Calculation of boundary layer thickness

[nalasimbha]

Hello All,

I would like to know how to calculate the boundary layer thickness for a flow through pipes (internal flow). I can find a lot of information of boundary layer thickness for flow over flat plate, but not for internal flow. Any help is appreciated

Thanks

 

[rb1957]

a quick google found this definition for boundary layer ... "a thin layer of flowing gas or liquid in contact with a surface (e.g., of an airplane wing or the inside of a pipe)."

try this ...

http://www.freestudy.co.uk/fluid mechanics/t3203.pdf

 

[btrueblood]

Most pipe flows have a velocity profile that can be termed "fully developed", i.e. the boundary layer extends across the entire pipe diameter. This occurs within a few 10's of pipe diameters from the entrance. The phenomenon is pretty well discussed in most Fluid Mechanics texbooks, e.g.

http://www.amazon.com/Engineering-F...9779/ref=sr_1_1?ie=UTF8&qid=1301336393&sr=8-1

 

[zdas04]

btrueblood,
You're kidding, right? Even entry-level fluids texts talk about the "no-flow boundary" that exists between any flowing stream and a "surface" that is moving at a different velocity (i.e., the pipe wall with v=0, or the control surface on an airplane with a velocity many orders of magnitude greater than the air it is flying through). My graduate fluid mechanics text was titled "Boundary Layer Theory".

These boundary layers are very thin, a few million molecules thick, but they have a profound effect on fluid dynamics.

I have a feeling that the OP was asking a classroom question, so I'm not going to answer his question, but "fully developed flow" is a condition that happens between the boundary layers (if it happens at all).

David

 

[CastMetal]

I think the OP and btrueblood refer to the boundary layer effected zone as the boundary layer. The area where velocity profile is effected by the boundary layer.

 

[btrueblood]

Zdas,

No, not kidding. What castmetal said. Think of laminar flow development in a pipe - in fully developed flow, the velocity profile is a perfect parabola. The whole flow is affected by the "boundary layer". Same thing happens in turbulent flow, though the profiles are fuller. BL flow theory mostly grew up from Couette flow, one of the few fully-solvable Navier Stokes problems...

Boundary layers can be as thin as you say, but can also be dang thick (e.g. the boundary layer on the tail of a WW1 zeppelin, I think the bl momentum thickness (99%) is in meters there, but I didn't do the calc, just new a gal who had . Really depends on the local Reynold's number.