Y+ which Velocity and reference Lenght
 

Hey Forum,
i want to calculate the flux over a spillway of a dam.
To calculate the right Y+ Layer height i need a few Basisvalues, f. e. the freestream velocity and the reference length.


Here is a video of one simulation run, without the (right) y+ layer.
https://www.youtube.com/watch?time_c...&v=opditCQ9TYk


I think this spot is responsible for the preformance of the spillyway.
https://www.bilder-upload.eu/thumb/3...1571838674.png
The velocity increases in order with the gradient of the spillway.

My idea is to get the freestream velocity from this area.
Here is a screenshot, seen from underneath the spillway.
https://www.bilder-upload.eu/thumb/d...1571839032.png


Can i take this velocity to calculate the Y+ Layer height ?

Another Point is the reference Lenght, f. e. this is the diameter of a pipe, vehicle lenght or a lenght of a plate.

I dont know how to convey it to my case, can i take the lenght of the spillway or the lenght of the section above ?



Is it even possible to calculate the Y+ like this, or is it completly wrong ?



Best regards
Kai

Hey,
the value is not exact, obviously.. The goal is to have an idea.
The speed is the freestream speed, i.e. 1.5m/s in your case.
The lenght should be the hydraulic diameter of your section, i.e. aproximately, you can use the lenght of the smallest side.

y+ is not a single value depending on one specific characteristic lenght!!


It is a function wherein the distance from the wall in normal direction provided the lenght as same as happens for the local Reynolds number Re_x.

Good morning, thank you Carlo_P and FMDenaro for you advice. @FMDenaro ok, but which length should i assume for it ? I. e. the caluclators at the internet uses only one value for Y+. I think, maybe its a simplification, to calculate it. The hydraulic diameter sounds reasonable at first :) Best regards Kai

y+ is a field function and not a single number.

Online calculators that calculate y+ (often misrepresented by the symbol Y+) are not actually calculating the y+ field but are doing either:
1) given a first cell height, calculating the y+ of that single cell
2) given a desired y+ for the first cell, calculating the cell height

What really goes into the y+ calculation is the friction velocity, which you can get from an estimate of the wall shear stress. The shear stress in these calculators will come from correlations from flat plats or pipe flows and that's why they'll ask for a reference length and reference velocity.

You also have to consider that there is more than 1 cell in your mesh and you'll have many many values of y+ for each of your wall adjacent cells.


Wherever boundary layers start to form (i.e. after an inlet), the shear stress and hence the y+ will be practically infinite no matter what you do.

Thank you for your detailed response LuckyTran,
i was wrong, yes of course they calculate the cell height not the y+ value.



i will try it, and report later.


Best regards
Kai

I dont get smart out of my values.


The first question is:

I like to calculate the right delta_s.
I made two slices, one at the beginning of the spillway and one at the end.
https://www.bilder-upload.eu/thumb/7...1572291346.png
Furthermore i made a SpreadSheetView of the Slices, and filter the first one, with alphaphase1 >= 0.8 (0 is Air, 1 is Water).
The average RMS (first slice) of the velocity is 0,929 m/s, there are 162 cells with ap1 >= 0,8.


At the second slice, the max ap1 at one cell is 0,52. I dont know why, do i lose water in my system ? Across the way to the second slice, there are only Wall boundarys with no inlet or outlet.



The second question is:

With this RMS velocitys, i get the reynoldsnumbers: 73 for the first slice and 206 for the second slice.

I use this equaton for the reynoldsnumber: ( rho (density) * v (velocity) * d (hyd. diameter) ) / eta ( kin. viskosity)



So im far away from the Re krit = 2300, in conlusion i got a laminar flow ?

Is that right ?



Best regards
Kai