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Message 1 of 8

Anonymous

1165 Views, 7 Replies

06-07-2013
07:45 AM

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Not applicable

06-07-2013
07:45 AM

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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.

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

Solved! Go to Solution.

Solved by OmkarJ. Go to Solution.

7 REPLIES 7

Message 2 of 8

06-07-2013
10:26 AM

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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

06-07-2013
10:26 AM

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

Message 3 of 8

Anonymous

in reply to:
Anonymous

06-07-2013
11:38 AM

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Not applicable

06-07-2013
11:38 AM

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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:

NoteThis equation is valid for flow with Reynolds number greater than 10*5*. 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

06-07-2013
11:38 AM

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 10*5*. 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

Message 4 of 8

06-07-2013
03:23 PM

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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

06-07-2013
03:23 PM

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

Message 5 of 8

06-10-2013
06:32 AM

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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

06-10-2013
06:32 AM

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

Message 6 of 8

Anonymous

in reply to:
Anonymous

10-14-2016
01:51 AM

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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

10-14-2016
01:51 AM

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

Message 7 of 8

10-14-2016
02:26 AM

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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

Jon Wilde

Technical Support Manager

CFD Knowledge Network | CFD Help | My Screencasts | Autodesk Simuation YouTube | CFD Tutorials

10-14-2016
02:26 AM

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

Jon Wilde

Technical Support Manager

CFD Knowledge Network | CFD Help | My Screencasts | Autodesk Simuation YouTube | CFD Tutorials

Message 8 of 8

10-14-2016
02:46 AM

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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

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

10-14-2016
02:46 AM

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

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