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Area-weighted-Average Pressure in a channel

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Old   May 15, 2008, 09:46
Default Area-weighted-Average Pressure in a channel
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Hi, I am simulating a simple flow in a channel with one inlet and one outlet. I want simply to calculate the Area-Weighted Average of the static pressure at inlet and outlet ; I have always zero for the outlet (same for same of facet values and mass averages) : what is wrong ? Thank for any helps .

Area-Weighted Average

Static Pressure (pascal)

inlet 0.49832925

outlet 0

---------------- --------------------

Net 0.24916463

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Old   May 15, 2008, 11:45
Default Re: Area-weighted-Average Pressure in a channel
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First check at what location you have given operating pressure. If this point is outside cfd domain, then fluent will fix it at outlet automatically. In that case your area wt avg static pressure will be zero. Interpret this zero as 0+operating pressure, so area wt avg pressure at outlet will be equal to operating pr. Similarly, inlet pressure will be 0.49833+operating pressure.

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Old   May 16, 2008, 09:45
Default Re: Area-weighted-Average Pressure in a channel
Andrew Clarke
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if you set a boundary condition of 0 for your outlet (default), you will get a reading of 0 for pressure, try instead measuring the weighted average pressure just before the outlet (i.e. if your model length is 50cm, then measure the pressure along created line surface (surface --> line/rake --> x0 and x1 =49.9cm, y0 = 0 and y1 = model height). You should get pressure greater than 0 when you get an xy plot of the new created line.
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Old   May 19, 2008, 04:11
Default Re: Area-weighted-Average Pressure in a channel
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Hi Thanks for the help In fact I am trying to check a force balance over the control volume of the simulation channel (the lenght of the channel is oriented along x) ; Basically In am trying to check the following : with Which forces are we dealing with ? (A is the area vector and orientated outward the domain):

Pressure forces: F_p_in = A_in *p_in (negative x) and F_p_out = A_out*p_out (positive x)

Friction forces: F_wall (positive x)

Momentum forces: F_M_in = rho * A_in * U_in^2 (negative x) and F_M_in = rho * A_out *U_ou^2 (positive x)

The final balance will lead in a scalar form to: A*p_in - A*p_out - F_wall + F_M_in - F_M_out = 0

Momentum forces can be calculated by momentum forces you have to use Report/Surface Integrals/ Flow Rate of X_velocity for the in- and outlet ; pressure with Report/Surface Integrals/Area-Weighted-Average .

For my simulation the equation above is not validated (meaning never equal to 0 (0,01... for example) I have try to calculate "by hand" each terms of the equations (instead to use report..) : it is still not right ; even if I used different surface than Inlet/outlet.

Is there a mistake somewhere ?

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Old   May 19, 2008, 05:31
Default Re: Area-weighted-Average Pressure in a channel
Andrew Clarke
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I had a similar situation with a model of mine where the mass flow rate difference between the inlet and the outlet was quite large. This can be caused if your convergence criterion kept at the standard e-6 for energy and e-3 for others. I find that this is not low enough. I instead reduce the energy criterion to around e-10 and others to e-7. The difference between the mass flow rates fell in my case from 0.1 to 0.00000000000000001 approximately in one of the models.

Therefore by decreasing your convergence criterion from the standard to a lower value, you should decrease imbalances. As your model is 3d, imbalances will be more apparent than in 2d. I had a friend who did a 3d model with mass flow imbalances of 0.01 approximately and he got his paper accepted for a journal... so may be on the right track.

If you are still unhappy with the imbalance, you could then examine closely the mesh to see if the mesh is sufficiently dense in areas of high gradients, low levels of skewness and not having any cells 2x bigger than corresponding adjacent cells particularly in the high gradients. Make sure that the increased density mesh areas are slightly outside where the high gradients begin ... this could help things also..

Let me know how you get on...

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