06.08.2013 Views

Sequential Methods for Coupled Geomechanics and Multiphase Flow

Sequential Methods for Coupled Geomechanics and Multiphase Flow

Sequential Methods for Coupled Geomechanics and Multiphase Flow

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

3.6. DISCRETE STABILITY OF THE NONLINEAR PROBLEM 53<br />

⎡<br />

For the undrained split, we have<br />

⎣ dun<br />

dp n<br />

⎤<br />

−→<br />

⎡<br />

dp ∗<br />

⎤<br />

−→<br />

⎡<br />

⎦ Au ud ⎣ dun+1<br />

⎦ Ap<br />

ud ⎣ dun+1<br />

⎤<br />

dp n+1<br />

⎦, where<br />

⎧<br />

⎪⎨<br />

⎪⎩<br />

A u ud<br />

: Div dσ = 0, δdm = 0,<br />

A p<br />

ud : ˙<br />

dm + Div dw = 0,<br />

dε ˙ = 0, dεp<br />

˙ = 0, dξ ˙ = 0,<br />

(3.89)<br />

which has homogeneous boundary conditions with no source terms. When we solve the<br />

mechanical problem A u ud<br />

dΨ d<br />

dt =<br />

<br />

<br />

Ω<br />

by the undrained split, we have<br />

<br />

dσ : ˙<br />

dε + dp<br />

ρf,0<br />

<br />

dm ˙ dΩ −<br />

<br />

dσ ′ : dεp<br />

˙ + dκ · ˙<br />

<br />

dξ dΩ<br />

Ω <br />

D<br />

<br />

d p≥0 = dσ : ˙ d<br />

dεdΩ − Dp Ω<br />

= −D d p ≤ 0. (3.90)<br />

Note that the operator <strong>for</strong> flow in the undrained split is the same as that in the drained<br />

split, A p<br />

ud is the same as Ap<br />

dr . Thus, the undrained split satisfies the contractivity condition<br />

<strong>for</strong> both mechanics <strong>and</strong> flow.<br />

3.6 Discrete Stability of the Nonlinear Problem<br />

As shown in the previous section (Armero, 1999), the undrained split honors the dissipative<br />

properties of the coupled problem. But, we need to find which algorithm (i.e. time inte-<br />

gration technique) provides unconditional stability - so called B-stability. By B-stability<br />

we mean the discrete counterpart of contractivity (Simo, 1991; Simo <strong>and</strong> Govindjee, 1991;<br />

Simo <strong>and</strong> Hughes, 1998), i.e.,<br />

<br />

dχ n+1 N ≤ dχ n N ∀n. (3.91)

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!