Hi, I have been working on a project which requires the evaluation of a third order space derivative term in my 1-D PDE, and this is done using the Coefficient Form PDE module. I found that COMSOL outputs the third order derivative d^3u/dx^3 as zero under the results plot.
I know this is clearly not the case since the plot of uxx clearly shows a curve and not a constant.
Since then I have search the internet regarding this issue and have tried other methods of specifying the derivative, including defining a new variable
F=uxx
and plotting d(F,x) in the results section still returns zero for all values of x.
I have also tried different order Langrange shape functions (up to septic) and yet the results are still the same. I am baffled by this as I even tested for a simple function such as
u(x)=4x^4 +3x^3-2x^2+x+7
and I am still unable to obtain the third order space derivative under Results.
Any assistance on this issue would be greatly appreciated. Thank you