Re–defining variables as
where, one obtains the following dimensionless form of the NS equation:
At the limit of , the 1st viscous term is neglected. However, the 2nd term is retained because
The dimensional form of the NS equation (for x-direction) may now be written for boundary–layer flow (at high Reynolds number) as:
one obtains the following dimensionless form of the NS equation:
, the 1st viscous term is neglected. However, the 2nd term is retained because 
momentum 
