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August 22, 2018, 10:32 |
pressure initialisation in a closed system
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#1 |
New Member
M. Fenster
Join Date: Dec 2015
Posts: 3
Rep Power: 11 |
Dear community,
I have a question about the static-pressure initialisation in CFX. The graphic below this post should clarify the situation. I want to run a CFD simulation of a mixer in a big cylindrical tank with a tube inside. In the following, a short summery of the setup in CFX-pre: - steady state simulation - incompressible fluid - ref. pressure 1 atm - buoyancy Model activ - isothermal - turbulence model SST The model is divided in two parts which are separated by the interfaces. The impeller domain is rotating with 5000 rpm and the tank-tube domain is stationary. On the top of the tank a "Free-Slip-Wall" boundary was choosen. 1) The first question I have is, how can I initilize a static-pressure field in this closed system? I started a CFD simulation with the setting above and I found the following text in the out-file: "Pressure has not been set at any boundary conditions. The pressure will be set to 0.00000E+00 at the following location: Domain : Mixer_SinglePhase Node : 1 (equation 1) Coordinates : (-6.20517E-05, 5.46441E-04,-2.61476E+00)." As you can see, on node 1 the pressure is set to 0. Is this value representative for the whole domain? After the simulation was finished, I made a contour-plot of the static-pressure and I was wondering why the pressure was negative in the tank domain, because actually I estimated that the pressure should be 0. 2) I have a problem to imagine the behaviour of the static-pressure field in a closed system with a roatating impeller inside. Maybe you have any idea? best regards M3tt Konzept.jpg Last edited by M3tt; August 23, 2018 at 02:39. |
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August 22, 2018, 20:18 |
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#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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As this is a compressible simulation you will need to set the pressure level correctly. It can be ignored in incompressible simulations, but in compressible simulations the density is a function of pressure so you have to get it right.
If the top free slip wall represents a free surface which is basically flat, then you could but a small section of that face as an opening with the local atmospheric pressure.
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August 23, 2018, 02:38 |
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#3 |
New Member
M. Fenster
Join Date: Dec 2015
Posts: 3
Rep Power: 11 |
Hi ghorrocks and thanks for your reply,
Sorry that was my fault ... the fluid is incompreesible not compressible. I will edit that post. You are right the free slip surface is representing an opening. In the end I am interested in the shaft-power, so I thought that the pressure level should be set correctly, in order to get the correct value for the torque. Best regards M3tt |
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August 23, 2018, 02:44 |
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#4 |
Super Moderator
Glenn Horrocks
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Location: Sydney, Australia
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If the flow is incompressible the pressure level does not matter, provided it converges. If your simulations are converging then it might be OK.
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Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. |
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August 23, 2018, 02:49 |
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#5 |
New Member
M. Fenster
Join Date: Dec 2015
Posts: 3
Rep Power: 11 |
Thanks for the fast reply and your efforts .
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closed system, initialisation, pressure |
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