Σχολή Χημικών Μηχανικών, ΕΜΠ · Some pictures will now be displayed which arise from the solution of a very simple fire-simulation problem. It is the one which

Υπολογιζηική μηχανική ηης αειθορίας

Μαρκάτος Νικόλαος

Καθηγητής ΕΜΠ

Σχολή Χημικών Μηχανικών, ΕΜΠ

Το παρόν εκπαιδεσηικό σλικό σπόκειηαι ζε άδειες τρήζης Creative Commons. Για εκπαιδεσηικό σλικό, όπως εικόνες, ποσ σπόκειηαι ζε άδεια τρήζης άλλοσ ηύποσ, ασηή πρέπει να αναθέρεηαι ρηηώς.

Άδεια Χρήζης

Outline of the argument

● “What will happen if…?”

is the most important question which a conscious being can ask.

Answering it rightly more times than not is what keeps most of us safe, healthy

and reasonably prosperous; for it allows us to:

▪ foresee, and so avoid, dangerous events;

▪ select, from available options, those which best promote happiness and well-

being;

and

▪ create opportunities which never before existed


● Society seeks, through education,

▪ to inculcate in the young the habit of asking this question;

and

▪ to convey how the right answers can be arrived at, which means to teach to all:

techniques of prediction.

● In essence, all such techniques are the same: examine the past; and if elements of

the present are seen there, suppose that what transpired is likely to happen again.

● Thus: “When last I pulled the tail of a cat, it scratched me; so, if I do it again,

another scratch is what I must expect”.

It is a sound principle.


● In ancient times, oracles were consulted on matters of importance, as best fitted

by age, experience or connections to foresee, what the past implied about the

impending future.

● That too was a sound principle, for those who could afford the oracle’s fees!

● How are these principles applied in Chemical Engineering?

▪ If the task in hand involves little novelty, as when one more reactor is to be built

for an established and satisfactory production line, simple repetition of past

actions is what the principle dictates.

▪ But when the performance requirements have changed, exceeding what the old

reactor is capable of, novelty is needed; and what is new has, by definition, no

past to be examined.

● What to do?


● There is however a more general encapsulation of the past, which we call

science; and for chemical engineers it takes the form of:

▪ laws of conservation of mass, momentum and energy, (Lomonosov, Newton,

Joule);

▪ laws of transport of those same entities by diffusion, viscous action and heat

conduction (Fick, Newton, Fourier);

▪ laws of deformation of solids in response to mechanical and thermal stresses

(Hooke);

▪ laws governing rates of chemical transformation, and electrical and magnetic

interactions (Arrhenius, Faraday).

● It is such laws, to which the chemical engineer must turn, whenever actions

without precedent are contemplated, in order to answer “what will happen if…?”

questions.

Contents of the lecture

The present lecture explains:

● how simulation-by-computer has become the engineers’ favored prediction

technique,

and

● how specialized software packages have become the oracles which they consult.

● Two things the modern oracles share with the ancient ones:

▪ they cost money (or sheep, oxen or other currency);

and

▪ Their pronouncements are never 100% reliable.

● The reasons for both will be explained.

Fire Simulation

• industry

• buildings

• public transport

• forests

!need for designing

reliable systems for

fire extinction

Fire simulation in a building

velocity vectors in a vertical cross-sectionisosurface of temperature

March 25-28,

Moscow, 2007

2. Example: What happens if

a fire starts in a building?

Some pictures will now be displayed which arise from the solution of

a very simple fire-simulation problem.

It is the one which has been prepared for display in the Internet-café

of this Exhibition.

The pictures show the scenario which is to be studied, namely:

• an office containing a standing man, some desks and some

computers.

• The store-cupboard from which chairs, desks and other objects

can be extracted is visible near the top of the picture .

March 25-28,

Moscow, 2007



Store-cupboard

Scene of the ‘virtual theatre’

March 25-28,

Moscow, 2007



Another man can be easily added to the ‘scene’ by clicking on the

appropriate image; here it is the object ‘People’.Object ‘People’

March 25-28,

Moscow, 2007



The scene can be viewed from any point of view.

