distributed resistance

[Anonymous]

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

[apolo_vanderberg]

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

[Anonymous]

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

[OmkarJ · Accepted solution]

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

[Anonymous]

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

[Anonymous]

Hello Pei-Ying,

It was informative to read your post. As the post is quite old, could you please share your experience with simulating perforated sheets using Sim CFD because I am also working with a similar application and it seems that using the FAR condition isn't sufficient.

Sanket

[Jon.Wilde]

Hey Sanket,

(Yup, I am everywhere )

If you run into other issues, please shout, we can help.

CFD will convert everything to a constant loss coefficient for the calculation, but FAR should be OK as long as you are smart with it.

Thanks,

Jon

[Anonymous]

Hello Jon,

Thanks for your reply. I was only wondering if Pei-Ying tried the 2 suggestions given by Omkar and how did they influence his results...

Sanket