Why is it important to have a Mach number=1 at the throat of Convergence-Divergence nozzle? What if the Mach number at the throat is less than 1? Is a modification in the nozzle design required then?
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Supersonic flow expands when accelerating while subsonic flow contracts. This is due to the increasing influence of density changes: While subsonic flow shows litte change in density over speed, in supersonic flow the density changes become dominant.
If the speed at the throat is still subsonic, it will drop further as the nozzle expands again. Only when it has reached Mach 1 in the throat, the flow will be able to accelerate further once the cross section expands downstream.
Peter Kämpf
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