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July 27, 2010, 10:49 
Velocity boundary condition: negative value (suction)

#1 
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Join Date: Nov 2009
Location: Germany
Posts: 96
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Hello,
i would like to use a negative velocity as BC for an outlet (the volume flow is known). Unfortunetaly the solution crashes or shows huge values. I'm using the simpleFoam solver with kepsilon. My guess is I have wrong settings: 0/U: Code:
in { type zeroGradient; } out { type fixedValue; value uniform (6 0 0); } Code:
in { type fixedValue; value uniform 0; } out { type zeroGradient; } Code:
in { type zeroGradient; } out { type fixedValue; value uniform 14.855; } Code:
in { type zeroGradient; } out { type fixedValue; value uniform 0.375; } Code:
in { type calculated; value uniform 0; } out { type calculated; value uniform 0; } Code:
in { type zeroGradient; } out { type fixedValue; value uniform 0; } Code:
in { type zeroGradient; } out { type fixedValue; value uniform (0 0 0 0 0 0); } By the way, the solution looks good when I switch to blowing (positive velocity at inlet). Regards, Toni 

July 28, 2010, 05:30 

#2 
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Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
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You are talking about an outlet at the beginning, about an inlet at the end.
What is it now? Can you tell us how it is aligned in respect to your coordinate system?
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July 28, 2010, 07:20 

#3 
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Join Date: Nov 2009
Location: Germany
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Sorry, I know it's confusing.
I attached a slice through the modell. The outlet called "in" is on the left side. The inlet called "out" is on the right side. Inlet and outlet is switched because i changed from blowing to suction. In the meantime i found out the solvers crashes because of the high velocity. I tried a smaller value and it worked. The value also has to be positiv. Otherwise it would be an inflow. The code for 0/U: Code:
in { type zeroGradient; } out { type fixedValue; value uniform (0.5 0 0); } 

May 7, 2012, 07:01 
icoFoam negative inlet velocity

#4 
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Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
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Hello the Forum and a nice week to you all,
I would like to specify a negative inlet velocity B.C. in icoFoam solver and am encountered a problem. I enclose the case, which is quite simple, in the attached file and also give a short description here. It's 2D case for flow simulation around a cylinder and the b.c.s are as follows, inlet: U fixedValue 1 or 1, p zeroGradient outlet: U zeroGradient, p fixedValue 0 at two sides, symetryPlane is applied and empty b.c. for in cylinder span direction. fvSchemes are specified as, Euler for ddt, Gauss linear for gradSchemes, Gauss cubic for divSchemes and Gauss linear corrected for laplacianSchemes. For U=1, icoFoam gives reasonable result. The force coefficients are shown in the attached Fig U=1. Strouhal number is 0.2, which agrees with experimental results. Drag coefficient is around 1, which is good. But when you change U=1, which is the case in the attached case, the simulation crashes at 95s. The force coefficients, as shown in Fig U=1, behaves normal at the beginning, but increases to a very large value when the simulation stops. I have no idea why I am experiencing such different results with only one change of the direction of the velocity. Can you please guide me to find out the reason? Any help would be greatly appreciated. The case can be found here, http://dl.dropbox.com/u/62722643/ico...donline.tar.gz And can be run directly with command Code:
icoFoam
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Kind regards, Albert 

May 8, 2012, 01:27 

#5 
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Yogesh Bapat
Join Date: Oct 2010
Posts: 43
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Hello,
You can try to use lower relaxation for momentum equation. Also another option is to ramp velocity at outlet in steps. Regards, Yogesh 

May 8, 2012, 23:40 

#6  
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Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
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Rep Power: 7 
Quote:
Thanks for your answer. Can you explain why and where I should lower the relaxation for momentum equation? Can you please also point out why there is such a great difference when the velocity direction changes?
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Kind regards, Albert 

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