# velocity increases extremely

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 January 30, 2010, 08:55 velocity increases extremely #1 Member   Axel Söhngen Join Date: Jan 2010 Location: Germany, Trier Posts: 31 Rep Power: 9 Hello, I'm trying to create an external flow over a cylinder. I made 2D mesh with blockMesh. First I try the case with a velocity of 1 m/s and Re=150. I'm using icoFoam but when I start the calculation the velocity increses with every timestep but it has to be constant. Whats the mistake?? Here are p and U dictionaries: p: dimensions [0 2 -2 0 0 0 0]; internalField uniform 0; boundaryField { frontAndBack { type empty; } inlet { type zeroGradient; } outlet { type fixedValue; value uniform 0; } lowerWall { type symmetryPlane; } upperWall { type symmetryPlane; } zylinder { type zeroGradient; } } U: dimensions [0 1 -1 0 0 0 0]; internalField uniform (0 0 0); boundaryField { inlet { type fixedValue; value uniform (1 0 0); } outlet { type zeroGradient; } frontAndBack { type empty; } lowerWall { type symmetryPlane; } upperWall { type symmetryPlane; } zylinder { type fixedValue; value uniform (0 0 0); } Thanks in advance

 January 30, 2010, 11:32 #2 Senior Member     Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 13 Are you doing a transient or steady-state simulation? __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

 January 31, 2010, 07:36 #3 Member   Axel Söhngen Join Date: Jan 2010 Location: Germany, Trier Posts: 31 Rep Power: 9 Hello sega, I'm doing a transient simulation!

 January 31, 2010, 07:54 #4 Senior Member   Claus Meister Join Date: Aug 2009 Location: Wiesbaden, Germany Posts: 241 Rep Power: 10 How does it themselves felt? I mean, do ya have, for instance, a CFL-drop or do you see it by doing pros-processing? Cheers

 January 31, 2010, 08:20 #5 Senior Member     Sebastian Gatzka Join Date: Mar 2009 Location: Frankfurt, Germany Posts: 729 Rep Power: 13 I'm not sure, but have you tried a different boundary condition for pressure on the outlet? Prescribing a pressure may be not always the right choice. __________________ Schrödingers wife: "What did you do to the cat? It's half dead!"

 January 31, 2010, 08:27 #6 Member   Axel Söhngen Join Date: Jan 2010 Location: Germany, Trier Posts: 31 Rep Power: 9 Hello Claus, the cfl increases extremely, and in paraview is shown that the velocity increases every timestep too. I try to simulate the karman vortex street with a Re of 100.

 January 31, 2010, 08:53 #7 Member   Axel Söhngen Join Date: Jan 2010 Location: Germany, Trier Posts: 31 Rep Power: 9 No I haven't tried another boundary for pressure at outlet! What else can I choose for the pressure at outlet? And is the pressure at the cylinder (uniform 0) the right choice? Thank you for your reply

January 31, 2010, 09:12
#8
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Sebastian Gatzka
Join Date: Mar 2009
Location: Frankfurt, Germany
Posts: 729
Rep Power: 13
Quote:
 Originally Posted by chelvistero And is the pressure at the cylinder (uniform 0) the right choice?
I don't think so. Think of the Bernoulli equation.
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 January 31, 2010, 09:25 #9 Member   Axel Söhngen Join Date: Jan 2010 Location: Germany, Trier Posts: 31 Rep Power: 9 Ok that's right. But the courant number and velocity still explode.

 January 31, 2010, 17:21 #10 Senior Member   Claus Meister Join Date: Aug 2009 Location: Wiesbaden, Germany Posts: 241 Rep Power: 10 Choosing a zeroGradient is the worse thing you can do, because of the pressure term in the Navier-Stokes-Equation, i.e. you have to specified the pressure at least in one point. Have you checked your mesh? A blow up is also related to singularities which can occur by bifurcation, so try a finer mesh or check if it fit the physical behavior of your scenario. Cheers

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