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"Pressure Inlets" VS "Velocity Inlet"

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Old   September 27, 2011, 20:51
Default "Pressure Inlets" VS "Velocity Inlet"
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Mohsin Mukhtar
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In my geometry, there are 2 inlets, both are at a Guage Pressure of Pg=200Kpa. The outlet is at a guage pressure of Pg-out=95Kpa. I am using "Velocity inlet" and a "pressure outlet" Boundary condition in FLUENT. As you know, in "velocity inlet BC" we do not specify Pressures so for pressure specification I used operating Pressure as 200Kpa (as it is also an important parameter in determining the density of incompressible flows). After residuals converge, the contour plots of "Absolute" or "Static Pressure" at the inlets show a pressure of 95Kpa rather than around 300Kpa. Dynamic pressure is fine here but the Static Pressure shouldn't be that low. May be in "velocity Inlet" boundary condition, FLUENT doesn't take into account the Static or Absolute Pressure, rather than it gives results only for "Dynamic Pressure"? It is just confusing me why is it like this??? Could anyone explain this?

Secondly, I tried "pressure inlet BC" instead of "Velocity inlet BC" at the inlets. The Absolute or Static Pressure was shown accurately but the contours of "velocity magnitude" at the inlet were giving very high velocity (50m/s) as opposed to 1 m/s of velocity in actual case.

There is a significant difference here in "Pressure inlet" and "Velocity inlet" boundary conditions. If I use "pressure inlet" boundary condition then velocity calculated is very high as compared to real case's boundary condition. On the other hand, if I use "velocity inlet" condition, then the pressures are not shown accurately. Could anyone explain both of this phenomenon in detail?
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