should y+ be upper than 30 in high-Re in all cells?
 

in my case I see in some areas yPlus is lower than 30 as attached.
but in most cells bigger than 30.is it a problem and I should raise yPlus by coarsening the mesh further?(its a narrow tube that furthur coarsening may lead to a not very good mesh)

you love posting... 8 topics in only 24 hours.

In traditional k-epsilon, the y+ should not go below 11. The limit for the viscous sub layer is often set at y+ = 11 (if you go to the code, you can see that the laminar limit is y+=11).

It is not good to have a y+ smaller than 11 because the flow is laminar in this region and since k-epsilon is only turbulent, your assumption become wrong. To avoid this, you can use K-epsilon RNG or Realizable because they are "extended" to the laminar region. However, it doesn't meet they are accurate in region with y+ < 11 ! They are just "better" and they avoid some crazy behavior (especially where 11 < y+ < 30.

And for your other topic where you cannot see the boundary layer, don't use the contour ! You need to use a curve plot perpendicular to the wall (use the filter of intersection in paraview and re-scale the data where you want).

Hi Heckmann
thanks.
no,I love more to solve my problems sooner!:D
thanks.but it seems I have forgotten that to say I'm using kOmegaSST.
does all cells have to be greater than 30 exactly or its sufficient that most be greater?
I didn't grasp what you said about BL.
whould you please answer in that thread more clearly?:)

If you use kOmegaSST, ALL your cells MUST have a y+ smaller than 5 !

kOmegaSST is used to solve the viscous sub layer. On this purpose, you must have at least one point in this region. To get good results with kOmegaSST, you should have a maximum y+ of 1 or smaller (I usually aim 0.5).

so can't possible to use kOmegaSST in high-Re form?
I think its revered!:confused:
see it:http://www.openfoam.org/mantisbt/view.php?id=835#c2167
any comments?

it's similar to plot on line,yes?

1) What is the point of using kOmegaSST in high-Re form (y+ > 30) ? Why not kOmega or even kEpsilon ?

2) yes, it's like plot over a line where you line start at the wall and ends just oustide of the boundary layer. You can get the profile.

1)i have read that kOmegaSST is better in separation phenomena,do you think its better to use pure kOmega?
2) then you mean I have to realize the limits of BL from the velocity profile,yes?there isn't maybe a more automatic way based on velocity or more dissipation or viscosity or something else?

1) in the case of a y+ < 5 only ! kOmega SST has been designed to work within the viscous layer, if you don't have a y+ < 5, you are outstide of the "domain of validity".

2) just draw a line that start outside your domain (1mm) and goes into your domain (1cm for example) and that is perpendicular to the wall. Then plot the velocity magnitude over this line. It will be equal to 0 at the wall and increase as it goes into the domain (and it will be null outside of the domain).

And if I remember well (need to check), kOmega SST works also for y+ > 30 because it has a condition that switch between two models. If you y+ is too big, kOmega SST should behave as kEpsilon does (and therefore, you get all the disadvantages of kEpsilon).

OK.thanks.I'm going to do some runs by y+<1.I'm doing now grid independence for y+>30 method.
how can I set y+<1?only one cell should be under y+<1 ?and if I fine the mesh for grid independence should y+'s be exactly the same(when bothare lower than 1)?
and if I set k as 1e-10 or 1e-11 then what value should I set for omega?
I read in two threads
1)one saying it should be omegaWallFunction with value 1e-10
2) in another it had came that it should be fixedValue with a high value by a formula.
which should I use?

All your wall cells must be under 1 to get a good accuracy no matter what happen. When you run a grid dependency, you should change the number of point in the domain. You can refine the lateral number of point, but it's hard to change the wall distance. To make simple, your y+ must be lower than 5 but it also should not be too small (let say 0.1 for example). You can make a grid with an average y+ = 1 and another one with y+ = 0.2.

The use of "fixedValue" for k comes from it's physical meaning. Since k tends to be infinitely small at the wall. It's not wrong to use the wallFunctions since the wallFunctions actually behave like fixedValue (or/and zeroGradient) that some users described, when the mesh is small enough. It's just that you do it manually. I always keep the omega wall function and sometimes when my mesh is fine enough, I use fixed k at 1e-12. No matter what, the wall functions are here to make your life easier so use them.

I want more to know about omega values
if I set:
what should I replace in x position?a very low value like 1e-12?or very high value like 1e10?
thanks.

Well, you should really study more because this kind of answer can be found quite easily... The value entry is here for two reasons. One it is for paraview that needs a value member to plot the data, and second, it is here because the wall function derive from a fixed value condition and therefore it must have the entry of the wall function but also from the boundary it is derived from.
I'm this case the value has no influence on the computation. It is just here to give a value to paraview. The wall function will give the good value by himself.

i have set it 1e-11.is it ok?
The run is too slow compared with high-Re.
How can find out flow really need a low-Re or not?my case is similar to shock tube case although very different BC's.

Open a boundary layer theory book and it will tell you how to "compute" the Reynolds number in the region close to the wall.

oh!
I certainly know how to compute Re number.:D
maybe you haven't heard about low-Re and high-Re grid?:confused:
I mean that type of mesh and related turbulent method.notice?:)

I'm not talking about the Reynolds number of the flow but about the Reynolds number of the boundary layer. It is slightly different...

I asked a clear question...

I certainly doubt that you can have a low Reynolds grid in a shock tube simulation since your cells are surely too large in the flow direction. The only way you can get "locally" a low Reynolds grid is in the case of an adverse pressure gradient that leads to a detachment (see the boundary layer Reynolds number). In this case, the boundary layer makes a reverse that locally slow down the fluid. Some cells can then be running at a "low Reynolds" regime as you say.

Ps. if you really have a "low reynolds grid" in a shock tube, just run it in LES not in RANS.

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.

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.

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:

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).

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

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?:)