Boundary Layer Thickness 2

[Sparweb]

Hello,
I'm trying to reconcile what I read about boundary layer thickness, from some sources, with the velocity distribution/profiles from other sources.
I've been using Anderson, Intro to Flight; White, Fluid Dynamics; Hoerner; Fluid Dynamic Drag; and a bundle of NACA reports, but I can't put the two together.

In some places, the BL thickness is given as a simple formula. Often like this for turbulent layer:

d = 0.37 x / (Re_x)^0.2 (Anderson eq. 4.99)
(it's 0.36 in Hoerner Figure 2)

That equation implies there is a distinct boundary layer; below you are in it, above you are out of it. I know that's not true.

Other sources give a velocity profile... parabolic-looking for laminar and logarithmic-looking for turbulent.
That suits what I understand about the BL better, but it is of course more time-consuming to calculate.

Is there a relationship between the two methods? IE; does the simple equation correspond to a particular part of the velocity distribution? Is it 90%? Is it 99%?

Hopefully someone can provide a written source that I can use to substantiate this.



STF

 

[WKTaylor]

SW... concur... I knew that...d'hoo......

I'd have suggested AGARD Flight Test & Instrumentation series IF I'd known that element was part of your overall interests, up front.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

No problem Wil! Leave me a trail of breadcrumbs, I'll find my way out of the woods. Thanks again.
Finally got the contract signed, and fleshed out the detailed program requirements with the customer (up until now we've been speculating on the scope of what they need, based on sketchy info from the request for proposals). Looks like they want instruments on the fuselage AND under the wing, after all. This will be even more fun than I thought. First phase is feasibility analysis, not drilling holes.


STF

 

[Andries]

Hi Sparweb,

As far as references are concerned, there is also the classic text of Hermann Schlichting: Boundary Layer Theory. I think it is now in its 6th edition.

Andries

 

[Sparweb]

Hello Andries,
It is a pleasure to hear from you. Thank you for the book reference.

PS
My condolences on the passing of your friend. To me, he was a mentor that shaped my career.
We had exchanged many letters for many years, and his training notes still have a place within reach on my desk.

STF