# Surge simulation

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 March 25, 2016, 16:55 Surge simulation #1 New Member   Piotr Join Date: Jan 2016 Posts: 6 Rep Power: 3 Hi ! I am going to perform surge simulation of centrifugal impeller with vaneless diffuser. I managed to calculate steady case and noticed big turbulance inside impeller. I also tried harmonics unsteady model, however it seems to be just for rotor/stator interaction (correct me if i am wrong) - results don't change with time. Is it possible to model surge in NUMECA turbo ? Which unsteady model should I use ? Does anybody have tutorial of such case ? Best regards, Piotr

 March 28, 2016, 05:19 #2 Member     Holger Dietrich Join Date: Apr 2011 Location: Germany Posts: 92 Rep Power: 7 Hi Piotr, you are right, non-linear harmonics unsteady method does not work in your case, because you have no rotor-stator or rotor-rotor combination(s) in your machine. Besides the NLH-method you can use the standard unsteady method.

March 29, 2016, 02:15
#3
New Member

Jessica Wei
Join Date: Mar 2016
Posts: 6
Rep Power: 2
Hi,

Thank you for your sharing.
Do you by any chance know if the NLH-method could work in a compressor with a volute, or it may only work in blade rows?

Quote:
 Originally Posted by DarylMusashi Hi Piotr, you are right, non-linear harmonics unsteady method does not work in your case, because you have no rotor-stator or rotor-rotor combination(s) in your machine. Besides the NLH-method you can use the standard unsteady method.

 March 29, 2016, 16:52 #4 Member     Holger Dietrich Join Date: Apr 2011 Location: Germany Posts: 92 Rep Power: 7 It works in a volute, too (compressor and turbine). But make sure to mesh just one blade passage of each row. The blade passage of each row needs periodic boundaries in circumferential direction. If you are interested in the background of this please ask, I could try to draw a sketch to explain the reason.

April 6, 2016, 15:45
#5
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Piotr
Join Date: Jan 2016
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Quote:
 Originally Posted by DarylMusashi Hi Piotr, you are right, non-linear harmonics unsteady method does not work in your case, because you have no rotor-stator or rotor-rotor combination(s) in your machine. Besides the NLH-method you can use the standard unsteady method.
Daryl,
Should I use domain scalling method or phased-lagged one ?

 April 6, 2016, 16:59 #6 Member     Holger Dietrich Join Date: Apr 2011 Location: Germany Posts: 92 Rep Power: 7 Hi Piotr, the advantage of the phase lagged method is a reduced CPU time needed for unsteady computations. But it is only allowed for one-stage machines (stator-rotor) or 1.5-stage machines with the same number of stators. Domain scaling is the only available unsteady method for you then.

April 7, 2016, 03:10
#7
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Piotr
Join Date: Jan 2016
Posts: 6
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Quote:
 Originally Posted by DarylMusashi Hi Piotr, the advantage of the phase lagged method is a reduced CPU time needed for unsteady computations. But it is only allowed for one-stage machines (stator-rotor) or 1.5-stage machines with the same number of stators. Domain scaling is the only available unsteady method for you then.

Daryl,
I am really grateful for your advices. I haven't checked Rotating Boundary Condition in Boundary Condition option. I used steady result for initial solution. In control variables i have to input physical time step. In NUMECA manual i found following formula:

As i understand omega is rotational speed (in my case 9000RPM), Nperiods (number of blades=21).
Could you please tell me how to calculate NOFROT ?

April 7, 2016, 16:26
#8
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Holger Dietrich
Join Date: Apr 2011
Location: Germany
Posts: 92
Rep Power: 7
Hi Piotr,
you are right, because you have no rotor-stator interface you have to calculate the physical timestep size manually. The following questions lead to the desired duration of the physical time step.

1. What is your rotational speed in [rad/s]?
2. How far in [rad] must one blade rotate for one blade passage?
3. How long does this take?
4. With how many time steps do you want to resolve one blade passage?

If you are lazy have a look at the attached file.
Attached Images
 physical_timestep.JPG (50.2 KB, 16 views)

Last edited by DarylMusashi; April 9, 2016 at 16:51.

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