Why does laminar flow turn turbulent(*) in external fluid flow and why doesn't a fully developed laminar flow turn turbulent in flow through pipes? (*:with increase in distance from the leading edge.)
I would appreciate if you give detailed explanation for the first question and then apply the same logic to answer the second.
 A: My understanding is as follows.
Fluid passing the leading edge nearest to the surface is slowed by viscous forces between the surface of the body and the free stream. Adjacent layers of fluid in the stream consequently move relative to this reduced flow, and are hence affected by viscous forces themselves. The result is that the adjacent layers of fluid flow most slowly closest to the surface, but are less affected at a greater distance therefrom. The boundary layer is essentially defined as the limit of where the fluid stream is (99%) unaffected. Differences between the velocities of the successive layers of fluid create variations in pressure and density, and at some point from the leading edge, these differences cause the flow to become turbulent.
In a pipe, the more distant layers merge. The velocities of the successive layers (note that these, of course, are conceptual rather than literal) still vary though, and turbulent flow still occurs at some velocity.
To address a subtlety in your question: laminar flow becomes turbulent with an increase in distance from the leading edge because the effect of fluid viscosity is progressive. Imagine the passing fluid being comprised of three adjacent layers - inner, middle and outer. The three layers in the stream reach the edge simultaneously. As the inner layer begins to flow over the surface, viscous forces slow it. However, the effect is not instantaneous; it must interact with the surface over some distance before it reaches its lowest velocity. The middle layer is then affected by viscous forces from the relative difference of the velocities of the two layers, inner and middle. Once again, the effect is not instantaneous. Finally, the outer layer is affected by viscous forces due to its difference with the middle. The result of these interactions is a boundary layer with a parabolic profile, compact at the end and broadening the further it travels from the leading edge. By comparison, a pipe necessarily controls the breadth of the boundary layer, and hence variations in pressure and density that causes turbulence.
