Undefined value found in the stiffness matrix, requested mesh quality cannot be achieved, and obstacle in the fluid channel

Dear All,

I am simulating a sphere flowing in a microchannel with solid traps (2D for simplification). Automatic re-meshing is activated. Two scenarios are considered: Scenario 1, the sphere will encounter a solid trap and get immobilized. Scenario 2, the sphere will flow in the channel beside the trap.

However, for Scenario 1, the simulation cannot go on (or get crushed) when the sphere draws near to the trap. The problem is 'requested mesh quality cannot be achieved', because the mesh near the trap changes so dramatically?

Moreover, how can I set a stop for the simulation, when the sphere achieves to hit the trap (obstacle)? or change the boundary condition?

For Scenario 2, the sphere cannot get through the narrow channel between the traps around it. The problem is 'Undefined value found in the stiffness matrix'. I guess this is also due to the mesh crushing? If I put the sphere at the crushed place and run the simulation again, the sphere can move again and get crushed at the next trap.

Could anybody find a way to solve these problem? Thanks a lot.

The files for scenario 1: www.dropbox.com/s/id9rp4l6ykuc2ba/Scenario1.mph
and for scenario 2: www.dropbox.com/s/fo2u3j4350nukv9/Scenario2.mph

Zimeng