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Valued Contributor
pei-ying.hsieh
Posts: 62
Registered: ‎10-31-2012
Accepted Solution

distributed resistance

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06-07-2013 07:45 AM

Dear Autodesk Simulation CFD experts,

 

I would like to model a thin plate with a bank of holes.  The thickness of the plate is 2 mm.  The diamter of the holes is 1.5 mm.  The distance between each 2 hole center is 4.8 mm.  It looks like I can use the K-Factor method.  But, how can I find out the K value?  Or is there any other method in CFD 2014 that will be more suited for this?

 

Thanks!

 

Pei-Ying

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apolo.vanderberg
Posts: 299
Registered: ‎08-31-2011

Re: distributed resistance

06-07-2013 10:26 AM in reply to: pei-ying.hsieh

To start you might look at the geometry and compute the Free Area Ratio

You could use this as input to the resistance model.

 

If you want to compute to a K the equation that would be used is

 

K = ( [0.707(1-FAR)^0.375 + 1 - FAR]^2 ) / FAR^2

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Valued Contributor
pei-ying.hsieh
Posts: 62
Registered: ‎10-31-2012

Re: distributed resistance

06-07-2013 11:38 AM in reply to: pei-ying.hsieh

Hi, Apolo,

 

Thanks again for the reply!

 

So, for a 5X5 holes:

     Total area = 24mm X 24 mm = 576 mm^2

     Free Area = pi x (1.5 mm)^2 x 25 /4 = 44.18 mm^2

==> FAR = 44.18/576 = 0.0767

 

Correct?

However, based on the wikihelp docuement,

-----------------------------------

The relationship between loss coefficient, K, and free area ratio, FAR, is given as:

 

NoteThis equation is valid for flow with Reynolds number greater than 105. The ratio of the flat portion of the hole length, l, to hydraulic diameter, Dh, is between 0 and 0.015:
 
-------------------------------------
Dh = 1.5 mm, plate thickness = 2mm, hence, I/Dh = 2/1.5 = 1.33.  Does this relationship still applies?  In addition, the note mentioned that this relation is valid for Re > 10e5 (or 105?).  I believe that in my case, Re is < 10e5.
 
Pei-Ying
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OmkarJ
Posts: 435
Registered: ‎10-02-2012

Re: distributed resistance

06-07-2013 03:23 PM in reply to: pei-ying.hsieh

pei-ying

 

You are right, the formula Apolo mentioned is for very thin sheets (t/d<0.015). Yours is a thick sheet and hence the formula will be different, 

 

You can get the constant k-factor by two methods:

 

1) Use Idelchik's handbook of hydraulic resistance to find the k-factor for your perforated sheet

2) Create a small unitary cell that is symmetrical, and simulate it for velocities, close to your operating velocities, and then calculate the value of k as : k=DP/ (0.5*rho*v^2)

 

where DP is pressure drop, rho is density of fluid and v is velocity.

 

OJ

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Valued Contributor
pei-ying.hsieh
Posts: 62
Registered: ‎10-31-2012

Re: distributed resistance

06-10-2013 06:32 AM in reply to: OmkarJ

Hi, OJ,

 

Thanks a lot!

 

I am in the process of getting the handbook.  In the meantime, I will try to do what you suggested in option 2.

 

Pei-Ying

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