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porous media velocity results

Question asked by Chris Pellegrino on Apr 17, 2018
Latest reply on Apr 17, 2018 by Chris Pellegrino

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How does Flow Simulation calculate the average velocity on the surface of porous media? If I look at a contour or isoline plot of velocity on the surface, the result values make sense. If I ask for the average velocity on the same surface, the average is an order of magnitude higher (even higher than the maximum the contour and isoline plots claim). Is it using a volume average (using the cells inside the porous media) as opposed to an area average on the surface? I noticed that on a vector plot, the velocities are higher inside the porous media (which numerically I can explain due to the small mathematical pores inside the porous media.

This disconnect is very annoying.

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Bill McEachern Apr 17, 2018 1:36 PM
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The velocity average should scale with the porosity anywhere on or in the porous media. So if your porosity is say less than 10% then you would see an order of magnitude increase in velocity due to the reduction in area to maintain continuity across the region assuming the flow is constrained to move through the porous media.
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- Chris Pellegrino @ Bill McEachern on Apr 17, 2018 2:25 PM
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  Thanks Bill - that's what I was thinking and that explains the velocity results in general, but why the disconnect between velocity contour plots and an average value? Basically the contour plots don't follow that logic like the average value does. That's my question.
  
  It seems like a coding issue. I can work around it by putting a solid just off the porous media surface (and of course disabling it), but SolidWorks (or maybe FloEFD in general), is pretty terrible at pulling results on surfaces that didn't exist during the run. I seem to be forced to rerun it, but then it doesn't reliably continue the solution nor even start a new one with the previous results. This is a MAJOR nuisance (flaw) in this CFD code which is another discussion...
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  - Bill McEachern @ Chris Pellegrino on Apr 17, 2018 2:30 PM
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    I don't understand what you are telling me. Can you post images?
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    - Chris Pellegrino @ Bill McEachern on Apr 17, 2018 2:49 PM
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      I can get images, but I have to pull them off another computer. Basically, looking at a contour plot on the porous media surface, the range of velocities may be from zero to 100. A hand calc shows the average velocity should be around 50, and if I ask for an average velocity a few inches off the surface, I get close to 50. If I ask for an average velocity on the media surface, it gives something like 300 or 400. Again, the issue is the disparity between the contour plot results and the average value result. This is really a question for the developers, but I was hoping someone already looked into this and knows a bit about it.
      
      I poked around and found someone complaining about the same thing a year or two ago (and we are on 2017, BTW, for our machine running Flow.) The only response mentioned an issue with the software, but I have no idea what "behind the scenes) knowledge and experience they have.
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      - Bill McEachern @ Chris Pellegrino on Apr 17, 2018 2:57 PM
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        what is porosity of the porous media?
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        Chris Pellegrino @ Bill McEachern on Apr 17, 2018 3:41 PM
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        It's only .05, but regardless, the issue is why contour results don't line up with the average.
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        Bill McEachern @ Chris Pellegrino on Apr 17, 2018 3:49 PM
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        Well it would make sense to me if the average velocity was 20 times the average outside of it as the flow area is reduced by a factor of 20. Does that line up?
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        Chris Pellegrino @ Bill McEachern on Apr 17, 2018 4:21 PM
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        No. The same surface is used to get both a contour plot and one average value. It makes no sense that the average of those contours is almost an order of magnitude larger than what the contours are showing. It shouldn't matter what my porosity value is. If porosity is affecting the velocity values that throw the average high, why is it not also throwing the contour plot high?
        
        At first I thought maybe the method of weighting the average might be the culprit, like perhaps with a volume weighted average (not area weighted as I believe the documentation somewhere states), it's pulling velocity values from inside the porous media, whereas the contour plot is only pulling velocity values on the surface. (I thought the values are all volume based with this code, but maybe I'm wrong.) In that case I can see the porosity value causing the discrepancy. I consider that a serious bug because I am using the delta P/velocity definition of porosity, yet it requires me to enter a porosity value, so I took a guess not knowing how it's going to use it (another major issue with SolidWorks - they do a terrible job with the detail behind things like this). Reporting results on the same surface but using different velocity values for each is wrong.
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porous media velocity results

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