How to fix error "Error using reshape To RESHAPE the number of elements must not change." when I am using pdegrad?

How to fix error "Error using reshape To RESHAPE the number of elements must not change." when I am using pdegrad?

I want to calculate du/dx and du/dy by using pdegrad:

[ux,uy] = pdegrad(p,t,u);

Then I got error above. I wrote edit reshape on the command line, and it tells me that when I want to reshape u using function uu=reshape(u,np,N), I have to make sure my u has np by N elements. I checked my u elements size is 832 by 41 where 832 is my solutions from the nodes points and 41 is my tlist. Then on pdegrad function, it defines N=size(u,1)/np, where size(u,1) is 832 and np is 832 since it only has 1 partial derivative equation so N is 1. Therefore my u has a size of 832 by 1.

Is there any way to fix this error or other way to calculate the du/dx and du/dy?

Thank you, Bella

Answer 1

[Alan Weiss]

I think that you need to compute pdegrad at each time separately. So try

[ux1,uy1] = pdegrad(p,t,u(:,1));
[ux2,uy2] = pdegrad(p,t,u(:,2));
% etc.

Alan Weiss

MATLAB mathematical toolbox documentation

[troxylon]

Hi Alan,

That works. Thank you so much. I know the ux and uy is the gradient on the centroid of the triangular mesh. How I convert it to be ux and uy on nodes? Thank you, Bella

[troxylon]

I tried to use pdeprtni

un = pdeprtni(p,t,ut)

then I got an error:

Undefined function or variable 'ut'.

[troxylon]

I got the solution. I used:

[ux1,uy1] = pdegrad(p,t,u(:,1));

unx1 = pdeprtni(p,t,ux1);

uny1 = pdeprtni(p,t,uy1);

Thank you.