[aspect-devel] Open boundaries + tracers

Wolfgang Bangerth bangerth at tamu.edu
Sun Dec 4 16:15:38 PST 2016

On 11/25/2016 03:43 PM, Rene Gassmoeller wrote:
> I recently added tracers to my models, they work great, thank you for adding
> them! I was trying to incorporate open boundaries (by Anne Glerum) into my
> models, they work just fine too. However, when I included both of them, I ran
> into a problem: the cells near the open boundary, through which constant
> inflow of material is happening end up not having enough tracers (and later
> not  having any tracers at all) (see attached screenshot). I decreased the CFL
> number from 0.5 to 0.1, but it didn't help. So I was wondering if you could
> tell me what the possible causes are?

Lev -- I don't think this question was actually answered. The cause is that 
you start with particles everywhere, and then you let these particles be 
transported along with the flow. If you have inflow, then you have material 
flowing across the boundary into the domain (but this new material does not 
bring with it any particles), and material that was close to the boundary 
flowing further inside (taking along the particles that were close to the 
boundary). The result is that you have no particles close to the boundary 
after some time -- independent of how large or small your mesh size, time 
step, or CFL number are.

What Rene and John describe is in essence a way to *generate particles in 
cells that have too few particles*, by "cloning" the ones that are in the 
vicinity. If all you care about is to use particles to visualize the flow, 
then that is appropriate. But if, for example, you wanted these particles to 
carry properties such as the accumulated strain, then this is not appropriate 
because the new particles will have the accumulated strain of the particles in 
the vicinity, not what would be appropriate for a new-born particle just 
making its way across the inflow boundary.

For these sorts of cases, we would have to implement a way to create particles 
along inflow boundaries from scratch, rather than interpolating from existing 
particles. But this is not currently implemented.


Wolfgang Bangerth          email:                 bangerth at colostate.edu
                            www: http://www.math.colostate.edu/~bangerth/

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