# Why cant there be a sonic velocity inside the convergent part of a nozzle? (ie upstream of the throat)

A convergent-divergent nozzle is typically used for accelerating or decelerating airflow to or from supersonic speeds. The typical configuration for such a nozzle is the de Laval nozzle that is depicted below. With red colour is denoted the place (throat of the nozzle) that according to the theory & experiments the sonic velocity is only possible to occur.

I do not understand why this cannot happen upstream of the throat as denoted with blue. Could anyone shed some light on the physical understanding of this please?

I am mainly concerned about why this scenario is not possible:

A subsonic airflow enters the nozzle at point (a.) and then due to the reduction of the area of the nozzle it accelerates and (supposing that this acceleration is enough) reaches a sonic speed at point (b.). I can understand that after reaching the sonic speed the fluid would have to decelerate again back to subsonic speed at point (c.) (because in order to keep accelerating the nozzle would have to be diverging), but this does not forbid the sonic speed to be reached at the previous point. This is where I get confused because if I follow the same reasoning the flow would be accelerated again and then decelerated back and forth which seems a weird outcome of my thought process.

Thank you for the help!

• It is not clear what about your question. Is it 1D, 2D or 3D flow, stationary or time dependent flow, ideal or viscous flow? Commented Jan 2, 2022 at 5:32

The Max number distribution along flow axis (left) and along line $$r=0.3$$ (right) for different time shown in Figure 2
As it shown in Figure 2 there are regions with $$M>1$$ and $$M<1$$ in a divergent part of nozzle.