Hi! I'm having trouble with the particle tracking in FLUENT 6.1 giving inaccurate results:

I'm trying to model raindrops falling through the air and hitting a surface (the ground or a building).

I drop a large number of particles in a steady-state flow field, using no turbulent dispersion and inert particles. I drop the particles approximately 70m, through a uniform flow field and then examine where they hit the ground.

Since the flow field I use is totally uniform, 3m/s everywhere, even by the ground, one would expect all the particles to fall the same way, and thus ending up in a pattern similar to how they were injected. This shows out to be almost the case, most of the particles falls as expected, but a few percent end up a few decimeters from their expected location on the ground.

I have tried decreasing the integration length scale down to 0.001m, but it doesn't help. And I guess that since the windfield is homogenous it shouldn't matter so much what length scale I use. I'm beginning to think that the mesh is affecting the particle tracking, even if it shouldn't considering the uniform flow field. I have also tried refining the mesh and using tetraedral as well as hexaedral cells, but without finding any clear cause to the inaccuaracy.

I would appreciate any help on how to get better accurracy when tracking particles. // David

Hi

Could you please tell me about where are located those particles that end up a few decimeters (you mean milimeters) from their expected location?

regards

Alex Munoz

Thanks for a quick response! I release the particles 50m above ground and calculate where they hit the ground. Some of the particles end up approximately 1-2 decimeters from where they should end up.

I could give the exact coordinates of some of the particles that show inaccurate trajectories, but since maybe a 100 out of 10000 released particles show this problem, I can't see how it would help. The only difference between the injections is the location. I therefore suspect that the trajectories are mesh dependent (even though the wind field is uniform).

I hope this makes it clear // David

Hi

I was wondering if the particles that get inaccurate position are located near to the walls

Regards

Alex Munoz

Hi again! The particles with inaccurate trajectories are evenly spread over the whole area, not only close to the walls.

I have now examined the problem further and come to the conclusion that the problem can be minimized by using a finer mesh. I still don't undrstand why though. Since the flow field is uniform, one would think it shouldn´t matter what cell size is used. In the integration scheme for particle tracking implemented in FLUENT 6.1, the flow field gradient at the position of the particle is used to calculate the flow field value at the next position of the particle. I'm beginning to think that the problem somehow comes from the method used to calculate this flow field gradient. // David

We have been using the DPM model for calculating erosion. We have found optimum accuracy to be achieved with a boundary layer at the surface, with the step length equal to the height of the first row of the boundary layer. Curtis.