## Simulation CFD

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# distributed resistance

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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

Solved! Go to Solution.

# Re: distributed resistance

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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

# Re: distributed resistance

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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:

*5*. The ratio of the flat portion of the hole length, l, to hydraulic diameter, Dh, is between 0 and 0.015:

# Re: distributed resistance

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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

# Re: distributed resistance

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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