y+(plus) for k-\omega SST and k-\eps varies by 100 order in same sims after postprocs
 

Dear Foamers,



As I had written many threads for my problem of Propeller case and still struggling to make it work for turbulent case. I had made the CAD geometry, innerCylinder_small, innerCylinder, outerCylinder same as that of propeller tutorial. As such I have changed only the input parameters of inlet velocity and \omega (in rad/sec). The laminar case seems to work properly but the turbulent case (using default k-\epsilon) seems to blow off after t=0.28 sec. Then many FOAMERS has suggested me to change my mesh according to the y+(plus) values, although using the wall function (not sure, if it works or not). And found that the y+(plus) for k-\epsilon and k-\omega SST are of the order of 3 and 300 respectively. The dat files originated from the pimpleFoam -postProcess -func yPlus is attached herewith. Could any one please comment on these values.


Also a very weird point, I used the negative angle of rotation which is clock-wise and the free stream velocity be -ve, exactly similar to that of the tutorial case. But surprisingly it rotates in opposite direction as that of the tutorial (off-course making the same view axes). Just wondering, if doing some thing very incorrect that these things are happening.

The values which you have posted are for time 0 so they obviously make no sense because the simulation didn't even start. You have to run the simulation until the flow develops to have some meaningful wall shear stress readings.

Hello Mark,


Many thanks for this input, I will update the value of y+ once the flow is flow is developed.

Hey Chandra,
if you want to make some tests with different yPlus (so, with different first layer cell), you can use refineBondaryLayer utilities.
In this way, you will refine the first cell.


The best way is to create a mesh with one big layer and then use different time refineBoundiarylayer to create different meshes with different number of layers.

Hello Carlo,
Many thanks for your post and help. But just wondering how to do that? For preparing mesh I am using snappyHexMesh and blockMesh. Is refineBondaryLayer utilities similar kind of utility as that of surfaceFeatureExtract in snappyHexMesh?

Sorry for this kind of question, also I am not very sure why the y(plus) value have an importance if I am already using the wall-function. How will y(plus) value will help me to simulate the case? Also, if I am having a sense of y(plus) in RANS will it also help me to get some sense of grid refinement if I am using the LES?


Again sorry for these set of questions, that may be may be related to the post.


Thanks!

Hello Carlo,


I got the value of y(plus) as (for the blade, (fixed blade)):
min: 2.51, max: 5.659346e+03 and average: 4.600024e+02


I am using k\omega and using wall functions.


Does it say something?


Thanks

Hey Chandra,

In snappyMeshDict, you define nlayer=1.
Then run blockMesh and snappy.
You will have a mesh with1 layer.
After that you can run refineBondaryLayer -option comand from terminal.
Now, you will have a mesh with 2 layers, run the simulation with high yplus.
Then, you can refine the bl with the same command refineBonadryLayer.
Now you will have 3 layers.
Run the simulation.
You will have a smaller yplus.
etc..
etc..


In this way you can try different firts high cell and so different yplus.

Hello Carlo,

I just tried the refineBondaryLayer and it sends me an error:
refineBondaryLayer: command not found


Something I am missing or not doing correctly?


Thanks for your time and help.