In the supersonic regime , the factor in the parenthesis of the lhs of equation (2.88) is positive. This leads to the fact that the velocity increases () as long as the cross-section increases (). On the other hand, in the subsonic regime, the velocity decreases () while the cross-section increases (). See right panel of 2.5.
If at the point of minimum cross-section (throat), two curves for and have an intersection. In this case, gas can be accelerated through the Laval nozzle. First, a subsonic flow is accelerated to the sonic speed at the throat of the nozzle. After passing the throat, the gas follows the path of a supersonic flow, where the velocity is accelerated as long as the cross-section increases.
Kohji Tomisaka 2012-10-03