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

#1 
Member
Join Date: Nov 2009
Location: Germany
Posts: 96
Rep Power: 8 
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 
Senior Member
Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
Rep Power: 12 
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?
__________________
Schrödingers wife: "What did you do to the cat? It's half dead!" 

July 28, 2010, 07:20 

#3 
Member
Join Date: Nov 2009
Location: Germany
Posts: 96
Rep Power: 8 
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 
Member
Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
Blog Entries: 1
Rep Power: 7 
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
__________________
Kind regards, Albert 

May 8, 2012, 01:27 

#5 
Member
Yogesh Bapat
Join Date: Oct 2010
Posts: 41
Rep Power: 7 
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  
Member
Albert Tong
Join Date: Dec 2010
Location: Perth, WA, Australia
Posts: 76
Blog Entries: 1
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?
__________________
Kind regards, Albert 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
vorticity boundary condition  bearcharge  Main CFD Forum  0  May 14, 2010 11:32 
boundary condition for derivative of velocity  Daiga  CFX  2  December 7, 2009 18:11 
maintaining a logarithmic velocity distribution  Morten Andersen  CFX  1  January 8, 2007 12:37 
Profile Data Velocity Boundary Condition Changes??  Maria Angelica  CFX  9  June 14, 2006 02:44 
Boundary condition for intermediate velocity  Yiling  Main CFD Forum  3  June 15, 2002 23:48 