How can I calculate the displacement thickness of a turbulent boundary layer?

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Discussion Overview

The discussion revolves around calculating the displacement thickness of a turbulent boundary layer, particularly in the context of transitioning from a laminar flow simulation using Thwaite's method. Participants explore various methods and formulas for predicting transition points and displacement thickness, with a focus on turbulence modeling and algorithm development.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks guidance on calculating the displacement thickness of a turbulent boundary layer after simulating the laminar part with Thwaite's method, mentioning "Head's-model" as a potential reference.
  • Another participant questions how the boundary layer transition is being determined, suggesting a semi-empirical formula relating Reynolds number to the shape factor.
  • A participant expresses skepticism about accurately predicting the transition point, noting that there is generally no reliable method to find Re_{tr} and that predictions may not align well with experimental results.
  • One participant references a specific equation from "Aerodynamics for Engineering Students," attributed to Smith and H. Gamberoni, indicating that they do not expect precise transition point predictions and are more focused on establishing a working algorithm.
  • Another participant suggests checking "Boundary-Layer Theory" by Schlichting and "Turbulent Flows" by Pope for additional information, while noting their own limitations in recalling specific details due to their work not involving turbulence.

Areas of Agreement / Disagreement

Participants express differing views on the predictability of transition points in turbulent boundary layers, with some acknowledging the challenges and limitations of existing methods. There is no consensus on a specific approach or formula for calculating displacement thickness.

Contextual Notes

The discussion highlights the complexity of accurately predicting transition points and displacement thickness in turbulent flows, with references to specific methodologies and literature that may have varying degrees of applicability.

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Lets go straight to the point.

I need to find a way to calculate the displacement thickness of a turbulent boyndary layer. The laminar part has been simulated with Thwaite's method but I need to go from there. I'v heard of "Head's-model" but can't find the solution for it. Anyone that can point me in the correct direction or know of a better way?
I'll be using it in combination with a simple panelmethod-algorithm.

Thanks!
 
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Well for starters, how are you determining where the boundary layer is transitioning?
 
boneh3ad said:
Well for starters, how are you determining where the boundary layer is transitioning?

I'm planning to use a semi empirical formula that puts Rex in relation to the shape factor. I't rather long so I won't write out all of it but it looks simple enough to use.
LHS is log(Rex) and RHS depends on the shapefactor alone. When LHS=RHS transition is predicted to occur.
 
Is this on a flat plate? I don't know what formula you are using off the top of my head but in general there is no way to find Re_{tr}. It won't compare too well with experiments.
 
boneh3ad said:
Is this on a flat plate? I don't know what formula you are using off the top of my head but in general there is no way to find Re_{tr}. It won't compare too well with experiments.

I appreciate the help!
I'v realized the problems involved in predicting transition from what I'v read. And no it's not a flat plate.
This particular equation can be found in Aerodynamics for Engineering Students. The book say the method was devised by Smith and H.Gamberoni at Douglas Aircrat Co. It's based on a more complex method of prediction that they found.
I don't expect to find the exact transition point and I'm not too concerned about that at the moment either. Finding the transition point is something that can be alterd later and knowing how innacurate the rest of the algorithm is... let's just say it doesn't matter too much. I just want something to start with and work from there when the basic structure of the algorithm is working.
 
Well at any rate, I can't really help you off the top of my head until I go back to work Monday. There are two sources that jump to mind that may have the information in there, which are "Boundary-Layer Theory" by Schlichting and "Turbulent Flows" by Pope. You may want to check there. I just don't remember off the top of my head because my work doesn't deal with turbulence and my copies of those books are at work.
 
Great, Thank you!
I'll check with the campus library if they have them.
 

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