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-   -   Schmidt Number setting in Buoyance model (http://www.cfd-online.com/Forums/cfx/98749-schmidt-number-setting-buoyance-model.html)

qsx4881 March 18, 2012 03:48

Schmidt Number setting in Buoyance model
 
2 Attachment(s)
Dear all,
I am simulating thermal stratification using k-e model with full buoyancy. But I am not sure how to set the buoyancy turbulence term in the Fluid models/Buoyancy Turbulence. The default setting is none, while other choices are production, production and dissipation, see attachment.

I have run some cases with the default setting and production and dissipation (Turbulent Schmidt Number=1.0 and Dissipation Coefficient=1.0), but the results really show big differences. For example, the maximum temperature difference through cross-sections is bigger while use production and dissipation. It seems that the Schmidt number and dissipation coefficient make the thermal stratification more seriously, see attachment. Besides, the Schmidt number also make the simulation difficult to converge,

So my question is what the influence of Schmidt number in buoyancy simulation? Does it relate to convection and buoyance? Which result is reasonable? Should I select production and dissipation?

Thanks in advance! Look forward to your replay.

ghorrocks March 18, 2012 06:20

I do not know those parameters in detail as they are of no importance to most simulations. After a quick read of the CFX theory manual they introduce new terms in to the production and dissipation equations to deal with buoyant effects.

If your simulation has found that they are important (and your results suggest they are) then you better do some reading of the CFX theory manual and the references quoted in it. You will have to understand these parameters to know how to apply them properly.

My suggestion is you probably want to activate this option and leave the Schmidt numbers at the default values. I would also compare results to the SST model which is a good baseline turbulence model. And hopefully you have some experimental results to compare to so you know if you are going in the right direction. If not you need a good benchmark case to compare against. And don't forget to do a sensitivity analysis on mesh, time step size and convergence tolerance so you know you are not kidding yourself.

qsx4881 March 18, 2012 12:05

Quote:

Originally Posted by ghorrocks (Post 350054)
I do not know those parameters in detail as they are of no importance to most simulations. After a quick read of the CFX theory manual they introduce new terms in to the production and dissipation equations to deal with buoyant effects.

If your simulation has found that they are important (and your results suggest they are) then you better do some reading of the CFX theory manual and the references quoted in it. You will have to understand these parameters to know how to apply them properly.

My suggestion is you probably want to activate this option and leave the Schmidt numbers at the default values. I would also compare results to the SST model which is a good baseline turbulence model. And hopefully you have some experimental results to compare to so you know if you are going in the right direction. If not you need a good benchmark case to compare against. And don't forget to do a sensitivity analysis on mesh, time step size and convergence tolerance so you know you are not kidding yourself.

Glenn,thank you for your earnest answer.

Just as you said, I want to active the option. I have read the theory manual, it doesn't help. In related papers, some active the option while others not.Professor Abdus Salam give a lector named "The.State.of.Turbulent.Mixing.and.Future.Directio ns.for.Research" http://v.163.com/movie/2010/7/I/A/M6...M6G40T8IA.html refers to Schmidt number, but I haven't understand. It really confused me.

Your suggestion almost include all possibilities, I am going to try. I have refined the mesh and tried little timestep (0.1s, now 0.5s). It seems doesn't matter. We are preparing the experiment but it need some time. So now I will find a benchmark to verify the option.

Thanks again.

ghorrocks March 18, 2012 17:57

Schmidt number is a key concept in turbulence modelling and there are several Schmidt numbers in the model. I recommend you get a good turbulence modelling textbook (eg "Turbulence Modelling for CFD" by Wilcox) and you get some background in this area.


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