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Old   August 16, 2013, 11:07
Question Fluent init-instantaneous-vel command, DES
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Sheng
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Dear all,
I have 3 questions not clearly understood.

1. As we can see from Fluent's guide for LES, if we continue from a steady RANS result, we must type the command "init-instantaneous-vel" to obtain an instantaneous velocity field. Question: does this apply to DES calculation? And, this option is not available for SA model?

2. What Fluent exactly does when we type "init-instantaneous-vel"? Can anyone render some hints? What's the formula or function?

3. I have one more question regarding the DDES model using SST turbulence model.

We know there're 4 options we can choose for SST-DDES in Fluent: F1 function, F2-function, DDES and IDDES. The user manual gives exact information about the F1 and F2 function based formulation. Question: what the exact formulation of the DDES and IDDES option? The user's manual just says it uses the same idea of SA-DDES, IDDES, with a constant changing from 8 to 20. Is there anyone know more about this?

Regards,
Sheng

Last edited by micro11sl; August 21, 2013 at 06:52.
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Old   August 26, 2013, 04:47
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Dear Sheng,

1) the option is always available, i don't remember if you first have to switch to unsteady and/or LES (which imply unsteady nonetheless) but then you can definitely switch again to any other model and, OF COURSE, Fluent can't drop off your actual data. Just give it a try

2) The method is the same as the spectral sinthesizer used for the inlet (check the manual). The difference is that no time variable is involved and, possibly, different constants are used

3) I'm not that much into DES and related methods. You should check some relevant reference
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Old   August 26, 2013, 06:48
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Quote:
Originally Posted by sbaffini View Post
Dear Sheng,

1) the option is always available, i don't remember if you first have to switch to unsteady and/or LES (which imply unsteady nonetheless) but then you can definitely switch again to any other model and, OF COURSE, Fluent can't drop off your actual data. Just give it a try

2) The method is the same as the spectral sinthesizer used for the inlet (check the manual). The difference is that no time variable is involved and, possibly, different constants are used

3) I'm not that much into DES and related methods. You should check some relevant reference
Thanks very much for the reply by sbaffini.

1) I tried and have got some data by far. I found under S-A model, there is no "solve/init/init-instantaneous-vel" command available, although from the user manual, it is stated the command is available for ALL RANS turbulence models.

2) I will investigate your reply further. It's interesting.

3) I look around some papers and have got some ideas. I will post my findings later.

I have a further question to ask sbaffini: do you use LES often more than DES?
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Old   August 26, 2013, 07:24
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Dear Sheng,

the main reason for the limitation you are facing may be due to the fact that, if i remember correctly, the method requires a velocity and a length scale to be specified in the domain to properly work. SA instead only provide one single scalar which is a combination of both. If you really need to use SA then you could try the following:

1) Once the solution with SA is obtained switch to k-eps; if the eddy viscosity is preserved go further (otherwise there is no chance)

2) Write a DEFINE_INIT UDF which will do the following:

- copy the velocity (and maybe pressure) as they are, i.e., with C_U(c,t)=C_U(c,t)

- for each cell define your own length scale L. This is somewhat arbitrary, but it is the only option. You might want to use some blending between the wall_distance and the local cell spacing (cubic root of cell volume). Take also a look at the DES,DDES descriptions for some other relevant definitions; still, neither of them will actually give you a proper length scale as required by the method for the initialization.

- with the eddy viscosity (NU_T(c,t) if i remember correctly) and the length scale above you can initialize the k and eps variables trough the proper definitions which should also be available in the Fluent manual.

3) Use such a UDF to initialize the flow

4) If everything worked correctly, you should now be able to use the initializer with some fields which are at least partially significant. The definition of the length scale might be strongly affecting the initialization process.

I never used DES and i almost exclusively work in LES
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Old   August 26, 2013, 07:36
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Quote:
Originally Posted by sbaffini View Post
Dear Sheng,

the main reason for the limitation you are facing may be due to the fact that, if i remember correctly, the method requires a velocity and a length scale to be specified in the domain to properly work. SA instead only provide one single scalar which is a combination of both. If you really need to use SA then you could try the following:

1) Once the solution with SA is obtained switch to k-eps; if the eddy viscosity is preserved go further (otherwise there is no chance)

2) Write a DEFINE_INIT UDF which will do the following:

- copy the velocity (and maybe pressure) as they are, i.e., with C_U(c,t)=C_U(c,t)

- for each cell define your own length scale L. This is somewhat arbitrary, but it is the only option. You might want to use some blending between the wall_distance and the local cell spacing (cubic root of cell volume). Take also a look at the DES,DDES descriptions for some other relevant definitions; still, neither of them will actually give you a proper length scale as required by the method for the initialization.

- with the eddy viscosity (NU_T(c,t) if i remember correctly) and the length scale above you can initialize the k and eps variables trough the proper definitions which should also be available in the Fluent manual.

3) Use such a UDF to initialize the flow

4) If everything worked correctly, you should now be able to use the initializer with some fields which are at least partially significant. The definition of the length scale might be strongly affecting the initialization process.

I never used DES and i almost exclusively work in LES
Thanks for a prompt reply.

That's what I am wondering. I wonder that step "init-instantaneous-vel" is not essential for DES simulation and inlet boundary condition doesn't require unsteady velocity fluctuations. IDDES may be different. I will investigate this further.
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