Calculating Shock Position in CD Nozzle

[@curtisrichards1903]

Wow! thank you so much for this clear and detailed explanation. I was lost on how we would locate the shock location till now (always seem to forget about iterative methods).

[@whalep]

All file names in capital letters...this guy is a MANIAC! Great vid man.

[@JoshTheEngineer]

Thanks! It's a little unconventional, but I like to write function file names in all caps so they're easier to identify in my directory and the code itself.

[@pradnyaabhang8844]

You explained it so well !

[@JoshTheEngineer]

Thank you!

[@GamePolitik]

Awesome - explained it really well!

[@JoshTheEngineer]

Thanks!

[@Classmates12]

thank you sir for making this video, sir can you add all topics of compressible flow, actually you teach in a very good way, so if you add all topics so it would be more beneficial . Thankyou !

[@JoshTheEngineer]

You're welcome! I'm planning on doing some more compressible flow videos in the future, but I haven't had any time lately. Hopefully sometime soon though.

[@Classmates12]

@@JoshTheEngineer thankyou!

[@randomlythings]

It is, really very helpful. Thank you very much, sir

[@JoshTheEngineer]

You're welcome!

[@eldencw]

I'm sure this question is a bit juvenile, but here goes. For a FFSC gas-gas rocket engine (think Raptor, a full-flow staged combustion engine), if it were able to have the gases fully mixed and burned within the say 4mm from the injector head, could you do away with the Convergent part of the nozzle and have only the injectors and Divergent section? (and get the same or better exit velocity?)

[@JoshTheEngineer]

Good question. The convergent-divergent design for rockets is pretty much as simple as physics will allow you to go. The reason we need to have a convergent section first is that the combustion products (after, like you said, they are fully mixed and burned) will be traveling slower than the speed of sound, or subsonic. In order to speed up a subsonic flow, you need to have a decreasing cross-sectional area. If we left out the convergent section, and just had a diverging section, then based on the area-Mach number relation (see my other video for its derivation), the flow would just slow down. So in order to get to supersonic velocities, and thus maximize the thrust from the engine, we need to first speed up the subsonic flow in a converging section, and then further increase the now-supersonic flow in the diverging section.

[@eldencw]

I guess to get over the hump of Mach 1, the gas flow out of the injectors would have to already be at Mach 1 or faster. So each injector would need to have a miniature convergent section directing the un-burned gasses and that would mean that the actual burn would have to occur while the gases are moving faster than Mach 1 which is extraordinarily difficult as I understand. It stikes me that there should be a better way, but I can't think of it..

[@eldencw]

I think where I get confused is....what is Mach 1 at the molecular level? Is it.... Due to heat, molecules are constantly bouncing off of each other in more or less a sphere of radius R, we will call that sphere a heat sphere...so sound in still room temp air is the collision of one molecule's heat sphere into the next molecule's heat sphere; imparting momentum to the next heat sphere. The first molecule returns to its original heat sphere at velocity Vm (neglecting that the speed is actually Sinusoidal). When the air is flowing, the first molecule returns to the heat sphere where it started plus some distance that the overall fluid has traveled at speed Vf. In sub sonic Vf the molecule's heat sphere returns such that it overlaps its previous heat sphere. As the fluid speeds up, Vf becomes greater than Vm. That is to say at Mach 1 and above, it's heat sphere's never overlaps it's previous heat sphere. ... but I have never seen an explanation like this....so I'm not sure its in the ball park.

[@Nighttdl]

@@eldencw You've never seen an explanation like that coz it's completely all over the place. Even your question about the convergent-divergent nozzle is symptomatic of how little you understood about the whole subject. There are pretty specific formulas and principles behind the definition of the speed of sound and behind the flow behaviour in a subsonic supersonic state, go check them out before flying too high with your fantasies

[@carloolivier1940]

Great video. Is there a simpler whay to calculate the shock location given the back pressure to reservoir ratio using the isentropic tables?

[@JoshTheEngineer]

There is a direct method (that I think I mentioned in the video at some point). This method avoids the iterations, but I didn't make a video about its derivation. It's in my MATLAB code that you can find in the link in the video description.

[@francovaldes4429]

Hello! Could you please mention which is the method that does not use iterations? Could you please recommend a video that teaches this method? Thank you very much, this video is incredible and helped me a lot@@JoshTheEngineer

[@azadehizadi5680]

thanks u helped me a lot by posting this video

[@JoshTheEngineer]

You're welcome!

[@nikohamil538]

"Reservoir" in french means "tank" and in english if i remember well, a large deposit of water or lake. In both cases it means "to store" liquid. Not to process or to combust.

