Great question—this is a super common point of confusion when diving into nozzle flow dynamics, so let's unpack this clearly.

First: Your core assumption is valid—stationary normal shocks do exist in converging-diverging nozzles under specific conditions #

When we assume a shock is stationary in the nozzle, this isn't just a mathematical trick for solving problems—it reflects real, observable flow behavior. Here's why:

• Reference frames are key: You're right that a normal shock propagates at supersonic speed relative to a stationary fluid. But in a nozzle, we're working in the reference frame fixed to the nozzle itself. If the upstream flow is moving at supersonic speed toward the shock, the shock can remain stationary relative to the nozzle. From the shock's own reference frame, the incoming flow is supersonic (matching the "shock moves faster than sound" rule you noted), so all physical laws hold perfectly.

• Stable shock conditions: When the nozzle's backpressure (the pressure outside the nozzle exit) falls within a specific range, a stable normal shock forms in the diverging section. Here's the sequence of events:

  1. Subsonic flow accelerates to sonic speed at the nozzle throat.
  2. Flow becomes supersonic in the diverging section, expanding and decreasing in pressure.
  3. The normal shock abruptly decelerates the supersonic flow to subsonic.
  4. The now-subsonic flow decelerates further in the remaining diverging section, increasing pressure to match the backpressure at the nozzle exit.
     This entire state is stable—if backpressure stays constant, the shock stays fixed in one position in the diverging section.

• Shock movement with changing backpressure: If backpressure shifts, the shock will move (e.g., higher backpressure pushes the shock closer to the throat; lower backpressure moves it toward the exit, or even out of the nozzle entirely). But as long as backpressure is held steady in the right range, the shock remains stationary.

To tie it all together #

Your observation about supersonic shock propagation is correct, but it doesn't conflict with stationary shocks in nozzles—it's just a matter of choosing the right reference frame. The stationary shock assumption is grounded in real fluid dynamics, not just a simplification for problem-solving.

内容的提问来源于stack exchange，提问作者john melon