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MathGroup Archive 2007

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Re: NDSolve doesn't stop

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74371] Re: NDSolve doesn't stop
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Mon, 19 Mar 2007 21:53:39 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <etikd4$j5c$1@smc.vnet.net> <etlchi$nrv$1@smc.vnet.net> <etlq6u$r9h$1@smc.vnet.net>

bar at ANTYSPAM.ap.krakow.pl wrote:
> Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com> wrote:
>>> notebook file is "not readable" so I put it on www
>>> http://robox.ap.krakow.pl/~bar/pde.nb
>> tmax = 20 ==> no warning message issued (indeed for any 0 < tmax <)
>> tmax = 21 ==> a warning message is issued: it might be worth reading it
>> =========
> 
>>   NDSolve::"eerr" : "Warning: Scaled local spatial error estimate of 
>> (159.0346060966196) at (t) = (21.) in the direction of independent 
>> variable (x) is much greater than prescribed error tolerance. Grid 
>> spacing with (15) points may be too large to achieve the desired 
>> accuracy or precision. A singularity may have formed or you may want to 
>> specify a smaller grid spacing using the MaxStepSize or MinPoints 
>> options. More...
> 
>  There is a problem, I can't see this message,
> message window i empty, noetebook i running continuously,
> without any message too,
> 
> (Mathematica 5.1
> Linux localhost 2.6.19-gentoo-r5 #10 SMP PREEMPT Tue Feb 20 21:34:30 CET 2007 i686 Intel(R)
> Pentium(R) 4 CPU 3.00GHz GenuineIntel GNU/Linux)
> 
> top shows 0%CPU, 0%Memory
> 
> When i try MaxStepSize->0.001
> thet top shows 99%CPU
> and %MEM is growing up to 100% (in about 5 minutes)
> and %CPU decreases to 0%
> 
> In this time I killed process ...
> 
> 
> There is a problem with my individual installation ?
> 
> The system of equations is not difficult, without nonlinearity,
> I can solve it "hand-made" , by reduction to ODE equation
> by separation t/x variable
> 
> I decided to use Math because i hoped to see solution
> in addition nonlinearity ...
> 
> Regards , Olaf
> 
> 
> 

Hi Olaf,

Of course, if you do not get any warning message, it is difficult to 
know what to do. Basically, the help page associated to the warning 
message advise you to adjust the options PrecisionGoal and 
StartingStepSize. (The higher precision you want, the smaller the step 
size must be.) For instance,

tmax=22;
solution =
NDSolve[{eq1,eq2,w1,w2,w3,w4,wt1,wt2,wt3,wt4},{ET,FI},{x,0,1},{t,0,tmax}, 
PrecisionGoal->6, StartingStepSize->0.0001]

Regards,
Jean-Marc


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