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Introduction to Boundary Layer
The concept of boundary layer was first introduced by L. Prandtl in 1904 and since then it
has been applied to several fluid flow problems.
When a real fluid (viscous fluid) flows past a stationary solid boundary, a layer of fluid '
which comes in contact with the boundary surface, adheres to it (on account of viscosity) and
condition of no slip occurs (The no-slip condition implies that the velocity of fluid at a solid
boundary must be same as that of boundary itself). Thus the layer of fluid which cannot slip
away from the boundary surface undergoes retardation; this retarded layer further causes retardation
for the adjacetit layers of the fluid, thereby developing a small region in the immediate vicinity
of the boundary surface in which the velocity of the flowing fluid increases rapidly from zero at
the boundary surface and approaches the velocity of main stream. The layer adjacent to the
boundary is known as boundary layer. Boundary layer is formed whenever there is relative motion
between the boundary and the fluid. Since zo = , the fluid exerts a shear stress on the
boundary and boundary exerts an equal and opposite force on fluid known as the shear resistance.
According to boundary layer theory the extensive fluid medium around bodies moving in
fluids can be divided into following two regions:
(i) A thin layer adjoining the boundary called the boundary layer where the viscous shear
takes place.
(ii) A region outside the boundary layer where the flow behaviour is quite like that of an ideal
fluid and the potentialflow theory is applicable.
7.1.1.1 Boundary Layer Definitions and Characteristics
Consider the boundary layer formed on a flat plate kept parallel to flow of fluid of velocity
U (Fig. 7.1) (Though the growth of a boundary layer depends upon the body shape, flow over
a flat plate aligned in the direction of flow is considered, since most of the flow surface can be
approximated to a Jlat plate and for simplicity).