[bergantz]

When we do a simple DEM particle settling run in a closed box with no fluid (granular only) it works just fine. However when we do that same run with a fluid, once the particles have settled they start "hopping" and moving in a non-physical manner even when there is no other fluid motion, the fluid should be static.

Has anyone else encountered this bizarre behavior?

Thanks

[ashokac7]

What do you mean by particles are settled. Does is mean that they are settled on the wall or hanging in domain. And what is desired physical behavior you are looking for ?

[bergantz]

By settled I mean sedimentation to the floor. Think of simple spherical particles settling under gravity. Once they are resting on the floor no more motion should occur. And the fluid velocity should go to zero. However what I find is that once the particles are resting on the floor some randomly start to suddenly rise a short distance, maybe 5 particle diameters, then fall back down. This is obviously non-physical. Sometimes the whole particle bed will start to do this.

Has anyone else encountered this?

[ashokac7]

Originally Posted by bergantz

By settled I mean sedimentation to the floor. Think of simple spherical particles settling under gravity. Once they are resting on the floor no more motion should occur. And the fluid velocity should go to zero. However what I find is that once the particles are resting on the floor some randomly start to suddenly rise a short distance, maybe 5 particle diameters, then fall back down. This is obviously non-physical. Sometimes the whole particle bed will start to do this.

Has anyone else encountered this?

Did you set the wall boundary condition for Lagrangian phase to "Stick"?. If they are stick once they come down to wall then they should not move. If not change this BC in Physics node for wall boundary. Even if they are not sticking to the wall then you can use Escape condition for particles at wall. But it may not be that accurate.

[bergantz]

Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.

[ashokac7]

Originally Posted by bergantz

Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.

Yes. But if wall BC is Rebound then coefficient of restitution would be 1 and particles will rebound back in domain. Imparting accurate value of restitution would be the key to accurate solution then. That's why we use stick character to force the particle stick to the wall. Which in my thinking more accurate than rebound for the cases like spraying or painting etc. You can relate your case accordingly.

[meangreen]

Originally Posted by bergantz

Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.

Have you checked your particle overlap? I wonder if when you turn on the fluid something changes in the solver and a large overlap aloud in granular phase turns to a large restoring force when continuum phase is coupled.