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-   -   Reversed Flow for a tenth slice of a rotor (https://www.cfd-online.com/Forums/fluent/94149-reversed-flow-tenth-slice-rotor.html)

 neptunemmx November 7, 2011 12:29

Reversed Flow for a tenth slice of a rotor

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 13:24

It is normal with Fluent, dont worry

 neptunemmx November 7, 2011 13:46

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 14:09

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 14:16

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 12:49

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 18:18

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 11, 2011 00:44

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

 neptunemmx November 15, 2011 23:11

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 13:03

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 14:51

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.

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