Hi,

Can anyone enlighten me about the physical importance of Y plus in general. Any references (Books) would be very helpful also. I coudnt find much on the web.

Thanks,

Dominic

Dear Dominic,

The y+ can be considered similar to the Reynold's number. It is a dimensionless quantity constructed out of the distance from the wall and the friction velocity. As such in turbulence, the y+ is a measure of the resolution of the viscous sub-layer. In general, closer the first layer, lower the y+ and finer the grid to resolve the laminar sublayer. This is the fact most turbulence models concentrate on. The resolution of the laminar sublayer directly influences the wall stress computations and hence the skin friction distribution. The linear profile of the laminar sublayer demands that y+<5 (approx.). However, different turbulence models have their own restricions on the y+ values and this could go down as low as 1. However, I have found that algebraic turbulence models like the Baldwin-Lomax model gives resonably accurate results even for y+ in 7-10 range ( for airfoil computations), though this may not be acceptable to others. For specific details on the y+ in turbulence modelling, you could look into Wilcox. Also, for the physics of y+, I believe a good reference is Tennekes and Lumley. You could also refer to other books on Fluid turbulence for more details.

Hope this helps

Regards,

Ganesh

hi..ganesh

To get the y+, the parameter of kinetic energy k is needed. It is easy for RANS turbulent model, but in LES how to do?

Thanks!

In addition to Ganesh: y+ is the most important parameter in turbulent boundary layers because depending on it the boundary layer can be divided in many layers. Each of this layers has a physical interpretation; for example there is the viscous sublayer where the viscous processes are greater than the other or the logarithmic region where a logarithimic law holds between the u+ and y+ .. and so on. This is explained in all the books regarding turbulence such as the Wilcox (the best book in the worlds for RANS simulations) or the Pope (much more physics and an overview of RANS-LES-DNS-PDF methods).

Regarding the y+ and LES the problem is not so simple because in LES you cannot only say " I have my first grid point at y+=1 and so it' all OK".

In LES you are resolving only filtered quantities and so the energy containing eddies and you are modelling all the subgrid eddies.

Away from the wall the size of the eddies you are modelling is related to the grid dimension.

Near the wall the problem is much more complicated because the layers I was speaking about earlier are present.

In principle if you simulate all these layers with the correct resolution you perform a DNS; if you do not simulate them (due to the grid dimension) you have to model what you are not simulating -----> Wall modelling in LES

hi.. mar " you have to model what you are not simulating -----> Wall modelling in LES "

Can talk about the wall modelling in LES?

Everything can be done in several ways!

For example have a look to:

www.onera.fr/congres/ti2006/definitivepapers/Medic.pdf

or the CTR-Stanford site

or just use scholar.google.com