Compressible Flow: Four Solved Example Problems (including Rocket Thrust Calculation!)

[@Ryan-og8gf]

Please keep making videos, all of them are great. Perhaps one about propulsion in a vacuum or low-pressure i.e. propulsion systems in space, hyperloop cold gas thrust propulsion etc.

[@VDEngineering]

Watch my rocket nozzle parts 1,2,3 where I talk about nozzle expansion

[@lalitdeshmukh8665]

Good VD keep it up . And try to improve audio quality

[@thobi6639]

Q1 Sonic Boom 0:30 Q2 Shockwave 2:03 Q3 Fanno Flow 4:52 Q4 Rocket Nozzle 7:55

[@obtsfan]

Hey, thanks for making this video, it looks great! Problem 4 has been very helpful. For problem 1, however, I am getting a different answer. I get that the jet will make a Mach cone with that angle and shape, but the time that you calculated is the time that it would take for the jet sound produced at that point in your image to reach the observer. But when the jet is directly overhead of the observer, it will still be producing sound that will be emanating in all directions, including straight down to the observer. Those sound waves will reach the observer with the sonic velocity, traveling the distance of just 1000 ft. The time I got was 0.904 seconds. Another way to think of it is, the vertical component of the sound wave velocity is going to be 337.2 m/s, whether the jet is stationary or traveling at Mach 5. So the horizontal speed shouldn't affect how long it takes for the sound waves produced when the jet is directly above the observer, to reach the observer. I think what you calculated is the time that it would take for the sound waves produced later on, to reach the observer, but earlier sound waves would have reached the observer long before that. This is kind of like a falling object trajectory. When an object is dropped from 10 m with zero horizontal velocity, it hits the ground in the same amount of time as the same object dropped from 10 m but with a large horizontal velocity. Intuitively, one might think that giving the object a horizontal velocity would make the time to hit the ground longer, because it travels a larger overall distance, but the vertical distance traveled is still the same (10 m), and the vertical component of its acceleration is still the same (9.81 m/s^2). In this case, the vertical distance that the sound wave travels is still 1000 ft, and the sonic velocity is still 337.2 m/s straight down, regardless of horizontal airplane velocity. I think a good problem to demonstrate the effects of a Mach cone would be to calculate how far the plane has flown past the observer by the time that the observer hears the plane. That distance would increase with jet velocity, and that's the concept of a supersonic jet that throws observers off. They aren't waiting longer than normal for the sound to get to them, it's just that the plane is long gone by the time they hear it because it's traveling faster than the sound can reach the ground.

[@obtsfan]

Also, shouldn't fL*/D have a 4 in front of it? In my notes from a gas dynamics class, Fanno tables, and wikipedia, it says 4fL*/D equals that equation. If the 4 should be there, then L* = 1.67 m < 2 m, meaning that the flow is choked. Or am I missing something?

[@obtsfan]

Oh. In some countries and professions, the f represents the Fanning friction factor, which is equal to 1/4th of the Darcy friction factor. Hence why we use 4fL*/D, or 4(f_F)L*/D where f has a subscript F for Fanning. Other countries and professions, the f represents the Darcy friction factor. Hence why they use fL*/D, or (f_D)L*/D where f has a subscript D for Darcy.

[@obtsfan]

At 14:38, the T1 looks like it was calculated with a T0/T1 ratio of 2.647, which is the ratio calculated if gamma = 1.4. But since gamma = 1.3, T0/T1 should be 2.236, giving a T1 of 1118 K. Then T2 would be 2423 K.

[@danielwalker5682]

Great work.

[@mamleshbohari2200]

Can u explain the complicated mechanics on a jet engine which regulate the fuel injection and such things?

[@motasembillah8763]

Anyone know where the formula sheet is posted? Couldn't find it in the video description or his website.

[@charlestran7309]

At the p0/pe =... equation, shouldn't there be a minus sign at the power so to the power of minus 1.5/1.5-1?

[@VDEngineering]

yes , typo

[@charlestran7309]

@@VDEngineering your welcome

[@obtsfan]

Actually p0/p should have a positive power, the only typo there (10:34) is that gamma is 1.3 so it should be 1.3/1.3-1. The p0/p equation at 12:54 is correct, with the positive power. But T0/T at 12:54 should not have a power of -1, it should have no power, or the power should be 1. When the fraction is static pressure (or temperature) over the stagnation pressure (or temperature), then the fraction is less than 1 so the power is negative (static values are always less than or equal to stagnation values). When the fraction is stagnation pressure (or temperature) over the stagnation pressure (or temperature), then the fraction is larger than 1 and the power is positive.

[@shutup1209]

do you have a group on facebook ,,??

[@VDEngineering]

I will make one at 5000 or 6000 subscribers

[@ssaikrishnasubudhi3373]

how did u calculate normal shock area.in the 4th question.... plzz rply sir... thanku..

[@panteleimonstamatakis787]

Hello Mr Vinayak . I have a question I want to ask but it it is quite long to be written here. Is it possible to personally contact you?

[@fredmaster5343]

Hello sir I am right now researching on the recent cryogenic rockets India has and I intend to upgrade some of those rockets but I am stuck with some formulas can you please help me in this?

[@josephtraverso2700]

Problem #3, why are you using fL/D instead of 4fL/D? I found ~0.3339 from the table as well, but if you use 4fL/D the flow is choked

[@kashyappadiya1476]

Hello sir! I am your fan from india and I am completing my bachlor thesis in germany and thesis is related to raptor rocket engine having methane oxygen fule. The my hand calculations are done as per the information available in internet about raptor engine fule inlet is 525.3kg/sec and my main problem in cfd simulation is what density should i use? (Simulation in ansys) as the mixture ratio of methane and oxygen is 3.8 i thought "volume weighted mixing law" density should be perfect and accordingly i got density value of 0.79(in inlet reference values) which i thought is good as methane and oxy will be gas,but when i calculated i didnt got the flow i want, and initially i tried with idel gas and i got the flow properly which i want but when i am using idel gas then the density reaches to 16kg/m3. Which i think is not correct so which is better for me? There are 9 ways for type of density selection. My mesh is very uniformed and will not have any problem with divergence detected etc.. i wlill be very happy if you guide me

[@kashyappadiya1476]

May be it is not problem with density, it can be problem with my geometry. Because Initially when i tried with idel gas and i got the flow at that time the geometry i used was having CURVED THROAT but later i changed the curved throat into a SHARP EDEGE throat between convergence and divergence area, so maybe that can be a problem.