# Boundary Conditions for External Flow

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 July 14, 2008, 07:23 I am solving for a flow past a #1 New Member   Anush Join Date: Mar 2009 Posts: 5 Rep Power: 10 I am solving for a flow past a cylinder in external flow in 2D using the icoFoam solver. It consists of a circle in a rectangular domain. I'm using the following boundary conditions: 1. Left side of the rectangle is the inlet with fixedValue velocity and zeroGradient pressure. 2. Right side of the rectangle is the outlet with fixedValue pressure and zeroGradient velocity. 3. fixedValue for velocity and zeroGradient pressure at the surface of the cylinder. I have the following doubts: 1. Are the above conditions I've used correct? 2. I don't know what conditions to use for the top and bottom sides of the rectangle. Should I treat them as outlets as well? 3. What would be the difference between specifying the walls of the cylinder as 'wall' or 'patch'? Does the difference come into play only in turbulence simulations? Thanks for the help. haow likes this.

 July 14, 2008, 08:06 1/correct 2/if inlet velocity #2 Member   Patrick Bourdin Join Date: Mar 2009 Posts: 40 Rep Power: 10 1/correct 2/if inlet velocity is parallel to bottom and top planes, you can use the 'slip' BC on these (however make sure bottom and top planes are far enough from the cylinder for this BC to be physically correct for external flow, otherwise you will spuriously accelerate the fluid over the cylinder). 3/It shouldn't make any difference for a laminar solver whether you use wall or patch in the boundary file

 July 14, 2008, 08:41 Hey thanks a lot, Patrick. #3 New Member   Anush Join Date: Mar 2009 Posts: 5 Rep Power: 10 Hey thanks a lot, Patrick. And also, Would 'slip' be a better condition to use than zeroGradient for both pressure and velocity? If so, why - does it not unnecessarily remove the normal component?

 July 14, 2008, 10:13 I don't know what is better. #4 Member   Patrick Bourdin Join Date: Mar 2009 Posts: 40 Rep Power: 10 I don't know what is better. The slip BC may stabilize the computation more than the other, try both settings, and see what happens. All in all, if both BCs work fine for your case, you should obtain very similar solutions (for top and bottom planes far enough from the cylinder).

 July 14, 2008, 12:18 Oh okay. Thank you very much! #5 New Member   Anush Join Date: Mar 2009 Posts: 5 Rep Power: 10 Oh okay. Thank you very much!

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