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Pressure outlet condition can't fix the pressure?

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Old   January 31, 2002, 07:31
Default Pressure outlet condition can't fix the pressure?
  #1
mei
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Dear Sir or Madam,

Now I am dealing with an unsteady problem with periodical inlet velocity. And I use a zero pressure as outlet condition. However under the monitor of each time step,the outlet pressure doesn't fix on zero.

Therefore I am wondering that if the pressure condition can not keep the pressure as certain value?

Hope you can do me a favor! Thanks a lot! Zhang Junmei
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Old   February 1, 2002, 09:43
Default Would you please to help me?
  #2
mei
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Hello,everyone!

Would you please to help me slove this problem? Thanks again! Mei
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Old   February 1, 2002, 11:55
Default Re: Would you please to help me?
  #3
Sharad Dugad
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Can you explain your problem in greater details?
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Old   February 2, 2002, 00:30
Default Re: Pressure outlet condition can't fix the pressu
  #4
mei
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Dear Sir or Madam,

I am very sorry to trouble you .However as a new user of Fluent, I need some help indeed.Sorry for bothering you indeed!

Now I am calculating the pusatile flow of blood in the vessels. To check the accuracy of result, first I build a straight cylinder,then give it pusatile inlet velocity and zero outlet pressure. The length of the cylinder is more than 10 times of the dimmeter. I think if the calculation is right, the velocity of the outlet will be equal to the inlet.

Yes,when the velocity is in the increasing state, the outlet velocity can keep up with the inlet velocity very well. However in the decreasing state, the outlet velocity can not keep up with the inlet velocity. And under the monitor of each time step,the outlet pressure doesn't fix on zero,it drops to negative when the inlet velocity in the decreasing state....

So I try to slove this problem in these days by using different outlet condition(outlet flow ratio equals to 1),increasing the iteration numbers of each time step and changing the relax factors of momentum and pressure. But the result is still bad similarly .

Have you ever happened to this problem before and how do you deal with it?

BTW, do you know why fixed outlet pressure can't fix the pressure of outlet?

Sorry to bother you a lot!

And thanks for your help very much! Regards Zhang Junmei
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Old   February 5, 2002, 13:38
Default Re: Pressure outlet condition can't fix the pressu
  #5
Amadou Sowe
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Can you tell us what function you used for your velocity inlet boundary condition?
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Old   February 6, 2002, 08:40
Default Re: Pressure outlet condition can't fix the pressu
  #6
mei
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Dear Amadou Sowe,

the inlet velocity I used is the function of both the radius and time, such as this:

F_PROFILE(f, thread, position) =(2-2*(y*y+z*z)/(.01*.01))*(0.2513-

0.0348*cos(5.026548246*t)+0.4440*sin(5.026548246*t )

-0.1863*cos(10.05309649*t)-0.1643*sin(10.05309649*t)

+0.0210*cos(15.07964474*t)-0.0437*sin(15.07964474*t)

-0.0136*cos(20.10619298*t)-0.0184*sin(20.10619298*t)

+0.0345*cos(25.13274123*t)-0.0030*sin(25.13274123*t)

-0.0024*cos(30.15928947*t)+0.0266*sin(30.15928947*t )

-0.0109*cos(35.18583772*t)-0.0007*sin(35.18583772*t)

-0.0052*cos(40.21238597*t)+0.0064*sin(40.21238597*t )

-0.0098*cos(45.23893421*t)-0.0105*sin(45.23893421*t)

+0.0063*cos(50.26548246*t)-0.0007*sin(50.26548246*t) ) ;

Thanks a lot! Zhang Junmei
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Old   February 6, 2002, 11:44
Default Re: Pressure outlet condition can't fix the pressu
  #7
Amadou Sowe
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Hi Mei,

The reason why I asked you for the inlet velocity is to find out whether the velocity is changing signs at the inlet. If your pipe is horizontal and is going from left to right and the inlet is on the left side, then positive velocity means that the flow is going into your domain. In this case the flow will be going out through your pressure outlet. After some time, let say the velocity changes to negative, this time the flow is going out at the velocity inlet the pressure outlet is now acting as an inlet. If the velocity is changing signs , this is probably why the pressure is not fixed.

Have you checked your velocity inlet to see if it changes signs? You can check this by monitoring your mass flow rate or velocity at the inlet.
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Old   February 6, 2002, 22:53
Default Re: Pressure outlet condition can't fix the pressu
  #8
mei
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Dear amadou,

Thanks for your help very much!

Yes the inlet velocity is changing periodically,first it increase to 0.9m/s, then it decrease to -0.2m/s,and then it increase to begin another period. Therefore there is a sign change in the inlet velocity.

And when the inlet velocity increase, the outlet velocity is also increasing. After the maxmium velocity(0.9m/s), the inlet velocity begins to decrease, but the outlet velocity decrease a little slowly. And some reverse flow also happens near the outlet wall, to keep the mass balance,the outlet velocity near the axial begins to increase, so the average velocity of outlet keeps above the zero during the whole decreasing phase,and not as same as the inlet velocity.

Do you think this kind of result is acceptable? And if it is not right,would you please to give me some advice on it?

Thanks a lot! Zhang Junmei
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