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DE

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Magma = liquide cristaux bulles. Magma visqueux = Bulles peu mobiles ... Tomograph: 1000 x 1000 x 1000. Modeling: 100 x 100 x 200. Volumes (voxels) Resampling ... – PowerPoint PPT presentation

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Title: DE


1
DE LA SIMULATION NUMERIQUE EN VOLCANOLOGIE
Alain Burgisser
CNRS Institut des Sciences de la Terre dOrléans
2
ECOULEMENT DE CONDUIT
MODELISER LA CHIMIE DU DEGAZAGE
3
ECOULEMENT DE CONDUIT
MODELISER LA CHIMIE DU DEGAZAGE
Magma liquide cristaux bulles
4
ECOULEMENT DE CONDUIT
Magma visqueux Bulles peu mobiles Eruptions
potentiellement (très) dangereuses
A. Burgisser
USGS
5
ECOULEMENT DE CONDUIT
6
ECOULEMENT DE CONDUIT
Cons. masse
Cons. quantité mvt
7
ECOULEMENT DE CONDUIT
Profondeur (m)
Pression (bar)
Viscosité (Pa s)
8
ECOULEMENT DE CONDUIT MULTIPHASE
9
ECOULEMENT DE CONDUIT
- Croissance des bulles à taux contrôlé
expérimentalement -gt Dégazage - Nombre
variable de bulles par cellule -gt
Coalescence - Entrainement variable
bulles/liquide -gt Perméabilité
10
ECOULEMENT DE CONDUIT
Porosité
Pression
11
ECOULEMENT DE CONDUIT LES PROBLEMES
- Taux de décompression irréalistes - Sous
relaxation
12
PERMEABILITE DU MAGMA
Comment le gaz sort-il des bulles ?
Eichelberger, 1995
13
MICROTOMOGRAPHY METHOD
X-ray Computed microTomography (CT)
Resolution size of 1 voxel 6 18 mm
Volumes (voxels)
Tomograph 1000 x 1000 x 1000
Working 400 x 400 x 800
Modeling 100 x 100 x 200
Resampling
W. Degruyter
Ketcham (2005)
14
MICROTOMOGRAPHY THE SAMPLES
Tricks to obtain a good thresholding of a
connected network
1. Use 3D filters to preserve seamless volumes
2. Start by underestimate bubble size, so that
walls are preserved
3. Then expand the selection while preserving
these walls
Frothy Pumice (14.4 mm high)
Tube Pumice (4.8 mm high)
15
MICROTOMOGRAPHY THE APPARATUS
1. Seal the four sides 2. Fill the volume with
low-viscosity liquid 3. Apply pressure difference
between top and bottom 3. Solve Navier-Stokes
equation
Issues Precision of the solution Keep the flow
laminar
16
MICROTOMOGRAPHY RESULTS
Frothy Pumice
Tube Pumice
17
CONCLUSIONS ON TOMOGRAPHY
Strength
Weakness
Large samples High precision (2)
In situ
Small samples
Tomography
Resolution of connections
Small samples
Complementarity
Use in-situ to calibrate thresholding, use
resulting volume for 3D morphology measurements
Use tomography for small samples (experimental
products)
18
MODELISER LA CHIMIE DU DEGAZAGE
La chimie des gaz change durant une éruption
M. Coombs
Que signifient ces changements ?
19
LINK BETWEEN DEPTH AND SURFACE
20
LINK BETWEEN DEPTH AND SURFACE
21
MODELLING THE C-O-H-S SYSTEM
Chemical equilibrium balance in the gas phase
Solubility law
Mass balance
22
CHIMIE DU DEGAZAGE LE PROBLEME
- Chiffres significatifs
23
CONVECTION OF FLUID PARTICLES
A S S U M P T I O N S
Modeling of independent motions of fluid and
crystals Changes in temperature influence only
the fluid density
Conduction
Convection
constant T (cold)
Infinite Sill
constant T (hot)
A S S E S S E M E N T
Heating vs. Cooling Heat transfer conduction
vs. Heat tranfer convection Convection cells
24
SILL COOLED FROM ABOVE
30 vol. crystal
Temperature distribution
BURGIS11
25
SILL COOLED FROM ABOVE
Temperature distribution
Crystal distribution
26
INITIATION OF CONVECTION
1. Sill cooled from above
Days
Temperature
Liquid volume fraction
7.5 m height Liquid viscosity 144 Pa s 30 vol.
crystals
Ttop 890 C Tbase 900 C
27
CONVECTION WITH CRYSTALS ALMOST SYMMETRIC
Steady state 100 days Crystal gradient 200-300
days Plume travel 2 days
Characteristic Times
28
HEAT TRANSFER
Nusselt number
Qconduction (W/m2) k DT/Dx Qconvection (W/m2)
H DT
heat flux convection heat flux conduction
Nu ---------------------------
29
CONVECTION CELLS
DT 10 C, H 7.5 m
5 vol. crystals
10 vol. crystals
30 vol. crystals
20 vol. crystals
30
HEAT TRANSFER
31
CONVECTION MAGMATIQUE LES PROBLEMES
1. Taille de cellule 30 cm 2. Pas de temps 1
semaine sans cristaux 1 minute avec
cristaux 3. gt45 cristaux blocage général
32
CONVECTION MAGMATIQUE LE MODELE
33
CONVECTION CELLS
Convection can appear in the most crystalline
part...
Days
Temperature
Liquid volume fraction
7.5 m height Liquid viscosity 144 Pa s Crystal
concentration gradient
Ttop 870 C Tbase 950 C Heated from below
34
CONVECTION MAGMATIQUE PERSPECTIVES
1. Sapprocher du seuil rhéologique 2. Etablir
une carte des comportements convectifs 3.
Etablir les temps caractéristiques
pour atteindre létat déquilibre pour le
transport de cristaux par des instabilités
35
CONCLUSION CE NEST PAS DE LA SIMULATION !
A. Burgisser
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