Aeroacoustics, turbulent parameters and convectiveOutlet
 

Hello dear Foamers,

I am rather new with OF, and I would like to share my case with you so that you might give me some hint on 2-3 questions...

I think I am not so far from what I could imagine are reliable results, but still I have some crucial questions (see under)



Here is my case :


preprocessing :

• I have used Salome to create a geometry (cross-section variable tube + radiating hemisphere), to mesh it and to define patches (inlet, outlet, fixed walls) on it.

boundary conditions :

• For the inlet, the velocity has a semi-paraboloid whose amplitude and small-axis length are time-dependent, according to a frequency fixed by the user, thanks to groovyBC.

• For the outlet, a convectiveOutlet is used, trying to simulate an open atmosphere with a convective velocity set as the sound propagation velocity in the air, for the sound components is what interests me in my case.

U :

inlet
{
type groovyBC;
variables "A=25;f=50;xp=pos().x;yp=pos().y;X=max(xp);Y=max(y p);";valueExpression "( (sin(2*pi*f*time())>0) && (sqr(xp/X)+sqr(2*yp/(Y*sin(2*pi*f*time()))))<=1) ? vector(0, 0, ( A*(1-(sqr(xp/X)+sqr(2*yp/(Y*sin(2*pi*f*time()))))) )) : vector(0, 0, 0)";
}


outlet
{
type convectiveOutlet;
convectiveVelocity uniform 343;
value uniform (0 0 0);
phi phi;
}


fixedWalls
{
type fixedValue;
value uniform (0 0 0);
}

p:

inlet
{
type zeroGradient;
}


fixedWalls
{
type zeroGradient;
}


outlet
{
type convectiveOutlet;
convectiveVelocity uniform 343;
value uniform 100000;
phi phi;
}

solvers :

• pisoFoam, for transient incompressible flow
• rhoPisoFoam, for transient compressible flow

turbulent parameters :

• Standard http://www.openfoam.com/features/img/menu24x.png model but I am wondering if I shouldn't try to use LaunderSharmaKE due to the supposedly low Re number...
• k = 0.00325
• epsilon = 0.000765
• NB : dimensions of the smallest diameter : 30 mm, total length of 338 mm (tube + hemisphere)

postprocessing :

• paraFoam, VisIt, probeToAudio (for the sound) and audacity (for the spectrum)

my questions :

1. How to set properly an open atmosphere boundary condition for the outlet, such as the far field should be uniform pressure (100000) and velocity (0 0 0)?

2. Is it worth using convectiveOutlet in order to create a non-reflective boundary condition for every wave going out of the domain, and sound-specific?

3. To what values should I set the turbulent parameters in the kEpsilon turbulent model? Is it worth using a low Reynolds model, such as LaunderSharmaKE in my case?

4. How to be sure it is aeroacoustics and not linear acoustics? How shall I plot the vorticity using either paraFoam or VisIt?



Best regards, and merry Christmas to all the foamers!!


Bertrand