[@mateigrigore833]

Hi Josh! Why is Pb = Pe? If there is a normal shock in the nozzle wouldn't the flow be overexpanded?

[@omarel-aajine8774]

Good lectures !

[@mostafaawad8241]

@josh, please try to reply me as soon as possible, I really need it for my research and I could not find any way to download it, so please provide me a link. thank you so much!

[@mostafaawad8241]

Please reply ASAP with another valuable link, Thank you so much for the understanding!!!

[@yousefammar4342]

i am also looking for this code, please let me know asap

[@JoshTheEngineer]

The website should be fixed now.

[@demsanemsi9387]

Your video gave me a lot of insight. I was trying to apply your method to to this problem ,but i am getting stuck big time . can you help please? :The back pressure to reservoir pressure ratio is 0.7 for a C-D nozzle, with an exit to throat area ratio of 2. Determine the cross sectional area location of a normal shock happening between the throat and the exit section for γ=1.4

[@demsanemsi9387]

For this particular question. do we also need to iterate as you said ?

[@JoshTheEngineer]

You will need to iterate in this problem since you are trying to find the area ratio of the shock location. You should be able to go through the same exact steps I use in this video to solve your problem. You can also download my MATLAB code (in the video description) and change the values to your values to solve the problem.

[@demsanemsi9387]

Thanks . Let me give it a go and see how it goes. Thanks agin

[@BillGates_Alex]

Bruh I'm trying to do this same problem from the book. Did you figure this out?

[@farouqal-sahara9831]

Hi demsa, do u happen to still have the codes?

[@gokceulugsu6708]

you saved lives

[@seepanapraveenkumar8649]

Great Video a lot of learning in this video...mean while I am also working in shock waves

[@JoshTheEngineer]

Thanks, and good luck with your work!

[@SamRogers]

Great video. Can you recommend sources for the design of rocket engine chambers? I have ‘Modern Engineering of Liquid Propellant Rocket Engines’. Also, where did you learn from, and do you work on similar systems day to day? Thanks.

[@JoshTheEngineer]

Thanks! Hmm, I haven't really looked into design of the combustion chamber before. Most propulsion books have chapters dedicated to combustion chamber design, but they might not go into as much specific detail as you want. A lot of them deal with the numerical analysis of the chambers, not the actual detailed design. You can check out chapters 5 and 8 in Rocket Propulsion Elements by Sutton and Biblarz. In chapter 10 of Rocket Propulsion and Spaceflight Dynamics by Cornelisse, Schoyer, and Wakker, there's a few pages on combustion chamber design considerations. There are other references out there, and you'll probably just have to mix and match until you get a full description of what you're looking for. I went to school for Mechanical and Aerospace Engineering, and I'm currently working on my Ph.D. I work with laser diagnostics for my graduate work, so I'm not really doing this gas dynamics stuff on a day-to-day basis. These videos are my projects that I work on over the weekend.

[@ing.anthony7097]

Hello Josh. I was wondering, if i have a big turbine, or a steam turbine or wtv, that's "intaking" air at a pressure of 10bars, and inside it the air will expand, lose some of it's temperature, and leave to the atmosphere. So, what if the expansion ratio was =2, and we have an exit air pressure of 5 bars. This will generate a shockwave because of the difference of pressure of the exit air 5bar and the atm 1 bar? Away from the shockwaves created by a high mach number and high velocities, i was just thinking there must a phenomena for this thing but i forgot it's name.

[@LizGeorge]

Hii, I'm not being able to access your website to get the Matlab code, is there any other way I can access the code?

[@JoshTheEngineer]

Sorry, sometimes my website goes down when WordPress has issues (which seems like a lot of the time). It should be fixed for now, so you can find the code on my website again. I'll probably try to upload all the code I have on my website to my GitHub page at some point too.

[@mostafaawad8241]

heeey did you find a way, if so could you reshare the link???

[@chaoszwc]

Thank you vey much for your videos bro!

[@JoshTheEngineer]

No problem!

[@abdullahyasinyagmur4687]

You are a legend

[@farouqal-sahara9831]

hi josh. I am unable to access the Wordpress website for the reference Matlab codes. Is the website down?

[@JoshTheEngineer]

It was down, but I just fixed it for now, so you should be able to access my website again.

[@farouqal-sahara9831]

@@JoshTheEngineer thanks josh!

[@mostafaawad8241]

heeey did you find a way, if so could you reshare the link???

[@manelsouguir8126]

is A*2 is different from A NS ? and why ? thank you

[@nikan4now]

Great video. Can you possibly not use the red pen...a bit hard to see those equations..thanks a lot

[@JoshTheEngineer]

I agree, it's pretty hard to see. I'll try to stick to my bolder colors in future videos!

