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Old   August 27, 2001, 04:54
Default the inlet boundary conditon
  #1
Luo
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I simulated a problem. But there is some mistake in the result. The value velocity in the inlet don't conform to what i fixed. And the features of my problem are: 1 Single phase 2 The turbulen model i applied is Mixlen
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Old   August 28, 2001, 07:01
Default Re: the inlet boundary conditon
  #2
kike
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Dear Luo

I can't infer what you want from you message. If you want some help try to:

1- Ask something. 2- Ty to be more clear describing your problem. 3- Try to explain what you have done into PHOENICS.

Nevertheless, reading your lines I suggest you deactivate the turbulence model and to run again your "case". Sometime to go step by step save time and energy.

Regards

Kike
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Old   August 28, 2001, 13:08
Default Re: the inlet boundary conditon
  #3
David Glynn
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I am not clear exactly what the problem is, but I gather you are having difficulty specifying the inlet boundary condition. I suggest you read the Lecture on Boundary Conditions, to be found in POLIS under "Lectures for Version 2". Note that most of the content of these lectures is still valid, the only problem with them is that they predate the VR interface. The way boundary conditions are implemented in Earth is still the same.

If you specify a fixed-mass-flow inlet, corresponding to prescribed density and velocity, this specifies what mass and momentum go into the cell through the boundary. When you say the velocity is wrong, do you mean the velocity in the first cell? If so, you should not expect this to be the same as the inlet velocity, which is by definition the velocity of the inlet flow, rather than a velocity within the solution domain.

I suggest you also look at the "NET SOURCES OF R1" in the RESULT file - this are mass flow rates in kg/s, and provide a very good way of checking whether you have got the boundary conditions right.

Good luck!
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Old   August 29, 2001, 04:19
Default Re: the inlet boundary conditon
  #4
luo
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sir
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Old   August 29, 2001, 04:26
Default Re: the inlet boundary conditon
  #5
luo
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dear Kike,

The results of my problem ,such as the velosity,in the inlet boundary don't equate with what i fixed. You knowe that is wrong. and i want to knowe what is the reason. please! regards luo
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Old   August 29, 2001, 06:45
Default Re: the inlet boundary conditon
  #6
Kike
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Dear Luo

No, it is not necessarily wrong. Of course the velocity you can see in your results is not the same you have imposed on the BC.

If you imposse the velocity, say in WEST face of certain region, the velocity you will have as "calculated velocity" for this cell correspond to the EAST face (using the defult solver option). So, both they are a cell size distant.

Moreover, if you are considering certain amount of turbulent kinetic energy for this BC then the resulting (calculated) velocity will differ from the inlet value according to it.

If your fluid density is constant over the inlet BC try to check what David had said about R1 source at this boundary.

Although, if the velocity values differ four or five order of magnitude then the problem is much more serious. How different they are?

Regards

Kike
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Old   August 29, 2001, 22:41
Default Re: the inlet boundary conditon
  #7
luo
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Dear Kike,

The value of the velosity that i imposed in the INLET BC is 25 m/s, but the calculated result is about 20m/s. And I assume that the fluid density is constant.

I am sure the reason isn't in the "cell distance" that you said, because the value,25m/s, of the velosity in the Bc in the domain is maximal in the domain and the maximal value in the result is only 20m/s.

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Old   August 29, 2001, 22:42
Default Re: the inlet boundary conditon
  #8
luo
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Dear Kike,

The value of the velosity that i imposed in the INLET BC is 25 m/s, but the calculated result is about 20m/s. And I assume that the fluid density is constant.

I am sure the reason isn't in the "cell distance" that you said, because the value,25m/s, of the velosity in the Bc in the domain is maximal in the domain and the maximal value in the result is only 20m/s.

Regards

luo

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Old   August 29, 2001, 22:59
Default Re: the inlet boundary conditon
  #9
luo
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Dear Dvid, The results of my problem ,such as the velosity,in the inlet boundary don't equate with what i fixed. You knowe that is wrong. and i want to knowe what is the reason. please! And what "the first cell" that you said mean?

regards

luo
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Old   August 30, 2001, 02:52
Default Re: the inlet boundary conditon
  #10
John C. Chien
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(1). For us to help you, it is helpful to know something about your problem definition, the Reynolds number, the mesh size. The more the better. (2). But it is up to you. (3). Any numerical analysis or simulation will involve some kind of approximation. So, you are getting something back in the same ball park, the right order of magnitude. (25 vs 20 m/s) (4). The accuracy is not good. (25-20)/25=20%, and that is well over the acceptable engineering accuracy of 5%. (5). In addition, if it is the fixed boundary condition, then something is wrong. (6). Let's assume that it is related to the numerical accuracy, then why not play with the mesh size first. Can you increase the mesh size, or double the mesh size, and check the results again? (7). If you have not yet performed 99 cases or calculations, then you have a lot to do next. (8). Tell us more about your fluid dynamic problem first.
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Old   August 30, 2001, 06:50
Default Re: the inlet boundary conditon
  #11
Kike
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Dear Luo

If I ask for help (I do it every day) I start to give all the information I can. The more info I give the better fitted the help I will receive. For instance, I like celtic music and I have several CD of it. If I want to ask to muy wife about one of them I should specify: the group (or player) and also the year (or concert) and if I can tell her the room where I think I could let it, it will be better. I agree with JCChien about the info we need to help you.

If you see into the momentum eqs. there are gradients and divergences, so the cell size (not the cell distance) is a key factor. Moreover, the reason you give about your "maximum velocity value" in your domain has not physical meaning for me (at least with the info I have about your problem). Depending on the physical system to model, pressure can do beautifull things with your "maximum/minimum" velocity values.

Regards

Kike
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Old   September 3, 2001, 23:19
Default Re: the inlet boundary conditon
  #12
Guanjian
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(1) As for my problem, it's fixed boundary conditon. If something were wrong, i want to knowe what I can do. (2) The Relouds number is about 2000.

The mesh size is about 10mm x 12mm x 13mm (3) my problem is:

1. the air conditon in the boiler.

2. single phase

3. the trbulence model is Kerng.

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Old   September 5, 2001, 06:53
Default Re: the inlet boundary conditon
  #13
luo
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Dear John, (1) As for my problem, it's fixed boundary conditon. If something were wrong, i want to knowe what I can do. (2) The Relouds number is about 2000.

The mesh size is about 10mm x 12mm x 13mm (3) my problem is:

1. the air conditon in the boiler.

2. single phase

3. the trbulence model is Kerng regards luo
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Old   October 4, 2001, 13:57
Default Re: the inlet boundary conditon
  #14
David Glynn
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To answer you, I need to know exactly what you mean by (1) "what I fixed" - in other words, how exactly have you specified the velocity, and (2) "the results of my problem, such as the velocity" - what exactly do you mean by this? You need to be more precise in your question.

By "first cell" I meant a cell into which the inflow is flowing.
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