[Getdp] Second order element peculiarity

[Jim Davis]

Hello getdp aficionados,

I am having trouble using second order elements in the solution of an electrostatics problem and I 
was hoping someone could tell me what I am doing wrong.  It seems that when I have a region 
surrounded by constrained surfaces (Dirichlet and homogeneous Neumann) the solution getdp gives me 
for second order elements contains nodal potentials higher than any of the constrained potentials in 
the grid.

As an example, I have attached the input files for a concentric cylinder problem.  The inner radius 
is 0.1 and is set to 0 volts while the outer radius is 0.5 and is set to 100 v.  The endcaps have no 
boundary conditions set and therefore default to the natural b.c. (homogeneous Neumann), right? 
Looking at the solution generated by getdp, there is a distinct rise in potential (117 v) prior to 
the outer radius.  If I use linear elements, I do not see this behavior.  That is, the potential 
increases monotonically from the inner radius to the outer radius as expected for linear elements.

In going to second order elements, I took the following steps:

              1.  Increased the number of Gaussian integration points for triangles from 4 to 6.
              2.  Increased the fill-in parameter in solver.par to cover the max bandwidth
              3.  Added the basis functions associated with element edges and added the 
corresponding constraints.

What am I missing?

Thanks very much for any insight you can provide on this problem.


Jim


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Jim Davis                              Telephone: (404) 894-7231
Signature Technology Laboratory              Fax: (404) 894-8515
Georgia Tech Research Institute            email: jd4 at prism.gatech.edu
Georgia Institute of Technology
Atlanta, GA  30332-0800

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