Help in porosity for knitted fabric??? Linear or Quadratic resistance coeff
Hi,
For knitted fabric having porosity of 30 and 50% similar to these photos http://www.arrigoni.it/img/agr/vento/vento8.jpg http://www.arrigoni.it/img/agr/vento/vento7.jpg 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.cfdonline.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.thermalwizard.com/tmwiz/ Regards 
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.

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? Regards 
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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. 
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:
http://www.thermalwizard.com/tmwiz/...ns/screens.htm Then continue with above approach? Quote:
Regards 
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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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Many thanks dear ghorrocks
I appreciate your great help. Just want to make sure about one correlation I found during my search: loss coefficient = (1porosity)*(drag coefficient) Best regards :) 
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.

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=0.81.2 it gave reasonable results, although it may not be accurate or even correct! Best regards 
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. 
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*(1f)+(1/f1)^2), where f is porosity 
Thanks A_Lyaskin for the formula, do you have a reference?
Best regards 
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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