Wall layer models

roadracer

Hi everybody. I have a doubt regarding wall layer models. I have always used the standard wall functions, and to let the model work properly the near wall cell size has to be set in order to have the resulting y+ value between 30 and 100. In engine transient simulations this is not possible, because the flow conditions varies a lot, so it is difficult to keep the y+ value at the same location within the prescribed values. Furthermore, there are time instants where the velocity is almost zero, so y+ goes to 0, but there is no boundary layer because the fluid itself is standing!! So I´m always in doubt when I have to set up the near wall cell size: what is a reasonable range of values of y+ that should I obtain?

In other words, are the wall functions model suitable for transient simulations? And what about separation?

Thank you in advance. Any help is welcome

Roadracer