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Old   December 21, 2007, 03:38
Default Area or Mass Weighted
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Emmanuel
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Hi,

I have a nozzle-ejector model and the mass-weighted and area-weighed velocity at the exit are very different. For example in one case, MW velocity is 47m/s and AW velocity is 30m/s. This is a compressible flow problem. I will appreciate any input.

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Old   December 21, 2007, 05:35
Default Re: Area or Mass Weighted
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Davide
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You cannot make a velocity area weighted average because it has no sense. You are going to give more wieght to higher velocity (cells where the mass flow is higher). To have the real velocity average you must use the area weighted that is the one that, multiplied by density and area, will give you the mass flow.

Bye Davide
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Old   December 21, 2007, 15:25
Default Re: Area or Mass Weighted
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Emmanuel
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Thanks David.

The mass flow fluents gives me is 0.64kg/s, that means it is using the area weighted average for the calculation.

m_dot = rho*v*area = 1.168*30*0.01823 = 0.64kg/s These values were obtained using Area-weighted average.

Using the Mass weighted average I obtain m_dot = rho*v*area = 1.162*47*0.01823 = 1.01kg/s This is clearly different from the fluent value for m_dot

Experimental result show that the correct exit velocity should be in the 37~38 m/s neighborhood. The fluid is air, I used the ideal gas model S-A model. I have about 100000 cells, and this is the axisymmetric case.

Is there anyone who has encountered such a problem? I am trying to obtain the thrust at the exit.

Emmanuel
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