numerical diffusion in tetrahedral grids
Hello everybody,
I made some cfdsimulations to get the mass fraction of a species passing a tube inside another type of species. Afterwards I was calculating kind of the ”spread” that the species experienced during this pass. Unfortunately the results differ a lot when I am using different grid types. When I am using a tetrahedral grid the spread is pretty low, when I am using a structured hexahedral grid the spread is about two orders of magnitude higher. Could this effect arise due to the numerical diffusion within the tetrahedral grid? We also found out that the larger the diffusion coefficient we are using, the smaller the spread of the species. But within a normal range of variation the difference in the spread of the species is not two orders of magnitude. Can the effect of the numerical diffusion have such a huge impact on the results of my simulations? It would be really nice, if somebody could help me! Thanks a million in advance, Lilly 
thet are intrinsically more diffusive than exa, and in addition thet grid are ususlly relatively coarser than exa given a fixed number of cells, becuase you need 8 thet to resolve a hexa.

minimise/avoid numerical diffusion in tetrahedral mesh
Thank you for your help, sail!
Does anybody know how I can minimise or (in best case) avoid this numerical diffusion? I am using the QUICK Scheme know which leads to a much better result, but is there a way to improve the results even more? I thought of maybe reducing the time step size to get a CFL number of about one? Does anybody has experience with this? Thank you in advance for your help! Lilly 
from my experience:
cfl between 1 and 12 y+ no less than 30 and no bigger than 100, higher or lower values increase the diffusion timestep as small as possible. grid must be aligned with the flow, and possibly refined as much as possible where you have the "phase change" at least second order in space, settle for first order in time, otherwise there might be some instabilities. take notice that i have never done pipes simulations so the parameters might vary. good luck 
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Thank you sail and baserinia for your help!
I will try to improve the cflnumber timestep size and y+. Unfortunately, my geometry has a bifurcation which makes it really hard to create a structured hexahedral grid aligned with the flow (I tried different mrthods in the bifurcation area, but failed). Do you have any idea how to circumvent this problem? Thanks a million for your advice! Lilly 
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I'm not sure what program you're using for generating your mesh. I usually use ICEM for these sort of things. 
Thank you for your help Baserinia!
I don't have access to ICEM, but to Gambit and Tgrid. Did you work with them or do you know, whether I can build such a butterflygrid using them? Lilly 
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