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using porous media to represent floor grating

Question asked by Dan Hofstetter on Oct 23, 2014
Latest reply on Oct 26, 2014 by Jared Conway

I'm performed some experiments in a ventilated room recently, and I'm trying to validate my simulation using measured gas decay data.  Below is an image of my model:Room.jpg

The Pit Fan supplies ventilation air to evacuate pit gases, and he barn outlet fans exhaust the room air.  There is also a perforated ceiling (not shown) with 1/2" diameter holes on a 6" x 6" grid that I represented using a perforated plate and environmental pressure.  The "totally-slotted floor" shown is actually made of 18" x 24" sections of grating with ~ 1/2" x 3" slots and 42% open area.  The grating is less than 1/2" thick.  To simplify my model and see if the trends come close to matching observations, I replaced the grating with long rectangular slots that have the same % open area.  Initially, there is 100% air above the slotted floor, and 150 ppm of contaminant gas below, separated by plastic sheeting to control the initial conditions.  At first glance, a manually-refined mesh produces reasonably correct results when I compare the point parameters from the simulation to the measured data at the same locations 12" below and 6" above the slotted floor.  However, there are a few places where the simulation greatly over or underestimates the gas concentration over time.  To try to improve the simulation results, it was suggested that I try to use the actual grating geometry, shown below:

Grating.jpg

The floor area covered by this grating is 9 ft wide by 36 ft long, and the grating is 1/2" thick in my model, so placing the real geometry in the model would result in a very large, fine mesh in the region of the flooring.  I set up a simulation using one section of grating in a wind tunnel with ideal walls and ran a parametric study to get the pressure drop across the faces of the grating, then defined a one-direction porous media to represent this flooring.  I then ran another simulation replacing the grating with a solid body with porous media, and got reasonably close results when comparing the pressure drop across the media.  Thinking the porous media would greatly reduce the mesh density in my larger model, I replaced the simplified flooring with a solid body that was 1/2" thick, and inserted porous media for that body.  When I meshed the model the first time, I noticed the number of mesh cells increased dramatically, probably due to the number of small cells needed to mesh that 1/2" thick body.  I wound up inserting a local initial mesh for the porous media body to make sure the mesh in that region appeared reasonable.

 

There isn't much guidance about actually using porous media in the help documentation or in the Flow tutorials.  Do I need to do anything special with regard to meshing the model when using porous media like this?  Really, a perforated plate would be much better for this model, but it isn't possible to use perf. plates except at the computational domain boundaries.  Any help is greatly appreciated!

 

Thanks,

 

Dan

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