Methods to optimise the grid resolution - FSMF.UK · 2018-06-02 · Methods to optimise the grid...
Transcript of Methods to optimise the grid resolution - FSMF.UK · 2018-06-02 · Methods to optimise the grid...
Methods to optimise
the grid resolution
London Fire Brigade's Headquarters, Union Street, UK
Fire and Smoke Modelling Forum, 3rd November 2017
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Giordana Gai
Francesco Saverio Ciani
Sapienza University of Rome, Italy
Guidi&Partners s.r.l., Italy
Outline
- Motivation, objective GG
- Methods GG
- Case studies FSC
- Conclusions FSC
London Fire Brigade's Headquarters, Union Street, UK
Fire and Smoke Modelling Forum, 3rd November 2017
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Giordana Gai
- MsC Structural Engineering
Sapienza University of Rome
- Ph.D. Candidate
Sapienza University of Rome
- Collaboration with the Italian Fire Brigades
(Corpo Nazionale dei Vigili del Fuoco)
- Fire Practitioner
- IFireSS 2017, Interflam 2016, IF CRASC ‘15
London Fire Brigade's Headquarters, Union Street, UK
Fire and Smoke Modelling Forum, 3rd November 2017
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Motivation
FDS LES (Large Eddy Simulation)
Navier-Stokes equations Filtered equations (Δ(x) = h(x) )
…FDS equations depend on h(x)
We choose the grid size h(x) considering:
• Theoretical background (correctness)
• Pragmatism (feasibility, time)
London Fire Brigade's Headquarters, Union Street, UK
Motivation
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Filter Grid size
Objective of the presentation
Review the methods currently used
D*/δx
Y+
MTR
Check them with examples
Add other global quantities for convergence check
London Fire Brigade's Headquarters, Union Street, UK
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Objective
Methods
Grid resolution = f ( characteristic fire diameter )
𝑄 = total heat release rate of the fire
D*/δx = 4-16 (FDS Validation Guide)
London Fire Brigade's Headquarters, Union Street, UK
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(A priori)
𝐷∗ = 𝑄
ρ∞𝑐∞𝑇∞ 𝑔
2/5
Coarse Fine
Methods
Methods
Grid resolution = f ( characteristic fire diameter )
NUREG 1824, 2007
London Fire Brigade's Headquarters, Union Street, UK
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(A priori)
Methods
Methods
Grid resolution = f ( characteristic fire diameter )
Computational cost
London Fire Brigade's Headquarters, Union Street, UK
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D*/δx Resolution δx (cm) T (hours)
4 Coarse 134 Tcoarse
8 Medium 68 16·Tcoarse
16 Fine 34 16·(16·Tcoarse)
Premises Civil
Fire size (MW) 5
D* (m) 5.4
Factor 16 = 23 · 2
CFL
δx
(A priori)
Methods
Methods
Near wall grid resolution = f ( Y+ )
FDS. LES with near-wall modeling with wall functions
(Smooth walls, Werner-Wengle wall model)
(Rough walls, Log law)
Wall models needs grid resolution near the wall to fall within a
range of y+, non dimensional distance from the wall expressed in
viscous units.
General guidelines: Y+ in the range 30-1000
London Fire Brigade's Headquarters, Union Street, UK
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(A posteriori)
Methods
Methods
Grid resolution = f ( measure of turbulence resolution )
Ksgs is the modeled energy (subgrid)
KLES is the calculated energy (LES)
MTR < 0.2
(Pope, 2004)London Fire Brigade's Headquarters, Union Street, UK
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(A posteriori)
0 (all solved)
1 (all modeled)
𝑀𝑇𝑅 =𝐾𝑠𝑔𝑠
𝐾𝑠𝑔𝑠 + 𝐾𝐿𝐸𝑆
Methods
Methods
Grid resolution = f ( measure of turbulence resolution )
1. Does «MTR<0.2» imply reliable results?
ASET predictions based on temperature, radiative heat flux gas,
visibility and FED
2. Are we able to guarantee «MTR<0.2» in the domain?
Near fire field, Far fire field
Ceiling, Human height
London Fire Brigade's Headquarters, Union Street, UK
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(A posteriori)
Methods
Methods
Grid resolution = f ( measure of turbulence resolution )
Pope, 2004:
As currently practised, LES is incomplete because the turbulence resolution length scale Δ(x) is specified subjectively in a flow-
dependent manner.
It can be made complete through adaptive LES.
The variation of Δ(x,t) is controlled (by grid adaption) so that a
measure M(x,t) of turbulence resolution (e.g. the fraction of the
turbulent kinetic energy in the resolved motions) is everywhere below a specified tolerance εm.
London Fire Brigade's Headquarters, Union Street, UK
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(A posteriori)
Methods
Case study. Fire compartment (20mx10mx3m)
London Fire Brigade's Headquarters, Union Street, UK
50 cm
25 cm
10 cm
2 MW 4 MW
Heat release rate of the burnerG
rid
sp
acin
gsiz
e
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Case study 1. Fire compartment
London Fire Brigade's Headquarters, Union Street, UK
Grid size: 50 cm
Y+ max: 879
Grid size: 25 cm
Y+ max: 400
Grid size : 10 cm
Y+ max: 195
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Case study 1. Fire compartment
London Fire Brigade's Headquarters, Union Street, UK
2 MW, z = 1 m
2 MW, z = 2 m
0.2
0.2
0.2
0.2
Y direction X direction
Y direction X direction
MTR
MTR
Case study 1. Fire compartment
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 100 200 300 400 500 600
London Fire Brigade's Headquarters, Union Street, UK
0
100
200
300
400
500
600
0 100 200 300 400 500 600
0
50
100
150
200
250
300
0 100 200 300 400 500 600
0.0
5.0
10.0
15.0
20.0
0 100 200 300 400 500 600
50 cm25 cm10 cm
2 MW, Temperature (z=1m) 4 MW, Temperature (z=1m)
2 MW, Rad.heat flux gas (z=1m) 4 MW, Rad.heat flux gas (z=1m)
Case study 1. Fire compartment
London Fire Brigade's Headquarters, Union Street, UK
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Francesco Saverio Ciani
- Fire Practitioner at Guidi & Partners s.r.l.
