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Old   January 30, 2010, 07:55
Default velocity increases extremely
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Axel Söhngen
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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
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Old   January 30, 2010, 10:32
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Sebastian Gatzka
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Are you doing a transient or steady-state simulation?
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Old   January 31, 2010, 06:36
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Hello sega,
I'm doing a transient simulation!
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Old   January 31, 2010, 06:54
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Claus Meister
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How does it themselves felt? I mean, do ya have, for instance, a CFL-drop or do you see it by doing pros-processing?

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Old   January 31, 2010, 07:20
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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.
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Old   January 31, 2010, 07:27
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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.
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Old   January 31, 2010, 07:53
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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
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Old   January 31, 2010, 08:12
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Quote:
Originally Posted by chelvistero View Post
And is the pressure at the cylinder (uniform 0) the right choice?
I don't think so. Think of the Bernoulli equation.
I would choose zeroGradient!
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Old   January 31, 2010, 08:25
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Ok that's right. But the courant number and velocity still explode.
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Old   January 31, 2010, 16:21
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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.

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