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Pressure averaging at the outlets.

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Old   November 21, 2007, 12:47
Default Pressure averaging at the outlets.
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Hi there,

For pressure specified outlet boundary conditions, fluent has one option 'Radial equilibrium pressure distribution'. While CFX-11 seems to have two options for the Pressure Averaging at the pressure outlets. 1) Average Over Whole Outlet 2) Circumferential.

Is 'Circumferential averaging' in CFX equivalent to 'Radial equilibrium pressure distribution' in Fluent?

I woould be very much thankful to you for your help !

Many Thanks,

Best Regards, KM.
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Old   November 21, 2007, 16:42
Default Re: Pressure averaging at the outlets.
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Hi KM,

With a Radial Equilibrium boundary condition, the specified pressure applies only to the position of minimum radius (relative to the axis of rotation) at the boundary. The static pressure on the rest of the zone is calculated from the assumption that radial velocity is negligible, so that the pressure gradient is given by

dp/dr = rho*v_theta^2/r

The Average Static Pressure boundary condition solves for the static pressure implicitely and will acheive an equilibrium static pressure profile. In the limit of zero radial velocity and swirling flow this is equivalent to radial equilibrium. A radial equilibrium static pressure profile is better than a fixed static pressure at swirling outlets, but it is still explicit and will result in a non-physical static pressure profile at the outlet. The CFX equivalent would be to specify the Static Pressure as an radial equilibrium expression (i.e. integrate the above equation), but I would suggest using the Average Static Pressure condition instead.

Other options: 1) Average over the whole outlet: The pressure is allowed to float over the whole outlet, or you can specify a specific region over which to evaluate the area averaged static pressure. There is no equivalent of this in Fluent

2) Circumferential: If this option is picked you will need to specify a one dimenstional spanwise profile. The solver will match/fix the circumferentially averaged pressure at each location, but allows the circumferential profile to float. A single value just means the circumferentially averaged profile is constant. There is no equivalent to this in Fluent.

Note that similar options are available for Mass Flow specified outlets. The default for Mass Flow outlets is also to let the static pressure and local mass flux to float, but match the specified mass flow. Options are also available to shift a pressure profile at a mass flow outlet, circumferentially match a pressure profile, etc.

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Old   November 23, 2007, 13:50
Default Re: Pressure averaging at the outlets.
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Dear CycLone, Thank you very much for helping me again with your detailed reply. It helps a lot. Many Thanks, KM.
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Old   March 31, 2021, 23:25
Default Hello Cyclone I have my radial velocity plot between stationary and rotating rings
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Please see the image attached. My solution is converged but the chart plot of the radial velocity doesn't make sense. the plot should reach zero near both walls due to the non-slip condition.
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Old   April 1, 2021, 01:05
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Might be caused by this
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Old   April 1, 2021, 05:24
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You can overlay your mesh on top of the radial profile and see how coarse the mesh is near the boundary.

If you plot the same profile using "hybrid" values for velocities, how does the profile look like?
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
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Old   April 1, 2021, 06:28
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You mention that you have a rotating ring. Assuming that you plot Hybrid values, isn't the value you see on the wall, the local velocity of the rotating part?
Also, make sure what velocity you are looking at. Is it Velocity or Velocity in Stn Frame?

In other words: there are various way of presenting the results. Make sure that you know what you are doing and take the one you need.
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