specify f coefficient in PDE

In my 3-D PDE model,the f coefficient is a spatial function(called by @ffunction),so I need to know the matrix of this coefficient. I have looked up the help manual of PDE coefficient，f=[NxNr],where N is the number of equations,one theory is that Nr is the number of tetrahedrons in the mesh，the other is that Nr is the number of points in the region that the solver passes,i.e. Nr=the number of points. I am confused with that. In previous test of 2-D PDE model,Nr is equal to the number of triangles and it run done. It will go wrong in the 3-D model when Nr is set to the number of tetrahedrons.

Answer 1

[Alan Weiss]

Depending on your MATLAB version, there are different values for the f coefficient. For recent toolbox versions, use this documentation.

For older versions, use this documentation, which specifies that Nr is the number of points that the solver passes to your function. There is a bit more information here.

I hope that this helps,

Alan Weiss

MATLAB mathematical toolbox documentation

[Xuejiao Li]

Hi,Alan! Thanks for your help. My version is MATLAB R2015b,whose toolbox is not useful for 3-D model. I used the syntax: "fcoeff = ffunction(region,state)"and "f=@ffunction" in my code,but I don't konw how to output the value of f at each point and plot it with "pdeplot3D" function.

[Alan Weiss]

Sorry, I don't understand you. Why do you want to plot the f coefficient in 3-D? But, in case this helps you in some way, see Plot 3-D Solutions, which gives some techniques for making 3-D plots. There are pointers in the first section to other MATLAB techniques for making 3-D plots, too.

Alan Weiss

MATLAB mathematical toolbox documentation

[ADSW121365]

In the context of an EM problem, the source coefficient may represent current sources which are useful to visulise as part of a system. Some example code as to how you could do this is given below:

C_n = findNodes(model.Mesh,'region','cell',1);  %Find nodes associated with a specific source
location.x = model.Mesh.Nodes(1,C_n);   %Manual location test vars
location.y = model.Mesh.Nodes(2,C_n);
location.z = model.Mesh.Nodes(3,C_n);
f_vals = f_coeffunction_2v(location,~) %In my case this is 3x number of elements in location

%Magnitude Plot:
f_mag = sqrt(f_vals(1,:).^2 + f_vals(2,:).^2 + f_vals(3,:).^2);
f_full = zeros(1,length(model.Mesh.Nodes));
f_full(C_n) = f_mag;
figure(); pdeplot3D(model,'ColorMapData',f_full,'FaceAlpha',0.5,'Mesh','off');