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where did the energy go?

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Old   April 7, 2009, 00:21
Default where did the energy go?
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I'm running some calcs with viscous heating in laminar regime. It a heating device, something like vent. It is a 2D problem with three vanes (spaced 120 degs). The overall power produced is supposed to be P=Moment*omega, that is supposed to be equal to the heat flux leaving the system, that is supposed to be equal to mu*((strain rate)^2). But it is not. Actually, if I try to calculate the moment around the vent axis, it produces two parts: pressure part and viscous part. If I use only viscous part - its working good, i.e. the energy ballances. While using the whole moment produces imballance in energy.
I've tried this with two concentric cyls, where there is no pressure part by definition. There everything works perfect and the produced heat equals mu*(dV/dr)^2 and equal to M*omega.
Why the ballance does not work in the vent case?
Please help.
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