March 25-28,

Moscow, 2007



It is possible to move and rotate objects, and

March 25-28,

Moscow, 2007



It is also possible to bring in a third man with a chair for him to sit

upon.

March 25-28,

Moscow, 2007



Here are some results of calculation.

The next pictures show colour contours of temperature and arrows

indicating air motion under normal conditions on

a horizontal and vertical planes.

March 25-28,

Moscow, 2007



Finally, an animation is shown of the spread of smoke and flame

when combustible material under a chair is suddenly set alight.

March 25-28,

Moscow, 2007



This rather trivial simulation was created in such a way that it could

be executed in a few minutes on a lap-top computer.

In engineering practice, of course, situations of much greater

magnitude are in question: and CFD simulations may take many

hours of computation on powerful computer clusters working in

parallel.

Such simulations are routinely

conducted for:

• multi-storey car parks;

March 25-28,

Moscow, 2007



• super-markets;

• concert-halls;

March 25-28,

Moscow, 2007



• under-ground railway stations;

• tunnels;

• airplanes; and

• ocean liners.

Fire simulation at a METRO station

Fire simulation in an airplane cabin

temperature contours

the BFC grid used

Fire simulation in a room

Steckler’s Room

1.04 0.72

2.80 m 1.25

0.3

2.80 m

0.3

2.18 m

0.3

62.9 KW

Fire simulation - Results

door centre w-velocity profile

0

2

4

6

8

10

12

14

16

-2 -1 0 1 2velocity (m/s)

j-c

ell

nu

mb

er 6-flux

discrete (24 rays)

no radiation

Steckler’s room

firedoor

y (18)

x (22 cells)

z (25)

door centre vertical temperature profile

0

2

4

6

8

10

12

14

16

300 310 320 330 340 350 360 370 380 390 400temperature (K)

6-flux

discrete (24 rays)

no radiation

Steckler’s room

firedoor

y (18)

x (22 cells)

z (25)

horizontal temperature profile at I=11, J=12

300

320

340

360

380

400

420

440

460

0 5 10 15 20 25k-direction cell number

tem

pe

ratu

re (

K)

6-flux

discrete (24 rays)

no radiation

Steckler’s room

firedoor

y (18)

x (22 cells)

z (25)

Water mist suppression system

• Fine water spray systems have

received much attention as a

fire suppression system

• Radiation is attenuated due to

absorption by the water

droplets

• Water sprays can even be

used as radiative shields for

protecting from possible

secondary ignition due to

thermal radiation

water sprays

fire: heat source

target

6 seconds

8 seconds

10 seconds

12 seconds

15 seconds

20 seconds

nozzles

outlet

Water mist

suppression system

for the extinction of

fire in the CARGO of

an AIRBUS

30 seconds

40 seconds

60 seconds

80 seconds

100 seconds

120 seconds

140 secondsSmall outlet

Water mist

suppression system

for the extinction of

fire in the CARGO of

an AIRBUS

Simulation of ejection of Inert Gas (ΝΔΑ-

ΑirLiquide) for the extinction of fire at the

CARGO of an AIRBUS

AFTER 1 SECOND AFTER 3 SECONDS AFTER 5 SECONDS

AFTER 10 SECONDS AFTER 16 SECONDS AFTER 20 SECONDS

AFTER 25 SECONDS AFTER 30 SECONDS

Simulation of an Oil-Spill

Prasologos island

(included in the NATURA 2000 network)

• The oil spill at the surface of the sea 3h after the accident

• The emulsion at the surface of the sea 3h after the accident


Lesvos island

• The oil spill at the surface of the sea 10s after the accident

• The oil spill at the surface of the sea 6.5h after the accident


Euboean gulf

• The oil spill at the surface of the sea 12h after the accident

• The emulsion at the surface of the sea 12h after the accident


0.000048

5.000048

10.000048

15.000048

20.000048

25.000048

30.000048

0.00E+0

0

1.00E-06 2.00E-06 3.00E-06

Volume fraction of oil spill(R2)

