Reversed Flow for a tenth slice of a rotor

neptunemmx · November 7, 2011, 11:29

I am modelling a 10 bladed rotating rotor, at 25m/s and rotating at 2800rpm. My grid is one tenth of a full cylinder and extends about 10 diameters in front of the rotor, and about 25 diameters behind the rotor. I am using periodic boundary conditions on the sides and pressure inlet, and pressure outlet on the inlet and outlet respectively. I am getting reversed flow on the outlet though. I have tried slowly increasing the rotation from 0 to 2800rpm yet I still get reversed flow at 1000rpm. Also I have tried solving this with the first order solver but I still get reversed flow before I switch to second order.

Any help would be much appreciated.

Far · November 7, 2011, 12:24

It is normal with Fluent, dont worry

neptunemmx · November 7, 2011, 12:46

For this particular case I am running the inviscid solver, which is why I am confused that I have reversed flow.

Far · November 7, 2011, 13:09

well for my case, I kept the boundary conditions, mesh, flow scheme, inlet & outlet boundaries and other things same/similar for CFX and Fluent but still I get reversed flow message in Fluent simulation

Far · November 7, 2011, 13:16

Quote:

extends about 10 diameters in front of the rotor, and about 25 diameters behind the rotor

are you modelling rotor in free stream? if not then place inlet at one chord upstream and outlet at 1.5 chord downstream

neptunemmx · November 10, 2011, 11:49

I am modeling it in the free stream at 25m/s. As I have it setup and using 1st order discretization I can converge 5 orders of magnitude(1e-5) without reversed flow. However using 2nd order I get reversed flow and I cannot converge more than 1 order of magnitude. Could it be an issue with the boundary conditions? I have setup my pressure inlet and outlet so that the flow is 25m/s (the desired frestream velocity), do I need to some how account for the affect the rotor will have on the pressure, i.e. increase or decrease the velocity?

bobmalaria · November 10, 2011, 17:18

Maybe you can lower the pressure relaxation factor in the solution control settings.
usually reversed flow can be tollerated if it is far enough away from the region you are interested in. maybe putting the outflow further away helps

Far · November 10, 2011, 23:44

did you specify the flow direction = normal or x = 1, y = and z = 0

neptunemmx · November 15, 2011, 22:11

I have specified the correct direction on my boundaries. Also I have extended my domain to 80 diameters (60 chord lengths) behind my geometry. I understand that reversed flow is ok, however I think that in my case it is stopping my solution from converging since the iterative residuals only converge to 1e-2.

Thank you for the help

emmkell · January 6, 2012, 12:03

Is reversed flow at the outlet normally a cause for non convergence? I want to reach a convergence of 1e-03 but the solution wont converge at the moment.

I know moving the outlet to a place where the reversed flow disappears is an option for removing it, but i have a high node count already and so woudl prefer to not add additional cells.

neptunemmx · January 30, 2012, 13:51

I think that as long as you are not assuming that the flow is inviscid(i.e. using a turbulence model) it is ok to have reversed flow.

November 7, 2011, 11:29	Reversed Flow for a tenth slice of a rotor	#1
neptunemmx New Member Michael Creaven Join Date: Jul 2009 Posts: 7 Rep Power: 16	I am modelling a 10 bladed rotating rotor, at 25m/s and rotating at 2800rpm. My grid is one tenth of a full cylinder and extends about 10 diameters in front of the rotor, and about 25 diameters behind the rotor. I am using periodic boundary conditions on the sides and pressure inlet, and pressure outlet on the inlet and outlet respectively. I am getting reversed flow on the outlet though. I have tried slowly increasing the rotation from 0 to 2800rpm yet I still get reversed flow at 1000rpm. Also I have tried solving this with the first order solver but I still get reversed flow before I switch to second order. Any help would be much appreciated.

January 6, 2012, 12:03		#10
emmkell New Member Emma Join Date: Sep 2009 Posts: 17 Blog Entries: 1 Rep Power: 16	Is reversed flow at the outlet normally a cause for non convergence? I want to reach a convergence of 1e-03 but the solution wont converge at the moment. I know moving the outlet to a place where the reversed flow disappears is an option for removing it, but i have a high node count already and so woudl prefer to not add additional cells. Last edited by emmkell; January 6, 2012 at 12:47.

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November 7, 2011, 12:24		#2
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	It is normal with Fluent, dont worry

November 7, 2011, 12:46		#3
neptunemmx New Member Michael Creaven Join Date: Jul 2009 Posts: 7 Rep Power: 16	For this particular case I am running the inviscid solver, which is why I am confused that I have reversed flow.

November 7, 2011, 13:09		#4
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	well for my case, I kept the boundary conditions, mesh, flow scheme, inlet & outlet boundaries and other things same/similar for CFX and Fluent but still I get reversed flow message in Fluent simulation

November 10, 2011, 11:49		#6
neptunemmx New Member Michael Creaven Join Date: Jul 2009 Posts: 7 Rep Power: 16	I am modeling it in the free stream at 25m/s. As I have it setup and using 1st order discretization I can converge 5 orders of magnitude(1e-5) without reversed flow. However using 2nd order I get reversed flow and I cannot converge more than 1 order of magnitude. Could it be an issue with the boundary conditions? I have setup my pressure inlet and outlet so that the flow is 25m/s (the desired frestream velocity), do I need to some how account for the affect the rotor will have on the pressure, i.e. increase or decrease the velocity?

November 10, 2011, 17:18		#7
bobmalaria Member Martin H. Join Date: May 2009 Posts: 31 Rep Power: 16	Maybe you can lower the pressure relaxation factor in the solution control settings. usually reversed flow can be tollerated if it is far enough away from the region you are interested in. maybe putting the outflow further away helps

November 10, 2011, 23:44		#8
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	did you specify the flow direction = normal or x = 1, y = and z = 0

November 15, 2011, 22:11		#9
neptunemmx New Member Michael Creaven Join Date: Jul 2009 Posts: 7 Rep Power: 16	I have specified the correct direction on my boundaries. Also I have extended my domain to 80 diameters (60 chord lengths) behind my geometry. I understand that reversed flow is ok, however I think that in my case it is stopping my solution from converging since the iterative residuals only converge to 1e-2. Thank you for the help

January 30, 2012, 13:51		#11
neptunemmx New Member Michael Creaven Join Date: Jul 2009 Posts: 7 Rep Power: 16	I think that as long as you are not assuming that the flow is inviscid(i.e. using a turbulence model) it is ok to have reversed flow.