[Getfem-users] Interpolation problem in Matlab interface

[Walker, Ryan T. (GSFC-615.0)[UNIV OF MARYLAND]]

Hi all,

I’m working on plate bending (i.e., bilaplacian) for a problem where I have observed data for the stiffness D.

I’m setting D using gf_mesh_fem_get to evaluate the Matlab interpolation function interp2 on the lagrangian mesh.

This produces an identically zero solution, as if D = 0 everywhere. However, when I use gf_compute to evaluate D on the solution nodes, I get the values I was expecting from the interpolation.

The simplest thing I can do to get this problem is modifying demo_bilaplacian.m to include this:

% create “data” on coarse grid

[xx,yy] = meshgrid(0:0.1:0.4,0:0.1:1.2);

foo = 1 + xx.^2 + yy.^2; 

% trying to interpolate onto the lagrangian mesh mfd

getD = @(x,y,z) interp2(xx,yy,foo,x,y);

D = gf_mesh_fem_get(mfd,'eval',{getD});

gf_model_set(md, 'add initialized fem data', 'D', mfd, [D]);

When I do this, I get the identically zero solution. But if I then do

nodes = gf_mesh_get(mfu,’pts’);

D_pts = gf_compute(mfu,D,’interpolate on’,nodes);

I get the values of D I was expecting.

If I use the same formula for D directly without interpolating like this:

getD = @(x,y,z) 1 + x.^2 + y.^2;

D = gf_mesh_fem_get(mfd,'eval',{getD});

gf_model_set(md, 'add initialized fem data', 'D', mfd, [D]);

I get the right solution (and the right values of D on the solution nodes).

What am I doing wrong here?

Thanks,

Ryan

**********************************************

Dr. Ryan T. Walker

Earth System Science Interdisciplinary Center

University of Maryland at College Park

Cryospheric Sciences Laboratory, Code 615

NASA Goddard Space Flight Center

8800 Greenbelt Road

Greenbelt, MD 20771 USA