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I'm trying to solve a set of coupled pde equations of functions C1[t,x], C2[t,x]. all works fine but I need to specify a conditional initial conditions of the form:

when (t=t.min) { C1 = if (x=x.min) 1 else 0 C2 = 0;

I've tried to set:

C1[0,x]==0, (C1[0, x] /. x -> 0) == 1

or use 

WhenEvent

but in either case I got:

NDSolve::bcedge: Boundary condition C1[0,0]==0 is not specified on a single edge of the boundary of the computational domain.

Is there any way around it?

Many thanks,

This question is a part of the post at http://community.wolfram.com/groups/-/m/t/1313375

edited:

Thanks. The code is:

Fp = 0.016666667; Vp = 0.07; Wp = 0.94; Dp = 10^-18; PSg = 0.05; Visfp = 0.35; Disf = 10^-18; L = 0.1; sol = NDSolve[{Derivative[1, 0][C1][t, x] == -Fp*L/Vp*Derivative[0, 1][C1][t, x] - PSg/Vp*(C1[t, x]/Wp - C2[t, x]) + Dp*Derivative[0, 2][C1][t, x], Derivative[1, 0][C2][t, x] == PSg/Visfp*(C1[t, x]/Wp - C2[t, x]) + Disf*Derivative[0, 2][C2][t, x], C1[t, 0] == Exp[-t], C2[t, 0] == 0, C1[t, L] == 0, C2[t, L] == 0, Derivative[0, 1][C1][t, 0] == 0, Derivative[0, 1][C2][t, 0] == 0, Derivative[0, 1][C1][t, L] == 0, Derivative[0, 1][C2][t, L] == 0, C1[0, x] == 1, C2[0, x] == 0}, {C1, C2}, {t, 0, 20}, {x, 0, L}] Plot3D[Evaluate[C1[t, x] /. sol[[1]]], {t, 0, 20}, {x, 0, L}]

I've tried:

\[Phi][x_] := Piecewise[{{1, x == 0.}, {0, x > 0}}];

but it freezes the kernel (calculations never stop).

The solution should look like:

enter image description here

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    $\begingroup$ Its hard to test without the exact set of equations you are solving, but you might try to put your condition in a Piecewise or UnitStep form. $\endgroup$ Commented Apr 3, 2018 at 22:28

1 Answer 1

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With a few changes we can get some answers.

Clear["Global`*"]

Data

Fp = 0.016666667; Vp = 0.07; Wp = 0.94; Dp = 10^-18; PSg = 0.05; Visfp = 0.35; Disf = 10^-18; L = 0.1;

PDE's

eq1 = Derivative[1, 0][C1][t, x] == -((Fp*L*Derivative[0, 1][C1][t, x])/Vp) - (PSg*(C1[t, x]/Wp - C2[t, x]))/Vp + Dp*Derivative[0, 2][C1][t, x]; eq2 = Derivative[1, 0][C2][t, x] == (PSg*(C1[t, x]/Wp - C2[t, x]))/Visfp + Disf*Derivative[0, 2][C2][t, x];

bc's and ic's

bc1 = C1[t, 0] == Exp[-t]; bc2 = C2[t, 0] == 0; bc3 = C1[t, L] == 0; bc4 = C2[t, L] == 0; bc5 = Derivative[0, 1][C1][t, 0] == 0; bc6 = Derivative[0, 1][C2][t, 0] == 0; bc7 = Derivative[0, 1][C1][t, L] == 0; bc8 = Derivative[0, 1][C2][t, L] == 0; (*ic1=Piecewise[[{{1,x==0},{0,x>0}}];*) ic1 = C1[0, x] == Exp[-10000*x]; ic2 = C2[0, x] == 0;

Note I have commented out the Piecewise condition. I got NDSolve to give answers with that ic, but the solution was fairly unstable and not satisfactory. I have replaced the ic with a steep exponential fn that closely approximates the ic you want. Numerical analysis by its very nature is approximate, and if you need to replace a sharply varying condition with a smoother function that is a reasonable approximation, that is a good idea. I do not use bc5 and bc6 because NDSolve can get those derivatives from the ic's making them redundant and with the exponential ic, bc5 is conflicting.

Continuing

sol = NDSolve[{eq1, eq2, bc1, bc2, bc3, bc4,(*bc5,bc6,*)bc7, bc8, ic1, ic2}, {C1, C2}, {t, 0, 20}, {x, 0, L}];

Check our solution as to bc's and ic's

Plot[C1[0, x] /. sol[[1]], {x, 0, L}, PlotRange -> All]

enter image description here

Plot[C1[t, 0] /. sol[[1]], {t, 0, 20}, PlotRange -> All]

enter image description here

Plot[C2[0, x] /. sol[[1]], {x, 0, L}]

enter image description here

Plot[C2[t, 0] /. sol[[1]], {t, 0, 20}]

enter image description here

The bc's and ic's look numerically reasonable.

Plot3D[C1[t, x] /. sol[[1]], {t, 0, 20}, {x, 0, L}, PlotRange -> All]

enter image description here

Plot3D[C2[t, x] /. sol[[1]], {t, 0, 20}, {x, 0, L}, PlotRange -> All]

enter image description here

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  • $\begingroup$ thank you so much! this helps a lot. $\endgroup$ Commented Apr 6, 2018 at 20:19

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