[immortality]

1)why are you saying the cells are too large?no I have set fine grid.
becuse Re number is high (its a flow in a channel like shock tube with running shock but the phenomena are very complicated than simple shock tube because of unsteady boundaries).I don't understand what you mean that only when the velocity be low we can use low-Re study.it doesn't have any relation to Re of boundary layer.when we have a turbulent flow a laminar sub-layer exists not important what is Re of boundary layer.low-Re region stands for very thin sub-layer at that viscosity is dominated to inertial forces.and when we name grid to high-Re we mean that the first cell center has located in logarithmic region that inertial forces are somewhat dominated to viscous forces.
otherwise correct.
2)in compressible flows should y+ be considered or y*?
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the grid needs to be fine enough in normal to wall direction not in the flow direction in RANS but in LES all should be very fine that increases the time of run too much.

[fredo490]

Well, there are other definitions of low and high Reynolds number grid... You can set a grid Reynolds number based on the velocity in the cell compared to the cell size in the direction of the flow.

You are talking about a normal to the wall spacing while there is another definition that is "parallel" to the wall. You use high aspect ratio quads near the wall because you assume that there is no strong adverse pressure gradient resulting in a detachment.

For the y+ and y* stories in compressible with/without wall functions, there are tens of topic explaining it. Use the search function of the forum and you will find your answer.

[immortality]

it takes too long in low-Re form and I don't think use it anymore.
I know there are different threads on y+ and y* and my question isn't on them.
I say whether we have to use y+ or y* in conditions (like <=1 or>=30 )
for compressible flows.
if anyone knows.
because one professor said me it:

For low-Re models, you have to set the physical value (fixedValue or
zeroGradient, depending on the variable), and ensure y* < 1
(compressible) or y+ < 1 (incompressible).

[fredo490]

Your answer is in the topics here.... For y+ below 1 without "wall functions" , the usual formula used in openfoam is not accurate anymore. So you have two choices, you can define a new function that is compatible (see other topics on the forum) or use the y star (because its definition is always true).

[immortality]

you mean that yPlusRAS -compressible is not appropriate?it uses y* formula that seems true for compressible cases if you agree.

[immortality]

Hi again
I wanted to say that when using low-Re time step turns too low in constant maxCo.then its not appropriate in an explicit solver like rhoCentralFoam that maxCo is limited to .5 as the creator's article.
then I don't continue it anymore(maybe in sonicFoam for comparison results by high-Re in rhoCentralFoam)
is there probably any suggestion?