Chapter 25 Compressible Fluids

cornsnicker3 · Jan 11, 2018

Good day everyone,

I just finished the problems in the MERM for this chapter, but really felt like it kicked my butt. I am especially weak with the T, T0, and T* concepts. I did Ok in the first part of the chapter, but the second part is really painful. Do you have any recommendations for really learning this stuff well? Should I just go through the example problems over and over until it sinks in?

Thank you,

Audi Driver P.E. · Jan 11, 2018

Sorry I can't be much help here. I had a combustion engines class in school that broke down compressible flow in ways that made it quite easy to understand and calculate. There was no text book for the course (at the time). All we used were the professor's hand written notes (compiled into a huge compendium spanning many topics). So, I guess the main point of the post is that there are alternate ways to calculate compressible flow problems that aren't quite the rocket science level. Perhaps you can find a text on it?

Ramnares P.E. · Jan 12, 2018

Fundamentals of Compressible Flow by Yahya is a good place to start.

If you're looking for compressible flow problems, I believe the 2500 Solved problems in Fluid Mechanics & Hydraulics should have some.

Slay the P.E. · Jan 12, 2018

For the purposes of the PE Exam, getting a whole book on compressible flow could be a bit of overkill (most of the stuff in such a book is grad school-level). The kind of problem you’re likely to encounter (1D isentropic nozzle flow and/or normal shockwaves, etc) don’t justify the expense or the anxiety.

The treatment of this subject in the Schaum’s Outlines Fluid Mechanics book by Merle Potter is more than adequate and provides plenty of exam-level problems (and more complicated than exam-level too). This book is just $23 in Amazon.

https://www.amazon.com/Schaums-Outline-Fluid-Mechanics/dp/0071487816

Audi Driver P.E. · Jan 12, 2018

Slay the P.E. said:
For the purposes of the PE Exam, getting a whole book on compressible flow could be a bit of overkill (most of the stuff in such a book is grad school-level). The kind of problem you’re likely to encounter (1D isentropic nozzle flow and/or normal shockwaves, etc) don’t justify the expense or the anxiety.

The treatment of this subject in the Schaum’s Outlines Fluid Mechanics book by Merle Potter is more than adequate and provides plenty of exam-level problems (and more complicated than exam-level too). This book is just $23 in Amazon.

https://www.amazon.com/Schaums-Outline-Fluid-Mechanics/dp/0071487816

While this is true, every good Mechanical Engineer should have a good reference for compressible flow that is understandable. If the section in the MERM is too high level, there are other sources out there that aren't.

Ramnares P.E. · Jan 12, 2018

Agree @Audi driver, P.E.. I was recommending the book as a good resource. For the purpose of the exam, working the additional problems should suffice.

Good recommendation @Slay the P.E. regarding Schaum.

casummerlin · Jan 13, 2018

I wouldn't worry about it. It'll be a lot of work for not many points on the exam. There's a reason it's usually thrown in as an afterthought in most fluids classes.

cornsnicker3 · Jan 14, 2018

It sounds like the MERM is adequate enough for this particular topic. I will just try to work the MERM problems a couple more times through.

Slay the P.E. · Jan 14, 2018

casummerlin said:
...not many points on the exam.

Indeed. However, the two questions you might get on gas dynamics will be absurdly easy (as long as you understand the topic). It would be a shame to let low hanging fruit go like this.

casummerlin · Jan 14, 2018

Slay the P.E. said:
Indeed. However, the two questions you might get on gas dynamics will be absurdly easy (as long as you understand the topic). It would be a shame to let low hanging fruit go like this.

I agree. And the MERM was sufficient for me.

Sent from my SM-G950U1 using Tapatalk

Audi Driver P.E. · Jan 15, 2018

casummerlin said:
I wouldn't worry about it. It'll be a lot of work for not many points on the exam. There's a reason it's usually thrown in as an afterthought in most fluids classes.

The reason it's "an afterthought in most fluids classes" is because it's comuptationally intensive to do it properly. Good approximations can be made using simple calculations, but to explain how they are derived (the purpose of undergraduate classes) could take up the entire course.

Chapter 25 Compressible Fluids

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