Connecting a rotating and a stationary domain

cfd940 · April 20, 2017, 13:44

Dear CFD-experts,

I´m a student at a german university and trying to simulate a uav (airplane) in Ansys CFX for a seminar paper. To include the effect of a turning prop I thought I could use an empty rotating domain connected to a stationary domain. I connected them by domain interfaces with following settings.

Interface Models:
- Option: General Connection
- Frame Change/Mixing Model: Frozen Rotor
- Pitch Change: Automatic

Now my problem is that the flux in the rotating domain stops rotating immediately at the end of the domain. Can you please give me instructions how to solve my problem?
Please compare my explanation to the attached picture of a simple test-simulation.

Test.JPG

ghorrocks · April 20, 2017, 19:48

You have missed the entire point of frames of reference....

A frame of reference can be seen as simply a grid system against which velocities are measured. Whether the frame of reference is rotating or not makes no difference to the actual flow, only on what you measure its velocity as.

And empty domain, whether rotating or not, has no effect on the flow. And that is what you are seeing. The corkscrew streamlines in the rotating domain is explained in this FAQ: https://www.cfd-online.com/Wiki/Ansy...f_reference.3F

If you want to introduce some swirl into the flow then use a momentum source term with a function which swirling momentum. Do not use a rotating frame of reference, this can be done in the stationary frame of reference the rest of your model is in.

cfd940 · April 21, 2017, 06:20

Dear ghorrocks,

thanks a lot for the fast response.

I have now added a momentum source to my simulation but i got a problem with the function which swirls the momentum. I searched the forum but i didn´t find good examples or a way to create such a function. I´ve got no experience with such things.

ghorrocks · April 21, 2017, 20:50

If the axis of the swirl is the origin, then

Theta = arctan2(y,x)
Radius = sqrt(x*x+y*y)
X_Mom_Source = C*Radius*sin(Theta)
Y_Mom_Source = C*Radius*cos(Theta)

Where C is the strength of the swirl in appropriate units. I forget if arctan2 is the correct function but it is something like that. You might also be able to do some simplification of it if you feel adventurous.

cfd940 · April 22, 2017, 08:31

I Got it. Thanks a lot!

April 20, 2017, 13:44	Connecting a rotating and a stationary domain	#1
cfd940 New Member Join Date: Apr 2017 Posts: 3 Rep Power: 9	Dear CFD-experts, I´m a student at a german university and trying to simulate a uav (airplane) in Ansys CFX for a seminar paper. To include the effect of a turning prop I thought I could use an empty rotating domain connected to a stationary domain. I connected them by domain interfaces with following settings. Interface Models: - Option: General Connection - Frame Change/Mixing Model: Frozen Rotor - Pitch Change: Automatic Now my problem is that the flux in the rotating domain stops rotating immediately at the end of the domain. Can you please give me instructions how to solve my problem? Please compare my explanation to the attached picture of a simple test-simulation. Test.JPG

April 20, 2017, 19:48		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	You have missed the entire point of frames of reference.... A frame of reference can be seen as simply a grid system against which velocities are measured. Whether the frame of reference is rotating or not makes no difference to the actual flow, only on what you measure its velocity as. And empty domain, whether rotating or not, has no effect on the flow. And that is what you are seeing. The corkscrew streamlines in the rotating domain is explained in this FAQ: https://www.cfd-online.com/Wiki/Ansy...f_reference.3F If you want to introduce some swirl into the flow then use a momentum source term with a function which swirling momentum. Do not use a rotating frame of reference, this can be done in the stationary frame of reference the rest of your model is in. alirezame and asanchez84 like this.

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April 21, 2017, 06:20		#3
cfd940 New Member Join Date: Apr 2017 Posts: 3 Rep Power: 9	Dear ghorrocks, thanks a lot for the fast response. I have now added a momentum source to my simulation but i got a problem with the function which swirls the momentum. I searched the forum but i didn´t find good examples or a way to create such a function. I´ve got no experience with such things.

April 21, 2017, 20:50		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	If the axis of the swirl is the origin, then Theta = arctan2(y,x) Radius = sqrt(xx+yy) X_Mom_Source = CRadiussin(Theta) Y_Mom_Source = CRadiuscos(Theta) Where C is the strength of the swirl in appropriate units. I forget if arctan2 is the correct function but it is something like that. You might also be able to do some simplification of it if you feel adventurous.

April 22, 2017, 08:31		#5
cfd940 New Member Join Date: Apr 2017 Posts: 3 Rep Power: 9	I Got it. Thanks a lot!