text: The layer in which the shearing viscous forces are significant, is called the boundary layer. This boundary layer is a hypothetical concept. It divides the flow in pipe into two regions:
  # Boundary layer region: The region in which viscous effects and the velocity changes are significant.
  # The irrotational (core) flow region:  The region in which viscous effects and velocity changes are negligible, also known as the inviscid core.
  When the fluid just enters the pipe, the thickness of the boundary layer gradually increases from zero moving in the direction of fluid flow and eventually reaches the pipe center and fills the entire pipe. This region from the entrance of the pipe to the point where the boundary layer covers the entire pipe is termed as the hydrodynamic entrance region and the length of the pipe in this region is termed as the hydrodynamic entry length. In this region, the velocity profile develops and thus the flow is called the hydrodynamically developing flow. After this region, the velocity profile is fully developed and continues unchanged. This region is called the hydrodynamically fully developed region. But this is not the fully developed fluid flow until the normalized temperature profile also becomes constant.
  In case of laminar flow, the velocity profile in the fully developed region is parabolic but in the case of turbulent flow it gets a little flatter due to vigorous mixing in radial direction and eddy motion.
  The velocity profile remains unchanged in the fully developed region.
  Hydrodynamic Fully Developed velocity profile Laminar Flow :  
  where  is in the flow direction.