Not working what does mean? Could you detail?

In isotropic turbulence there is no characteristic domain lenght, one uses the Taylor microscale. Do you correctly make the variables non dimensional?

When you say linear forcing, do you mean in physical space or spectral space?

Dear Prof. Filippo and Dr. Farzad,

Thank you for showing interest in the question and clearing the doubt.

Here is some more information on my problem.

1. I am working in physical space.

2. By correct simulation I mean I have a fluctuating velocity field with mean approximately zero. By incorrect simulation I mean the velocity field is not fluctuating rather decaying.

3. I have added linear forcing term A*(rho*(U-Ubar)) in the NS equation. A is a constant. Ubar is the spatial mean.

4. I am using ABC flow as my initial condition.

Ux=cos(2*pi*y/L)+sin(2*pi*z/L)

5. The following are the observations:

- For scaled domain of box less than 2*pi, the velocity field is always decaying. I have tried altering values of A and fluid viscosity keeping density 1 kg/m3

- For 2*pi (in m) domain with viscosity 1 N.s/m2 and division 32 also the velocity field decaying. However if viscosity is reduced to 1e-3 it is found to be fluctuating.

- Simulation is grid dependent as for the above mentioned case with division 64 the velocity field is fluctuating.

What I seek is how to know how can I predict a priority for what property and grid size I will be getting turbulent flow field coz trying out different permutation and combinations for this is inefficient.

Thank you once again.

With Regards

Krishna Kant

Dear Krishna Kant

Is it a in-house code or a software like openfoam or etc?

Thanks,

Farzad

Dear Dr. Farzad Farajidizaji,

I am working in OpenFOAM only.

Thank You

Krishna Kant

Can I ask which version? And what solver you are using?

Farzad

I am using version 4. The solver is a customized solver of interFoam. However, there is no two-phase problem I am trying to do right now, and having vof 0 throughout the domain. My base solver is validated well for several problems.

Krishna Kant

Sorry for my late response. Did you get any success?

Thanks,

Farzad

Yes.

I got a homogeneous isotropic turbulence field in a scaled domain with some trial and error for the value of A in the linear forcing term Au.

Thanks and regards

Krishna Kant

I am also doing similar thing, and I have used LES with the same AU added to momentum source where A comes from Lundgren(2002) article. My problem is that turbulence decays first and then reach a steady solution and then increases a little bit and I do not know how to have stable turbulence(HIT) with linear forcing.

It should first increase and then decrease and then become stable.

http://basilisk.fr/src/examples/isotropic.c

How are you modelling this with LES?

Krishna Kant Thanks for the reference. I am using the kEqn LES model from openFoam;

https://www.openfoam.com/documentation/guides/latest/doc/guide-turbulence-les-k-eqn.html

I see that you are using Rosales(2005) method where I have used Lundgren(2002) method. I should try Rosales(2005) method. just two other questions;

1- What is your initial prescribed power spectrum? CBC or Karman Howarth?

2- Is there any way to integrate "http://basilisk.fr/src/examples/isotropic.c" to OpenFoam or should I implement it myself?

Thanks,

Farzad

Farzad Farajidizaji I am initializing the field as ABC flow following the reference "http://basilisk.fr/src/examples/isotropic.c". It's a straightforward implementation in OpenFoam.

Thanks

Krishna Kant

Thanks

Dear Krishna Kant

I developed my version of Forced HIT in OpenFOAM, just there is a problem when I add gravity. If I use interFOAM without gravity(g=0), code works fine, but when I add g=-9.81, a mean flow in gravity direction starts to develop which is wrong. In my forcing I add A*(U-Umean) to the momentum equation which I believe it must cancel the effect of Umean at each time step, but unfortunately I still see mean flow. Have you had any experience on this issue?

Thanks,

Farzad

Dear Farzad Farajidizaji

I have not tried this problem with the gravity effects. So I had no experience in this regard.

Thanks

Krishna kant

Thanks