendors results in stagnation.

• For example, they mostly use scientifically obsolescent models of turbulence because

stagnation is more profitable than pioneering.

An illustration follows.

Turbulence models in the commercial CFD packages

Two kinds of instability can lead to turbulence:

1. Kelvin-Helmholz (shear-generated); and2. Rayleigh-Taylor (body-force generated).


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Commercial CFD;stagnation (contd)

When the commercial packages were first created, models existed for the first kind (Kelvin-Helmholz) only.

They were therefore adopted by all vendors.

Since then, models have been invented for turbulence of the second kind (Rayleigh-Taylor).

Such turbulence arises in turbo-machinery, heating and ventilating, combustion processes, the atmosphere and elsewhere.

However, few package vendors have paid any attention to these new models.


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Thus, few CFD packages can simulate the experiment first performed by my student Lewis Stafford in 1978, recently kindly repeated for me by our colleagues S.Sapozhnikov and V.Mityakov:• Take a glass-sided vessel. • Fill the lower half with salty water, coloured by some drops of dye.• Carefully fill the top half with clear fresh water to create a sharp interface.• At each end place electrodes, connected to a battery.• The temperature of the better-conducting salty water rises more rapidly than that of the fresh. • So, from being heavier, the salty water becomes lighter.


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• The consequent Rayleigh-Taylor instability causes rapid turbulent mixing to occur.• Within a second, the vessel appears to be filled with coloured fluid.

That process is easy to understand; less easy is what follows if you switch off the electric current as soon as mixing starts and then walk out of the room. Returning a few minutes later, you will see, perhaps to your amazement, that the fluids have returned to their original state: colourless above and coloured below. They appear to have ‘unmixed’.

How? I will leave you to think about it, giving just one clue: the molecular diffusivity of salt is much less than the thermal diffusivity of water. Now I shall show a video.


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Experiment, the start


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Experiment, its middle

+


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Experiment, the end


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Experiment - video


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Example:

turbulent unmixing

The ‘unmixing’ process can be simulated quantitatively by a Rayleigh-Taylor turbulence model. Here is the result of such a simulation:

Сontours of the volume fraction of salty fluid are displayed.

At the start (on the left), the volume fraction is unity in the bottom half and zero in the top half.

Later (in the middle) fragments of salty fluid rise, and even begin to concentrate at the upper surface.

Later still (on the right), the heating has stopped; so the salty fragments, lose heat to the fresh water and fall down to the bottom again.

The next slide shows more detail.


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Example

(end)

Calculated salt concentration contours;

blue = 0.0; red = 1.0


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How to end the stagnation

The role of CFD users

Don’t blame the CFD-package vendors!The vendors follow fashion, and should not be expected to innovate.

Society did not expect

• the originators of hammers and chisels to sculpt Michelangelo’s David.

• nor the inventor of the quill pen to write Shakespeare’s plays;


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The role of CFD users (contd)

• the manufacturers of grand pianos to compose Beethoven’s sonatas;

• Herren Otto and Diesel to compete in Formula 1 races.

It is skilled users of the new techniques who make the advances; and they are the unforeseeable few among the miscellaneous many.

Those therefore who wish the world to profit from the potential of CFD should try to increase the number and variety of users.

Society did not expect


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What CFD now needs is a secondary (‘spin-off’) market place.

There is nothing new in this idea.

Thus:

• Michelangelo was paid for his sculptures; and art dealers continue to profit from his hammer and chisel.

• Shakespeare was a shareholder in his company; and today’s actors, stage-hands, theatre owners and critics buy and sell their services.


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• Composers, whether Beethovens or Beatles, collect royalties; and performers (concert pianists, opera singers and divorce lawyers) collect their fees.

• Taxi- and bus-drivers, for whom Otto is a comic-character and Diesel a kind of fuel, earn their livings from two of the innumerable secondary markets which encircle the internal-combustion engine.


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How to end the stagnationThe role of CFD users (contd)

Secondary markets flourish whenever:

1. The necessary implements (hammers and chisels, pens and paper, harps and pianos, automobiles and motor-cycles) become plentiful.

2. Working rules are agreed:

• Shakespeare learned to spell (more or less).

• Beethoven used Guido d’Arezzo’s musical notation invented in 1050 AD !

• Drivers stop at traffic lights.

3. Quality is measured (each fuel has its Octane Number).


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How to end the stagnationThe role of CFD users (contd)

4. Sellers offer goods and services (sheets of music, gramophone records, rides to the airport).

5. Buyers exist for what is offered for sale.


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How to end the stagnationThe role of CFD users (end)

The secondary CFD market will flourish when:

1. The cost of CFD packages becomes negligible (“code-crackers” do perform a public service).

2. A uniform language is invented for prescribing CFD problems (package-specific input languages will die out).

3. Users learn to understand that all CFD simulations are unreliable, but some are more so than others.

4. Users of different kinds become both buyers and sellers of:

• Input files, with explanations and exemplary results;

• Display and interpretation utilities which they have developed;

• Educational material making use of one or other package.

5. Web-sites exist where the material is seen, bought and sold.


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What can we do, participants in the Sixteenth Leont’ev School-Seminar, or rather those of us who are or wish to become practitioners of CFD?

Why not start a Russian CFD-Advancement Club (or Society, or Shop, or Market)?

This could then create a website for the purposes of:

• letting individual members display the interesting results of their CFD simulations;• allowing interested visitors to purchase the corresponding input files;• processing the orders,• collecting the money, • delivering the goods to the purchasers and • transmitting the money to the creators of the goods.

How to end the stagnationWhat can we do?


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What can we do? (contd)

If one of you had something to sell, how could you do so?

One way would be to post a page to CHAM’s website, looking perhaps like this.

Anyone could

• see it;• read about it;• run the simulation;• send you the money;• download whatever you are selling.

The mechanisms for all this exist.


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What can we do? (contd)

What kinds of Input Files would I hope that the members would wish to sell in this way? Any which interest them, of course. But my suggestions would include:

• simulation of turbulence of the neglected Rayleigh-Taylor kind, in particular, modelling of the ‘turbulent-unmixing’ process, especially if compared with experimental data;

• heat-exchanger simulations which calculate the flow patterns rather than presuming them (as is done still by most commercial design programs);

• simulations in which stresses in solids and the flow of heat and fluid are computed simultaneously;

• combustor simulations in which a Rayleigh-Taylor model is exploited;


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What can we do? (end)

• genuine multi-phase-flow phenomena, such as diesel-engine-fuel sprays with many distinct groups of droplets;

and especially

• code-independent input files employing a numerics-free formulation of the situation, which could be written by any engineer and understood by any CFD software package.

Results of simulation of fuel allocation in the characteristic zones in combustion chamber of the medium speed diesel: S/D=260/260, spray angle 1500, RPM=1000, BMEP=16 bar.


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Concluding remarks

I hope that what I have said about the Past and Present has interested you intellectually;

but it is my hope that what I have suggested about the Future will still more engage your practical interest.

I remind you finally of the title of this presentation: “CFD: Past, Present and Future”.

cfd: past, present and future by

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