goals and intent of cflow explosivity of lava domes estimate of gas overpressure heterogeneity of...
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GOALS AND INTENT OF CFLOW
EXPLOSIVITY OF LAVA DOMES
ESTIMATE OF GAS OVERPRESSURE
HETEROGENEITY OF GAS CONTENT IN FLOWS AND DOMES
GAS LOSS THROUGH CONDUIT WALLS
2-D PRESSURE STATE IN THE CONDUIT
MT UNZEN, JAPAN
MT ST HELENS, USA
CFLOW H. Massol, C. Jaupart
VISCOUS ANDCOMPRESSIBLEFLOW
INCOMPRESSIBLEFLOW
BUBBLY SECTION
MAGMA CHAMBER
Exsolution level
INTEGRATIONDOMAIN
SCHEMATIC VIEW OF A VOLCANIC CONDUIT
ORIGINALITY: 2-DMETHOD: Finite Element
= -2 e + 23
( .v) + Pg - K ( .v)
Pg: Gas pressure Shear viscosityK: Bulk viscosity
Pg = P + K ( .v)
RHEOLOGY (1)
VISCOSITY+COMPRESSIBILITY
GAS OVERPRESSURE
DOME EXPLOSIVITY
bR
Pg
o , p
f
Pml
K,
RHEOLOGY (2)
= pg -
2b
- 4µl
˙
RR2
b 31 -
b 3
R3[ r r ] r=R ( )
K =43µ
l
1 -
[ r r
] r=R = pm
- 3KR
R
p m
= p b-
2b
0
z
ra
h
H
zz = p s u = 0
zz = p atm
u = 0or rz = 0
u = 0w = 0
u = 0
rz = 0
DOMAINAND BC
BASIC EQUATIONS
Artificial time
Mass lumping
Petrov Galerkin weighting
[ D ] U = SU
[ D ] W = SW
[ M ] = S
Conservation of momentum
Conservation of mass
Criteres d’arret
10-2 10-8
Criteres d’arret
10-2 10-8
CAPABILITIES
VARIABLE MELT VISCOSITY
VARIABLE CONDUIT GEOMETRY
HORIZONTAL AND VERTICAL VELOCITY COMPONENTS
VARIABLE COMPRESSIBILITY
ASSUMPTIONS AND LIMITS
EQUILIBRIUM DEGASSING
ONLY VALID BEFORE FRAG. LEVEL
ANALYTICAL SOLUTION
HYPOTHESES
- No horizontal velocity - Constant compressibility - Constant viscosity
BUT: Gas pressure varies in both directions
Numerical model benchmark
RESULTS
PARABOLIC PRESSURE PROFILE ACROSS THE CONDUIT
DIMENSIONLESS NUMBER, D
PH-PaP0-pa
= P K+4/3
a2
H2= D
EXAMPLERESULT (1)
U = 0
= 106 Pa.s
x0 = 0.5 Wt%
P(0,H) = 0.56 MPa
OVERPRESSURE AT THE CENTER OF THE CONDUIT EXIT
Compatiblewith theanalytical solution
EXAMPLERESULT (2)
x0 = 0.5 Wt%
P(0,H) = 0.56 MPa
Variable viscosity(Hess and Dingwell, 96)
rz = 0
P(a,H) = 1.5 MPa
EXAMPLERESULT (3)
x0 = 0.5 Wt%
P(0,H) = 0.34 MPa
P(a,H) = 1.1 MPa
CONCLUSIONS
GAS PHASE IS OVERPRESSURED / DOME EXPLOSIVITY
HORIZONTAL PRESSURE GRADIENT / VITRIFIED MARGINS, HETEROGENEITY IN GAS CONTENT IN FLOWS AND DOMES
IMPORTANCE OF THE EXIT BOUNDARY CONDITIONS / CREASE STRUCTURE
FUTURE WORK
BOUNDARY CONDITIONS
- CONDUIT WALLS AND - COUPLING WITH FLOW
CRYSTALS
Fragmentation Level
Laminar Flow
Turbulent Flow
Nucleation of Bubbles
NUCLASCENT
1-D Finite differenceCylindrical GeometrySteady state
Variable viscosityNon-equilibrium degassing
(H. Massol, T. Koyaguchi)
EVOLUTION OF DISSOLVED WATER IN THE MELT
H=5000 ma = 50 m0 = 106 Pa.sx0 = 4wt%= 0.02 N m-1
D = 10-11 m2 s-1
H=5000 ma = 50 m0 = 106 Pa.sx0 = 4wt%= 0.02 N m-1
D = 10-11 m2 s-1
EVOLUTION OF PRESSURE AND NUMBER OF BUBBLES
OUTPUT OF THE MODEL
BUBBLE SIZES BUBBLE DENSITY
PRESSURE INSIDE BUBBLES
NEXT STEP: CONTINUOUS BUBBLE SIZE DISTRIBUTION