k and omega boundary conditions.
 

Hi,

I am relatively new with OpenFoam and I would be grateful if some one can help me please. I am trying to set up a case using simpleFoam and the kOmegaSST model (for a subsonic flow around and airfoil) but I have difficulties defining the boundary conditions for k and omega.


My question is what should be the value of the term “value” in the following script ?
And also what should be the value for internal field ?


For k:
dimensions [0 2 -2 0 0 0 0];

internalField uniform ????;

boundaryField
{

airfoil
{
type kqRWallFunction;
value uniform ???;
}
farfield
{
type turbulentIntensityKineticEnergyInlet;
intensity 0.9;
value uniform ????;
}
frontAndBackPlanes
{
type empty;
}
}




For Omega:
dimensions [0 0 -1 0 0 0 0];

internalField uniform ???;

boundaryField
{

airfoil
{
type omegaWallFunction;
Cmu 0.09;
kappa 0.41;
E 9.8;
value uniform ????;
}
farfield
{
type turbulentMixingLengthFrequencyInlet;
mixingLength 0.005;
k k;
value uniform ????;
}
frontAndBackPlanes
{
type empty;
}
}


Thank you in advance for your consideration and time.


Sincerely,


A.D.E

k is the specific turbulent kinetic energy and omega the turbulence length scale. In general it is hard to know how much they are.

If you are simulating a wind tunnel with inlet speed U and turbulence intensity x%, then you can guess k = x/100 * U^2 at the inlet boundaries and in the domain. For omega I cannot give you tips, but I read that often omega smooths out independently of the BC's.

Not stated is the fact that "internalField" represents the initial condition or initial guess for the flow field. I suspect the "value" field does not need to be specified in your 0/ directory for many if not all of the boundary conditions you list. On subsequent time steps, that variable may be output and contain the current value of the cell faces on the boundary.

Hello,


Thank you both for your reply. I am currently running a simulation after considering your comments. Can I also ask what does the value of mixingLength represent? For a simulation of a flow in a wind tunnel what value can I use?


Thank you,


A.D.E

Hello

Next time try reading the OpenFOAM User Guide first please:

http://www.openfoam.com/docs/user/ca...#x5-120002.1.3

Under this title you should find hopefully your answers:

2.1.8.1 Pre-processing


Best regards

Cp plots
 

Hello I would like to plot the Cp curve around an aerofoil in paraFoam can anyone tell me how I can go about it?

Thank you in advance

A.D.E

Thank you.

Hi A.D.E.


hope this above can help you


k=3/2*(Ve*I)^2
I=0,16/R_e^(1/8)
epsilon=C_mu^(3/4)*k^(3/2)/Dh
omega=(k)^(1/2)/Dh


for more information see the user guide.

On the left side, in Mesh Regions, select only the airfoil surface to be visible. Then make a plane normal to the spanwise direction. Right click on your plane and Add filter -> Data Analysis -> plot on sorted lines. Once this done toggle visibility of only pressure and you have your pressure curve (note i forgot to mention that you can use the calculator option and calculate your cp as p - pinf/1/2 rho uinf^2 and for a free stream velocity of 1 and a density of 1 a freestream pressure of 0, this should just be cp = 2*p)