[sharonyue]

Hello guys,

This question is quite general, Im not familiar with LES model. AFAIK, no matter which kind of model I use, I should predict the same results which is the same with experiments.

For example: pls see this video:

https://www.youtube.com/watch?v=eP_eytKCCFY

We can see LES and RANS predicts instant velocity field is quite different, Why? I wish its the same. All predict the results which can be comparable with experiments.

I think I must misunderstand something about this, pls correct me. Thanks.

Best,

[agustinvo]

They are not definitively the same! RANS and LES are just turbulence models, and they work by using different assumptions. For this reason, as you say, the instantaneous velocity field is not the same. RANS models are used to simulate steady flows, unlike URANS (I don't know alot about them, but they include time dependency on their turbulent equations).

In case of LES, you resolve ALL the flow EXCEPT the sublayer region, which is modeled. And this turbulence model is time dependent. Maybe some LES experts can assess you better than me, but the idea is that you will not get the same results in both models. Once you have the LES results, you can average and you could retrieve the same fields than in RANS.

And in relation with the experimental results, it will depend on how you set up your RANS model. In case of the LES model, I have not a lot of information about how it works.

I hope to have helped you.

[sharonyue]

Originally Posted by agustinvo

They are not definitively the same! RANS and LES are just turbulence models, and they work by using different assumptions. For this reason, as you say, the instantaneous velocity field is not the same. RANS models are used to simulate steady flows, unlike URANS (I don't know alot about them, but they include time dependency on their turbulent equations).

In case of LES, you resolve ALL the flow EXCEPT the sublayer region, which is modeled. And this turbulence model is time dependent. Maybe some LES experts can assess you better than me, but the idea is that you will not get the same results in both models. Once you have the LES results, you can average and you could retrieve the same fields than in RANS.

And in relation with the experimental results, it will depend on how you set up your RANS model. In case of the LES model, I have not a lot of information about how it works.

I hope to have helped you.

Hi bro,

that really clarify something for me.

So from your experience, LES is kind of like DNS except the sublayer region. This is REAL transient simulation.

Even using transient algorithm, RANS provides steady-state flow fields. Thats why there are different in the result.

however, so if RANS is just steady-state simulation, is there a necessary to run simulation with transient RANS model? From this tutorial, after a long time, the transient simulation using RANS looks like the same with simpleFoam's result which is rather faster.

Best,

[agustinvo]

Yes, we can say that LES is a kind of DNS. The difference is that you don't refine close to the walls, which is the part the most computational resources need.

however, so if RANS is just steady-state simulation, is there a necessary to run simulation with transient RANS model? From this tutorial, after a long time, the transient simulation using RANS looks like the same with simpleFoam's result which is rather faster.

From the theory behind RANS, you divide each field in a mean term, and a fluctuating term, x=X+x'. When you average (Reynolds Averaged Navier Stokes), your system of equations will give you only the mean terms. So if in this example the results you obtain are the same is because this flow becomes steady, and since you are only obtaining the mean part of the velocity, you will see the same profile after a long time in the case of the unsteady simulation, and this will be the same than you steady simulation. In other cases, you will never get the same, such as a Rayleigh Benard cell, which is always a time-depending flow.

[sharonyue]

Thanks Agustin, that really helps alot.