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Before changing density, estimate approximate velocity (No 4), also check thermal boundary conditions. Bests, |
Thank you for your reply Amir,
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I have also simulated the results with constant density. The results are similar to compressible flow. Also, the thermal conditions used were also checked and seems to have no problems. Constant temperature is used at the inlets for the flow of N2. Thanks again for your valuable responses. |
It think the problem is resolved if you do this:
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Bests, |
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Dear Mr. Amir
Please have a look at this short issue. I have posted pictures also. As, I have already descirbed in the previous posts, I have 2 inlets and 1 outlet. The lower inlet is having a "velocity inlet" BC with a velocity magnitude of 4.678m/s. The upper inlet is also having a "velocity inlet" BC with a velocity magnitude= 0.3 m/s. The operating pressure is 297458 Pa (as the flow is considered incompressible and this value is used to compute the density of the flow, hence, this value is set according to mean flow pressure of the inlets). The outlet is creating a suction and it has a Pressure outlet" BC with a guage pressure value of -196623.3 Pa (i-e Absolute pressure=100834.7 Pa, as guage pressure will be added to operating pressure for absolute pressure). After simulation: The solution gives appropriate velocity contours but the pressure contour plots for the inlets are strange. Picture 1: (Contours of Guage Static Pressure at Lower inlet) The contours of guage static pressure are giving negative values (similar to the outlet Pressure values). Picture 2: (contours of Guage static pressure at Upper Inlet) Here, also the contours of static pressure are giving negative value (similar to the outlet Pressure values) Picture 3: (contours of Guage static Pressure at outlet) Here the contours are giving negative values as specified pressure outlet Boundary condition.So, it seems to be fine. Question 1: Shouldn't the contours of static pressure be similar to operating pressure value (which is 297458 Pa-mean flow pressure)? What is actually operating pressure? Does operating pressure is the pressure outside the domain? Why is the contour plots at the inlets are giving pressure values relative to outlet pressure values and not the one specified in operating pressure. To check this confusion, I calculated the solution with "pressure inlet" BC at inlets and "pressure outlet" BC at outlets. In this case, As, it is pressure Inlet boundary condition, so pressure values were given at inlets. The simulation results show the static pressure values to be appropriate but the velocity values at the inlets were calculated very high (i-e 100m/s, actually it should be 4.678 m/s according to the flow rate given to me). Question 2: how is FLUENT calculating velocity magnitude for Pressure inlet case? through Bernouli equation? I'll be grateful for your guidance. Mohsin |
Dear Mohsin,
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Bests, |
Dear Amir, Thank you so much for your kind consideration.
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You mean that Absolute pressure or Guage static pressure are not important in incompressible flow. So, the values of static pressure obtained at the inlets should not be seen. Only the difference of static/dynamic/absolute pressures should be noted. As in my case, the static pressure contours, at the inlets, are very strange (giving negative values; picture 1 and 2 above), the results may be ok as the difference is important and not the absolute values. Could you please clarify why absolute values are not important and only pressure difference is important in incompressible flows? |
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Thank you so much. |
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Bests, |
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Dear Amir, thank you very much for you valuble discussion.
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The momentum equation is shown in picture. Just out of curiosity, what do you mean by "Constant" in this Quote:
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Dear Mohsin,
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But the meaning of your statement is generally correct but it depends on different applications. Note that you can easily add arbitrary values to your pressure field in post processing stage via "custom field functions". Quote:
Bests, |
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Hi,
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Bests, |
Thank you so much. I am so glad that people, like you, spare their time for this kind of valuable discussion. Everything regarding my long awaited puzzle is clear now.
have you came across or can you recommend any FLUENT manual/guide/book (other than the FLUENT USER GUIDE) which can give more insight into its solution procedures? |
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If you want to dig further into numerical procedures basically I can recommend these famous and valuable CFD books:
Hope that help you. Bests, |
Hello!
I am confused at your discussion. what is the difference between static gauge pressure and total gauge pressure? I have to apply vacuum pressure at the one side of outlet. I set operating pressure 101.325 kPa. i have to apply 96.825 kPa absolute pressure. so guage pressure will be -4.5 kPa that i should set? Am i right? |
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-4.5 kPa is what you need to set at the outlet, that is correct. |
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