Dep

th(m

)

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

0.00E+

00

5.00E+

02

1.00E+

03

Oil DensityV

olu

me f

racti

on

of

oil s

pill

Series1

Series2

W = 6 (m/s) inGr./Re = 9.29x 10

2 -2

ΔΤ=5

τήμα 13

Air pollution - street canyon

W = 6 (m/s) inGr./Re= 1.86x 10

-2

ΔΤ=1

W = 6 (m/s) inW = 1 (m/s) inGr./Re = 6.69x10

2 -1

ΔΤ=1

Air pollution - street canyon

ΑΝΑΘΔΗ ΔΡΓΟΤ: ΠΔΣΡΟΛΑ ΔΛΛΑ Α.Δ.Β.Δ.

ΔΘΝΗΚΟ ΜΔΣΟΒΗΟ ΠΟΛΤΣΔΥΝΔΗΟ

ΣΜΖΜΑ ΥΖΜΗΚΩΝ ΜΖΥΑΝΗΚΩΝΜΟΝΑΓΑ ΤΠΟΛΟΓΗΣΗΚΖ ΡΔΤΣΟΓΤΝΑΜΗΚΖ

ΔΜΠ - ΖΜΔΡΔ ΔΡΔΤΝΑ

& ΣΔΥΝΟΛΟΓΗΑ 2000

ΓΙΔΡΔΤΝΗΗ ΔΠΙΠΣΩΔΩΝ ΣΟ ΠΔΡΙΒΑΛΛΟΝ ΑΠΟ ΣΗΛΔΙΣΟΤΡΓΙΑ ΓΙΤΛΙΣΗΡΙΩΝ - Η ΠΔΡΙΠΣΩΗ ΣΗ ΠΔΣΡΟΛΑ

Σχήμα: Εώνες ημερήζιφν ζσγκενηρώζεφν SO2 - Τθιζηάμενη καηάζηα ζη (άνεμ ος δ σηικός - 2m/s, κλάζη ε σζηάθει ας: Α, ύυος αναζηροθής 400m)

1km

>200>200

125-200

100-125

80-100

65-80

50-65

40-50

30-40

20-30

10-20

5-1 0

0.25-5

[μg/m]3

1 km

Σχήμα: Εώνες ημερήζιφν ζσγκενηώζεφν SO2 - Μελλονηική καηάζηαζη (άνεμος δσηικός- 2m/s, κλάζη εσζηάθειας: Α, ύυος αναζηροθής 400m)

1km

1km

> 200> 200

125-200

100-125

80-100

65-80

50-65

40-50

30-40

20-30

10-20

5-10

0.25-5

[μg/m]3

Environmental impact of an Oil Refinery

SO2 daily concentration zones.

Existing situation.

(westerly wind at 2 m/s, equilibrium class A,

Inersion height 400 m).

SO2 daily concentration zones.

Future situation.

(westerly wind at 2 m/s, equilibrium class A,

Inersion height 400 m).

The tank geometry considered.

Fuel-tank Fires

Velocity Vectors (YZ plane)

EnthalpyEnthalpy

Enthalpy Contours (XZ plane)

Fuel-tank Fires

EnthalpyEnthalpy

Enthalpy Contours (YZ plane)

Concentration of CO vs. the height at a

position of 5 km from the flow axis.

Fuel-tank Fires

Configuration of risk zones I and II.

Ππόηςπο Κηίπιο-Ιδεαηόρ Τόπορ

Ν

Αξαηνθαηνηθεκέλε Πεξηνρή

Πεξηνρή Απαιιαγκέλε από

Θεξκηθέο Πεγέο

Ηκηαζηηθό Πεξηβάιινλ

Κηίξην Δμνρηθνύ Τύπνπ

(rural-type building)

Computational domain and grid

Πεδίν Ρνήο/Βηνθιηκαηηθνί ΧάξηεοΕπιηάτσνζη ροής

PMVs ↓, PD ↑

ζηο άνοιγμα

Αποδεκηές ζσνθήκες

-0.88<PMV<0.88

AND

PD<15%

Venturi Effect!!!

