[aspect-devel] Open boundaries + tracers

[Louise Kellogg]

We should consider implementing a scheme for inflow of particles to accompany an inflow boundary condition.  This would be useful for all sorts of problems. The density and properties would have to be specified with the boundary conditions. 

> On Dec 4, 2016, at 4:15 PM, Wolfgang Bangerth <bangerth at tamu.edu> wrote:
> 
> 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.
> 
> Best
> W.
> 
> -- 
> ------------------------------------------------------------------------
> Wolfgang Bangerth          email:                 bangerth at colostate.edu
>                           www: http://www.math.colostate.edu/~bangerth/
> 
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