156. How is it that gas moving very rapidly is unable to "communicate" with gas or surfaces in front of it?
When gas is moving slowly through a channel, it can respond to obstacles by flowing around them. For example, when the gas encounters a constriction in the channel, it speeds up to flow quickly through the narrowing and its pressure drops. But when the gas is moving very fast through the channel, it has trouble avoiding obstacles and behaves differently at a constriction. Instead of speeding up to flow smoothly through the narrowing, the gas collides with the walls of the constriction and is pressure rises. It just wasn't able to "sense" the presence of the constriction before it actually hit the constriction. When gas moves faster than the speed of sound in that gas, it can't anticipate changes in its environment and it doesn't follow Bernoulli's equation. That's why the nozzle of a rocket flares outward to handle the supersonic gas that emerges from the nozzle's throat. That high-speed gas experiences a pressure drop as it spreads out into the broad portion of the nozzle. The gas's density drops and its pressure goes down.