Βάζη Γεδομένυν-Μεηαβληηέρ

ειζόδος/εξόδος

Γz

θ

Occupied

volume

Stored TCIavg

Μ=70W/m2, Height O.V.=1.1-1.8m



Βήκα Πξνζαλαηνιηζκνύ: Γζ=30ν

Βήκα Ύςνπο Αλνίγκαηνο: Γz=0.275m

63 (+ 10) δεύγε Μεηαβιεηώλ Δηζόδνπ/Δμόδνπ γηα θάζε

δξαζηεξηόηεηα!

Γηάλπζκα εηζόδνπ:

Γηάλπζκα εμόδνπ:

[ ] Tx z

[ ( * ) ]T

NVy P M V P M V P M V S E T P D

No. of data PMV PMVNV PMV(SET*) PD (%) θ (degrees) Δz (m)

1 1.4785 1.0159 1.5506 5.7482 0 2.2

2 1.4278 0.9618 1.3633 7.2892 30 2.2

3 1.2340 0.8107 1.0907 14.7745 60 1.925

4 1.3651 0.9146 1.2710 12.9148 90 1.375

- - - - - - -

- 1.2325 0.8270 0.9893 22.0853 120 1.1

- - - - - - -

- 1.2629 0.8579 0.9426 21.6496 150 0.825

- - - - - - -

63 1.5157 1.0460 1.6218 4.2040 180 0

Βάζη Γεδομένυν-Μεηαβληηέρ

ειζόδος/εξόδος

Γz

θ

Occupied

volume

Stored TCIavg




Βήκα Πξνζαλαηνιηζκνύ: Γζ=30ν

Βήκα Ύςνπο Αλνίγκαηνο: Γz=0.275m

63 (+ 10) δεύγε Μεηαβιεηώλ Δηζόδνπ/Δμόδνπ γηα θάζε

δξαζηεξηόηεηα!

Γηάλπζκα εηζόδνπ:

Γηάλπζκα εμόδνπ:

[ ] Tx z

[ ( * ) ]T

NVy P M V P M V P M V S E T P D

No. of data PMV PMVNV PMV(SET*) PD (%) θ (degrees) Δz (m)

1 1.4785 1.0159 1.5506 5.7482 0 2.2

2 1.4278 0.9618 1.3633 7.2892 30 2.2

3 1.2340 0.8107 1.0907 14.7745 60 1.925

4 1.3651 0.9146 1.2710 12.9148 90 1.375

- - - - - - -

- 1.2325 0.8270 0.9893 22.0853 120 1.1

- - - - - - -

- 1.2629 0.8579 0.9426 21.6496 150 0.825

- - - - - - -

63 1.5157 1.0460 1.6218 4.2040 180 0

Εκπαίδεσζη/Επαλήθεσζη

Τετνηηού Νεσρωνικού Δικηύοσ

Σύνολο Ελέγτοσ Ακρίβειας T.N.Δ.

Kennard-Stone

( )y f x

Γομή Τεσνηηού Νεςπυνικού

Γικηύος

1

ˆL

j

j

y w f

i i jx x - c

( )i jf x c

Αποηελέζμαηα/Ππόβλετη

Μ=70W/m2

Windward opening height (m)

Win

d incid

ence a

ngle

(o)

