# LES,Periodic Boundary Conditions

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 January 15, 2003, 12:55 LES,Periodic Boundary Conditions #1 Marco Guest   Posts: n/a Hi, I'm conducing a study to insert in an existing CFD code developed in my department a computational model based on LES. Specifically, this is the " Combined LES and time dependent RANS" proposed by Speziale in 1997 (a reference: "A combined Large-Eddies Simulation and Time-Dependent RANS Capability for High-Speed Compressible Flows",C.G.Speziale,Journal of Scientific Computing, Vol 13,No3 ,1998). Briefly: subgrid scale models are dimensionless functions of the computational mesh size times a Reynolds stress model based on a two equations model. A LES go continuously to a RANS computation as the mesh becomes successively more coarse or the Reynolds number becomes much larger ; the level of dicretization is parameterised by the nondimensional ratio of the mesh size to the Kolmogorov length scale. I have to write a new FORTRAN algorithm to insert in the old code, than to test it in the classic backward facing step. Have you got any experience of this model ? Furthermore, I have some doubt about the lateral boundary conditions in the channel: I must apply periodic boundary conditions, but does this simply mean that the variables (velocity,pressure,ecc.,) are identical but unknowed in the two opposite walls ? And in LES should I make any particular adaptations to impose this conditions? Thanks for Your help.

 January 19, 2003, 08:03 Re: LES,Periodic Boundary Conditions #2 Tom,L Guest   Posts: n/a hello,I'm a beginer in LES,and i wrote my code using fortran language.now my code does not work well furthermore,i didn't read the paper Combined LES and time dependent RANS?proposed by Speziale in 1997. i use the periodic boundary in my code in spanwise, and set the velocity(pressure,etc) of front as the same as the behind,that is to say the first grid==the lastest grid. Perhaps it's not real! you can read the book "Turbulence:the legacy of Komogrove".

 January 21, 2003, 06:44 Re: LES,Periodic Boundary Conditions #3 Marco Guest   Posts: n/a Hi ,Tom I think that the method of Speziale is not the best for a beginner, to understand the LES. I'm also beginner,but i have before studied the theory of the LES,and now i begin the applications. I use periodic conditions in spanwise, but i think that this mean , for example in a channel, that the lateral wall are identical. If the first grid = lastest grid, we have a streamwise periodic condition. Thanks