Does density vary with time in a steady flow? Steady flow is defined as the type of flow where density, pressure and velocity don't change with time, as far as my understanding goes. However, in the video below, they say that the density in a steady flow varies with time. Shouldn't it vary with space, but not time?
Why do they call it steady flow, then?
https://www.youtube.com/watch?v=QBGRknV63qA&list=WL
Thank you for your time!
 A: Imagine you have gas flowing in a wide pipe that feeds into a narrow pipe.
Steady flow means that at some position in these pipes, the density, pressure, and velocity aren't changing in time.
But if you imagine a particular group of molecules in the gas, flowing along the pipe, and more or less moving together, then as those molecules move from the wide pipe into the narrow pipe, they'll speed up and (I'm guessing because I'm an EE, not a fluid dynamics guy) they'll get compressed together, increasing the density of the gas around those particular molecules.
And that somewhat imaginary "group of molecules flowing together" is more or less what is meant by the fluid element.
More succincty: Steady flow means that at each position in the system, the density pressure, and velocity aren't changing. But a fluid element will move around the system, so its density, pressure, and velocity will vary as it moves.
A: At time 2:01 he says Density is NOT a function of time.  He is talking about at a single location (not a moving element) the density is constant.  Later he talks about the element at a different location which is ALSO at a later time having a different density.  It moved to a new location with a different pressure and, therefore, a different density.
Makes perfect sense. I thought he was very clear.  When it moves it can change density.
