# Issue on zeroGradient BC for system rotation

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February 21, 2017, 19:26
Issue on zeroGradient BC for system rotation
#1
New Member

eco
Join Date: Jan 2017
Posts: 5
Rep Power: 8
Dear formers,

i have an issue with the zeroGradient BC at flow outlet combined with system rotation for a 2-D (xy) incompressible steady state problem. The whole domain is about to be rotated among the z-axis.

1. Whats working so far:
Plane-turbulent-channel flow setup, good results without system rotation using either SRFSimpleFoam or a modified simpleFoam with Coriolis force in the momentum equation. Both solvers work fine with arbitrary two-equation-turbulence-models (e.g. kEpsilon, kOmegaSST) and give the same results.

BCs velocity/relative velocity:
inlet = fixed value
BCs pressure:
outlet = fixed value 0

2. The issue:
Starting from a fully converged non rotation state, as soon as the system is rotated among the the z-axis in positive or negative direction the outflow shows unphysical behavior, not holding the condition of zeroGradient for the relative velocity (and absolute velocity plus turbulence quantities, whereas the pressure is fine). The result is the same for the modified simpleFoam and SRFSimpleFoam.

(Picture see attachment)
Top case, positive rotation among z-axis.
Bot case, negative though.

3. My comment:
The issue seems to be independent of the solver and turbulence model (since the used turb-models dont even represent rotational dependency anyways). Furthermore, the picture illustrates the impact of the rotational direction which shows that there might be a problem with the understanding of absolute and relative velocity at the outlet BC.
As far as i understand the "type" of a BC is independent of the basic flow quantities such as U,p,k,omega what ever, it just "zeros" the normal gradient of the quantity it is applied to, right? In my understanding this should not be the problem in the case of "Urel" though.

I am tankful for every advice!

Best regards,

Magnus
Attached Images
 zoomed-view-outlet.png (42.8 KB, 38 views)

Last edited by magnushaese; February 21, 2017 at 19:34. Reason: highlighting the problem with color...

 February 28, 2017, 07:54 #2 New Member   eco Join Date: Jan 2017 Posts: 5 Rep Power: 8 solved: new boundary condition with the pressure gradient balances the Coriolis force. grad(P) = -2 Omega ^ U Last edited by magnushaese; March 8, 2017 at 11:47.

 January 28, 2022, 14:43 #3 Member   George Join Date: Dec 2020 Posts: 31 Rep Power: 4 Hello magnushaese, Did you use SRFSimpleFoam for this simulation? Best, George