Boundary Layer Thickness
- Since
, it is customary to select the boundary layer thickness
as that point where
approaches 0.99.
- From Table 28.1,
reaches 0.99 at η= 5.0 and we can write
(29.5) - However, the aforesaid definition of boundary layer thickness is somewhat arbitrary, a physically more meaningful measure of boundary layer estimation is expressed through displacement thickness .
Fig. 29.1 (Displacement thickness) (b) Momentum thickness
- Displacement thickness
: It is defined as the distance by which the external potential flow is displaced outwards due to the decrease in velocity in the boundary layer.
Therefore,(29.6) - Substituting the values of
and
from Eqs (28.21a) and (28.19) into Eq.(29.6), we obtain
 |
or,
 | (29.7) |
Following the analogy of the displacement thickness, a momentum thickness may be defined.
Momentum thickness ( 
): It is defined as the loss of momentum in the boundary layer as compared with that of potential flow. Thus
): It is defined as the loss of momentum in the boundary layer as compared with that of potential flow. Thus  |
  | (29.8) |
and 
from Eg. (28.21a) and (28.19), we can evaluate numerically the value of 
for a flat plate as
 | (29.9) |
have been shown in Fig. 29.1.