Output a function of the solution vtk

[cjham]

Hello, Sorry this is probably a very basic question but I haven't been able to get it to work. I'd like to output the solution, u, to vtk which I can do but I'd also like to output the derivative in the x direction of the solution. I am not sure how to make this happen. I have done various things with Dx(u,0) but none have worked!

[connorjward]

I think your issue is likely that Dx(u, 0) is a symbolic expression whereas for visualisation you want a Function. I think you can get what you want by doing

deriv = Function(V).interpolate(Dx(u, 0))

# now plot deriv

[cjham]

This worked, thank you.