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-   -   Helicopter Rotor in Hover - Pressure Farfield Boundary Conditions (https://www.cfd-online.com/Forums/fluent/115876-helicopter-rotor-hover-pressure-farfield-boundary-conditions.html)

 mjforsteruk April 8, 2013 16:13

Helicopter Rotor in Hover - Pressure Farfield Boundary Conditions

I am trying to replicate one of the cases from the Caradonna & Tung paper. I have a symmetric mesh similar to this paper with a semi-cylndrical far-field and am using a density based solver in Fluent 14.

I think the velocity at the far-field boundaries should be zero, however I am confused about the gauge and operating pressures. I imagine the top, bottom and cylindrical far-field should all be set as pressure-far-field and the pressure there should be equal to the atmospheric pressure. Does this mean I should have a zero operating pressure and gauge pressure as 101325, or the other way around?

I have tried a stationary reference frame with the blade rotating and also with a rotating reference frame but neither seems to give any reasonable results.

Any help would be much appreciated.

 CDollarsign April 8, 2013 23:53

Are you considering compressible flow? Assuming that the blade tips at the rotor can reach speeds nearing Mach 1 you should include compressibility. Gauge pressure and operating pressure are used to calculate density through the Mach number. Either the Fluent theory guide or user manual states for Mach > .1 (I think) to use '0' as an operating pressure, thus making the gauge pressure what ever you are assuming for atmosphere, as well as initial gauge pressure on your BC. You may want to use 'Pressure Inlet' instead of 'Pressure Far-Field' for your BC...

 mjforsteruk April 9, 2013 03:54

Thanks for your help. I am using an ideal gas with sutherland law due to compressibility and pressures as you suggest. I will try using a pressure inlet and see how it goes.

hello mjforsteruk
I know it's been a long time since last post in this thread but i hope you would see it & could help me.

I am working on exactly the same case. as you mentioned pressure far field boundary is used (since blade tip Mach number is around 0.8 & the flow is compressible). now my questions are:

1- in the pressure far field boundary condition you should enter Mach number, is this related to blade tip Mach number and i should enter M=0.8 or this is far field Mach number and i should enter M=0 (since my case is in hover)?

2- I don't need flow field far from the blade so i just want to model near blade region but as it's said in fluent user guide when you use pressure far field BC your region radius must be 20 cord length! what can i do about this? if i want to reduce my solution domain(to save computation time) is there any other BC i can use instead of Pressure far field ?(note that flow is compressible in my case)

any help would be greatly appreciated

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