Emergent Anisotropy and Flow Alignment in Viscous Rock by Hans Mühlhaus, Louis Moresi, Miroslav...
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Transcript of Emergent Anisotropy and Flow Alignment in Viscous Rock by Hans Mühlhaus, Louis Moresi, Miroslav...
Emergent Anisotropy and Flow Alignment in
Viscous Rock
by
Hans Mühlhaus,Louis Moresi, Miroslav Cada
May 5-10
Outline
•The last time…. (Hakone): Folding instabilities in layered rock using director theory combined with pressure solution, mobile and immobile phases, novel computational scheme
Publications: - Louis Moresi, Frédéric Dufour, Hans Mühlhaus, Mantle convection models with viscoelastic/brittle lithosphere: Numerical methodology and plate tectonic modeling, PAGEOPH, submitted 2001- Muhlhaus,H-B, Dufour,F, Moresi, L, Hobbs, BE (2001) A director theory for viscoelastic folding instabilities in multilayered rock (30 pages) submitted to the Int. J. Solids and Structures-H-B Mühlhaus, L.N Moresi, B. Hobbs, and F. Dufour (2000)Large Amplitude Folding in Finely Layered Viscoelastic Rock Structures, PAGEOPH, submitted 2001-Hobbs, B.E., Muhlhaus,H-B, Ord, A and Moresi, L. (2000) The Influence of Chemical migration upon Fold Evolution in Multi-layered Materials. Vol. 11, Yearbook of Self Organisation. Eds H.J. Krug and J.H. Kruhl; Duncker&Humblot , Berlin , 229-252
•Today: Oriented materials and emergent anisotropy in simple shear and natural convection; thermal coupling in simple shear and …convection
Finite AnisotropyDirector evolution
n : the director of the anisotropyW, Wn : spin and director spinD, D’: stretching and its deviatoric part
)( kikjkjkiijn
ij DDWW
ij nin j
jn
iji nWn
Rotations
Spin of an infinitesimal volume element
1,2v 2,1v
2,11,22
1vv average
Spin of microstructure
n n
Undeformed ground state:
n.Wn.0v
v0n n
1,2
1,2
Anisotropic Viscous Rheology
If the director is oriented parallel x2:
122112
1111
2
,2
D
and
pD
S
,2 2222 pD
ijklijklSijij pDD '' )(22
General case; n notparallel x2:
Microstructures in Polycrystalline Materials during Deformation
Moving integration points
nodepnn
tp
ttp xNvtxx )(
We interpolate the nodal velocities usingthe shape functions to update theparticle positions.
t is chosen “small” for accuracy purpose.
The material history and stress ratesare stored on particles.
Orthotropic folding (click picture to play movie)
Example 1
Evolution of folding in anisotropic visco-elastic layer. Isotropic embedding material has viscosity 0.001, layer has shear viscosity 0.001, normal viscosity 1. Results are shown for perturbation to the director orientation with wavenumber 2q and 10q . Notice change in dominant mode
for 10q ;Deborah number De=VL
Flow Alignment in Simple Shear
Simple shear, same boundary conditions as in previous example however the director orientations are initially random: every particle of our advection scheme gets its own spatially randomly distributed director orientation; = angle between the x2 and the director orientation ranges between and -The top row are contour lines of . The deformation increases from (a) to (b). The green line (bottom) row, Figures (a)-(c) show initial and displaced particle positions. The parallel lines in (b) and (c) follow from the periodic boundary conditions
Extension with-and without yielding (click
picture to play movie)
…Nonlinear rheology, taken in the broadest sense, may be the single most important aspect of the behaviour of earth materials…Schubert, Karato, Olson, TurcotteFrom Outline of IMA Workshop Nonlin. Cont. Mech., Rheology and the Dynamo
Shear Histories simple shear and shear alignment
with shear heating and temperature dependent viscosity
maxv
maxT
10
12
22
maxT
ntdisplacemetop
alignment
maxv
maxT
12
22
5
0Tand0vv:0x
1TTand0v;1:1x:s.'c.B
1HlayershearofWidth
ntdisplacemetop
2,212
ref2122
maxv
0
190
p0
0p0
0
RT
Qexp108400
D
HVcPe79.0
HTc
VDi
0
Shear-Heating:Director Field and
Temperature Contours
10.1ntdisplacemeTop
Shear Alignment with Shear Heating and Temperature Dependent Viscosity
12
22
12
22
maxT
alignment
ntdisplacemetop
Director Models
Liquid Crystals: de Gennes & Prost, 1972, 1993Geophysics: U Christensen, 1984 (post –glacial rebound, mantle convection)
Director Evolution(U CH.):Transforms as line element
Present Model:Transforms as surface normal vector
nvnvn )()(t,
))()(t, nvnvn
Director ModelsSteady State
The director evolution equation has a steady State solution in which the director is point-wiseoriented normal to the velocity vectors.Solution maybe non-unique however…….
Proof that
0nnthat
followsit0vvandv
vcesin
if0)nv()nn(v
nvnv
i,kk,i
2,21,11
2
i,kki,kk,ik
ki,kk,ik
n
vn
nv is a particular solution for steady states:
Stability of Normal Director Solution
Represented are 2 solutions:One assuming director normal to velocity and one where the 1st
10 steps are run assuming normality and subsequent steps are integrated using full
director evolution equation.
ntdisplacemetop
maxv
maxv
10
15
5
nDissipatio
maxT
61053.0Ra,27.0Di
dtvmax
Convection with Shear HeatingFull director evolution ; Di=0.25;
Ra=1.2x106
maxvx5.0
alignment
ndissipatiox5.0
dtvmax
Director Alignment
05.0dtvmax
05.0dtvmax
Degree of Alignment
nv1221 vnvn
sin
Director Alignment in ConvectionRa=0.5x106
)(
2 3
SNa
ThqR
Conclusion
• Rheology for layered materials as a basic unit (building stone) for more complex rheologies, modelling of crystallographic slip planes etc• director orthogonal to velocity vector in steady state •Orthogonal solution seems stable in convection•Mean shear strain of approx 6 required for alignment in simple shear•Examples include thermal coupling and influence thereof on alignment in simple shear, various convection studies•Codes used: Fastflo, Ellipsis
Seismic Anisotropy
Convection with Ra =500.000
Isotropy
Isoterms
Anisotropy
Velocity Field
Streamfunction
)(
2 3
SNa
ThqR