I am a geotechnical engineer. I want to use MFiX to simulate the dissolution of methane hydrate within the soil. The pressure of soil is an important influential factor for this problem. So I want to find some ways to exert boundary pressure on particle assembly in DEM.
The first equivalent way I think is to use supergravity, i.e., increase the gravity value, to increase the pressure of the particles. One flaw of this method is that there is a pressure gradient within the soil. And I can’t limit the CFD domain in a layer of the particle assembly.
The second equivalent way is to use some heavy particles covering the assembly. However, the heavy particle will sink down.
So I wonder if there are other equivalent ways to exert pressure on particle assembly in DEM. At the same time, the pressure within the CFD domain is relatively uniform.
So it sounds like you want to pre-consolidate a soil specimen before investigating dissolution of the methane hydrate, is that correct?
I can think of a few things you can try. One would be to use a supergravity condition that is a function of soil depth (i.e. g = f(y)) so you can impose a pressure gradient. The DEM rolling drum tutorial has details about implementing a position-dependent gravity condition. The other is you could create a “wall of particles” that move a fixed distance to consolidate your specimen until you achieve the desired pressure gradient (i.e. compress the soil over some fixed distance and then hold their position). The DEM screw feeder tutorial shows you how to prescribe a fixed motion to a series of particles for use as a boundary condition.
As you identified, you don’t want to also impose a pressure gradient on the fluid phase unless there really is one. The supergravity condition would be an example where you will need extra lines of code if you want to apply it to just the particles and not the fluid.
Thanks very much for your valuable instruction.
For the particle wall method, I want to know if I can keep the z coordinates of the particles in the wall the same during the simulation and make the wall particles move up and down together under gravity?
You can assign the x, y and z velocities of the wall particles to anything you want. So if you want to assign the z-velocity to be 0 and the y and x velocities to something else, you can do that. If you only want to overwrite the z-velocity and leave the y and x velocities to be determined by the momentum equations, you can also do that.
Just be aware that with this approach, you are decoupling the movement of the wall particles from the physics of the process. Watch out for unphysical behavior and make sure you are imposing a realistic boundary condition on your particles.
One other thing I should have mentioned - instead of a wall of particles, you can also use a moving STL file as of MFIX 20.2: MFiX 20.2 Release Announcement - Multiphase Flow Science Group at NETL
This would be more similar to the “servo-wall” BC you mentioned in a previous post.
Thanks very much for your valuable instruction.