modify ccmp for initialisation

Thomas Baumann · August 12, 2009, 11:06

Hello all,

I'd like to modify my postprocessing results from a ccmp file.
I simulate a large eddy simulation of a channel flow with cyclic boundary conditions.

I begin with simulating the channel with RANS-Modells to get initialisation values for my LES. That works without any problems.
But I get a laminar profil in the LES. It is a channel flow with a Reynolds number about 50000. So it should be turbulent.

I think the problem is the initialisation profil, because there are no fluctuations of the velocity and pressure because of turbulence. Is there a possibility to change the values in the *.ccmp data I use for initialisation of the LES?

Thanks, Thomas

vishyaroon · August 12, 2009, 11:33

Are you sure it is a problem with the restart?

Are you running LES on a smooth channel? In case of unsteady RANS models you still might develop into a turbulent flow in a plane channel, but with LES if you do not have some kind of a turbulence generator, then your flow is bound to relaminarize. Can you check if this is what is happening. This should not be a problem in RANS as the turbulence equations take care of it, but the LES turbulence has to be artifically generated.

Thomas Baumann · August 13, 2009, 02:39

It's a channel with walls on top, bottom, left and right.
Solving with normal RANS the velocity-profils are turbulent. Using these results for initialisation it relaminarizes.

RANS: u_max/u_mean=1.1, sign for turbulent profil
LES: u_max/u_mean=2, sign for laminar profil

That's why I want to modify the data to initialize fluctuations for the veloicity and pressure datas in the LES.

I can modify the registers of the post-datas, but I don't know how to save them in the *.ccmp.
The simulations are solved with starcd 408. Does anyone know the command or anything that can help?

vishyaroon · August 13, 2009, 08:48

Hello Thomas,

If you want to run LES on a smooth channel (which is periodic) even initialization will not help. This is not a problem with the software or your restart file. It is a commonly observed characteristic of solving turbulent flows using LES.

You need to trip the boundary layer to keep it turbulent. If you just initialize the flow is quickly going to laminarize unless it is in the highly turbulent regime.

Regards
Aroon

Thomas Baumann · August 24, 2009, 09:35

Hello,

I made the LES of the channel like described in the tutorial 16 at the Star-CD Tutorial Guide. I initialised big vortices with a user-function and it seems to work. I get nice vortices and I hope the channel will stay turbulent by the following iteration steps ...

I couldn't find a solution for overwritting the *.ccmp-file, but it's actually not necessary.

Regards Thomas

August 12, 2009, 11:06	modify ccmp for initialisation	#1
Thomas Baumann Member Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 98 Rep Power: 16	Hello all, I'd like to modify my postprocessing results from a ccmp file. I simulate a large eddy simulation of a channel flow with cyclic boundary conditions. I begin with simulating the channel with RANS-Modells to get initialisation values for my LES. That works without any problems. But I get a laminar profil in the LES. It is a channel flow with a Reynolds number about 50000. So it should be turbulent. I think the problem is the initialisation profil, because there are no fluctuations of the velocity and pressure because of turbulence. Is there a possibility to change the values in the *.ccmp data I use for initialisation of the LES? Thanks, Thomas

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August 12, 2009, 11:33		#2
vishyaroon Senior Member Aroon Join Date: Apr 2009 Location: Racine WI Posts: 148 Rep Power: 16	Are you sure it is a problem with the restart? Are you running LES on a smooth channel? In case of unsteady RANS models you still might develop into a turbulent flow in a plane channel, but with LES if you do not have some kind of a turbulence generator, then your flow is bound to relaminarize. Can you check if this is what is happening. This should not be a problem in RANS as the turbulence equations take care of it, but the LES turbulence has to be artifically generated.

August 13, 2009, 02:39		#3
Thomas Baumann Member Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 98 Rep Power: 16	It's a channel with walls on top, bottom, left and right. Solving with normal RANS the velocity-profils are turbulent. Using these results for initialisation it relaminarizes. RANS: u_max/u_mean=1.1, sign for turbulent profil LES: u_max/u_mean=2, sign for laminar profil That's why I want to modify the data to initialize fluctuations for the veloicity and pressure datas in the LES. I can modify the registers of the post-datas, but I don't know how to save them in the *.ccmp. The simulations are solved with starcd 408. Does anyone know the command or anything that can help?

August 13, 2009, 08:48		#4
vishyaroon Senior Member Aroon Join Date: Apr 2009 Location: Racine WI Posts: 148 Rep Power: 16	Hello Thomas, If you want to run LES on a smooth channel (which is periodic) even initialization will not help. This is not a problem with the software or your restart file. It is a commonly observed characteristic of solving turbulent flows using LES. You need to trip the boundary layer to keep it turbulent. If you just initialize the flow is quickly going to laminarize unless it is in the highly turbulent regime. Regards Aroon

August 24, 2009, 09:35		#5
Thomas Baumann Member Join Date: Apr 2009 Location: Karlsruhe, Germany Posts: 98 Rep Power: 16	Hello, I made the LES of the channel like described in the tutorial 16 at the Star-CD Tutorial Guide. I initialised big vortices with a user-function and it seems to work. I get nice vortices and I hope the channel will stay turbulent by the following iteration steps ... I couldn't find a solution for overwritting the *.ccmp-file, but it's actually not necessary. Regards Thomas