DistMesh giving a singular FEM matrix?-Hi



Anyone here with some experience of using DistMesh in finite element code? I m solving the scalar Helmoltz equation in an annular region in 2D:



rho_1 < rho < rho_2



where rho_1 is the radius of a Perfect Electric Conducting cylinder and rho_2 is where the mesh is truncated using an Absorbing Boundary condition.



Now, I ve written code to mesh the region myself (by dividing the region into annular rings and picking a fixed number of points on each ring) and I ve written another program which uses DistMesh to mesh the region. I get a nice solution using my own meshing code but the FEM matrix becomes singular when I use DistMesh. This is the DistMesh Code I used:



　Circle with hole

rABC=1.5

rCyl=0.5

fdstring=sprintf( ddiff(dcircle(p,0,0, f),dcircle(p,0,0, f)) ,rABC,rCyl)

fd=inline(fdstring, p )

box=[-2,-2 2,2]

[p,N]=distmesh2d(fd,@huniform,0.04,box,[])



I tried changing box and the 0.04 value (initial edge length). But every time, the matrix becomes sin