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Old   February 28, 2001, 12:15
Default Negative presures
  #1
Newbie
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Hi all. I,m trying to make very simple simulations in a bending channel with gas as a fluid in turbulent flow using k-e model. Fluent 5 keeps on reporting negative pressures even if I've change the operation pressure.

I wonder if it tells me that there is something wrong with my simulations or I'm just missing some physics.

Thanks

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Old   February 28, 2001, 16:55
Default Re: Negative presures
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John C. Chien
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(1). Flow around a very simple bend is not simple at all. (2). As a matter of fact, currently the state of the art of turbulence modeling is such that it could not be modelled accurately and must be done individually. (3). You should ask your support engineer about the error message of the "negative pressure". It simply says that the code is not smart enough to handle your problem, or you are not smart enough to use the code. (4). If you take a look at the technical journals, you will find that researchers are still working on the flow around a simple bend as PhD dissertation. (5). My suggestion is: try a straight channel first, then after you have obtained the desirable solution, try to bend it slightly and see what you can get from the code. A 90 degree bend is likely to create flow separation.
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Old   March 1, 2001, 23:42
Default Re: Negative presures
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Jin-Wook LEE
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What kind of desnisty option are you using ? Do you use constant density or ...... ?

Sincerely, Jinwook

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Old   March 1, 2001, 23:46
Default Re: Negative presures
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I use the simplest case, I believe, its air at constant density. But as Mr. Chien stated maybe is quite complicated.

Thanks for your interest.

Arturo
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Old   March 2, 2001, 08:20
Default Re: Negative presures
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cfdfans
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I think you can refer to the ˇ¶Turbulent Flow and Heat Transfer in a Mixing Elbow ˇ·of FLUENT 5 Tutorial Guide. Good luck
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Old   March 2, 2001, 11:50
Default Re: Negative presures
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keith
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If you are talking about gage pressure (which Fluent reports) then of course pressures can become negative. For example, if you are using an operating pressure of 101 kPa, you still have positive pressure even if Fluent is reporting -100 kPa.

Of course, if you are using constant density, then absolute pressure is unimportant to the simulation anyway. Setting the operating pressure is equivalent to setting some arbitrary reference. I'm guessing you could even make the operating pressure itself negative, it shouldn't matter to the computation.

Keith
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Old   March 2, 2001, 15:25
Default Re: Negative presures
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John C. Chien
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(1). This seems to be the most confusing part in using the code(s), based on my experience. (2). The users must double check the results and make sure that there is no mistake made in reading the "pressure". (3). If the result is gauge pressure, then the code must print "gauge pressure". (4). One can easily find out whether this is still a problem, by selecting the perfect gas law option. Or run the compressible flow module.
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Old   March 5, 2001, 00:01
Default Re: Negative presures
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Thanks Keith and Jhon, it is as you say. I have to confess that I read the manual twice looking for the kind of pressure fluent reports and I couldn't found it. I agree with John, is quite confusing. Thanks to all that posted such a trivial question.

Sincerely Arturo
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Old   March 5, 2001, 05:13
Default Re: Negative presures
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Jin-Wook LEE
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Dear Newbie

Of course, you are right(constant density). For the flow of temperature dependent density(ideal gas law), negative pressure sometimes observed. Because you are not the case, I think that it's only numerical convergence problem. According to your message, your case is very simple one.

I assume your case is 2D. Check your grid quality. As I somtimes said, 4 side mesh as orthogonal as possible is best. And avoid sudden expansion of the grid size along each direction and keep the aspect ratio of each grid as close as possible to the unity. Everything is impossible. But as close as possible.

Sincerely, Jinwook

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