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How to calculate mean pressure in a pipe flow 

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August 30, 2001, 08:22 
How to calculate mean pressure in a pipe flow

#1 
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I want to find the mean pressure in a cross section But how to get the mean pressure Please tell me step by step Anyway thank for your help


August 30, 2001, 14:30 
Re: How to calculate mean pressure in a pipe flow

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(1). I don't know how to do it, but I know that there are different types of "mean pressure". (2). So, the first thing to do is : define the "mean pressure" first in terms of algebraic equation. Then you can find out whether you can write a piece of code to do it, or you need to contact the support engineer. (3). Sometimes, the solution is rather simple, depending on the geometry.


August 30, 2001, 14:59 
Re: How to calculate mean pressure in a pipe flow

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Hi John,
please answer those question where you have someting to contribute. Otherwise be quiet. Now for the mean pressure: If you have a celllayer which forms your section then it is simple. Put those cells into a cset. Then load the pressure ("getc p") and type "summ cset". So you should see some average value for register 4. You can also do a cellvolume based averaging. Then you also have to load the cellvolume into another register and multiply ..... Just use the "operate" command for this purpose. 

August 30, 2001, 19:32 
Re: How to calculate mean pressure in a pipe flow

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(1). If I follow your suggestions, what kind of mean pressure would I be getting? (2). Could you be more specific? Is that a simple algebraic mean pressure? (which is usually not very useful). Or Is that an area weighted average pressure? (in that case, you need the the 2D area of the cell on that particular cut (or crosssection) There is no such thing as the volume averaged pressure on a crosssectional plane. (3). Well, normally we are interested in the massflow averaged pressure or total pressure. In that case, the averaged density and velocity of that particular cell cut also must be available. (4). At the inlet plane or the exit plane, it might be relatively straightforward because the cell faces are normally on a flat surface. In the middle of the pipe, in general, when you make a crosssectional cut, it will not go through an existing flat cell surfaces. (5). I had the same need before, when I was using CFXTASCflow. And I had to use hand integration using graph paper, because the mesh was 3D. (6). Since I am no longer using the code, I don't have the need to know whether it can be done or not.


August 31, 2001, 04:06 
Re: How to calculate mean pressure in a pipe flow

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Define the section you need using spoi and snor (as for a section plot). Make sure the section only cuts the cells you want to calculate the mean for. Enter command "inte cset". You get the area and the integrated values for normal vector (A*VN), scalar (A*S) and normal vector*scalar (A*VN*S) for the section. For the mean of the scalar value, you must divide the integrated scalar (A*S) value by the area (A).
(Another application of this command: load the velocities and the density (getc all dens). Then the "inte cset" command gives you the mean velocity (A*VN / A) and the massflow (A*VN*S) through the section.) 

August 31, 2001, 04:09 
Re: How to calculate mean pressure in a pipe flow

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I forgot something: of course, you must first load the pressure.


August 31, 2001, 08:33 
Re: How to calculate mean pressure in a pipe flow

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Since you have access to all variables you can do any sort of averaging you want.
If you want to do an averaging about an arbitrary area (also a curved one) then you have to create a shell mesh which describes this section and use "pmap" to map the post results to this shells. Then you can use the "summ" or "integate" commands. 

August 31, 2001, 09:21 
Re: How to calculate mean pressure in a pipe flow

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August 31, 2001, 09:59 
Re: How to calculate mean pressure in a pipe flow

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Damn you John C Chien, a guy asks a question in a group, we get two good answers from knowledgeable users (Joern and Stefan) and then you pop up to pump hot air into the forum, if you don't know squat be quiet!
My mother always taught me it was rude to knock someone down and not try to help them afterwards so let's give a quick lesson in html tags: if you want a list of points
then type it like this <ol> <li>First point</li> <li>Second point</li> <li>Third point</li> <ol> 

August 31, 2001, 13:00 
Re: How to calculate mean pressure in a pipe flow

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(1). If someone has to ask a question about how to compute mean pressure when using a commercial code, then we know that the code is not very userfriendly. (2). And if the vendor think that two good answers is enough to solve the problem, then it answered the question why this code is not as popular as the other. It looks like that the people behind are not userfriendly. (3). The better answer is to give the macro to the reader so that he can compute the mean pressure. He is not interested in your mother's teaching at all. (4). So, is it possible to list the macro "nicely in your expert HTML form here", so that we can all verify the result? A stepbystep macro instruction to get the mean pressure in a pipe here, please.


October 11, 2001, 12:19 
Re: How to calculate mean pressure in a pipe flow

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Of course you should load also the velocity vector. The following prostar commands are needed:
load,, !load postfile getc all ptot !get velocity and total pressure spoint .... !define section point snorm .... !define section normal cset .... !define cell set to cut integ cset !>see stefan's message 

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