Best outlet BC for instationary results

harly · August 16, 2009, 04:39

Hi,

I am using OpenFoam for a while now and I am getting some fair results with it, but I have to admit, that my boundary conditions are mostly taken from suggestions from this message board.

I am trying to resolve unsteady vortex shedding behind a bluff body on a shorter grid and therefore I am looking for a "better" outlet condition than a pressure out - which I used until now.

Perfect would be a BC which would allow in- and outlet but I am not quite sure how to apply such an BC.

inletOutlet or outletInlet seem to be a start but how do they differ ? and what else is there to specify in the BC patch ?

Thanks for your advise.

henrik · August 16, 2009, 05:26

Dear Daniel,

having an outlet with flow coming back is a very bad idea and should be avoided by extending the domain and keeping the cell count in bay using aggressive cell size increase towards the outlet.

However, there are cases where this is difficult or where you simply want to stabilise a steady-state run to get over the first few iterations.

@1: inletOutlet is zeroGradient where the flow goes out and fixedValue where the flow comes in --> This should be used on an outlet boundary. OutletInlet is just the opposite.

@2: My suggestion for an outlet patch without corners:

totalPressure on p
inletOutlet on U, k, epsilon

This can be improved, but see how is works for you first.

Henrik

August 16, 2009, 04:39	Best outlet BC for instationary results	#1
harly Member Daniel Harlacher Join Date: Mar 2009 Location: Davis, CA, United States Posts: 60 Rep Power: 17	Hi, I am using OpenFoam for a while now and I am getting some fair results with it, but I have to admit, that my boundary conditions are mostly taken from suggestions from this message board. I am trying to resolve unsteady vortex shedding behind a bluff body on a shorter grid and therefore I am looking for a "better" outlet condition than a pressure out - which I used until now. Perfect would be a BC which would allow in- and outlet but I am not quite sure how to apply such an BC. inletOutlet or outletInlet seem to be a start but how do they differ ? and what else is there to specify in the BC patch ? Thanks for your advise.

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August 16, 2009, 05:26		#2
henrik Senior Member Henrik Rusche Join Date: Mar 2009 Location: Wernigerode, Sachsen-Anhalt, Germany Posts: 281 Rep Power: 18	Dear Daniel, having an outlet with flow coming back is a very bad idea and should be avoided by extending the domain and keeping the cell count in bay using aggressive cell size increase towards the outlet. However, there are cases where this is difficult or where you simply want to stabilise a steady-state run to get over the first few iterations. @1: inletOutlet is zeroGradient where the flow goes out and fixedValue where the flow comes in --> This should be used on an outlet boundary. OutletInlet is just the opposite. @2: My suggestion for an outlet patch without corners: totalPressure on p inletOutlet on U, k, epsilon This can be improved, but see how is works for you first. Henrik