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Help in porosity for knitted fabric??? Linear or Quadratic resistance coeff

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Old   September 15, 2011, 15:50
Default Help in porosity for knitted fabric??? Linear or Quadratic resistance coeff
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Hi,
For knitted fabric having porosity of 30 and 50%
similar to these photos



Any suggesions of resonable values for
Linear or Quadratic resistance coefficients?
or permeability
I need these values to define the porous material.
I have spent considerable time searching for any useful literature for this issue.

What is the range of values you expect would be appropriate for such fabric?
For example: Quadratic resistance coefficients = 300 kg/m^4?

Is it possible to calculate these values?
I have gone through almost all threads regarding above parameters in this forum,
I have been in this link too:
http://www.cfd-online.com/Wiki/Fluen...e_drop_data.3F

but the problem is that I don't have and can't perform experiment to obtain the required data.

Could obtaining the loss coefficient K_loss and head loss be of any help?
using this link
http://www.thermal-wizard.com/tmwiz/

Regards
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Old   September 15, 2011, 18:57
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Glenn Horrocks
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You can estimate the resistance of a single isolated cylinder quite accurately, the drag of a cylinder is well known. If you search the literature you can probably find the drag/resistance of a bank of cylinders. You should be able to define a resistance coefficient from this.
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Old   September 16, 2011, 01:29
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Thanks for your response ghorrocks

Is this what you are meaning by isolated cylinder (second and last objects)
http://www.centennialofflight.gov/es...nts/TH12G8.htm
found through this:
http://www.centennialofflight.gov/es...ients/TH12.htm

From Wikipedia I found this
http://upload.wikimedia.org/wikipedi...4ilf1l.svg.png
http://en.wikipedia.org/wiki/Drag_coefficient

But which resistance is defined through the drag coefficient and how?
Also, the knitted fabric is very thin, would that effect the cylinder assumption?

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Old   September 16, 2011, 06:36
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Quote:
Is this what you are meaning by isolated cylinder?
Yes, that is correct. The drag coefficient is actually a function of Re, but if you are lucky enough to be in the constant drag coefficient regime then a constant is fine.

Quote:
But which resistance is defined through the drag coefficient and how?
Convert the drag coefficient into a pressure drop versus flow velocity curve. Then this can be implemented as a porous resistance or source term.

Quote:
Also, the knitted fabric is very thin, would that effect the cylinder assumption?
I do not think you understand what I am getting at - the fabric is a whole bunch of little cylinders next to each other. The total drag is the sum of the drag of each little thread, and each thread can be assumed to be a cylinder.

But the drag of an array of cylinders next to each other is different to an isolated cylinder. You will have to take this into account.
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Old   September 16, 2011, 09:17
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Thanks again dear ghorrocks

What i'm trying to do is to examine the effect of the fabric porosity on blowing air wind (So it is an open area not pipe for instance)

I found the following link that provides a way to determine the resistance loss coefficient in (m^-1)
(Is it enough to define the porosity of the fabric accurate enough?)
http://jullio.pe.kr/fluent6.1/help/html/ug/node236.htm
It is located under this title:
Quote:
Deriving Porous Media Inputs Based on Superficial Velocity, Using a Known Pressure Loss
So how about determining the head loss coefficient for a screen with required porosity:
http://www.thermal-wizard.com/tmwiz/...ns/screens.htm
Then continue with above approach?

Quote:
Convert the drag coefficient into a pressure drop versus flow velocity curve. Then this can be implemented as a porous resistance or source term.
Any source that provides the needed correlations of drag coefficient? From my search I didn't reach any helpful thing.

Regards
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Old   September 16, 2011, 09:28
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Quote:
So how about determining the head loss coefficient for a screen with required porosity:
http://www.thermal-wizard.com/tmwiz/...ns/screens.htm
Then continue with above approach?
By taking another thought, the above link for head loss in screen is for pipe, so it won't be accurate for the case of open blowing air?

Regarding the drag force, how can I use it to obtain the pressure drop, or head loss coefficient? any helpful equation?

Sorry for the "shallow" questions but i'm structural engineer and lots of what I learned in fluid has vanished
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Old   September 17, 2011, 07:15
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Quote:
Deriving Porous Media Inputs Based on Superficial Velocity, Using a Known Pressure Loss
This approach looks good. I don't have time to read it in detail but it looks promising.

Quote:
drag force, how can I use it to obtain the pressure drop
If you have the drag force of the membrane then you can divide it by the membrane area to get the pressure drop across the membrane.
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Old   September 17, 2011, 09:32
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Many thanks dear ghorrocks
I appreciate your great help.

Just want to make sure about one correlation I found during my search:

loss coefficient = (1-porosity)*(drag coefficient)

Best regards
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Old   September 18, 2011, 07:42
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I have no idea about that. It does not seem to be applicable to the CFX approach as CFX does not use a loss coefficient of that form.
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Old   September 18, 2011, 15:27
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I see,
I found it in this publication
http://journals.tdl.org/ICCE/article/viewFile/4173/3854
equation 8 in 3rd page.

When I used it with the assumption of cylinder having Cd=0.8-1.2 it gave reasonable results, although it may not be accurate or even correct!
Best regards
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Old   September 18, 2011, 18:26
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Again, I do not have time to read your link.

But it appears that they are simple grossing up the individual cylinder drag to the local porosity. This sounds like a very approximate (ie not very acurate) approach to me as it does not take into account proximity effects. But if you have nothing better and do not need very accurate results then you could use it anyway.
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Old   September 19, 2011, 05:11
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There is an empirical formula for meshes, which can also be used for fabric, I guess:

delta_P/(0,5*rho*V^2)=1,62*(1,3*(1-f)+(1/f-1)^2),

where f is porosity
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Old   September 21, 2011, 14:55
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Thanks A_Lyaskin for the formula, do you have a reference?

Best regards
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Old   September 22, 2011, 11:43
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It's from "Handbook of Hydraulic Resistance" by Idelchik. There is a chapter about grids and meshes and etc. The formula is for wattled mesh.
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