Using Air Permeability in STAR-CCM+

C72 · October 4, 2021, 11:11

Hi guys, I've recently been looking at porous media tutorials in Star and seen that porous media are defined using the Darcy-Forchheimer equation.

I'm attempting to model a building as permeable to some degree by defining it's walls and roof as porous media using this equation. After doing quite a bit of online research I've been able to find some typical air tightness/ air permeability values in m3/(h.m2)@50Pa for different building types from certification standard documents.

I am just wondering if anyone knows if there is a way to relate these values to the Darcy-Forchheimer equation to get the inertial and viscous porous resistance values that I need to define these porous regions in Star?

Any feedback would be massively appreciated.

Thanks!

LuckyTran · October 4, 2021, 12:01

It's tricky because the air tightness coefficient is a 0D parameter. You need more info and then you'll end up building your own model of what is a building.

First you would probably have to assume that the Forchheimer coefficient is 0 because there just isn't enough data to figure out what it is. But if you take the air permeability metric and divide it by the average building wall thickness that would yield the Darcy coefficient (and divide that also by the differential 50 Pa).

C72 · October 5, 2021, 09:28

Thanks for the response LuckyTran, this helps a lot!

I had been familiar with Darcy's Law beforehand so it went over my head a bit when i saw this additional Forchheimer term.

I have read that this extra term is to account for non-linear turbulence within the porous region. Is the lack of data on this coefficient simply down to lack of gathered data from material experiments?

Also, if i set this term to zero, effectively using Darcy's equation only, will it still be acceptable to model using turbulent flow solvers etc?

Thanks again,

C72

LuckyTran · October 5, 2021, 11:57

The extra term is nothing to do with turbulence. It's the inertial term which is basically a crude attempt to account for deviations from the basic Darcy's law. It doesn't hurt or help turbulence modeling, porous media modeling has nothing to do with turbulence.

You need data for pressure drop vs velocity over a very large range to accurately curve fit it for both the Darcy and Forchheimer term. Having only a single data (e.g. just the air permeability) you can only get one of them.

A ton of ppl do porous media modeling with only the Darcy term because they have your same issue, they don't have data for the Forchheimer term. In some other non-Star CFD solvers, the Forchheimer term isn't even included. You'll be fine.

C72 · October 5, 2021, 12:04

No problem. This is a massive help so thanks again!

C72

C72 · December 3, 2021, 10:00

Hi again, I just have another question regarding use of the basic Darcy's Law for my application.

Is this not only valid when velocities are small enough to neglect the inertial term? i.e. Re<<1. My Re is currently around 11,000,000

I understand i have no choice but to neglect this term anyway to avoid curve fitting and view the building pressure drop as the independent variable as this is what i am trying to do. I am just trying to justify my choices a bit better for neglecting this.

October 4, 2021, 11:11	Using Air Permeability in STAR-CCM+	#1
C72 New Member Calum Join Date: Sep 2021 Posts: 14 Rep Power: 4	Hi guys, I've recently been looking at porous media tutorials in Star and seen that porous media are defined using the Darcy-Forchheimer equation. I'm attempting to model a building as permeable to some degree by defining it's walls and roof as porous media using this equation. After doing quite a bit of online research I've been able to find some typical air tightness/ air permeability values in m3/(h.m2)@50Pa for different building types from certification standard documents. I am just wondering if anyone knows if there is a way to relate these values to the Darcy-Forchheimer equation to get the inertial and viscous porous resistance values that I need to define these porous regions in Star? Any feedback would be massively appreciated. Thanks!

October 4, 2021, 12:01		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 66	It's tricky because the air tightness coefficient is a 0D parameter. You need more info and then you'll end up building your own model of what is a building. First you would probably have to assume that the Forchheimer coefficient is 0 because there just isn't enough data to figure out what it is. But if you take the air permeability metric and divide it by the average building wall thickness that would yield the Darcy coefficient (and divide that also by the differential 50 Pa). C72 likes this.

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October 5, 2021, 09:28		#3
C72 New Member Calum Join Date: Sep 2021 Posts: 14 Rep Power: 4	Thanks for the response LuckyTran, this helps a lot! I had been familiar with Darcy's Law beforehand so it went over my head a bit when i saw this additional Forchheimer term. I have read that this extra term is to account for non-linear turbulence within the porous region. Is the lack of data on this coefficient simply down to lack of gathered data from material experiments? Also, if i set this term to zero, effectively using Darcy's equation only, will it still be acceptable to model using turbulent flow solvers etc? Thanks again, C72

October 5, 2021, 11:57		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 66	The extra term is nothing to do with turbulence. It's the inertial term which is basically a crude attempt to account for deviations from the basic Darcy's law. It doesn't hurt or help turbulence modeling, porous media modeling has nothing to do with turbulence. You need data for pressure drop vs velocity over a very large range to accurately curve fit it for both the Darcy and Forchheimer term. Having only a single data (e.g. just the air permeability) you can only get one of them. A ton of ppl do porous media modeling with only the Darcy term because they have your same issue, they don't have data for the Forchheimer term. In some other non-Star CFD solvers, the Forchheimer term isn't even included. You'll be fine.

October 5, 2021, 12:04		#5
C72 New Member Calum Join Date: Sep 2021 Posts: 14 Rep Power: 4	No problem. This is a massive help so thanks again! C72

December 3, 2021, 10:00		#6
C72 New Member Calum Join Date: Sep 2021 Posts: 14 Rep Power: 4	Hi again, I just have another question regarding use of the basic Darcy's Law for my application. Is this not only valid when velocities are small enough to neglect the inertial term? i.e. Re<<1. My Re is currently around 11,000,000 I understand i have no choice but to neglect this term anyway to avoid curve fitting and view the building pressure drop as the independent variable as this is what i am trying to do. I am just trying to justify my choices a bit better for neglecting this.