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-   -   Pressure difference at outlet (http://www.cfd-online.com/Forums/cfx/20298-pressure-difference-outlet.html)

 Nessan April 13, 2004 20:44

Pressure difference at outlet

Dear friends,

In my simulation the reference pressure was set to zero and the outlet was also set to zero pressure. Then hydrostatic pressure was applied in the domain. After some iterations, I check the pressure at the outlet (in CFX-Post), it is less than the specified pressure. In addtion, both the total pressure and pressure (I guess it is the relative static prssure in CFX-Post). I am doing incompressible fluid simulation and 2nd differencial scheme was used.

Thanks,

 Nessan April 14, 2004 01:51

Re: Pressure difference at outlet

Hello, everyone,

I even checked the example result file - StaticMixer_001.res - which was located at Example folder under CFX 5.6 installtion folder. I found that the calculated pressure is totally different from that specified at Out boundary. In the example, reference pressure is zero and the relative static pressure at OUT boundary is set to zero. However, if you use calculator, you will find the average pressure is 579.786 Pa and average total pressure is 8438.77 Pa. How does the difference between calculated pressure nad specified pressure come from? (I would think the Pressure in CFX Post as static pressure).

In my simulation case, the static pressure at top surface should be zero theoretically because hydrostatic pressure is applied and the top surface was set to zero static pressure (reference pressure is zero), however, I got negative static pressure at the top surface after convergence.

Anybody has any idea about that!

Thanks,

Nessan

 Robin April 14, 2004 12:36

Re: Pressure difference at outlet

Hi Nessan,

By default, the conservative control volume values are loaded into Post. If you calculate the pressure on the conservative values, the resulting pressures will not be the same as the boundary condition, as they represent the values slightly inside the domain. Rather, you should load the "Hybrid" values (through the variables editor or using the button on the calculator panel). The result may not be exactly your boundary condition, but should be within round-off.

Similarly, when you are plotting values on the boundary, select the hybrid option to view the actual boundary values.

Regards, Robin

 Nessan April 18, 2004 00:20

Re: Pressure difference at outlet

Hi, Robin,

Thanks for your response. Actually I draw a contour of pressure on the pressure boundary in Post using Hybrid option. Does this have anything to do with mass-pressure coupling?

Thanks,

Nessan

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