# CFX Pre. Definition of rotating/non-rotating boundaries for a radial compressor

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 October 12, 2015, 04:31 CFX Pre. Definition of rotating/non-rotating boundaries for a radial compressor #1 New Member   Anton Join Date: Oct 2015 Posts: 7 Rep Power: 2 Hi, I am a newbie in ANSYS/CFX/CFD, so sorry for very basic questions I have a compressor wheel (Domain), with Subdomains: Inlet, Outlet, Hub, Schroud etc. I have defined everything through QuickSetup Mode. The wheel (Domain) is rotating at 37000 rpm. The thing I can't clearly understand is why some parts boundaries are rotating, some are static and some are counterrotating. The logic says if the wheel (domain) is rotating, then I define "domain motion" at 37000 rpm, and everything is static in that coord. system, except of the shroud, which is "counter rotating", which makes it static in absolute coord. system. Or, alternatively, if domain coord. system is static, then all parts should be rotating, and the shroud is static in both domain and absolute coord. system. But the shouldn't be all 3 options at the same time - rotating, static, counterrotating. Where am I wrong?

 October 12, 2015, 06:19 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,278 Rep Power: 88 I do not quite understand your question - but the concept is reasonably straight forward. Obviously the rotor spins and the shroud is still. But a better way of thinking about it is to imagine a rigid mesh (ie not deforming and only able to spin) describing the fluid volume around the rotor and shroud. The mesh around the rotor obviously spins with the rotor and the mesh in the shroud is stationary, but you can see that there is some flexibility in where you can define the transition from rotating to stationary. Then on the rotating frame of reference you can either have a stationary wall (which rotates with the reference frame) or a counter rotating wall (which is stationary in the absolute frame). And on the stationary frame you can either have a stationary wall (which is stationary in the absolute frame) or a rotating frame (which rotates.. obviously).

 October 12, 2015, 10:26 #3 New Member   Anton Join Date: Oct 2015 Posts: 7 Rep Power: 2 I mean, when I am doing CFX-Pre it automatically (New Case -> Simulation Type -> Turbomashinery) I get all 3 types - static, rotating and counter rotating. So what is the sense of making counter rotating part on a rotating frame, instead of a simply stationary part?

 October 12, 2015, 19:35 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 11,278 Rep Power: 88 As I said, think about what the mesh has to do to describe the motion. That tells you what needs to rotate and what needs to be stationary. Then you assign static, rotating and counter rotating based on that.

 Tags coordinate system, domain, rotation

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