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smcg1849 August 29, 2013 10:23

Open Rotor Propeller / Fluid-Fluid Interface problem
Hi guys,

at the moment i am working on a student project which is about a open rotor turboprop. In this project i have to study the air flow in front of the inlet to the engine.

Anyway the propeller has 9 blades. To get faster results of the first simulations, iam working with periodic conditions of course. In this case I defined a rotating domain which contains the blade + hub and a stationary domain which simulates the far field around the blades. This is shown in the image below.
Please have a look to the fluid-fluid interfaces.
My question is related to the red surrounded fluid-fluid interface:

Is this interface numerical valid? Iam asking this question because in my lecture interfaces are supposed to be orthogonal to the flow, which is not given in this case (interface surface is tangential to the inlet flow).

Am I right about this? If yes, how can I solve this problem? Which Interface settings have to be made in this case? I am really confused about this. Can you help me?

Thanks a lot in advance! :)


ghorrocks August 29, 2013 18:38

Your proposal looks OK. Interfaces can be aligned with the flow direction.

Is the bottom a wall? Is the ground or something? If this thing operates a long way away from stationary boundaries (eg an aircraft in the air) then you can ignore the stationary frame of reference and use a single rotating frame of reference for the entire simulation. Then you do not need any interfaces.

smcg1849 August 29, 2013 21:33

hi ghorrocks,

first of all: thank you for answering :)

The bottom is no wall or something. It is nothing physical there. Maybe the picture in my first post is a bit confusing.
For better understanding please have a look at the updated picture.
Also have a look to this picture:

The area between the 2 curves on the red surface is copied 9 times in radial direction (periodic condition). In this case only 1 blade is shown.
All this geometry is located in the rotating domain. It rotates about the z-axis. And it is operating far away from stationary boundaries (its an aircraft propeller).

But in this case I have to consider the flow far away from the propeller (about 5x blade radius) aswell. The problem when using a single rotating domain is the very high peripheral speed, which is (r x omega). Due to this high peripheral speed the air velocity far away from the rotational axis is supersonic, which isnt allowed.

If I add a stationary domain, which is simulating the farfield and which is connected with the rotating domain by an interface i can solve this problem.

My problem is now:
Is this interface automatically valid or do i need to make specific settings? Which interface settings have to be made to make this physically realistic?

ghorrocks August 30, 2013 06:54

The solver can handle the rotating frame of reference moving at supersonic speed but the fluid is subsonic (at least I think it can). If you suspect this is a problem then use the "alternate rotation model" which puts a few terms in the stationary frame of reference and might help in this case. Search the forum and CFX documentation for more details on this option.

If there is no stationary geometry then I definitely would just use a single rotating domain. Then you have no need for interfaces at all.

If you insist on using the domain interface you have shown - there is no special settings you need to activate. It should take care of itself.

smcg1849 August 30, 2013 14:09

Hi glenn,

I would prefer using a single rotating domain aswell but in reality this high supersonic peripehral speed does not exist. And if it does not exist in reality but iam using this in my modeling, i will get bad similation results, right? How can i handle this problem by using a single rotating domain then?

Btw: If I use an interface should I use a stage or frozen rotor interface? I was thinking about using a stage interface if the interface is not to close to the bladetip. Which interface settings would you recommend me to use in this case?

ghorrocks August 30, 2013 22:21

I already answered your question on the supersonic speed in my last post, and also suggested how to mitigate it.

For the interface types, read the documentation for the implications of the different types of interfaces.

smcg1849 August 30, 2013 22:42

Okay i will have a try at using a single rotating frame with the "alternate rotation model" setting activated.

Thank you for helping!

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