Post Processing: Measuring Boundary Layer
 

Hey guys.

I'm simulating the flow around a Formula One Halo. As I want to implement small scale Vortex Generators (50% of the Boundary Layer thickness), I need to know the Boundary Layer thickness before the separation occurs.

First I did the meshing and solving, I made sure the Y+ values were between 0 and 5 to ensure a good resolution and capture of the boundary layer. But now, I'm not too sure if there's a more "measurable" way of measuring the Boundary Layer thickness.

What I did was plotting the velocity vectors, zoom into the surface, adjusting the vectors size until I detected the flow separation. Then I went to the cell before, and used the Ruler to measure from the surface until the first vector that has the same size as the next. The result I got was 1.62mm which doesn't seem crazy but I was expecting a little bigger given I'm using 35m/s air flow around a cylinder shaped Halo! But I wanted to ask if there's any other more "scientific" way to measure the Boundary Layer.

Here's a picture of how I got the result!

https://imgur.com/Y6P8FhK

https://imgur.com/Y6P8FhK

There’s probably too many scientific ways to estimate boundary layer thickness. :)

Start here, there’s a few links to offsite pages that will help.
https://www.cfd-online.com/Tools/yplus.php

Thats what I found before, but this is not what I'm looking for.

Because I don't want to calculate the theoretical BL thickness. I already have the simulation done, my Wall Y+ values are between 0 and 5, I want to specifically measure the BL thickness on the post processor!

Because my CAD file is a Halo, it's not a plane, it's not that easy or accurate to calculate the boundary layer as if it was a straight plane!

Did you use All y+ Wall Treatment? ;)

Think about the boundary layer theory and about the "results" you want if there is an impact. Check you physics in the cells where (or next) to the Y+ is higher and then - think again about...

Yes, All Y+ Wall Treatment was one of the Models selected! (Actually it selects automatically!)

I'm not too sure if I'm not being able to explain what I need, or is that I'm not understanding your helps.

I don't want to calculate the theoretical boundary layer wall thicness to assign to my Prism Layers. That was already done, and with wall y+ values of 0 to 5, I think I'm having a good capture of the Boundary Layer.

What I want to know, is at a specific point of the geometry (right before the flow separation occurs), what is the físical boundary layer at that point, because I need to inser vortex generators, and these are given in % of the boundary layer thickness!

How can I measure the BL thickness at a specific point without using the ruller?

Do it oldschool. You know the total height and the amount of layers and you know your stretching factor.

A suggestion!
 

Hi Andre,

If I understood correctly, you want to measure the boundary layer thickness on a surface. My suggestion is to create a line probe normal to the surface at the point where you need to measure the thickness. Then make an xy plot for the velocity choosing the line probe as part. By definition, boundary layer thickness is the normal distance where u = 99% of V_free stream.

Hope this helps.

That would make me calculate the Thickness of my Prism Layers, not the thickness of the Boundary Layer at a specific point!

Thanks! That's indeed a very good idea and indeed a much more precise way of measuring the thickness! Thank you!

In general that's not correct. Instead, It is: , Where is the asymptotic velocity which varies in the streamwise direction .