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 pato January 31, 2012 06:49

pressure drop

Hi, i´m sorry to ask again but i n would like to ask you what you think the best solution for my problem is.
I just modelled a perforated plate(1cm thick), obtaining the velocity profile along the plate.
Considering that now i have to get the same results but replacing the 1cm plate for a porous media, what boundary conditions would you change in order to get the same velocity profile??

i´m trying to write a pressure drop throug the interface of the porous media, but don´t really know what equation should i use, finding different information depending on the website i search.

 Dimeflow January 31, 2012 08:11

Hi Pato,

If you've explicitly modelled your perforated plate, then you could measure the pressure drop across, and divide by the the plate thickness.

This will give you your (-deltaP/deltaX) value which you can assign as a General Momentum Source term.

If the perforations are only in one direction, you can apply a large number in the other two (10^6)

Cheers

EDIT: you'll want to set this up as a subdomain

 pato January 31, 2012 11:24

yes, the only problem is that the pressure on the perforated plate was not constant.

 Dimeflow January 31, 2012 11:44

but is the pressure drop across it constant?

 pato January 31, 2012 11:52

no, it is not constant, that is the main problem.
how to obtain an specific velocity profile in the upper part of my porous media. In the simulation with preforated plate i had an irregular drop pressure along the plate, which caused an specific velocity profile. My task is to reach this velocity profile with porous media.

 Dimeflow January 31, 2012 12:44

obvious suggestion, but does the the pressure drop change discretely (ie if the number/size/arrangement of perforations changes discretely). If so you could set various subdomains with individual pressure drops assigned.

 pato January 31, 2012 12:53

All perforations have the same size, and are placed at the same distance from each oher. nevertheless the drop pressure through the last perforations is much bigger than at the beginning. that is tmay problem, how to modelate this drop pressure.

 pato February 1, 2012 10:51

do you have any idea how to solve it?? i´d be really grateful if someone could help me. I´m completely stucked and can´t go any further.
Thank you all

 Dimeflow February 1, 2012 12:46

Hi Pato. If estimating the pressure drop properties via loss coefficients is not an option, it seems what you're after doing is defining a pressure drop profile from your explicit model(s) results, and assigning this to your porous model.

There's every chance there's a far easier way to go with this... but one option could be to set up a number of simple test cases with an inlet positioned directly perpendicular to a portion of your perforated plate from original model. Running at different speeds will allow you to measure the varying pressure drop across the medium and build a deltaP v Vel curve, which you can then use to create a function in CFX, which can in turn be referred to in a CEL expression in the Momentum Source/Sink term.

Sound a realistic option?

Has been a while since I've done something like this so will have a quick read tomorrow and post anything that may at all be of help. Like I said, there may be an easier way to go about this.

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