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Stage Interface (Stage Average velocity vs Constant Total Pressure)

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Old   April 1, 2016, 07:28
Default Stage Interface (Stage Average velocity vs Constant Total Pressure)
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Dear all,

can someone give a comprehensive explanation about the Stage Interface (Static average velocity vs Constant Total Pressure)?

The CFX guides says that the when the Constant Total Pressure is chosen, the downstream velocity (i.e on Side 2 of the interface) is calculated based on the average band total pressure and the direction in the relative frame.

What does this mean "the direction in the relative frame"?

Also, it says that the in this case (Constant Total Pressure), the frame type will be rotating. Isn't it obvious that the frame type for Stage interface for a rotor domain is always rotating?

Any clarification on these topics would be helpful.

Regards,
Juzer
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Old   April 1, 2016, 08:10
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you already have posted in topics that explain more about stage interface and frozen rotor: http://www.cfd-online.com/Forums/cfx...machinery.html

maybe to following pictures help:

1. Frozen Rotor, velocity profile in the interface ("what the downstream flow sees")

--> FR shows _one_ specific flow pattern since the rotor is fixed.

2. Stage Interface, velocity profile in the interface ("what the downstream flow sees")

--> stage interface shows an 'averaged' flow (velocity) profile
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Old   April 1, 2016, 08:24
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Thanks for the reply.

I understand the difference between stage and frozen rotor.

But within Stage there are two ways of calculating the downstream velocity.

1) Stage average velocity
2) Constant total Pressure

I would like to know the difference. What does Constant Total Pressure mean?

Does it mean that my total pressure is constant during my simulation and only my velocity changes or adjust and therefore my dynamic pressure changes?

Regards,
Juzer











Quote:
Originally Posted by -Maxim- View Post
you already have posted in topics that explain more about stage interface and frozen rotor: http://www.cfd-online.com/Forums/cfx...machinery.html

maybe to following pictures help:

1. Frozen Rotor, velocity profile in the interface ("what the downstream flow sees")

--> FR shows _one_ specific flow pattern since the rotor is fixed.

2. Stage Interface, velocity profile in the interface ("what the downstream flow sees")

--> stage interface shows an 'averaged' flow (velocity) profile
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Old   April 1, 2016, 10:32
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By default, the stage interface (i.e., the mixing plane) averages static pressure across the two sides. The option is about velocity. If you choose Constant Total Pressure, total pressure will be averaged to calculate velocity with the transferred flow direction. It is only for rotating frame. (The stage interface is also available with stator-to-stator cases.) The default averages velocity, too.
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Old   April 1, 2016, 10:53
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Quote:
Originally Posted by turbo View Post
By default, the stage interface (i.e., the mixing plane) averages static pressure across the two sides. The option is about velocity. If you choose Constant Total Pressure, total pressure will be averaged to calculate velocity with the transferred flow direction. It is only for rotating frame. (The stage interface is also available with stator-to-stator cases.) The default averages velocity, too.
Thanks for the clarification.

When you mean "with the transfered flow direction", does that mean in the rotating frame of reference (for rotor)?

Also, Total Pressure in Stn Frame for rotor remains constant or averaged?

The option "Constant Total Pressure" is confusing.

Thanks,
Juzer
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Old   April 1, 2016, 11:07
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Quote:
Originally Posted by juzer_700 View Post
Thanks for the clarification.

When you mean "with the transfered flow direction", does that mean in the rotating frame of reference (for rotor)?

Also, Total Pressure in Stn Frame for rotor remains constant or averaged?

The option "Constant Total Pressure" is confusing.

Thanks,
Juzer
I am not representing Ansys, but my understanding is, (a) of course flow direction from the rotor because the option is how to set the downstream velocity (b) local total pressure profiles at both sides are averaged in tangential direction. They would be absolute ones (because of downstream stator). I have never used that option. Manual says it could be helpful for a tight coupling like a very small gap between two.
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Old   April 1, 2016, 11:42
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Not sure what version you are using, but in the past the Frame Type parameter allowed the Stationary option as well.

The Stationary option was removed in later releases since the predictions were not better that Average Velocity, nor Constant Total Pressure with Flow Direction in the Rotating Frame.

At a frame change interface, you are on different frames depending on what side you are looking from.

"Constant Total Pressure" at the frame change interface means "no loss of total pressure in the selected frame type during the crossing of the interface". That is, if you compute the Total Pressure in "Frame Type" on both sides of the interface, they should be theoretically the same. If you do a similar calculation for the solution obtained using Average Velocity, you should see a drop of Total Pressure in "Frame Type".

Hope the above helps,
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Old   April 4, 2016, 03:25
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Quote:
Originally Posted by Opaque View Post
Not sure what version you are using, but in the past the Frame Type parameter allowed the Stationary option as well.

The Stationary option was removed in later releases since the predictions were not better that Average Velocity, nor Constant Total Pressure with Flow Direction in the Rotating Frame.

At a frame change interface, you are on different frames depending on what side you are looking from.

"Constant Total Pressure" at the frame change interface means "no loss of total pressure in the selected frame type during the crossing of the interface". That is, if you compute the Total Pressure in "Frame Type" on both sides of the interface, they should be theoretically the same. If you do a similar calculation for the solution obtained using Average Velocity, you should see a drop of Total Pressure in "Frame Type".

Hope the above helps,
Thanks for the clarification. Now I get it. Just to strengthen my understanding, when you meant "Frame Type", I understood as:

Total Pressure in Stn Frame@(Interface Side 1) = Total Pressure in Stn Frame@(Interface Side 2)

I hope that is correct.
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Old   April 4, 2016, 10:35
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Correct !!!
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Old   August 29, 2019, 06:01
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Quote:
Originally Posted by juzer_700 View Post
Thanks for the clarification. Now I get it. Just to strengthen my understanding, when you meant "Frame Type", I understood as:

Total Pressure in Stn Frame@(Interface Side 1) = Total Pressure in Stn Frame@(Interface Side 2)

I hope that is correct.
I have the same problem with the rotor-stator interface in an axial compressor. and:
Total Pressure in Stn Frame@(Interface Side 1) is NOT equal to Total Pressure in Stn Frame@(Interface Side 2).
The set up for the mixing plane is interface with “stage” frame change and constant total pressure, and I used “ggi stage groups max=200” with fine mesh.
Is this problem related to CFX itself? Is there any solution to correct this jump in total pressure in the sides of the interfaces?
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