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I am simulating a turbine and I have some question about standard wall function in FLUENT.

Standard wall function requires $y^+$ value between 30 to 60 on the blade. but the following quote states:

When using wall function models, the $y^+$ value should ideally be above 15 to avoid erroneous modelling in the buffer layer and the laminar sub-layer [Source]

Why $y^+$ value should ideally be above 15 by using wall functions? Can massive separation be modeled with $y^+$ value between 30 to 60 on the blade? because it is mentioned according to FLUENT help:

Traditionally, there are two approaches to modeling the near-wall region. In one approach, the viscosity-affected inner region (viscous sublayer and buffer layer) is not resolved. Instead, semi-empirical formulas called "wall functions'' are used to bridge the viscosity-affected region between the wall and the fully-turbulent region. The use of wall functions obviates the need to modify the turbulence models to account for the presence of the wall.

In other words, I think that there is a velocity profile in viscosity-affected inner region as a wall function that it can almost model boundary layer and massive separation. For example, wall functions with 30

I studied this article but I don't understand the following sentences:

The logarithmic law for mean velocity is known to be valid for . In FLUENT, the log-law is employed when . It should be noted that, in FLUENT, the laws-of-the-wall for mean velocity and temperature are based on the wall unit, $y^*$ , rather than $y^+$. These quantities are approximately equal in equilibrium turbulent boundary layers.

What is difference between $y^*$ and $y^+$? What is the exact meaning of "equilibrium turbulent boundary layers"?

Generally, my question is that when wall function approach is used, viscosity-affected inner region (viscous sublayer and buffer layer) is computed by using semi-empirical formulas with good accuracy. Is this statement correct?

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Wall function solves buffer layer and outer layer, but not viscous sublayer. see law of the wall explanation from wiki.

For flow separation modelling, sub-viscous Boudnary layer (yplus<5) need to be resolved, thus yplus < 2, use SST k-omega RANS, or more advanced models.

y* is almost identical to Y+, detail see "wall function and near wall treatement" in theory guide of fluent.

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