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On the pressure far-field boundary condition

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Old   March 24, 2005, 09:13
Default On the pressure far-field boundary condition
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mini
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Hello everyone, I am simulating the jet flowfield. I set the pressure far-field boundary conditions for the outside region, but the flow (outside) is not symmetric. while not setting the symmetry boundary conditions, I think the flow should be also right. Why ? I doní»t know the reason, please help me. Thanks.
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Old   March 24, 2005, 09:32
Default Re: On the pressure far-field boundary condition
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Jason
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I'm not entirely sure what the problem is... from the sound of it you have asymmetric flow when using the pressure far field, and you think your flow should be symmetric even though you didn't use a symmetry boundary condition... is this close?

Pressure far field condition needs to be far away from any geometry so that it is seeing clean flow. Problems arise when upstream or downstream effects of the geometry intersect the pressure far field. For an airfoil the general rule is 5 to 10 chords in all directions. If separation is going to occur, then you probably want to consider having at least 10chords downstream. In high speed flow, with shocks for especially, you probably want to consider being about 10 or more chords perpendicular to the flow so that the shock can dissipate before intersecting the BC.

Your grid can also cause assymetries. Quick example, lets say you have a symmetric airfoil with a coarse mesh on the underside and refined mesh on the topside. This is going to cause an assymetry in the calculation of the flow and you'll get some lift value on the airfoil even though you're at 0deg AoA. If your mesh isn't set up properly, it doesn't take much of a difference in the the upper and lower mesh to create a slight pressure difference, which leads to asymmetric loads.

I hope something in this was helpful.

Goodluck, Jason
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Old   March 24, 2005, 21:28
Default Re: On the pressure far-field boundary condition
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I see, thanks. I will try to modify it.
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Old   March 25, 2005, 16:48
Default Re: On the pressure far-field boundary condition
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Riaan
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Also check what boundaries touch the PFF boundary faces....E.g. you cannot have a pressure inlet/outlet touching a PFF wall.

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