Torsten Hennig <Torsten.Hennig@umsicht.fhg.de> wrote in message <735098493.260351.1275405399232.JavaMail.firstname.lastname@example.org>...
> > "Hello,
> > I'm trying to solve a system of 3 partial
> > differential equations (3 diffusion equations, 2nd
> > law of Fick) using ode15s! First of all, I
> > transformed my system in an ODE system using the
> > finite difference method to discretize each equation.
> > After, I attached the boundary conditions , that
> > after the discretization are become algebraic
> > equations , and so in the end I' m obliged to solve a
> > DAE system. From my readings I understood that I have
> > to use ode15s to solve M*y'=f(t,y)! I defined the
> > matrix M, by putting 0 where ever I have an algebraic
> > equation, and I 've created a separate function file
> > for f(t,y), in which I defined the coefficients of
> > the variables. The function f(t,y) has the form
> > B(n,n)*C(n), where B is the matrix with the
> > variable's coefficients and C is the variable vector!
> > After the first run of the code I obtained negative
> > results so I thought that it's best to
> > adimensionalize the variables, and I did so, and now
> > I'm confronting with a warning message :'Warning:
> > Failure at t=3.124127e+00. Unable to meet integration
> > tolerances without reducing the step
> > size below the smallest value allowed (7.105427e-15)
> > at time t. '' . I know that the coefficient 's matrix
> > is ill conditionned but what I don't know is how to
> > handle this problem!""
> > I forgot to mention that the equations are coupled
> > and that I'm trying to simulate the transfer of a
> > specie through three different phases ( that's why I
> > have 3 basic diffusion equations).
> Did you try MATLAB's pdepe ?
> Best wishes
So I did what you told me and the answer form matlab is :
1.0000e+00 1.0000e+00 1.0000e+00 1.0000e+00 1.0000e+00
1.0000e+00 1.0000e+00 1.0000e+00 9.9999e-01 9.9357e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9996e-01 9.8743e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9990e-01 9.8149e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9983e-01 9.7574e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9974e-01 9.7013e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9963e-01 9.6466e-01
1.0000e+00 1.0000e+00 1.0000e+00 9.9950e-01 9.5932e-01
1.0000e+00 1.0000e+00 9.9999e-01 9.9936e-01 9.5414e-01
1.0000e+00 1.0000e+00 9.9999e-01 9.9920e-01 9.4951e-01
== 1.0000e+00 1.0000e+00 9.9999e-01 9.9903e-01 9.5065e-01=== 11th time step
1.0000e+00 1.0000e+00 9.9999e-01 9.9894e-01 1.0235e+00
1.0000e+00 1.0000e+00 1.0000e+00 1.0166e+00 1.4504e+01
1.0000e+00 1.0000e+00 1.0003e+00 1.2295e+00 1.7303e+02
1.0000e+00 1.0000e+00 1.0039e+00 3.9296e+00 2.1823e+03
1.0000e+00 1.0001e+00 1.0499e+00 3.8166e+01 2.7658e+04''
it seems that is working well until it reach the 11th time step! Anyway I know that my J matrix is not stable because the rcond is 1.475e-10, but from my point of view my equations are written correctly.
I tried to use' pdepe ' but I have problems to impose two conditions at the same boundary ( that are the flow continuity and an equilibrium condition; the flow continuity is like : D_liq*dcliq/dx=D_gaz*dcgaz/dx (x= liquid height ) and cgaz=P*cliq, where P is a partition coefficient)!
Thank you very much for answering me and if you have any other ideas please tell me!