[@paulboughosn6174]

Just thank you !!

[@JoshTheEngineer]

You're welcome!

[@abdolkarimmehrparvar6583]

Hi Dear josh I have a question, last night my friend and I were disgusting about the weight of something which is suspended in the air, does a bird which is flying in the airplane has an effect on the weight of the airplane? Thank you for your attention and thanks for your amazing video

[@davidsandell7833]

How about if you were standing in the plane and it went into a zero gravity dive. Would that change the weight of the plane?

[@junuhunuproductions]

You are a legend, lol

[@m.muzammilshahbaz4015]

how to find unknown ratios by rearranging the known ratios. is there any rule to do this? please make a video on this. i am facing problems in solving numericals.

[@JoshTheEngineer]

You just need to make sure all the ratios you use cancel to the final one you want. You'll find these come in handy for aircraft engine analyses too (turbojet, turbofan, etc.). There's no real rule, just think about trying to break up an unknown ratio into simpler components that you can solve for more easily.

[@Sunny-jp3io]

Bro, the link for MATLAB code is not working. Can you pls look into it and share updated link ASAP? Appreciate your reply!

[@Sunny-jp3io]

Can you plz share that code ASAP?. Also can you plz mention how to do pressure ratio plots along its length?

[@mostafaawad8241]

heeey did you find a way, if so could you reshare the link???

[@Vikram21-2K]

what do you mean when u say shock and what is importance of isentropic flow?

[@JoshTheEngineer]

The shock is a normal shock wave, which in the idealized version of the flow, instantaneously changes properties across the shock. Across a normal shock, the entropy increases. This means you can't assume isentropic flow all the way through the nozzle. However, upstream and downstream of the normal shock, you can assume isentropic flow (independently).

[@mattspurgeon2663]

What pages of Modern Compressible Flow can I find the direct method?

[@JoshTheEngineer]

In the third edition, the direct method explanation is given on pages 215-217, with an example on pages 217-218.

[@nick357d]

Slacking on studying for my gas dynamics exam by watching this video....

[@JoshTheEngineer]

I would loosely consider this studying. There are worse things you could be doing with your time!

[@aaronboyer883]

This is amazing thank you

[@hervenb]

good video except Its hard to read the orange expo marker one the whiteboard :(

[@JoshTheEngineer]

Thanks! Yea, I've stopped using the orange marker. It looks okay to me when I'm writing on the board, but in the video it doesn't show up as clear.

[@miladbayernejad9672]

great video, how can i get this matlab code?

[@JoshTheEngineer]

Thanks! The code is available on my website: www.joshtheengineer.com/2018/01/24/normal-shock-in-converging-diverging-nozzle/

[@jamesmoses4810]

This page no longer active?

[@thesamikshashow]

The MATLAB code does not open

[@chrismontgomery2199]

Why does the shock wave occur in the first place?

[@navsquid32]

Isentropic flow, when the flow is supersonic in the diverging section, cannot meet the boundary condition where pressure at the exit is equal to the back pressure, like in supersonic flows. In order for this to occur, a normal shock develops, which is a highly non-isentropic process where the properties of the flow change suddenly. This allows the pressure to begin to rise again in the diverging section of the nozzle. It's like a discontinuity in the flow. It's a rapid negative acceleration of the flow, which becomes subsonic. At higher back pressures, the normal shock will be in the nozzle. At lower back pressures, you can get shocks outside of the nozzle.

[@amatoallahouchen5894]

your videos are really amazing, just one remark you speak too fast! thanks a lot!

[@JoshTheEngineer]

Thank you! Yes, I tend to speak pretty fast, but that's just the way I've always been so it's hard to change.

[@PeterKocic]

x0.75 speed

[@rearea784]

@@JoshTheEngineer It is just good

[@engmustafarafidain8829]

Thank u from Iraq

[@JoshTheEngineer]

You're welcome!

[@jiaweizhang7894]

Hii, I'm not being able to access your website to get the Matlab code, is there any other way I can access the code?

[@mostafaawad8241]

heeey did you find a way, if so could you reshare the link???

[@JoshTheEngineer]

Sorry, there was an error on my website that I just fixed. The code should be available again (I know this is 6 months late).

[@mostafaawad8241]

Hii, I'm not being able to access your website to get the Matlab code, is there any other way I can access the code?

[@mostafaawad8241]

i tried all the other methods you mentioned and all of them are not working!

[@JoshTheEngineer]

Hi Mostafa, I just fixed my website, so you should be able to download it from there. Thanks.