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88 Aeronautical Engineer’s Data Book
Values of Re for 2000 < Re < 2300 are gener­
ally considered to result in transition flow.
Exact flow regimes are difficult to predict in
this region.

5.4 Boundary layers
5.4.1 Definitions
The boundary layer is the region near a surface
or wall where the movement of the fluid flow
is governed by frictional resistance.
The main flow is the region outside the
boundary layer which is not influenced by
frictional resistance and can be assumed to be
‘ideal’ fluid flow.
Boundary layer thickness: it is convention
to assume that the edge of the boundary layer
lies at a point in the flow which has a velocity
equal to 99% of the local mainstream
velocity.

5.4.2 Some boundary layer equations
Figure 5.9 shows boundary layer velocity
profiles for dimensional and non-dimensional
cases. The non-dimensional case is used to
allow comparison between boundary layer
profiles of different thickness.

Dimensional case Non-dimensional case
Edge y Edge
∼
y 0.99 U∼U 1 of BL δ = y u = 1.0 of BL

u δ u
u = 1.0
�� y y
∂u
∂y y=o
u u
= u
U 1
Fig. 5.9 boundary layer velocity profiles