Viscoelasticity in mantle convection

Transcription

1 Mgr. et Mgr. Vojtěch Patočka Supervised by: RNDr. Ondřej Čadek, CSc. 5th June 2015

2 Content Method of implementation Model Method Testing the method Thermal convection Mechanical deformation Results Numerically instaneous behaviour Time step decoupling Elastic episodes Construction of the constitutve equation 3D analogs Entropy production maximization

3 I2ELVIS - Taras Gerya staggered grid, markers regional studies, benchmarks 3D version not elastic yet - I3VIS Ellipsis - Moresi, Muhlhaus finite elements, markers used as integration points regional studies StagYY - Paul Tackley staggered grid, global studies, 3D, spherical shell not elastic yet

4 Model Extended Boussinesq - nondimensionalised equation of motion + continuity + thermal equation rheology p + σ = Ra T e z v = 0 T t = 2 T v T σ + D σ = 2ηD η(p, T ) = exp( A(T T ref ) + B(p p ref ))

5 Method Constitutive equation σ + D σ = 2ηD D(p, T ) := τ rel κ h 2 = ηκ Gh 2 σ := σ + ( v ) σ + (σw Wσ) a(dσ + σd) t advection co-rotation co-deformation Discretization (mixed euler, 1st order accurate) σ n = dt dt + D 2ηDn + D dt + D σn 1 + D dt dt + D (Wσ σw+a(dσ+σd))

6 Method σ n = dt dt + D 2ηDn + D ( ) σ n 1 + dt (Wσ σw + a (Dσ + σd)) dt + D σ n 1 := σ n 1 + dt ( Wσ σw + a ( Dσ + σd ) ) n 1/n σ n = 2η n num D n + Dn dt + D n σn 1 ; η num := dt dt + D η Viscoelasticity parameter Z Z := dt dt + D ; σn = 2 Z η n D n + (1 Z) σ n 1

7 Method dt n 1, n = stability( v n 1, D, d); dt a ; dt el T n = advection-diffusion (T n 1, v n 1, dt a ) σ n 1 = σ n 1 + deadvection (σ n 1, v n 1, dt a ) η n = η(t n ); η n num = η n num(η n, dt el, D n ) rhs n = ( D n dt el +D n σ n 1 ) Ra T n e z no decoupling: dt a = dt n 1, n dt el = dt n 1, n Stokes(η n num, v n, p n ) = rhs n σ n = σ n ( σ n 1, η n num, v n )

8 Thermal convection Harder - Journal of Non-Newtonian Fluid Mechanics, 39 (1991) 67-88

9 Thermal convection Co-rotational derivative (Jaumann, a = 0)

10 Thermal convection Upper convected derivative (a = 1)

11 Thermal convection Viscous case

12 Mechanical deformation Bending of an elastic slab (under gravity for 20 Kyr) ρ 1 = 4000; η 1 = 1 e27; G 1 = 1 e10; ρ 2 = 1; η 2 = 1 e21; G 2 = 1 e20;

13 Numerically instaneous behaviour Onset of convection is related to numerically instaneous elastic behaviour for Deborah number above certain critical value. Ra = 10 4 D critical =

14 Numerically instaneous behaviour Example of velocity singularity

15 Numerically instaneous behaviour Jaumann - parametric study, failed runs

16 Time step decoupling dt n 1, n = stability( v n 1, D, d); dt a ; dt el T n = advection-diffusion (T n 1, v n 1, dt a ) σ n 1 = σ n 1 + deadvection (σ n 1, v n 1, dt a ) η n = η(t n ); η n num = η n num(η n, dt el, D n ) Gerya / Moresi decoupling: multiple advection rhs n = ( D n dt el +D n σ n 1 ) Ra T n e z Stokes(η n num, v n, p n ) = rhs n σ n = σ n ( σ n 1, η n num, v n )

17 Time step decoupling dt n 1, n = stability( v n 1, D, d); dt a ; dt el T n = advection-diffusion (T n 1, v n 1, dt a ) σ n 1 = σ n 1 + deadvection (σ n 1, v n 1, dt a ) η n = η(t n ); η n num = η n num(η n, dt el, D n ) rhs n = ( D n dt el +D n σ n 1 ) Ra T n e z convection decoupling: interpolation needed Stokes(η n num, v n, p n ) = rhs n σ n = σ n ( σ n 1, η n num, v n )

