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Actual Velocity Values in Porous Media

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Old   May 1, 2018, 17:20
Default Actual Velocity Values in Porous Media
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Narendra
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Hi,
I have simulated flow through porous media and created a new surface (line) then I exported velocity values using File-> Export -> Profile option. Should I multiply/divide this exported velocity with porosity of porous media or not to get actual velocity values? or exported values are Darcy velocity or actual velocity? In cell zone condition, I used "Physical Velocity" for porous formulation but I believe this selection doesn't make any difference when we export velocity. Please suggest me.
Thanks.

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Old   May 1, 2018, 21:33
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There is a superficial velocity, which is the velocity as if the fluid flows through the porous media as if it is not there (but there is a pressure drop).

If you want the faster, or accelerated velocity in between two pores, then you want the physical velocity.

There is a Fluent option to switch back-n-forth between the two. If you do not do anything, Fluent gives you the superficial velocity , so you need to turn on the physical velocity option if that's what you want. But if you forget to use the option....

The superficial velocity is the physical velocity times the fluid porosity. The fluid porosity here is volume occupied by the fluid divided by total volume. Note sometimes you call solid porosity the solid volume over the total volume. So be careful and remember to do a quick sanity check.
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Old   May 1, 2018, 23:45
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Hi, Thanks for your reply.
My doubt is whether exported velocity values for porous media are superficial velocity values or physical velocity values?

-------------------------------- Free Surface (river flow)
-> free flow

------------------------------- Interface
........porous sand bed...............
................flow >......................

In my case, I am simulating free fluid and flow through porous media together. You can imagine river flow with sandy bed. In river flow i.e. free flow, velocity decreases as we move toward river bed. Near the bed (interface) but inside the free flow, velocity is non zero which is called slip velocity. The slip velocity occurs when surface is permeable. As soon as we enter in porous bed (near interface), in first few cells velocity increases. In first few cell in porous bed, velocity is higher than slip velocity but ideally velocity should be less in porous media than the velocity in free flow near the interface. Is it correct that velocity in porous bed near the interface higher than the slip velocity? After these cells, it starts dropping till reaches to Darcy velocity or physical velocity.
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Old   May 2, 2018, 00:35
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No change to my response. Your question has already been answered.

For any given superficial velocity, the velocity between the pores will be greater because the same flow must accelerate to squeeze through the reduced area/volume.
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Old   May 2, 2018, 12:30
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Hi Lucky Tran,
I totally understand what you are saying. But my doubt is something different. We have two velocities for porous media- Physical velocity and superficial velocity. My doubt is which values does Fluent export when we export velocity by File-> Export -> Profile option. In other words - Whether exported velocity values are physical velocity or superficial velocity?? I want correct numerical values of actual velocity.
Thanks.
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