[oozcan]

Hi all,

I am working on 3d heat transfers over 3d multi geometry. 2 Fluid zones and 1 solid one.


I would like to show you total geometry that is found attached.

Air is passing through a channel that is seen attached.

Air meets complexity solid geo outer faces. I have uploded air top view to be seen easily.

I could not use y+ calculator as I dont know what boundary length is,instead, I have calculated, skin friction coeff. (schlichting), wall shear stress, friction velocity and last one first layer thickness,

The reason why I do above,that is not correct Reynolds number in y+ calculator output. (as boundary lenght value results in it)

Where does Boundary length dimension match up in latest attached photo? (length, width or height)

Kind Regards,

[LuckyTran]

The friction velocity is the best way to estimate the first layer thickness. If you know the skin friction coefficients and can get the friction velocity, use it!

Boundary layer calculators are usually based on developing boundary layers in flat plates. The length in these calculators is the distance from the leading edge of the plate. If you already know the friction velocity, then you don't need this approach.

[oozcan]

Originally Posted by LuckyTran

The friction velocity is the best way to estimate the first layer thickness. If you know the skin friction coefficients and can get the friction velocity, use it!

Boundary layer calculators are usually based on developing boundary layers in flat plates. The length in these calculators is the distance from the leading edge of the plate. If you already know the friction velocity, then you don't need this approach.

As far as I know, developing means that required L distance couldnt be reached for fully developed.

Well, I have attached calculator and is to see detail as you inform me

Re<10^9 on a flat plate.

I took a formula regarding skin friction coefficient ''http://www.cfd-online.com/Wiki/Skin_friction_coefficient '' (Schlichting) and then, wall shear stress,friction velocity and first layer thickness have been calculated,respectively.

There is no enough distance to attain ''fully developed turbulent flow '' and Reynolds number is already is about 3000, it is compatible ''calculator''. That is why I look up to calculator and calculated first layer thickness by giving y+=1.

if I have no enough information, which dimension should I use it for calculating it ? (screenshots is to be seen in my first post.)

[LuckyTran]

The length in y+ calculators is the same x that you put into the Schlicting formula when you need to calculate Rex.

This x is the local distance to the leading edge of the plate where the boundary layer growth started. y+ calculators of course can't tell you what to do in complex geometry where boundary layers get interrupted, separate, reattach, regrow, blah blah blah. These calculators only help you to get within an order of magnitude.

Each different location in your domain will have a different first layer thickness needed to achieve a desired y+. You need to decide how conservative you want to be. Mesh it, roll with it, run the simulation, and then tune your mesh to achieve your goal.

[oozcan]

Originally Posted by LuckyTran

The length in y+ calculators is the same x that you put into the Schlicting formula when you need to calculate Rex.

This x is the local distance to the leading edge of the plate where the boundary layer growth started. y+ calculators of course can't tell you what to do in complex geometry where boundary layers get interrupted, separate, reattach, regrow, blah blah blah. These calculators only help you to get within an order of magnitude.

Each different location in your domain will have a different first layer thickness needed to achieve a desired y+. You need to decide how conservative you want to be. Mesh it, roll with it, run the simulation, and then tune your mesh to achieve your goal.

Which way is suitable one in order to find '' friction velocity''?
I compare two ways with each other.

First way, if attahced screenshots has been used, firt layer thickness is different from the other

Or, wall shear stress is calculated by using skin friction coeff. from Schlichting, then, friction velocity and first layer thickness, respectively, I always choose the last one. if the first one is best one, I need to correct myself to find a proper choice.