again: nonlinear fit...
- To: mathgroup at smc.vnet.net
- Subject: [mg52100] again: nonlinear fit...
- From: db at ict.fhg.de (julia)
- Date: Thu, 11 Nov 2004 04:51:58 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Hello,
I've posted some days before and i didn't get an answer.
I think my question was not detailed enough and therefor I'll
try it again.
My experimental data have to be fit to this model:
lamkonv[x_, {v_, Dz_, L_}] :=
1/(2*v^2*x^3)*((Dz*x/Pi)^0.5*(L*Exp[-L^2/(4*Dz*x)] - (L + v*x)
*Exp[-(L - 2*v*x)^2/(4*Dz*x)]) + 0.5*(L^2 + Dz*x)*
(Erf[L/(2*(Dz*x)^0.5)] - Erf[(L - 2*v*x)/(2*(Dz*x)^0.5)]))
where Dz,v,L are the parameters.
(It is a 2-dim. convective-diffusion model)
I've tried an own code, nonlinearFit, NonlinearRegress and other
optimization
algorithms (quasiNewton..) ad I couldn't work it out.
1.The fit runs in a physically wrong range or
2.I get imaginary values for the parameters or
3. I get the "nrlnum" -message or
4. The fit does nothing
Especially the parameter Dz behaves very strange. The value of Dz i
about 10^3 times smaller than the other parameters. I've read in a
posting before, that this may cause problems.
Can anybody help me with that?