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I want to solve a system of nonlinear two differential equations (say, in $\theta(x,t)$ and $\phi(x,t)$ with NDSolve, but I want to stop the evaluation when one of the functions ($\phi(x,t)$) reaches zero, or, for instance, a certain value $\bar{\phi}$.

Because then I would like to study how, where and (of course) when this pinch-off happens.

What is the best way of doing this, in your opinion?

Putting a constraint before running NDSolve?

I tried with WhenEvent, adding a line WhenEvent[\[Phi][x, t] == 0, Plot[\[Phi][x, T], {x, 0, L}] in the code, but it gives me

NDSolve::litarg:
To avoid possible ambiguity, the arguments of the dependent variable in  WhenEvent
(...)should literally match the independent variables

Here is the link to the shortest code I can manage to give:

https://www.dropbox.com/s/4t5o53lppb2itk8/mma3.nb

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Use WhenEvent –  Szabolcs Mar 20 at 15:26
    
How can I deal with the fact that I have two variables? I mean, if I write WhenEvent[$\phi$[x,t]==0, Tsing=t], I don't think it would work... –  usumdelphini Mar 20 at 15:40
    
Wouldn't it be better to use Method -> {"EventLocator", "Event" -> phi[x,t]==0}] –  usumdelphini Mar 20 at 15:43
    
WhenEvent is the new way to do it, introduced in v9. It should do everything that EventLocator did and more. Sorry, no time to check the details right now ... –  Szabolcs Mar 20 at 15:52
1  
The problem here is that the integration proceeds along the t axis (Mathematica uses the method of lines), so asking when f[x,t]==0 is not a well defined question. I guess you mean f[x,t]==0 for all x, but that is unlikely to happen. So the condition should probably be something else. –  Szabolcs Mar 20 at 20:10

2 Answers 2

up vote 3 down vote accepted

What Silvia wrote about the method of lines is basically true. The problem is that NDSolve tries as much as possible to "hide" the specifics about the discretization procedure so you can't use something like u[t,x] in a WhenEvent for a PDE as x would mean "any x" and that would be inifintely many tests. But for exactly that use-case there is the possibility to get a list of values at the actual discretization points instead which makes the formulation of the desired WhenEvent almost trivial and quite fast. The trick is to use "DiscretizedMonitorVariables" -> True for the "MethodOfLines" method. u[t,x] will then in fact be a vector of as many values as the method of lines uses in the spatial direction (and I think it should handle all cases with nontrivial spatial discretizations as well):

NDSolveValue[{
  D[u[t, x], t] == D[u[t, x], x, x],
  u[0, x] == 0,
  u[t, 0] == Sin[t],
  u[t, 5] == 0,
  WhenEvent[Min[u[t, x]] < -10^-4,
   tEnd = t; "StopIntegration"
   ]
  }, u, {t, 0, 10}, {x, 0, 5},
 Method -> {"MethodOfLines", "DiscretizedMonitorVariables" -> True}
 ]

For more information, see tutorial/NDSolveWhenEvents#24434 in the documentation center (or search for "Avoiding Wraparound in PDEs" here) and of course tutorial/NDSolveOverview is always a good place to learn about the details of NDSolve...

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Thanks a lot, it was enlightening! –  usumdelphini Mar 21 at 16:22
    
This is a true answer! –  Silvia Mar 21 at 17:02

I haven't seen your particular PDEs, but I don't think your can numerically evaluate the target functions in the NDSolve scope, as the numerical solutions (expressed as InterpolatingFunction objects) hasn't been constructed.

If it were me, I may consider doing the plotting afterward:

sol = NDSolve[{
                y''[t] == -1, y[0] == 1, y'[0] == 0,
                WhenEvent[
                           y[t] == 0,
                           tEnd = t; "StopIntegration"
                         ]
              }, y, {t, 0, 2}];
Plot[y[t] /. sol, {t, 0, tEnd}]

Update:

I think I overlooked the question, and the real difficulty here might be related to the numerical method of lines, which is the way how NDSolve deals with this kind of PDEs.

According to the documentation, with the NML, the directions of boundary-value problem are considered as space dimensions, and the direction of initial-value problem is considerd as time dimension. All space dimensions are discretized, but the numerical integrations are only taken along the time dimension. So it's many lines start from some points in the space space at time zero, and evolve over time, thus the internal integrations are all one-dimensional and the "StopIntegration" action can only be taken along time. This is the reason you got the error in your comment.

One possible (and only partial) solution is specifying all space variables in WhenEvent, leaving only the time variable. Here is a simple demonstration:

Module[{sol},
       sol = NDSolve[{
                      D[u[t, x], t] == D[u[t, x], x, x],
                      u[0, x] == 0,
                      u[t, 0] == Sin[t], u[t, 5] == 0,

                      (* the space variable x, i.e. #, is fixed value: *)
                      WhenEvent[u[t, #] - .1 == 0,
                                tEnd = t; "StopIntegration"
                               ]

                      },
                      u, {t, 0, 10}, {x, 0, 5}];
       Plot3D[
              Evaluate[-(#/10) + u[t, x] /. sol], {t, 0, tEnd}, {x, 0, 5},
              PlotRange -> All, 
              PlotStyle -> ColorData["Rainbow"][1 - Rescale[#, {0, 5}]], 
              Mesh -> False]
      ] & /@ Range[0, 5, .5] // 
 Show[#, AxesLabel -> (Style[#, 20, Italic, Bold] & /@ {t, x, u}), Lighting -> "Neutral"] &

integration can only be stop along time

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It gives me always NDSolve::litarg: To avoid possible ambiguity, the arguments of the dependent variable in WhenEvent (...)should literally match the independent variables I think that this might be related with having 2 independent variable –  usumdelphini Mar 20 at 17:27
    
@usumdelphini Sounds like syntax problems to me. Could you add your whole PDEs and mma code in the question? –  Silvia Mar 20 at 17:33
    
I've just linked a minimal .nb, unfortunately I cannot do any better, but essentially I do not think it is matter of syntax, because if I remove the line "WhenEvent[...]", everything works fine. –  usumdelphini Mar 20 at 17:45
1  
@usumdelphini Looks like you're using mma 7 while WhenEvent is introduced in version 9. Sorry for the confusion... I don't have a version 7, will delete this answer soon. –  Silvia Mar 20 at 17:49
1  
Thanks, now it actually gives me another error (sorry to bother you so much), NDSolve::nbnum1: The function value InterpolatingFunction[{{0.,10.}},{4,7,0,{25},{6},0,0,0,0,Automatic},<<1>>,{Devel‌​oper`PackedArrayForm,{0,<<25>>},{2.005,2.00496,2.00483,2.00462,2.00433,2.00397,<<‌​13>>,1.99603,1.99567,1.99538,1.99517,1.99504,1.995}},{Automatic}][x]==0 is not True or False when the arguments are {0.,<<3>>,{-0.000916735,-0.000908904,-0.000885598,-0.000847226,-0.00079441,-0.00‌​072804,-0.000649232,<<11>>,0.000649232,0.000728807,0.000795892,0.000849321,0.0008‌​88165,0.000911767,0.000919698}}. >> –  usumdelphini Mar 20 at 19:11

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