CFX and gravity direction

dibloff · March 19, 2018, 10:59

Hello Analysts

I’m running a complex model in CFX with multiphase, rotating domain and gravity. the domain is initialized with air, and then coolant is being introduced through the coolant inlets. The CFX solver will not run in steady state mode if the gravity vector is perpendicular to the axis of rotation. the ANSYS guys confirmed that this is a solver limitation. I was told it’s the same in fluent too. I want to run a steady state analysis compared to transient, because my fluid time scale is ~4 ms, while my thermal time scale would be in 10’s of seconds.
to simplify the analysis I ventured into using a negative pressure boundary @ the outlet, to “lure” the fluid there, but I’m not sure what should be the pressure I would have to use, since it’s 2 phase flow. the volume does not have a breather hole, or other opening on it. so if I apply a negative pressure boundary at outlet it’ll create a relative vacuum and it’ll facilitate the oil flow from the oil inlet(s).
I’ve done a transient analysis to see what the (stabilized) pressure would be @ the outlet (over a 1 sec run time) but it did not converge. I’ve done numerous small scale studies to investigate this but at the end of the day I’m not sure I can believe the numbers.
I’m wondering if anyone has a better idea on how to tackle this problem.
Any suggestion is appreciated,
thanks.

A.

Gert-Jan · March 19, 2018, 11:18

My fall back scenario is to extent the domain with a knock-out drum where oil and gas separate. And then create a top outlet for gas and bottom outlet for oil.

Opaque · March 20, 2018, 10:55

You may have misunderstood the support answer, but it is not possible to run a steady state simulation with a rotating domain and the gravity direction normal to the axis of rotation.

It is not a limitation of ANSYS Fluent, ANSYS CFX or any other code. It is not mathematically possible to solver such a problem because it is incorrectly posed.

In the rotating frame the gravity vector is moving opposite to the rotation direction; therefore, the problem is always transient for the observer in the rotating frame.

Hope the above helps,

March 19, 2018, 10:59	CFX and gravity direction	#1
dibloff New Member Attila NAgy Join Date: Apr 2012 Posts: 1 Rep Power: 0	Hello Analysts I’m running a complex model in CFX with multiphase, rotating domain and gravity. the domain is initialized with air, and then coolant is being introduced through the coolant inlets. The CFX solver will not run in steady state mode if the gravity vector is perpendicular to the axis of rotation. the ANSYS guys confirmed that this is a solver limitation. I was told it’s the same in fluent too. I want to run a steady state analysis compared to transient, because my fluid time scale is ~4 ms, while my thermal time scale would be in 10’s of seconds. to simplify the analysis I ventured into using a negative pressure boundary @ the outlet, to “lure” the fluid there, but I’m not sure what should be the pressure I would have to use, since it’s 2 phase flow. the volume does not have a breather hole, or other opening on it. so if I apply a negative pressure boundary at outlet it’ll create a relative vacuum and it’ll facilitate the oil flow from the oil inlet(s). I’ve done a transient analysis to see what the (stabilized) pressure would be @ the outlet (over a 1 sec run time) but it did not converge. I’ve done numerous small scale studies to investigate this but at the end of the day I’m not sure I can believe the numbers. I’m wondering if anyone has a better idea on how to tackle this problem. Any suggestion is appreciated, thanks. A.

March 20, 2018, 10:55		#3
Opaque Senior Member Join Date: Jun 2009 Posts: 1,811 Rep Power: 32	You may have misunderstood the support answer, but it is not possible to run a steady state simulation with a rotating domain and the gravity direction normal to the axis of rotation. It is not a limitation of ANSYS Fluent, ANSYS CFX or any other code. It is not mathematically possible to solver such a problem because it is incorrectly posed. In the rotating frame the gravity vector is moving opposite to the rotation direction; therefore, the problem is always transient for the observer in the rotating frame. Hope the above helps, dibloff likes this.

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March 19, 2018, 11:18		#2
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,835 Rep Power: 27	My fall back scenario is to extent the domain with a knock-out drum where oil and gas separate. And then create a top outlet for gas and bottom outlet for oil.