[Rob Wilk]

I did a Civil Engineering course some years ago and from my textbook I had a question on boundary layer thickness.
So far I have been able to calculate the transition distance, but I am not sure how to calculate the boundary layer thickness from there.
The purpose of this question is to work out how to calculate the boundary layer thickness from what I have said.

Scroll Down to see the question as an image below.

[LuckyTran]

Look in your text for the a correlation for the boundary layer thickness. For example, a laminar BL has a displacement thickness that looks like 4.91/sqrt(Rex)

[Rob Wilk]

Originally Posted by LuckyTran

Look in your text for the a correlation for the boundary layer thickness. For example, a laminar BL has a displacement thickness that looks like 4.91/sqrt(Rex)

For the turbulent part, based on velocity profile and wall shear stress equations given, my textbook has :

boundary layer thickness = 0.37 (Rex)^1/5 * x^0.8


I have not worked out the Boundary layer thickness for the laminar part just yet , my textbook has the same velocity profile as this example, but a different wall shear stress.
When I get the chance I will calculate it .


For this question that I have, I don't think they really allow for the overlap, when the flow goes from laminar to turbulent.

I say this because , in this Mixed Boundary Layers Calculations
video https://www.youtube.com/watch?v=_QwcffD0APE&t=199s, 9 minutes in, they say in real life you have to allow for the overlap for a small distance before it goes to turbulent.

Laminar distance = x lam = x transition = 0.69 m (what I have calculated)

Turbulent distance = x turb = L - x transition + x overlap


What is important important for me to understand LuckyTran is how this overall boundary layer thickness works when it goes from laminar to Turbulent flow.

[aero_head]

Originally Posted by Rob Wilk

For the turbulent part, based on velocity profile and wall shear stress equations given, my textbook has :

boundary layer thickness = 0.37 (Rex)^1/5 * x^0.8


I have not worked out the Boundary layer thickness for the laminar part just yet , my textbook has the same velocity profile as this example, but a different wall shear stress.
When I get the chance I will calculate it .


For this question that I have, I don't think they really allow for the overlap, when the flow goes from laminar to turbulent.

I say this because , in this Mixed Boundary Layers Calculations
video https://www.youtube.com/watch?v=_QwcffD0APE&t=199s, 9 minutes in, they say in real life you have to allow for the overlap for a small distance before it goes to turbulent.

Laminar distance = x lam = x transition = 0.69 m (what I have calculated)

Turbulent distance = x turb = L - x transition + x overlap


What is important important for me to understand LuckyTran is how this overall boundary layer thickness works when it goes from laminar to Turbulent flow.

Are you asking how to calculate the boundary layer thickness in the transition region, or are you asking how to calculate the length of the transition distance? Or are you simply asking for some equations in the turbulent region?

[Rob Wilk]

Originally Posted by aero_head

Are you asking how to calculate the boundary layer thickness in the transition region, or are you asking how to calculate the length of the transition distance? Or are you simply asking for some equations in the turbulent region?

Hi aero_head

I have already calculated the length of the transition distance, it's 0.69 m.
This is the length that the laminar flow goes for.

So now I am trying to work out what the question wants ( boundary layer thickness at a distance equal to twice the transition distance from the leading edge ).

[aero_head]

Originally Posted by Rob Wilk

Hi aero_head

I have already calculated the length of the transition distance, it's 0.69 m.
This is the length that the laminar flow goes for.

So now I am trying to work out what the question wants ( boundary layer thickness at a distance equal to twice the transition distance from the leading edge ).

Hello Rob,

Thanks for clearing that up. So, I believe the next step would be to calculate what the local Reynolds number is at a distance of (2 * 0.69) m, then calculate the boundary layer thickness depending on if you are in the transition or turbulent region. My guess is that it would be turbulent, due to the length used coupled by the fact that you are not provided with an equation for the transition region.
As for your comment about the shear stress, that is the 'normal' situation to use that equation, please see the attachment. It just means you don't need to derive the boundary layer equation from the momentum integral equation/first principles. So you would use the boundary layer equation in the attached image (yours had x^0.8 (?), should just be x).