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When is a particle considered "touching" a boundary

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Old   June 4, 2010, 14:16
Question When is a particle considered "touching" a boundary
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In the Fluent 6.3 UDF Manual 2.5.1 it says that DEFINE_DPM_BC is "executed every time a particle touches a boundary of the domain"
I would like to know what the definition of "touches" is. Is it when the particle is within one radius of the boundary or is it when the center of the particle is touching or beyond the boundary.

Here is why I am asking:
I tracked a number of particles with the DPM boco of trap but there were so many particles that I could not tell where the deposition was occuring. I decided to use a UDF instead so that I could view where they were depositing. The UDF would write, to a file, the position of the partilce that "touched" the wall and caused DEFINE_DPM_BC to execute. However, the number of particles written to the file and the number of particles that were reported as trapped by Fluent were not the same. There were many more particles trapped than written to the file. This begs the question what is "touching" and what is "trapped"?
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