[Sunxing]

Originally Posted by umer.chaudrey

Hey everyone,

I have a query regarding K-epsilon values too.

I am using a transient compressible rhoPimpleFoam solver, RAS turbulence model but without any wall functions for k and epsilon.

Whatever I read in various threads on this forum, I understand that initial values of internal fields of k & epsilon are independent of the solution.

But however in my case, if I use different values my solution behaviour and values change. Can anyone please comment on that?

I had written on another thread too, but no one replied regarding specific k & epsilon intial values for internal field. I am not using a velocity boundary condition, rather pressure difference.

Thanks, I will really appreciate some advice.

Hi umer.chaudrey

I am also simulating a case using the rhoPimpleFoam solver, RAS turbulence model. In my case--wall-jet flow, I set the inlet velocity as fixed value. However, I don't know how to set the the outlet condition for velocity and inlet/outlet condition for pressure. I wander that how do you set these condition in your case?

Regards,
Sunxing

[sdharmar]

Hi Sunxing,

You can use zeroGradient BC or inletOutlet BC for U at the outlet.
For p you can use zeroGradient at the inlet and fixedvalue at the outlet. I am also a new member so please double check my suggestions.

If any senior members can support/oppose my comment that would be really appreciated.

BR,
Suranga.

[Sunxing]

Hi Suranga,

Thanks for your reply. I have already tested this condition that you have suggested, however it has turned out to be a bad setting.

regards,
Sunxing

[idefix]

Hello,

at the moment I have some problems with one simulation.
k and epsilon have very high values at the wall. I think that there is something wrong with the wall function.
But I am not familiar with this topic.
I read this sentence here in the discussion.

Yes. If you have a look in the source code, you will see that the wall function for k simply impose a zeroGradient condition at the wall, while the while function for epsilon calculate (for each timestep) the first grid point value by using an algebraic expression derived from the classical logarithmic law-of the wall approach.

Where can I find this mentioned source code?
Are there any literature explaining the wall function model used in OF in k epsilon-model?

Sorry for this simple question, but I am very confused at the moment and I am not very good in C++.

Thanks for your help
Idefix

[cfdman_aero]

hi, I have a difficulty with discretization of source terms of k and epsilon. for example we have (du/dy)**2 in the k source term. my code is on FVM and explicit. Please guide me through.

[cutter]

Please don't hijack other threads! What have you done so far and which discretization scheme are your trying to employ? What's your intention - writing an own solver?

Have a look at the book "Essential Computational Fluid Dynamics" by Oleg Zikanov. It covers the basics of FVM discretization pretty thoroughly. Many other CFD books do as well.

cutter

[cfdman_aero]

Originally Posted by cutter

Please don't hijack other threads! What have you done so far and which discretization scheme are your trying to employ? What's your intention - writing an own solver?

Have a look at the book "Essential Computational Fluid Dynamics" by Oleg Zikanov. It covers the basics of FVM discretization pretty thoroughly. Many other CFD books do as well.

cutter

Thanks for reply. I am writing FVM for incompressible turbulent flow utilizing K-Epsilon. The scheme is explicit and I do not know how to discretize the nonlinear production in wich we have i.e. (du/dy)**2. I use some methods but the code doses not run and exit with "floating point error: invalid". I wonder if there is any method to linearized form of those terms.

[payal05]

helo
I am relatively new in OF and would be grateful if some one can help me please. I am trying to set up a case using rhopimplefoam solver and the kOmegaSST model but I have difficulties defining the boundary conditions for omega. frankly speaking i dont have any clue about omega BC"S.

in my case i have inlet , axis , upperwall, outlet so hw to define BC's for that ?

[joneill053]

Hi All!

I would like to possibly revive this thread as I have a few questions regarding setting up k and omega for the SST model.

I am simulating moderate flow over a bumpy airfoil (Re =300,000) and I have calculated values for both k and omega based on a length scale of 7 percent of my chord length (ie 0.07*0.201768 meters) and an initial guess of turbulence intensity of 1%. I know most literature says for moderate flows to use 1-5% but I am unsure of how to adequately guess where I fall within that range.

The values I calculated with this become k = 0.083622 and omega = 37.38255.

(1) Should these values be initialized as the "internalField uniform" values or as the value uniform for the inlet?

(2) I have an inlet outlet condition with the inletValue uniform set to 0 for both k and omega so if there is any reverse or inlet flow in theory there should be little or no effect on my flow traveling over my airfoil if there is any reverse or back flow appearing(please forgive me if I misunderstand how the inletOutlet type works...I am relatively new with OpenFOAM). With a zero gradient at the outlet, would these be proper BC's to set?

Any help at all would be appreciated!

Thanks,

Joe

[Anand895130]

Hello Everyone

I'm using this code for running my Rotating wall,

bump
{
type rotatingWallVelocity;
origin (958.85 1755.17 0);
axis (0 0 1);
omega constant 230.3834; //rad per sec using 13200 deg/s=2200 rpm value;


In this what will be the reynold's number. and how to change the reynold's number if I want to ?

Thanks in Advance

Any reply will be helpful

[igne]

What if I use my initial velocity and calculate k and epsilon values?

[vava10]

Hey,

I am simulating a kayak for velocity 4.5 and 5.5 m/s and I am using K-omega SST. I found the formulas for finding the values as shown in the image. I am not an expert in CFD. I am getting really high value for omega. I don't know if I am making any mistakes consept.
But when I try these formula on some on the examples cases, I am not getting the correct answer

length 5.19

Following are the values

Re = 23355000
I = 0.0191
k = 0.0111
L/sqr(Re) = 1.073e-3
l =4.29e-4
omega =488

in some of the papers which I read they just assume the value of intensity I as 0.5%


can some one help?

thanks in advance
vava10