I am working on a two-dimensional granular system composed of particles with diameter sigma and mass M. My study focuses on the deposition of these granular particles within a rectangular region without periodic boundary conditions. The system is modeled using the L2 model as described in Phys. Rev. E. 64, 051302 (2001). For particle-wall interactions, I employ the wall/gran hooke/history model on the bottom and lateral walls, treating the interaction similarly to particle-particle interaction. The particles are subjected to a gravitational field, which I have set to 1.
For the deposition process, I use the following fix deposit command:
fix 1 all deposit 100 1 500000 1111 region boxdepos global 1.1 1.2 near 1.5 units box
Here, I am depositing 100 particles of type one every 500,000 simulation steps. The particles are randomly positioned above the highest current atom in the simulation box, at a distance randomly chosen between 1.1 and 1.2 sigma. The “near” parameter is set to 1.5 to prevent overlaps.
Although the simulation runs, I find a weird behavior: each particle deposition drastically disturbs the structure developed by the previously deposited particles. Specifically, all particles scatter in all direction upon each new deposition. As a result, the structure formed by earlier depositions is completely disrupted, preventing me from studying the gradual evolution of force distribution in the system.
Has anyone experienced a similar problem or can offer me some guidance? Perhaps I am neglecting an important aspect.
Can you clarify exactly what about the observed behavior is “weird?” The force from a particle falling on other particles will of course cause them to move. Are you certain the impact velocity isn’t just larger than you want (or possibly greater than should be allowed by the timestep size)?
I am sharing a short video to demonstrate the weird behavior I mentioned. I ran a short simulation with a time step of 1e-4, depositing particles every 5e5 simulation steps. Each frame is saved every 1e4 steps. As shown in the video, I have 4 particles when a new one is introduced. This particle, even though it doesn’t collide with any others initially, starts to move at high speed. It then collides, causing the other particles to bounce all over the simulation box. The initial velocity of all disks is set to zero when they are introduced into the system.
Thanks, that’s useful. I think your deposit command is simply inserting an atom that overlaps heavily with the vertical wall, causing it to shoot sideways. I’d recommend shifting the bounds of boxdepos so inserted atoms do not initially contact walls. Hope this helps.