# Passive scalar is diffusive without mesh motion

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 July 23, 2015, 08:59 Passive scalar is diffusive without mesh motion #1 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,661 Blog Entries: 6 Rep Power: 48 Hi all, I have a question to all of you. Maybe you are more familiar with that problem. The case is very simple. A 2d rectangular with an outlet velocity of 1m/s, p is fixed at the inlet with fixedValue; the walls are zeroGradient for U and p. The passive scalar is set to zeroGradient at each boundary. With setFields I generate an area with values = 1 for the passive scalar (everywhere else its zero). Case 1 has no mesh motion Case 2 has mesh motion due to the velocity field at the inlet (flux after the iteration = 0 at the inlet face). The position of the scalar is the same (as expected) but why is without mesh motion the scalar diffusive? The transport of the scalar is only done by time and convection (no diffusion term). In the mesh motion I added the mesh flux to the solver, but can this be the reason for less diffusion? But if I check the code: Code: ```fvm::ddt(S) +fvm::div(phi, S)``` There should be no diffusion in the case with no mesh motion. S is only transported by the flux, which is everywhere constant. :/ Here you can check the results: www.holzmann-cfd.de/cfd-online/passiveScalar.avi Any hint is appreciated. Thanks in advance, Tobi __________________ Keep foaming, Tobias Holzmann

 July 23, 2015, 09:46 #2 Senior Member   Join Date: Oct 2013 Posts: 395 Rep Power: 16 Are you sure there should be no diffusion at all? As far as I know, there is always some artificial, numerical diffusion (though I'm not sure if this requires that an actual laplacian is present in the equation). I have no idea why the diffusion is smaller with mesh motion, but I have no experience with dynamic meshes at all.

 July 23, 2015, 10:32 #3 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,661 Blog Entries: 6 Rep Power: 48 Hi chriss, you are right. There is always Diffusion, hence there is only convection. But like you mentioned, I also don't know why I get the better result with mesh motion. Maybe it's due to the fact that the mesh flux reduces the convection term itself. (U-Umesh) which is smaller within the mesh motion case. __________________ Keep foaming, Tobias Holzmann

 July 23, 2015, 12:45 #4 Senior Member     Kyle Mooney Join Date: Jul 2009 Location: San Francisco, CA USA Posts: 323 Rep Power: 16 I would say that the moving mesh case has less diffusion because there is near zero relative advection taking place. Like you said, with (U-Urel)~=0, there really isn't much math going on to even allow for numerical diffusion.

 July 31, 2015, 04:18 less diffusive scheme #5 Senior Member   Fabian Roesler Join Date: Mar 2009 Location: Germany Posts: 213 Rep Power: 16 Right, I agree with kmooney. The relative advection in the mesh motion case is smaller and so the artificial diffusion is smaller as well. What schemes did you apply and have you tried with a less diffusive scheme? Cheers Fabian

 July 31, 2015, 05:18 #6 Super Moderator     Tobias Holzmann Join Date: Oct 2010 Location: Tussenhausen Posts: 2,661 Blog Entries: 6 Rep Power: 48 Hi Fabian, at the beginning I was wondering why its less diffusive but the reason for that is exactly what we are talking about. (U - Urel) ~ 0. I also tried with different schemes, second order, 1st order, not much difference. Thanks for all feedbacks, __________________ Keep foaming, Tobias Holzmann

 August 5, 2015, 09:51 #7 Member   Hannes Join Date: Apr 2009 Location: Schleswig, Germany Posts: 38 Rep Power: 15 __________________ FluidEngineeringSolutions