PMV

0.9

1

1.04

1.04

1.171.17

1.17

1.17

1.17

1.31.3

1.3

1.3

1.3

1.431.43

1.43

1.43

1.43

1.43

1.561.56

1.56

1.56

1.5

6

1.56

1.5

6

1.69

1.6

9

1.69

1.8

2

1.8

2

1.69 1.69

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

0.6

0.7

0.7

0.7

0.80.8

0.8

0.8

0.8

0.90.9

0.9

0.9

0.9

0.9

11

1

1

1

1

1.11.1

1.1

1.1

1.1

1.1

1.1

1.2

1.2

1.2

1.2

1.3

1.3

1.2


Win

d incid

ence a

ngle

(o)

PMVNV

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

0.6

0.80.8

0.8

0.8

0.8

11

1

1

1

1.21.2

1.2

1.2

1.2

1.2

1.41.41.4

1.4

1.4

1.4

1.4

1.61.61.6

1.6

1.6

1.6 1.6

1.8

1.8 1.8

1.8

1.8

1.8

2


Win

d incid

ence a

ngle

(o)

PMV(SET*)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180 0

3.8

3.8

3.8

3.8

3.83.8 3.8

7.67.6

7.6

7.6

7.67.6 7.6

11.4

11.4

11.4

11.4

11.411.4 11.4

15

.2

15.2

15.2

15

.2

15.2 15.2

19

19

19

19 19

22.822.8

22.8

22.822.8

26.6

26.6

26.6 26.6


Win

d incid

ence a

ngle

(o)

PD (%)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

PMV<0.88

PD>15%

PMV≈1

PD<15%


Μ=60W/m2

0.4

0.60.6

0.6

0.80.8

0.8

0.8

11

1

1

1

1.21.2

1.2

1.2

1.2

1.2

1.4

1.4

1.4

1.6

1.6

1.4

1.4

1.4


Win

d incid

ence a

ngle

(o)

PMV

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

0.2

0.3

0.3

0.40.4

0.4

0.50.5

0.5

0.5

0.60.6

0.6

0.6

0.6

0.70.7

0.7

0.7

0.7

0.7

0.9

0.9

0.9

0.9

0.9

1

1

1

1

1 0.9


Win

d incid

ence a

ngle

(o)

PMVNV

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

-0.2

0

0

0.20.2

0.2

0.2

0.40.4

0.4

0.4

0.60.6

0.6

0.6

0.6

0.80.8

0.8

0.8

0.8

11

1

1

1

1

1.21.2

1.2

1.2

1.2

1.2

1.2

1.41.4

1.4

1.4

1.41.4

1.4

1.6

1.6

1.6

1.6

1.8


Win

d incid

ence a

ngle

(o)

PMV(SET*)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

3.53.

5

3.5

3.5

3.53.5

7

7

7

7

7

7 7

10.510.5

10.5

10.5

10.5 10.5

14

14

14

1414

17.517.5

17

.5

17.5

17.5

21

21

21

21

24.5

24.5

24.5

28

28

7


Win

d incid

ence a

ngle

(o)

PD (%)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

PMV<0.5

PD>15%

PMV<0.8

PD<15%

-1.5

-1.25

-1.2

5-1.25

-1

-1

-1

-1

-0.75

-0.7

5

-0.75

-0.75

-0.5-0.5

-0.5

-0.5

-0.5

-0.2

5

-0.2

5

-0.25

-0.25

0

0

0

0

0.2

50.2

5

0.5

0.5

0.25 -0.25

0.25


Win

d incid

ence a

ngle

(o)

PMV

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.2

30

60

90

120

150

180


Μ=40W/m2

-2.1

-1.8

-1.8

-1.8

-1.5

-1.5

-1.5

-1.2

-1.2-1

.2

-1.2

-0.9-0.9

-0.9

-0.9-0.9

-0.6-0.6

-0.6

-0.6

-0.6

-0.3

-0.3

-0.3

-0.3

-0.3

0

0

00

0

0.3

0.3

0.3

0.3

0.60.6

-0.30.6 0


Win

d incid

ence a

ngle

(o)

PMV(SET*)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

3.5

3.5

3.5

3.5

7

7

7

7

7

7

10.5

10.5

10

.5

10.5

10.5 10.5

14

14

14

14

14

17.5

17.5

17

.5

17.517.5

21

21

21

21

24.5

24.5


Win

d incid

ence a

ngle

(o)

PD (%)

0 0.275 0.55 0.825 1.1 1.375 1.65 1.925 2.20

30

60

90

120

150

180

PMV<-0.6

PD>15%

PMV>-0.6

PD<15%

The cost of CFD

● The new oracles demand their sacrifices

What are they?

