Please, how can I output global displacement (U) in Comsol? I want to calculate the compliance of my model. (Please, see the attached Comsol code).
compliance = U transpose x Global stiffness matrix x U
I have used these lines of code to obtain the global stiffness matrix (Kf).
MA = mphmatrix(model, 'sol1', 'out', {'K','E'})
K = MA.K; % stiffness vector
Kf = full(K); % global stiffness matrix
Thank you.
Dear Samuel,
you should use the mphgetu command to extract the solution vector. I also suggest to have a look at the way you can solve the problem from the the constrained system, as this gives you and hint on how to use the uscale matrix (not sure if you need this).
% solve directly from constrained matrices
MA = mphmatrix(model ,'sol1', ...
'Out', {'Kc','Lc','Null','ud','uscale'},...
'initmethod','sol','initsol','zero');
Uc = MA.Null*(MA.Kc\MA.Lc);
U0 = Uc+MA.ud;
U1 = U0.*MA.uscale;
is in the mathlink user manual.
Cheers,
Fabio
Thank you so much, Dr Fabio for this respond.
The code worked.
I also tried exploring mphgetu:
t = mphgetu(model)
But t returned a zero vector. Please, what do you think I am doing wrong? Please, find the attached code. Thank you.
Samuel,
mphgetu() provides the solution after you execute model.run(). While, the procedure for solving the problem by using direct matrix computations stores the solution in U1.
That you called compliance seems to be the (potential) energy density
U = (grad u)^T S (grad u ), in this case K = grad^T S grad (S is the stiffnes tensor)
Note that this is true JUST when you deal with the homogeneous Navier equation,
div sigma = 0 , where sigma = S : gamma (gamma is the strain or deformation)
There could be other example (for instance reaction-diffusion equation) where the matrix K contains other terms, so please be aware of this.
You can check your by-hand computation by running your problem with the user interface.
Cheers
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