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Pressure Boundaries for contactless seal with thread (pump)

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Old   August 24, 2016, 09:32
Default Pressure Boundaries for contactless seal with thread (pump)
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Luke Miller
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Hello,

again, I have some trouble to define pressure boundaries and calculate the correct pressure in CFD post.
I’m trying to simulate a contactless seal with a thread, like a labyrinth seal. The fluid is “pumped” by the rotating thread from side A to B (See picture 1)




In my simulation, I let the wall rotate in opposite direction, that’s easier to simulate.
First attempts look good, the flow direction is correct (picture 2)





On both ends I defined an Opening with “Opening Pressure = 0 Pa” (Normal to Boundary condition). So I have no real Inlet and Outlet conditions.(picture 3)





But the pressure result seems not to be correct, compared to analytical calculations from a book.

Regardless from my results, how would you define the boundary conditions?
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Old   August 24, 2016, 20:02
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You define the boundary conditions to describe the conditions at that location which exists in the system. I have no idea what is upstream or downstream or this device so I cannot make a suggestion.

If the pressure field is wrong can you post an image of what you are getting and another image showing what you expect to get?
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Old   August 25, 2016, 09:00
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Okay, I’ll try to explain in a different way. Imaging a threaded rod or a screw, which rotates in a hollow cylinder. The gap between the thread an inner wall of the cylinder is very small (0,2mm) compared to the diameter of the screw (30mm). When the screw rotates, it can pump from one side to the other. In this case the fluid is oil. The mechanic is used for contactless seals. And I want to simulate the pressure, which this “pump” can produce.
The analytical formula says, that a max. pressure of 147658 Pa can be produced with a mass flow of 0,00898 kg/s
My results in CFD Post are:


Area Average of Total Pressure on Inlet
806.233 [Pa]


Area Average of Total Pressure on Outlet
1460.77 [Pa]


Mass Flow on Outlet
-0.00698017 [kg s^-1]




Like you see, the mass flow is almost the same. But pressure is far away from the other result.
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Old   August 25, 2016, 18:59
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What does the analytical formula assume? Are you sure the analytical formula is right?

Then there are issues about basic simulation accuracy, discussed in the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

In this case mesh resolution would be a key factor I think.
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