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[Koranten]

Hi everyone,

I am trying to solve a 1-st order differential equation in space with Aspen Custom Modeler and I am having a problem with my boundary condition.

Since my ODE is 1-st order, I should need only ONE boundary condition, for instance T(0)=100, but ACM tells me that my problem is underspecified by one fixed variable. It seems ACM needs a second boundary condition. I do not understand that.

For instance, I could solve my ODE using Excel by setting only T(0)=100 with explicit Euler method, so mathematically speaking, this is ok. I don't get it.

When I want to solve a 2-nd order ODE, I am typing two boundary conditions (one Dirichlet and one Neuman), this is ok, and this works.

So what is the problem with my 1-st order?

Here is the code, very simple, but underspecified by one fixed variable :

***

n as integerparameter (20);
L as realparameter;
z as LengthDomain(highestorderderivative:1,
length:L,spacingpreference:L/n);

T as Distribution1D(XDomain is z) of temperature;

T(0)=100;

T(z.interior).ddx = 1;

End

***

Could someone help me?

Thanks a lot