simple coil with core

Hello,

I started to work with Flux. I modeled an unmeshed coil with an iron core inside. I used the application ‘’magneto static 3D’’ and the 'automatic formulations'. The dimensions of the coil are L_coil=0.4m (length), R_coil=0.05m (radius) und N=1600 (number of turns). The dimensions of the core are L=0.4m, R=0.02m, my_r=1200 (reltive permeability of the material). In Flux the material is described by the magnetic property 'linear isotropic'. The current through the coil is I=0.5A. To verify my solutions I made a calculation for the magnetic flux density and the magnetic field in the middle of the coil with the simple formula: H=I*N/sqrt(L_coil²+(2*R_coil)²)=1940.285A/m and B=my_r*my₀*H=2.926T. But with my simulation model I get the values H=110.58A/m an B=0.167T. These are high differences. Does somebody know what my fault could be? I checked all input Parameter and my calculation. When I modeled the same coil without a core the calculation accorded with the simulation. 

Thanking you in advance.