Consider a flat smooth surface such as a plate moving through a viscous fluid. Because of the motion some fluid near the plate travels with it. After some distance from the plate, the fluid has no motion, only the plate along with the layer travels. This layer which is moving along with the plate is called as boundary layer or frictional wake.
The movement of the fluid in the wake takes two forms. One assumption is made here: "The fluid next to the surface has no motion relative to it, but is carried with it."
The two forms being either laminar or turbulent.
Laminar means the fluid travels in a series of layer in-between the wake without mixing. For example: the movement of oil is laminar.
Next is turbulent which means there will be intermixing of fluid in between the wake because of the eddies. For example: the movement of water is turbulent.
The type of flow depends upon the inertia forces and the viscous forces. The ratio between these two forces is known as the Reynolds number.
Reynolds number Rn = vl/γ, where
v - is the speed in m/s,
l - is the length of the surface in m,
γ - is the co efficient of kinematic viscosity of the fluid in m2 / sec.
Here the value of the γ depends upon the temperature ( deg Celsius ) . The values of the γ are expressed in terms of cst, (ie) centistokes. γ for seawater = 1.190 cst = 1.190 x 10-6 m2 /sec, γ for fresh water = 1.140 cst = 1.140 x 10-6 m2 /sec.
