Dealing with mass of void elements(Eulerian FEM)

Boni

Hello,

I would like to simulate a collapsing water wolumn by using my own Eulerian FEM code (incopressible viscous).

Problem: I have problems calculating the global pressure of the system. Since I need the inverse of the nodal mass, which is "zero" at the nodes of void elements.

Does somebody know how to deal with the mass of void elements?

I tried to used 1.0d-6 as void mass but it leads to huge numerial problems.

Thank you in advance for any help!

Boni