[chelvistero]

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

[sega]

Are you doing a transient or steady-state simulation?

[chelvistero]

Hello sega,
I'm doing a transient simulation!

[idrama]

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

[sega]

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.

[chelvistero]

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.

[chelvistero]

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

[sega]

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.
I would choose zeroGradient!

[chelvistero]

Ok that's right. But the courant number and velocity still explode.

[idrama]

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