# Boundary Layer Thickness for Flat Plate

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.

Try using the following formulas for boundary layer thickness over a flat plate. These formulas are empirically derived.

For Laminar Boundary Layer

For Turbulent Boundary Layer

Sources: Fundamentals of Aerodynamics by John D. Anderson

edit: Updated the format of my answer as recommended by a moderator.

• Welcome to Engineering! So as to avoid link-rot, please expand your answer to include the relevant information from the link provided, at least the most essential points. Feel free to quote directly from the source. – Wasabi Sep 28 '19 at 11:39
• Hi Rishi Is this correct. For the transition distance of 0.69 m there is a laminar boundary layer and for the next 0.69 m there is a turbulent boundary. How do I deal with the fact that the flow is a mixed boundary layer going from laminar to turbulent flow. Do we have to calculate the boundary layer thickness separately for laminar and turbulent layers and then add them or is there another way. This is the main concept I am having difficulty understanding, once I have this understood then I should be able to get the answer. – Rob Wilkinson Apr 17 '20 at 23:38
• Hi RIshi So do we add both the boundary layer for laminar and turbulent boundary layers together to work this out ? Also you have the same x for Reynolds number and boundary layer thickness formulas, I'm just thinking is the x in Reynolds number based on m width, so you don't need it. Please get back to me when you can, because I think I may hopefully have this if I understand all this stuff. It is important that you get back to me while I am in the mood to get this sorted. – Rob Wilkinson Apr 19 '20 at 8:01
• Hi Rishi I haven't heard back from you in a while man. How does this work with the laminar boundary layer changing to turbulent for a combined boundary layer flow. What I mean is do I approach the question like this: Treat it to start with as Turbulent boundary layer thickness for twice transition distance, subtract turbulent boundary layer thickness for transition distance and then add boundary layer thickness for transition distance. Are you able to clarify with me how I should calculate this based on these formulas that you showed me ? – Rob Wilkinson May 7 '20 at 4:40