Sad that despite all the computer-generated graphics and advances in knowledge we have now, this 1950s documentary manages to outclass everything. Amazing content.
Great video, definitely cleared some things up. Something I found very cool, with just the air hose blowing high speed air at 12:00 you can see dancing diamonds. I first saw these learning about jet engines, when you see the dancing diamonds in the thrust of a jet engine it's running properly. Thanks for posting this video.
And most engines are designed in such a way that you should see 11 of the diamonds to see it is running perfect. The SR-71's mach diamonds are really easy to count, maybe because of the highly unique fuel or something? Also, if the mach diamonds are 'dancing,' that is very, very bad as it indicates instabilities in something that should be running smooth with nearly no changes occurring to the internal physics of gaseous or liquid fluid dynamics.
With reference to Zamaca band comment one month ago - after such an eloquent reasoning... the last paragraph should read: "At high altitudes the speed of sound is lower because of the lower air density."
In gases the speed of sound is actually mostly determined by temperature and molecular mass. higher temperatures and lower molecular masses lead to higher speeds of sound. The air is colder at high altitude, which makes the speed of sound slower than sea level. Hydrogen has the lowest molecular mass, which is why light gas guns (cannons that fire small pellets at hypersonic speed, used for researching the effects of micrometeor hits on spacecraft) use hydrogen as the working fluid that pushes on the projectile. The fundamental reason for this is that the temperature of a gas is proportional to the kinetic energy of the molecules it is made from, so when the temperature is higher (more energy) the molecules move faster and thus can transmit changes in pressure more quickly. Lighter molecules have a higher velocity for a given energy level, which is why gases with a lower molecular mass have a higher speed of sound. The above does not hold true for solids or liquids. In solids the main factor is Young's modulus (and Poisson's ratio), as a result extremely stiff materials like Beryllium or Diamond have extremely high speed of sound, while materials with very low Young's modulus (like rubber) correspondingly have very low speeds of sound. Steel (as in a train track) is relatively stiff, and thus sound travels through it very fast, much faster than air (though much slower than Diamond). For liquids the determining factor is bulk modulus (or compressibility).
Somebody help. The speed of sound depends on material or fluid density: the denser the material the higher the speed of sound (think putting your ear on a train track...). At sea level the air is denser and therefore the speed of sound is higher than at high altitude (any altitude) because the air pressure is less. Yet at 3:00 - and this is not the only youtube location where I find this stated (!), it is said that the speed of sound depends only on temperature, and because the temperature is lower, so is the speed of sound. The opposite is true: the lower the temperature the higher the density, so the higher the speed of sound. At high altitudes the speed of sound is higher because of the lower air density (the temperature may have a negative but not linear.contribution, I am sure there are plentiful studies).
Starting with an incorrect model will lead to incorrect conclusions, even if your logic is otherwise sound. The claim that the speed of sound increases with density (all other things being equal) simply is not correct. For solids, the sound speed squared depends on the ratio of the stiffness to the density. Following your example, the train track has a high sound speed in reality due to the extreme stiffness of metal, not due to its high density. Regarding altitude effects, you are neglecting to consider the Ideal Gas Law and isentropic relation in your analysis. From an x-momentum analysis of an isentropic wave, it can be shown that the speed of sound (squared) is equal to the partial derivative of pressure with respect to density at constant entropy. If p=rho*R*T (ideal gas) and p=C*rho^gamma (isentropic), then it turns out that this partial derivative is just equal to gamma*p/rho=gamma*R*T, where gamma is the specific heat ratio Cp/Cv (Note: this is only true for weak waves! Strong pressure waves like shock waves travel faster than this speed). So in a sense, the speed of sound in air depends on the RATIO of pressure to density, but this is directly proportional to the temperature, which is why we say it depends only on one variable. Hopefully this clears some things up, let me know if I did a bad job explaining anything.
@@AluminumOxide Thanks! I see the video is titled now to where I could figure that out. Apparently, it was not when I asked the question seven years ago. And, with that information I easily found Parts 2 & 3: watch?v=ciIv_7WkPxQ
because every point of the wing is hit by air molecules many times a second. It's like the diapason shown at the beginning, exept instead of hitting it on the table, it's hit by a very fast air molecule.
You did a great job. I very rarely see such good illustrations. I cant thank you enough. I rather see your video first then read a text book. I liked the example where you show how the pressure wave changes the stream line upstream.
