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Particle tracking trapped or incomplete

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Old   May 25, 2007, 11:06
Default Particle tracking trapped or incomplete
  #1
Abigail
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I am using the discrete phase model to track particles from size 1 to 50 microns in a room and looking at where they deposit on surfaces. I am injecting 6700 inert particles with one size for each injection spread across a line within the room. I have my outlet set to escape and walls trapped.

The problem I have is with sizes 10 and 20 microns I am receiving a high number of incomplete particles. When I view particle tracks the majority of these look like they have hit a surface. When colouring the line with particle residence time (blue 0 to max time at red) the line is only coloured blue, so it appears to be trapped early on. If I increase the max number of steps (by several orders of magnitude) it is still marked as incomplete.

I wonder if this is a problem with my cells nearest the wall. My boundary layer was set up with the cell closest to the wall being 1cm deep. Because of my y+ values after running the solution I refined y+ so it was less than 5 and changed to using enhanced wall functions. Would this possibly prevent the particles from impacting on the surface? I tried running the same set up with standard wall functions (although I know y+ is very low) and I still get the same problem.

I have tried using different tracking schemes but I still get the same problem.

Can anyone help?
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Old   May 30, 2007, 12:31
Default Re: Particle tracking trapped or incomplete
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margaret mkhosi
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Hi, You need to increase the max. number of steps in DPM. The default is 500, but you can go to a maximum of 1e+09. It seems that your particles do not complete the trajectory.
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Old   May 30, 2007, 13:10
Default Re: Particle tracking trapped or incomplete
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Abigail
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I have tried increasing my number of steps, by a large amount and still get the same results, and visually it looks like the particles are hitting the floor after a short amount of time.

If I look in detail at what is happening on a plane 11mm above the floor the particles are moving up and down small amounts in this area. I think this seems unlikely to be realistic, and more likely to be a problem with my boundary layer. My cell closest to my wall is 10mm deep. This is similar to work I have read in literature that consider particle transport in room air flows.

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Old   May 30, 2007, 14:33
Default Re: Particle tracking trapped or incomplete
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Madhukar
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hi

What type of injection u r using? also did u activate dispersion random walk model. If not try that with different values in the number of tries field

regards

Madhukar
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Old   May 31, 2007, 11:19
Default Re: Particle tracking trapped or incomplete
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Abigail
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Thanks for your message. My injection was a group injection 6700 particles. Along a line across the room. One particle diameter for each injection (injection1 - 10microns, injection2 - 20 microns), inert particles.

I did not activate turbulent dispersion before. I have tried switching it on now and the particles are impacting on the surfaces!

With the 10 microns there are now about 900 particles hitting the floor. 700 hitting the ceiling and 1300 hitting the wall.

This seems odd to me (gravity is definitely turned on!). I know the particles are very small but I would have thought more would be impacted on the floor than the ceiling. For 20 microns the 600 hit the walls and 5000 the floor and non hit the ceiling which seems more realistic so maybe it is just that the 10 micron particles are too small to be greatly affected by gravity.

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