- Temporary Research Fellow
Alma Mater Studiorum, University of Bologna
- M.Eng Building Engineering
Alma Mater Studiorum, University of Bologna
- IFireSS 2017
Fire and Smoke Modelling Forum, 3rd November 2017
London Fire Brigade's Headquarters, Union Street, UK
Case Studies
Pearson correlation coefficient (r)
-1 10-0.25-0.75 0.750.25
strong strongintermediate intermediateweak weak
no relation
Directindirect
London Fire Brigade's Headquarters, Union Street, UK
Case Studies
0
100
200
300
400
500
600
700
800
900
1000
0 100 200 300 400 500 600 700 800 900 1000
Tem
pe
ratu
re _
me
sh 2
5 c
m [
°C]
Temperature _ mesh 50 cm [°C]
4 MW _ z = 2m
y_6m__z_2m x_11.5m__z_2m y_8m__z_2m x_11.5m__z_2m
r Pearsoncoefficient
r2 Coefficient of determination
Factory Building Case Study: Description
London Fire Brigade's Headquarters, Union Street, UK
A simplified version of areal factory building
Factory Building Case Study
Factory Building Case Study: Description
London Fire Brigade's Headquarters, Union Street, UK
MAIN BURNER CHARACTERISTICS
Italian Fire Prevention Code
(2015)
• RHRMAX= 50 MW• Fire Growth Rate= 75 s
(ultra-fast)• Soot Yield= 0.18 kg/kg
The Burner
Factory Building Case Study: Description
London Fire Brigade's Headquarters, Union Street, UK
Three Grid resolution were tested
0
50
100
150
100 cm 50 cm 25 cm
PC hours
ho
urs
The devices were placed in60 different positions in themodel.
Factory Building Case Study : y+ @ boundaries
London Fire Brigade's Headquarters, Union Street, UK
Cell dim: 50 cm
Y+max: 1300
Cell dim: 25 cm
Y+max: 750
Cell dim: 100 cm
Y+max: 3000
Y+ @ boundaries
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Leve
l : 7
,5 m
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 7
,5 m
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Leve
l : 7
,5 m
Dev
cso
n t
he
Y p
lan
e
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Leve
l : 7
,5 m
Dev
cso
n t
he
X p
lan
e
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Leve
l : 7
,5 m
Dev
cso
n t
he
X p
lan
e
Factory Building Case Study : MTR
London Fire Brigade's Headquarters, Union Street, UK
Leve
l : 1
,5 m
Leve
l : 4
,5 m
Leve
l : 7
,5 m
Far
Fiel
d
0.2
0.2
0.2
Grid Sensitivity AnalysisCase Studies Results: Grid Sensitivity Analysis
London Fire Brigade's Headquarters, Union Street, UK
240 different devices were placed inthe model:• 60 Temperature devcs;• 60 Rad. Heat Flux Gas devcs;• 60 Visibility devcs;• 60 FED devcs.
0
0.2
0.4
0.6
0 200 400 600 800
Radiative Heat Flux Gas100 cmRad_X99.5_Y1_Z4.5
50 cmRad_X99.25_Y1_Z4.75
25 cmRad_X99.125_Y1_Z4.625
We need to filter thisdata!!!!
240 Gridsensitivity
assessments
The FilterCase Studies Results: The filter
London Fire Brigade's Headquarters, Union Street, UK
100 cm 50 cm 25 cm
Find the Pearson’scoefficient
Find the Pearson’scoefficient
50 cm
Find the meanerror
Find the meanerror
100 cm 25 cm
PC1<PC2
Yes
Convergent
NoPC2>0.75
Yes
NotConvergent
NoEM1>EM2
(EM1) (EM2)(PC1) (PC2)
NoNot
Convergent
Yes
Convergent (EM1)(EM1)
Results : Percentage of ConvergenceCase Studies Results: Grid Sensitivity Analysis
London Fire Brigade's Headquarters, Union Street, UK
88%
88%
72%
82%
0% 20% 40% 60% 80% 100%
FED
Visibility
Radiative Heat Flux Gas
Temperature
FED Visibility Radiative Heat Flux Gas
Temperature
Conclusions -Factory Building Case Study-
• The Y+ assessment points out that the 100 cm grid spacing
should be avoided in the near-wall field
• The MTR assessment points out that the grid should be
refined in the upper layer volume and in the fire-plume zone
• The pearson correlation coefficient could be a good
parameter in order to assess if the solutions don’t have
appreciable differences
• The grid Sensitivity study of the Factory Building model
shows that a 50 cm grid spacing could be used.
Conclusions
London Fire Brigade's Headquarters, Union Street, UK
Future Developments
• The «convergence filter» should be improved in order to
evaluate if the grid spacing is on the safe side considering
the objective of our PBD approach
• Considerations on the computational cost might be added
to the filter
• This filter should be tested in a lot of different case studies
in order to find if there are limits for the use of the
correlation Pearson coefficents
Conclusions
London Fire Brigade's Headquarters, Union Street, UK