Gravity volume force in non-isothermal laminar flow

Hello

I'm trying to simulate non-isothermal laminar flow through a tube, and I use 2D-axisymmetrical to decrease the computing time. The model works fine with laminar flow and heat transfer, but when I add gravity, volume force, in the laminar flow interface (-g_const*spf.rho) a solution cannot be found. I get the following error:

- - -

Failed to find a solution.
Singular matrix.

There are 16 void equations (empty rows in matrix) for the variable mod1.p.
There are 1 void equation (empty rows in matrix) for the variable mod1.spf.pmfinl1.
Returned solution not converged.
- Feature: Stationary solver 1 (sol2/s1)
- Error: Failed to find a solution.

- - -

A finer mesh made no difference, but decreasing the gravity to "-0.1*g_const*spf.rho" returned a converged solution.

Any ideas to what might be the problem would be appreciated.

/Simon Tågerud