Entrance length

In this image we can see that the Shear Stress is maximum right at the entry point and becomes uniform as the fluid flow develops.

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.

单词	Entrance length
释义	Entrance length 英语百科 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.
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