       Re: Solving Ordinary differential equations by NDSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg104975] Re: [mg104962] Solving Ordinary differential equations by NDSolve
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Sun, 15 Nov 2009 20:47:27 -0500 (EST)

```It makes no sense to enter Pi to two decimal places. In general, enter all constants exactly and let the subsequent processes define the overall precision.

d = 2*10^-2;
A = d^2 Pi/4;
Po = 5*101325;
Pa = 1*101325;
rho = 1000;
V0 = 5*10^-3;
gamma = 114/100;

sol = V /. 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][];

Use Chop to eliminate the numeric noise (imaginary values smaller than your accuracy and precision).

v[t_] := Chop[sol[t]]

Plot[v[t]*1000, {t, 0, 9},
PlotRange -> All]

Grid[Table[{t, v[t]}, {t, 0, 9, 1}]]

Bob Hanlon

---- Allamarein <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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