1. The software

In the first two decades of the CFD industry, licences to use the software packages

could be sold for tens of thousands of Euros.

Nowadays their price is dozens times less.

Industrial organizations can purchase them easily, academic ones not so as yet.

The cost of CFD

2. The hardware

● Nowadays hardware costs have decreased dramatically and parallel-computing

facilitates the use of more adequate computational grids

● Another possibility is to use remote clusters via Internet paying for actual

services.

In this case it is only a laptop that a user needs to solve great multifactor

problems.

The Cost of CFD

3. The personnel

● Nowadays, therefore, it is the cost of hiring CFD-literate personnel which is the

most serious impediment to the extension of computer simulation.

● Suitable people are those,

▪ who have experience of several packages

▪ who recognize that most of their claims to superiority are ill-founded,

▪ who understand the limitations from which they all suffer

▪ who possess well-balanced commonsense, and

▪ who are pragmatically skeptical and when the occasion arises, they can

conclude that

“A particular computer simulation simply must be wrong”

Why the “Oracles” are not completely

reliable

● Grids are inadequate. If a hundred million control cells had been used, these cells

would still be hardly small enough to represent a continuous problem as discrete.

Only the largest computer clusters in the world would have been able to handle so

many; and the computation would take for too long for its outcome to remain of

interest.

● Engineers like to think that fuel and oxygen combining form “combustion

product” gases but chemists have discovered that a great many of intermediate

products are formed in combustion, and the information is too immense.

● Therefore, engineers create simplified combustion models and their accuracy

always raises doubts.

● The same is true for many physical phenomena such as radiation and turbulence


reliable

● Although turbulence has been much studied, none of the current representations

are known to correspond with reality in all circumstances.

● This regrettable fact seems likely to remain until some Newton reduces chaos to

order.

● Until then all predictions of turbulent flows must be regarded as no more than

probable forecasts of “What will happen if…?”

● When chemical reaction and two-phase effects are present the margin for error

widens.


reliable

What is to be done?

● The optimists are impressed by the plausible-seeming attractively-coloured

images they produce.

● The pessimists argue that all packages use the same dubious models of

turbulence, etc, and all are compelled to use far-too-coarse grids.

Aristotle’s advice is here appropriate: The best lies between the extremes

It entails recognizing that

1. the CFD-based predictions are no more than indicators of probability, but

2. they are immensely better than the mere guess-work which is mankind’s only

alternative

Conclusions

● The use of CFD has been increasing steadily in the last three decades for the

design of equipment and processes of modern engineering

● Its use seems certain to continue to grow

● It is therefore obvious that the educational systems of the world should prepare

their students to participate and to contribute to it.

Χρηματοδότηση

• Το παρόν εκπαιδεσηικό σλικό έτει αναπηστθεί ζηα πλαίζια

ηοσ εκπαιδεσηικού έργοσ ηοσ διδάζκονηα.

• Το έργο «Ανοικτά Ακαδημαϊκά Μαθήματα Ε.Μ.Π.» έτει

τρημαηοδοηήζει μόνο ηη αναδιαμόρθωζη ηοσ

εκπαιδεσηικού σλικού.

• Το έργο σλοποιείηαι ζηο πλαίζιο ηοσ Επιτειρηζιακού

Προγράμμαηος «Εκπαίδεσζη και Δια Βίοσ Μάθηζη» και

ζσγτρημαηοδοηείηαι από ηην Εσρωπαϊκή Ένωζη

(Εσρωπαϊκό Κοινωνικό Ταμείο) και από εθνικούς πόροσς.

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