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Cyclic boundary giving wrong gradient

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Old   November 15, 2013, 09:41
Default Cyclic boundary giving wrong gradient
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Ralf Kuehn
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I'm trying to simulate a simple test case. A fully developed laminar flow through a channel with simpleFoam using cyclic BC on the inlet and outlet (simple 2D channel, length 0.5 m height 0.05 m, viscosity 100 Pas, rho 1000 kg/m3).
I set up the average cyclic velocity ubar (0.05 0 0) into the fvOptions dict and setup 'type cyclic' for the inlet and outlet boundaries.
After solving the case i get the expected parabolic velocity profile.

Looking into the log for the pressure gradient, I got a number with the correct magnitude (expected -24.0), but the wrong sign (.. "Pressure gradient source: uncorrected Ubar = 0.05, pressure gradient = 23.666" ...)
As the channel flow is driven by the pressure gradient, it should be negative ???!
Can anybody give me a hint, what's going wrong?

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