flow through expansion of pipe why it must fill all area of cross section of pipe at steady state

[gsunil]

in a flow through expansion of pipe suddenly , why does fluid expand whole pipe,at steady flow it fills whole pipe and velocity decreases. why cant fluid flow straight without filling entire cross section as it did before keeping the air above or below which is at the same pressure and exit without change in velocity.
when air gets disturbed also as it has space to flow without being disturbed why does it fills and reaches steady state.
i want to know why and how it happens as it attains steady state.

 

[MortenA]

Is this a physical "problem" or are you referring to a simulation using a specific software package (and perhaps the error log)?

 

[BigInch]

1.) It must because, there is no air present at that location. Where did the air come from? If there was no air and the pipe was flowing partially full, it would be an indication that the "empty" space is actually full of water vapor. That means that the pressure would have to be less than the water's vapor pressure. Water's vapor pressure is so low that it doesn't happen everywhere, although it happens a lot outside of a pipe. So, if the pressure is higher than water's vapor pressure, the pipe is forced full of water, given that there is no air. Total vacuums are not allowed next to any fluid.
2.) It does not have to, if there is air present at that location.
3.) It must because, there is no air present at that location and the pressure is greater than the water's vapor pressure. Therefore the water will not vaporize to fill the entire diameter.
4.) It must because, the hydraulic grade line; total energy of the water at that point, is greater than the energy required to fill the pipe.
5.) It does not have to, when the hydraulic grade line is lower than the top of the pipe. There is only enough energy to fill the pipe part way.

Independent events are seldomly independent.