Wall function in les.
 

Hi, can anybody help me, by telling ...how the wall function is implented in LES. I Read....but i could not understand , how to impelement. i am using staggered MAC algorithm for N-S solver..

Thanks in advance.

Re: Wall function in les.
 

Hi Pankaj

Wall functions are calculated in very similar manner to RANS modelling. In order to switch on wall functions you just switch them on in CFX-Pre. I guess thats not what you are asking. Your question is not very clear. If you explain your problem a bit more some one might be able to help.

Re: Wall function in les.
 

I thought LED didn't use any wall functions, but DES (Detached) does - the same ones as RANS.

Re: Wall function in les.
 

Wrong, Phil!

Pankaj, you can read this two article :

- Piomelli, U. and Balaras, E., Wall layer models for large eddy simulation. Annual Review of Fluid Mechanics. v34. 349-374.

- Nikitin, N.V., Nicoud, F., Wasistho, B., Squires, K.D. and Spalart, P.R., An approach to wall modeling in large eddy simulations. Physics of Fluids. v12. 1629-1632.

Some method a very near to RANS method, but some over are different.

Re: Wall function in les.
 

Thanks phill and gaama... actually i am working on code..writtten by me. i am running stagared grid....and simulating channel flow. so, near the wall , right now i am using no slip b.c..

so, i am just putting vel=0... i.e---u(i,j,k)=-u(i,j+1,k), for any i,k but j=1 as, i am using stagarred grid and j, is the wall normal direction..and j=1 is the fictious cell..j=2 is solution domain...wall is between j=1,2..

Now, cal u tell ..how to use wall fucnction code it...

thanks in advance...also, i will see the paper..

if any body did some worked out hands material, please send to my mail..

thanks..

Re: Wall function in les.
 

Hello Pankaj,

A wall function is used to emulate turbulent stresses in the boundary layer which normally would require a fine resolution to capture. The boundary layer is made of the viscous sub-layer(1), closest to wall and then a log-law layer(2). The velocity in these two regions is described by:

(1) u+ = y+ (2) u+ = ln (y+) + A

You will find these definitions in any standard text on turbulence. The above equations are emperical and can be used in flow simulations such as channel flows. In LES, what you need is information at the first point away from the wall. , usually in the log-law later (eqn, 2) In this equation, the unknown u_tau (friction velocity) appears on LHS and RHS and an iterative procedure has to be used. Once u_tau is known, the velocity at this point is known. Read book/literature to find more.

A good introduction to wall functions (also RANS) and how those are implemented in OpenFOAM, can be found here:

A Thorough Description Of How Wall Functions are implemented in OpenFOAM