force fluctuations

Dear all，

I conducted a multi-phase flow simulation similar to a dam-break , with a slit dam in the center of the tank. When running OpenFoam or LIGGGHTS separately, the results seem normal, but the forces monitored in OpenFoam fluctuate significantly after coupling (the upstream face of the dam). I have considered factors such as boundary conditions, mesh size, and particle gradation, but the problem remains unresolved. Turning off the void fraction model reduces the fluctuations, but the issue still exist.

Finally I tested a stimulation (10m*.6m*1.5m, cell size of 0.1 m, particle radius of 0.0135 m, and 20° slope), but abnormal fluctuations still occur，even negative values. Could anyone give me suggestions? Thank you very much!

Best wishes
Lee