What happens to air in a convergent duct when it reaches supersonic speed?

When air flows through a convergent duct and reaches supersonic speeds, its behavior aligns with specific principles from fluid dynamics. As air accelerates through the narrowing of the duct, it experiences a reduction in pressure. This acceleration leads to an increase in air velocity as it transitions from subsonic to supersonic speeds.

In the context of compressible flow, particularly when dealing with supersonic speeds, the behavior of the air can be described by the principles of conservation of mass, momentum, and energy. As air accelerates, the local pressure decreases in a converging duct. This phenomenon is consistent with the relationship outlined by the Bernoulli principle, which indicates that as the velocity of a fluid increases, the pressure within the fluid decreases. Therefore, when air achieves supersonic speeds within a convergent duct, its velocity increases while the pressure decreases, contradicting the notion that pressure would increase.

This understanding clarifies why the other options are not consistent with the physical behavior observed in a converging duct as air transitions to supersonic speeds.