Fluid Mechanics Lesson 15B: Compressible Flow and Choking in Converging Ducts

Fluid Mechanics Lesson Series - Lesson 15B: Compressible Flow and Choking in Converging Ducts.

In this 14-minute video, Professor Cimbala examines compressible flow from a pressurized tank into a converging duct. He explains why this flow cannot go supersonic, but must remain subsonic throughout the duct. He shows that the only place where the flow can be sonic is at the exit plane of the duct. He then defines equations for critical conditions, which is another way of saying sonic conditions. He describes how flow through a duct chokes when the back pressure is low enough. Finally, he derives the equation for mass flow rate through the duct and does an example problem.

This video incorporates material from Sections 12-2 and 12-3 of the Fluid Mechanics textbook by Cengel and Cimbala, Edition 4.

An Excel file listing of all the videos in this series, along with the annotated notes generated during filming of the videos, can be found at


You can also watch all related short videos with one click by going to one of Dr. Cimbala's playlists:

[kierancaldwell]

You are a god amongst men. My Gpa is single handedly being saved by these lectures

[kadirdoganoymak]

Blessed are those who do their jobs well. Thank you Mr. CIMBALA

[ZayyDogg]

I have to retake compressible because my professor is a bit cocky and also extremely hard to sit and listen to after 3 other classes that day. However you did such an amazing job explaining choked flow/unchoked flow, I'm going to have to binge this during the summer. Thank you!

[lamifcb]

thank you for the videos, they've been so helpful. would really appreciate if you can finish the rest of the chapter soon

[jacobw..]

Thank you for the video, there's one confusion I have, how did you get the p*/po to be 0.5283 in the example when calculating p*? Is there a property table? My college program is not covering these particular concepts.

Also, I'm building a self landing rocket with CO2 cartridges as a propellant source for school capstone project. I'm trying to calculate the mass flow out of those small cartridges, and the choked flow that basically puts a cap on how much thrust force I can get out of them. The thing bothers me the most is the pressure of them (800psi to 3000psi) and the freezing cold temp.(~ -70C) when it's being released. I doubt that these are nearly isentropic nor adiabatic.

Will it still be worth it to try to figure out the choked flow when I have to assume and accommodate tons of properties.

[miguelmartins]

At 1:29 you say the flow must remain subsonic in a converging duct. What if we have a converging duct with supersonic flow, followed by a diverging duct (after the throat) with a subsonic flow? Is it even possible? I saw it in an exercise and it isn´t making much sense to me... Thank you in advance!

[raniaraihan]

Good day, sir. What will be the case when Ma>1? Do we equate the critical properties to the exit values?

[leoygdf]

whats the difference in back pressure and exit pressure? Back pressure what causes the fluid to flow due to the pressure gradient but I don't get how thats different from pressure at the exit. is pressure at the exit not also the same thing?

[halarmad]

How does it work when the pressure in the container is lower than the ambient air? Thank you

[michaelgarcia]

Dont you have to establish that the stagnation conditions in the tank (e.g. pressure, exit area) will be sufficient before analyzing for compressible flow conditions? What are these conditions?

[Sam-dhfn]

I would like to ask in the nozzle is it possible that the Ma is greater than 1?

[Gingerneer]

What happens in the nozzle if you achieve choke flow before the throat? How sensitive are nozzles to this occurring?