# Open atmosphere boundary

I am simulating air flow over an obstacle, in may case an ocean wave. The project is called
Flow over an ocean wave
I am struggling with the top boundary condition which is representative for the boundary to the free atmosphere.
To wrap it up I have following boundaries:
bottom-wall setup as this represents the ocean surface.
inlet (left hand side)- constant velocity inlet
outlet (right hand side)- pressure outlet.
and now the top:
I tried the “symmetry” condition as this reflects an zero net-exchange with the atmosphere, but there is also no outflow . As I have an obstacle, which decreases the diameter of the air path, this increases the air flow. In the “real world” this would not happen as the air can, to a certain extend, avoid the obstacle by rising (in this case out of the simulation box).
Then I tried also the pressure outlet boundary condition. This has the advantage that the air which is trying to avoid the obstacle also has the chance to rise out of the simulation box.
My question is now:
As I have a pressure outlet condition on top, does the lost air somehow influence the results?
I am losing air to the top which is not replaced, or only replaced by the inlet to the left. In the “real world” the lost air would be also replaced by some flux from the top, right?
Is there a pressure inlet and outlet condition which depending on the pressure difference acts as an inlet or as an outlet?

Hi @azavarsky. Absolutely there is, if you change velocity inlet to free stream or even change boundary to custom and define it as an inlet - outlet velocity it will use the values for inlet places and set zero gradient for outlets.

However, it would always be best to just increase the height of the bounding box so that it doesn’t influence results, this IMO leads to more stable simulations.

Hope this helps,
Darren

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Hi @1318980,
thanks a lot for the info.
I set now the Inlet to freestream. The outlet is a pressure outlet with a fixed value zero.
I changed the top condition to custom:
velocity freestream
pressure fixed value zero
dissipation and turbulent kinetic energy to symmetry.
I always get the error:
Inconsistent patch and patchField types found

Do you have any ideas why? It is a patch type boundary.
What should I out in into the custom setting?

Hi @azavarsky, have you got a project link you could share?

Many thanks,
Darren

Sure,
Flow over an ocean wave
I am currently working in the simulation designer on the “trans k omega sst” simulation

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Hi @azavarsky, I have altered the boundary so it looks like this:

This allows air in and out the walls.

Can I also ask what you expect from a transient simulation over a steady state? It would certainly be better to set up on a steady state simulation then move onto a transient. But I can’t help but think that it should remain a steady state?

Kind regards,
Darren

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Hi @1318980,
you are right for this simulation a staeady state is better. But I also have simulations where I start with low wind speed 1 m/s and ramp it up to 10 m/s. I would like to see the transition. But you are right for testing this here steady state should be fine

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ah ok, makes sense. Good luck!

Darren