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PimpleFoam odd behavior, any suggestions?

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Old   March 20, 2013, 08:38
Default PimpleFoam odd behavior, any suggestions?
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NJG
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Nick Gutschow
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Hello Community,

I am running a slight mod of buoyantBoussinesqPimpleFoam and encountering some odd behavior. I will first explain my modification, then my run details, and then my strangeness. Also, I have attached a gif at the bottom showing what I am talking about. It shows the (x,z) plane for mole fraction CO2 on a log scale.

My solver is just buoyantBoussinesqPimpleFoam to which I have added mole fraction of CO2 and added density shifts with regards to the body forces for this changing mole fraction. So, just as the base solver alters p_rgh by beta*(T - Tref) to take into account heat related density changes for body forces, so here I add the additional term betax*(x - xref) for composition related density changes for body forces. I also had to add molecular diffusivity, and then a wall function to calculate turbulent molecular diffusivity, but otherwise the original solver is in tact.

My case is one in which I have a roughly 3x3x3.5 meter "room" (z is the vertical direction) in the center of which coming up from the floor is a 0.4x0.4x1 meter pillar (as you can see it does not go all the way to the ceiling). The very top of the pillar is a 0.04x0.04x0.04 cube. On one face (+x) this cube has a constant velocity in the +x of about 6 cm/s. This face also has a constant temperature of about 310 K, and a constant mole fraction CO2 of 0.05. The room ambient temperature is about 293 with the walls, ceiling, and floor set at this as constant values, and the ambient mole fraction begins at 0.0004 with all boundaries being zero flux. Thus this +x face blows in hot CO2 rich air, and I expect a plume coming from it. Also note that this +x has a p_rgh BC of zero gradient (which I am not so sure about...)

To enforce a mass balance on the room, 4 of the other faces on the cube (+z, +y, -y, -x) have been allowed to be "outlets" of a kind, having zero gradient velocity BCs and uniform 0 values for p_rgh (once again, not sure about this one...). Also, these faces are zero gradient in temperature and mole fraction.

As for my results, in a nut shell, the plume shoots out in a direction opposite what I expect (see gif below). As the velocity out of the face is +x, I would expect the plume to come out of the +x. But instead a wavering plume in the -x can be seen,

So, any ideas what is going on? Are my BCs problematic? Any input or new ideas are most welcome.

-NG
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Old   March 20, 2013, 08:52
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NJG
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Nick Gutschow
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Well turns out that gif is too large. Hopefully my description is enough. Otherwise, I can send any interested parties the gif directly. Thanks for your time.

-NG
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