Choking explanation for pressure variation/reduction

Decreasing the pressure ratio or in actuality the back pressure, results in increase of the entrance and the exit velocity until a maximum is reached for the exit velocity. The maximum velocity is when exit Mach number equals one. The Mach number, as it was shown in Chapter (4), can increases only if the area increase. In our model the tube area is postulated as a constant therefore the velocity cannot increase any further. However, for the flow to be continuous the pressure must decrease and for that the velocity must increase. Something must break since there are conflicting demands and it result in a ``jump'' in the flow. This jump is referred to as a choked flow. Any additional reduction in the back pressure will not change the situation in the tube. The only change will be at tube surroundings which are irrelevant to this discussion.

If the feeding nozzle is a ``converging-diverging'' then it has to be differentiated between two cases; One case is where the $\frac{4fL}{D}$ is short or equal to the critical length. The critical length is the maximum $\left.\frac{4fL}{D}\right\vert _{max}$ that associate with entrance Mach number.

Figure: The pressure distribution for a long 4fL/D