18 Time step decoupling Realistic case: separate viscous and viscoelastic parts of the domain Z := dtel dt el + D ; σn = 2 Z η n D n + (1 Z) σ n 1 bulid-up relaxation Desired behaviour: viscoelastic material does not move across many cells within elastic time step (viscous material can - elastic time step does not play any role in viscous part of the domain). D = ηd 0 ; dt el = 0.01 τ rel κ h 2 = 0.01 D 0; Z = η ; 0.01

19 Time step decoupling Jaumann - failed runs, decoupled

20 Time step decoupling Upper convected - failed runs, decoupled

21 Time step decoupling Jaumann - not trusted runs, decoupled

22 Time step decoupling Critical Deborah, time step decoupled

23 Time step decoupling Critical Deborah, time step decoupled: elastic strain rate

24 Time step decoupling Critical Deborah, time step decoupled: upper convected

25 Elastic episodes

26 Elastic episodes Undercritical Deborah number - time step/scheme convergence

27 Elastic episodes Overcritical Deborah number - time step convergence

28 Elastic episodes Overcritical Deborah number - time scheme convergence

29 Elastic episodes Time step analysis - overcritical Deborah number

30 Four ways to derive our rheological equation 1D analogs objective rate not specified Entropy production maximization dissipation needs to be prescribed objective rate specified 3D analogs - is it new? objective rate specified Microscopical model e.g. dumbbells diluted in newtonian fluid We need to know how the equation was derived for interpretation: elastic strain rate, elastic energy, dissipation...

31 3D analogs 3D analogs Geodynamical texts often use σ ij = 2η ε ij - incorrect Four possible strain tensors: eulerian/lagrangian, full/linearized ε lin := 1 2 ( u + ( u)t ); ξ lin := 1 2 (GradU + (GradU)T ) ε := 1 2 (I F T F 1 ); ξ := 1 2 (FT F I) which have following material time derivatives ε lin = D 1 2 (( u)l + LT ( u) T ) ε = D (εl + L T ε) ξ lin = D 1 2 (( u)t L T + L( u)) ξ = F T D F

32 3D analogs Maxwell rheology is based on this idea: Elastic and viscous deformations add together, but stress is the same for both elastic and viscous parts of the deformation τ = p + σ; D = D vis + D el σ = 2ηD vis = 2Gε el Choice of ε or ξ or ε lin or ξ lin gives us the relation ε el D el and thus specifies the rate used in our rheology (only for ε we get an objective rate - lower convected one)

33 Rajagopal Entropy production maximization Motivation - interpretation of physical quantities: what is dissipation, elastic strain rate, elastic energy... σ : v = σ : D = σ : σ 2η + σ : σ 2G Dissipation? Rate of change of elastic energy?

34 Rajagopal Oldroyd B model - ICC Thermodynamic considerations e(δ, ρ, B Kp(t)) = e 0 (δ, ρ) + G 2ρ (trb K p(t) 3 log det B Kp(t)) ζ = (T G(B Kp(t) I)) d : D d + G(C Kp(t) I) : D Kp(t) Prescribed dissipation Resulting model ζ = 2η 2 D 2 + 2ηD Kp(t) C Kp(t) : D Kp(t) T = pi + 2η 2 D + G(B Kp(t) I) B Kp(t) = G η (B K p(t) I)

35 Rajagopal After setting η 2 = 0, we get Maxwell model T = pi + G(B Kp(t) I) B Kp(t) = G η (B K p(t) I) T = pi + σ σ + τ rel σ = 2ηD But now we now the dissipation of our model, we prescribed it So the question is ζ = 2ηD Kp(t) C Kp(t) : D Kp(t) 2ηD Kp(t) C Kp(t) : D Kp(t)? = σ : σ 2η

36 Rajagopal By using σ = 2ηD τ rel σ and σ = G(BKp(t) I), this question reduces to (kinematic consideration regarding the natural configuration needed): (F Kp(t) D Kp(t) F T K p(t) ) : (F K p(t) D Kp(t) F T K p(t) )? = which is clearly not satisfied? = (D Kp(t) F T K p(t) F K p(t)) : D Kp(t)

37 Rajagopal Conclusions We used a grid based method to implement viscoelastic rheology into a simple thermal convection model Time step decoupling which could be trusted in realistic cases was proposed Simulations show that locally high stresses are produced by viscoelasticity, which means that viscoelasticity could trigger plasticity in visco-elasto-plastic code

38 Rajagopal Thank you