MODELLING AND SIMULATION

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MODELLING AND SIMULATION OF HEAVY CLOUD
DISPERSION IN SEMI-CONFINED URBAN AREAS
A. Busciglio, A. Brucato, F. Grisafi, F.
Scargiali Department of Chemical Engineering
University of Palermo
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INTRODUCTION
LARGE USAGE OF TOXIC AND/OR FLAMMABLE CHEMICALS
? DANGER DUE TO AN ACCIDENTAL RELEASE
POSSIBILITY OF FORMATION OF A HEAVY CLOUD
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WHAT MAKES A CLOUD HEAVY?

• HIGH MOLECOLAR WEIGHT
• (i.e. Chlorine, Cyclohexane,etc.)

• LOW TEMPERATURE

• PRESENCE OF AEROSOL

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HEAVY CLOUD FEATURES
Flattened shape
Tendency to downwards stratification
High pollutant ground concentrations, where the
threat to human safety is highest
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AVAILABLE MODELS
SIMPLIFIED MODELS More than 100, different
nature and complexity

• Oversimplifications

Large computational savings for simple geometries
Unsuitable for simulating articulated geometries
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CFD MODELS
(Computational Fluid Dynamics)
SOLVES THE HEAVY GAS DISPERSION PROBLEM
STARTING FROM THE BASIC CONSERVATION EQUATIONS
MINIMUM APPROXIMATION IN THE SIMULATION OF
COMPLEX TERRAIN
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PRESENT WORK
APPLICATION OF A GENERAL PORPOUSE CFD CODE (AEA
CFX 4.4) TO THE SIMULATION OF DENSE CLOUD
DISPERSION INSIDE URBAN AREAS
RELEASE POINT
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CONSTITUTIVE EQUATIONS (turbulent)

• Continuity

• Momentum balance

• Transport equation for the scalar

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TURBULENCE MODEL
Buoyancy-related turbulence energy generation term
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WEAKLY COMPRESSIBLE APPROXIMATION
low Mach Number (Mlt0,3)
(AEA CFX 4.4)

• No sound waves are allowed
• (infinite sound speed)

• Kinetic energy negligible with respect to
• internal energy

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Preliminary validation
(Ayrault et al. J.Flow Vis. Im. Proc, 1995 and
97)
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Horizontal mean concentration profiles downwind
the fence
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INTERMEDIATE CONCLUSION
THE BUOYANCY TREATMENT ADOPTED APPEARS TO BE
EFFECTIVE IN THE DESCRIPTION OF BUOYANT FLOWS OF
THE TYPE OF INTEREST IN THIS WORK
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COMPUTATIONAL DOMAIN
SIMPLE NETWORK OF CITY ROADS WITH BUILDINGS ON
THEIR SIDES
500.000 cells
RELEASE POINT
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SIMULATION STRATEGY

• FLOW FIELD SIMULATION BEFORE THE RELEASE

SINGLE BLOCK WITH ONE BUILDING

• HEAVY CLOUD (CHLORINE) RELEASE SIMULATION

TOTAL DOMAIN OF 24 BUILDINGS
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SINGLE BLOCK SIMULATION

• Steady state simulation with constant wind
  velocity
• on the top plane (u?) and periodic planes in
  the
• four vertical sides

30.000 CELLS DOMAIN
u?
periodicplanes
A
B
u? 5 m/s, at 100 m
RELEASE POINT
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FLOW FIELD VECTOR PLOT
VERTICAL PLANE ACROSS THE BUILDING
WIND DIRECTION
Wind speed m/sec
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FLOW FIELD VECTOR PLOT
HORIZONTAL PLANE ACROSS THE BUILDING
WIND DIRECTION
Wind speed m/sec
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FLOW FIELD VALIDATION
Vertical profiles of normalized velocity
A
B
Water channel
(Hanna et al., Atm. Env. 2002)
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TOTAL DOMAIN SIMULATION
THE FLOW FIELD OBTAINED IN THE SINGLE BLOCK WAS
SUITABLY REPLICATED IN ORDER TO OBTAIN THE
INITITIAL FLOW FIELD OVER THE TOTAL DOMAIN OF
24 BUILDINGS
AN INSTANTANEUS RELEASE OF 100 Kg OF CHLORINE
WAS THEN SIMULATED
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
t15 sec
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
RELEASE POINT
T 1 min
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
RELEASE POINT
t 2 min
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
25 ppm
RELEASE POINT
1000 ppm
t 3 min
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
25 ppm
1000 ppm
RELEASE POINT
t 4 min
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RESULTS
Building height H 12 m (H/W 0,6) buildings
area coverage f 14
25 ppm
RELEASE POINT
1000 ppm
t 5 min
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INFLUENCE OF NORMALIZED HEIGHT
Building width W 20 m face to face distance S
32 m
H/W 0
H/W 0.3
H/W 0.6
H/W1.2
25 ppm
t30 sec
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INFLUENCE OF NORMALIZED HEIGHT
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INFLUENCE OF BUILDING FRACTIONAL AREA
Building width W 20 m Building height H 12
m
f 0
f 14
f 20
t30 sec
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INFLUENCE OF BUILDING FRACTIONAL AREA
Cmax vs distance from the release point
f building area coverage
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CONCLUSIONS

• A SIMULATION PROCEDURE FOR DENSE CLOUD DISPERSION
  ABLE TO EXPLOIT GENERAL PURPOSE CFD CODES HAS
  BEEN DEVELOPED

• THE BUOYANCY TREATMENT ADOPTED WAS SHOWN TO BE IN
  VERY GOOD AGREEMENT WITH WIND TUNNEL EXPERIMENTAL
  DATA FOR HEAVY PLUME DISPERSION AFTER A FENCE

• THE PROCEDURE WAS THEN APPLIED TO A SIMPLIFIED
  GEOMETRY MIMIKING A URBAN CANOPY
• THE BEFORE-RELEASE FLOW FIELD OBTAINED WAS FOUND
  TO BE IN EXCELLENT AGREEMENT WITH LITERATURE
  EXPERIMENTAL DATA

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CONCLUSIONS

• THE BUILDINGS HEIGHT AND FRACTIONAL AREA COVERAGE
  WERE FOUND TO STRONGLY INFLUENCE CLOUD DISPERSION
  DYNAMICS BY INCREASING VERTICAL AND SIDE
  DISPERSION RATES

• RESULTS INDICATE THAT FLAT-TERRAIN SIMULATIONS
  TEND TO OVERESTIMATE THE CONCENTRATION LEVELS
  REACHED DOWNSTREAM THE RELEASE POINT

• THE SIMULATION PROCEDURE HERE DEVELOPED MAY BE
  EMPLOYED TO GENERATE PSEUDO-EXPERIMENTAL
  INFORMATION FOR ASSESSING SIMPLIFIED MODELS
  ACCURACY

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THANK YOU FOR YOUR ATTENTION