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Re: pde toolbox
Posted:
Mar 16, 2013 1:10 PM


Hi Bill,
There is a follow up question to the one listed below. I am using the following method to solve my elliptic problem:
I create a geometry 'g'. I then mesh it: [p,e,t] = initmesh(g); Then I create my boundary matrix using [Q,G,H,R] = pdebound(p2,e2); I then assemble other matrices using [K,M,F] = assema(p,t,c,a,f); Finally I solve the problem using u = assempde(K,M,F,Q,G,H,R);
I get the following error from assempde  ??? Error using ==> plus Matrix dimensions must agree.
Error in ==> assempde at 245 KK=K+M+Q;  As per the link below the size of the Q matrix in the boundary file is (N^2 ne) which for a onedimensional system is (1 ne), where ne is the number of edges in the mesh.
http://www.mathworks.com/help/pde/ug/boundaryconditionsforscalarpde.html
However the size for the K and M matrices is (Np Np) where Np is the number of nodes in the matrix.
So I see that the K matrix and Q matrix are not of the same dimensions and hence the error. I am not able to figure out how to fix this problem. Any help will be appreciated very much.
Thank you.
"Bill Greene" wrote in message <khfgev$sgb$1@newscl01ah.mathworks.com>... > Hi, > > "Sashankh Rao" wrote in message <khdu9o$j3c$1@newscl01ah.mathworks.com>... > > Using the Matlab pde toolbox I obtain a solution to the Poisson equation for a given geometry using dirichlet boundary conditions. First, I want to determine the gradient at all the boundary nodes. Is this possible using the toolbox? Second, I want to use these gradient values as an input boundary condition for the same boundary to solve a different equation (same geometry). Is this possible to do using the toolbox? Thanks. > > Yes, this is definitely possible. I'll try to point you in the right direction. > > The function pdegrad can be used to calculate the gradient of the solution at the > element centroids. > http://www.mathworks.com/help/pde/ug/pdegrad.html?searchHighlight=pdegrad > Then the function pdeprtni can be used to interpolate these centroid values back > to the nodes. > http://www.mathworks.com/help/pde/ug/pdeprtni.html > > Using these gradient values in the boundary conditions will require you to write your > own boundary condition function, referred to as a "boundary file" in the PDE > Toolbox documentation. > http://www.mathworks.com/help/pde/ug/pdebound.html > > This documentation page has some examples of how to write such a function. > http://www.mathworks.com/help/pde/ug/boundaryconditionsforscalarpde.html > > Bill



