       Re: Solving Ordinary differential equations by NDSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg104985] Re: [mg104962] Solving Ordinary differential equations by NDSolve
• From: Matteo <matteo.diplomacy at gmail.com>
• Date: Sun, 15 Nov 2009 20:49:22 -0500 (EST)
• References: <200911151055.FAA09206@smc.vnet.net> <dcedc2b0911150628k67132db4o960a87b99556247b@mail.gmail.com>

```Could you report here the working nb file?
Thank you for youexplanation: this complex part in ODE annoyed me in my
problems.

Matteo

Pratip Chakraborty ha scritto:
> Hi,
> There is nothing to be panicked about the complex values of V.
> Just use Chop to get rid of the complex tail which is nothing but a
> artifact generated during numerical computation.
>
> You can see it yourself
> Plot[Evaluate[V[t]/.sol//Chop]*1000,{t,0,9},PlotRange->All]
> Grid[Table[{t,v[t]//Chop},{t,0,9,1}]]
> And also to see the residual error of the solution generated by
> NDSolve evaluate the following
> Er=(V'[t]==A Sqrt[2 (Po (V0/V[t])^gamma-Pa)/rho])/.a_==b_->
> (a-b)^2/.V[t]-> v[t]/.V'[t]-> v'[t];
> Grid[Table[{t,Er//Chop},{t,0,9,1}]]
> Now you will realize that there is nothing wrong with the solution.
> Regards,
> Pratip
>
> On Sun, Nov 15, 2009 at 11:55 AM, Allamarein
> <matteo.diplomacy at gmail.com <mailto:matteo.diplomacy at gmail.com>> wrote:
>
>     I'd solve this ODE:
>
>     V'[t] == A Sqrt[2 (Po (V0/V[t])^gamma - Pa)/rho
>     IC: V == V0
>
>     I wrote this code:
>
>     d = 2*10^-2 ;
>     A = d^2  3.14/4;
>     Po = 5 *101325;
>     Pa = 1*101325 ;
>     rho = 1000 ;
>     V0 = 5*10^-3 ;
>     gamma = 1.14;
>     sol = NDSolve[{
>        V'[t] == A Sqrt[2 (Po (V0/V[t])^gamma - Pa)/ rho],
>        V == V0},
>       {V}, {t, 0, 9},
>       MaxSteps -> 1000000, AccuracyGoal -> 10, PrecisionGoal -> 10];
>     v[t_] := V[t] /. sol[];
>     Plot[Evaluate[V[t] /. sol]*1000, {t, 0, 9}, PlotRange -> All]
>     Grid[Table[{t, v[t]}, {t, 0, 9, 1}]]
>
>     If it can be useful, I can underline units of these variables:
>     d [m]
>     P0, Pa [Pa]
>     rho [kg/m^3]
>     V [m^3]
>     gamma [--]
>     t [sec]
>
>     Running this code, V has got comlex part. This is impossible, because
>     it's a volume.
>     I should re-write my ODE in order NDSolve can digest better or I can
>     set an option in my code where I suggest " V must be positive and
>     real"?
>
>

```

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