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December 8, 2004, 00:04 
Boundary Condition for LES

#1 
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Hi,
I am trying to incorporate strandard model of LES into an existing code, the "SmagorinskyLilly Model", and I am trying to validate the code using a simple channel flow case, with Velocity INLET and an OUTFLOW boundary condition. As I have read that there should be a fluctutation velocity that should be given to INLET at the begining of every timestep, I am finding difficult to implement the same, in the code, as one of the models suggest to develop a ramdom number, and multiply the same with the modulus of veloctiy, and add it to the freestream. Is this model, fair enough or should I use some other way of generating the fluctutation velocity. What material can I refer to get some idea. Your Help in this Regard will be appreciated. Thanks in Advance, Krishna. 

December 8, 2004, 00:49 
Re: Boundary Condition for LES

#2 
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if you have access to fluent then they have explained how they have implemented it ..if you do not have access to fluent drop me an email i shall send you relevent text


December 8, 2004, 02:46 
Re: Boundary Condition for LES

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Hi Zxaar,
Could you please send me the relevant text on les and also the steps to run les using fluent. My email is sencal@hotmail.com. Thannks for your help. sencal 

December 8, 2004, 04:07 
Re: Boundary Condition for LES

#4 
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HI Zxaar,
Thanks for ur reply, I am trying to implement a LES model in my Code same as that of FLUENT version 6.1, although I doubt its accuracy, but I think it will be fair enough for me to start with that, so I can proceed further with later models, so all the things that I have mentioned in the previous post, refer to FLUENT ones. FLUENT uses a Random number to generate the fluctuation velocity, if I am not wrong!!!!! Krishna. 

December 8, 2004, 05:07 
Re: Boundary Condition for LES

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drop me an email.. i wished to talk more on this,


December 8, 2004, 05:07 
Re: Boundary Condition for LES

#6 
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note my email from the this message


December 8, 2004, 09:37 
Re: Boundary Condition for LES

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The most common inlet/outlet boundary condition for channel flow is periodic which lets the turbulence become fully developed. You must allow the mass flow to vary to balance the wall shear stress. The flow is driven either by adding a body force or by modifying the pressure solver (assuming incompressible flow solver) to introduce a periodic boundary condition plus a constant difference between the inlet and outlet. The latter is neater but the former simpler. There are other approaches such as specifying an intial mass flow and maintaining an instantaneous global force balance.
Random numbers at the inlet are almost never sensible boundary conditions. What goes on in boundary layer flows is strongly controlled by the shear stress. Random numbers usually give zero shear stress and the inlet motion very rapidly disappears. 

December 8, 2004, 21:44 
Re: Boundary Condition for LES

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Hi Andy,
I can understand your point, but speaking in the coding terms, how do you allow the mass flow to vary to balance the wall shear stress ? Can u advice me or suggest me some material ? And the second method that you have stated refers to periodic boundary condition (copying parameters from Outlet back to Inlet), that is I hope is by adding a fluctutating term along with the mean at the Inlet at every time step. Correct me !!!! Thanks, Krishna. 

December 9, 2004, 01:32 
Re: Boundary Condition for LES

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December 10, 2004, 06:53 
Re: Boundary Condition for LES

#10 
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your email addy has a problem ... mails keep bouncing ... coefficent of resituition is 1


December 11, 2004, 21:13 
Re: Boundary Condition for LES

#11 
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Ummm. You may need correcting but I am not sure I am the best person to do it or that this forum is the best means of doing it. Do you have a supervisor or an experienced researcher you can talk to about simulating turbulent flow? The questions forum can be useful for pointers like which boundary condition is best to try first but is not much good at providing the details.


December 12, 2004, 20:34 
Re: Boundary Condition for LES

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Have a look at this paper. It could be useful,
"A digital filter based generation of inflow data for spatially developing direct numerical or large eddy simulations" Klein et. al. Journal of computational physics, (186), 2003, pp. 652665. 

December 14, 2004, 23:04 
Re: Boundary Condition for LES

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In FLUENT 6.2 (to be released soon  beta releases are available), the random number generator will be superceded by two new turbulence synthesizers  Random Flow Generator (Try Google search) and vortex method.


December 20, 2004, 16:48 
Re: Boundary Condition for LES

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These papers may interest you. As a matter od fact, this is a hot topic in LES. There are many other papers on this.
Lund et al, 1998, "generation of turbulent inflow data for spatiallydeveloping boundary layer simulation", JCP, 140, p233 Xiong, et al, 2004, "Simple method for generating inflow turbulence", AIAA, 42(10), p2164 

December 22, 2004, 21:54 
Re: Boundary Condition for LES

#15 
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HI Woody,
Thanks for the Reply, I already had a look at the 1st paper you have mentioned, the second one sounds interesting, I am trying for the same, If u have one copy, do u mind mailing me!!!!! krishna. 

January 7, 2005, 18:42 
Periodic Boundary Condition for LES

#16 
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Hi, I'm working on "SmagorinskyLES" model and trying to validate the code using a square duct with periodic boundary condition. I implemented a fully developed turbulent flow plus fluctuations as inlet condition and the priodic velocity B.C. at inlet and outlet. For pressure preiodic boundary condition I need to know other's idea. I'm wondering if you can guide me in this regard.
Thanks in advance, Fereshteh 

January 30, 2005, 14:21 
Re: Periodic Boundary Condition for LES

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sorry, i dont have an answer for u, but maybe u have an answer for me.. i need to find out the velocity field for air flowing inside a square duct. if u are validating your code with such a duct it means that u have experimental data!! if it is so i'll be very glad if u could send me them or tell me how can i have them cus i really dont know how to do it!
tanks in advance if u can do something for me enrico. 

February 1, 2005, 22:05 
Re: Periodic Boundary Condition for LES

#18 
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Hello Fereshteh,
I am not exactly sure what is your problem but I if you have access to FLUENT or CFX packages you can quickly check your code. SmagorinskyLilly and Large Eddy Simulation models are available in FLUENT. There are two types of periodic boundary conditions available also. The first type is referred to as "cyclic" boundary and does not allow a pressure drop across the periodic planes. The second type allows a pressure drop to occur across translationally periodic boundaries, enabling you to model "fullydeveloped" periodic flow. If you like to discuss it more, give me some info about your geometry and I might be able to do a quick run in FLUENT for you. Good luck! Kambiz 

February 17, 2005, 17:56 
Re: Periodic Boundary Condition for LES

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Hello Mr. Kambiz, Could you please tell the difference between cyclic and translational periodic boundary conditions and how they influence the pressure boundary conditions.
Thank you Suresh 

February 17, 2005, 19:31 
Re: Periodic Boundary Condition for LES

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Rotationally periodic boundaries are boundaries that form an included angle about the centerline of a rotationally symmetric geometry. Translationally periodic boundaries are boundaries that form periodic planes in a rectilinear geometry. Let me know if you need more help.


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