Final answer:
In a steam nozzle, when the steam velocity reaches the speed of sound at the throat, the flow is said to be sonic, corresponding to option (c) Sonic.
Step-by-step explanation:
When steam velocity in a nozzle reaches the speed of sound at the throat, the flow is said to be sonic. This is a critical point in fluid dynamics, particularly in the context of nozzle design, where the flow transitions from subsonic to sonic conditions. The speed of sound is the speed at which sound waves propagate through a medium, and when a fluid reaches this speed, it is moving at Mach 1.
The phenomenon where the source exceeds the speed of sound and no sound is received by the observer until the source has passed, leading to a mixing of sounds from the approaching and receding source, results in the creation of a sonic boom. This is relevant because it shows the impact of reaching and exceeding sonic speeds in practical applications, such as jet flight. Supersonic speeds are those greater than the speed of sound, but a flow that exactly meets the speed of sound at a certain point is specifically termed sonic.
To directly answer the student's question, when steam velocity in a nozzle reaches the speed of sound at the throat, the flow is sonic, which would correspond to option (c) Sonic in a multiple-choice question.