Entrance length

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.