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The entrance region refers to that portion of pipe until the velocity profile is fully developed. When a fluid is entering a pipe at a uniform velocity, the fluid particles in the layer in contact with the surface of the pipe come to a complete stop due to the no-slip condition. Due to viscosity of the fluid, this layer in contact with the pipe surface, resists the motion of adjacent layers and slows them down gradually. For the conservation of mass to hold true the velocity of middle layers of the fluid in the pipe increases (since the layers of fluid near the pipe surface have reduced velocities). This develops a velocity gradient across the cross section of the pipe. ==Boundary layer== The layers in which the shearing viscous forces are significant, is called 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. When the fluid just enters the pipe, the thickness of the boundary layer gradually increases from zero as we move in direction of fluid flow and eventually it reaches the pipe centre and fills the entire pipe. This region from the entrance of pipe to the point where the boundary layer covers the entire pipe is termed as the hydrodynamic entrance region and 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 termed the (hydrodynamically) fully developed region. But this is not the fully developed fluid flow until the normalised 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 :'' (where is the streamwise or flow direction). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Entrance length」の詳細全文を読む スポンサード リンク
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