I agree. I just thought we did not have to go back that far. I thought this was a given.Ogge wrote:Standing waves DONT move from the tailpipe! Standing waves DONT MOVE, they are standing! ONLY, only, only traveling waves move down the axis!
Ogge wrote:The traveling waves are the only axial moving energy source. Standing waves generate transverse mechanical vibration of the walls. The standing waves is dampening the energy of the traveling waves by radiating it externally where it energizes the air boundary arround the tube. This tube vibration creates longitudinal waves (in the air) perpendicular to the waveguide on the OUTSIDE of the waveguild as SOUND.
Umâ€¦ We do need to go back, it seems. Thereâ€™s a misunderstanding here that needs to be resolved. The term â€˜standing waveâ€™ is a contradiction in terms. It really is standing, but it is not a wave.
But â€“ and it is a big â€˜butâ€™ -- it is the indication that real, moving waves are about. We are talking of the standing wave, all the time knowing that it is just a representation of something else â€“ of a certain combination of waves moving to and fro along a conduit. Itâ€™s a kind of shorthand.
This is nothing unusual. We employ the same kind of shorthand when we talk of the centrifugal force, which does not really exist either, but is just a representation of something else.
I find it strange to hear of the standing wave described apart from the traveling waves, as if one were transversal and the other axial. We have just concluded that there is no such thing as a standing wave, havenâ€™t we? It is just a concatenation of moving waves.
Furthermore, those moving waves are anything but axial. They may look axial while they are moving down a conduit, but they are really spherical. Given the chance, they will move in all possible directions. Look at a Schlieren photo of a blast coming out of a pulsejet tailpipe and you will see a perfectly spherical bubble.
I agree. Better to say, there is plenty of gas movement â€“ otherwise there would be neither thrust not aspiration -- but it is not attributable directly to the passage of the wave. The major movement of gas is the result of equalization of gross imbalance of pressure along the conduit. Thereâ€™s great pressure at one end of the duct and no pressure at the other. So, gas flows from the high-pressure end to the low-pressure end.Ogge wrote:There is no movement from node to antinode to node
That said, however, the standing wave (i.e. the combination of moving waves!) still plays a great role. It regulates the process, setting its timing and giving it additional energy. It takes energy from the combustion process, stores it and releases it back into the process. It is like the flywheel on the piston engine.
The real picture of events is very complex. We are watching at least three interweaving phenomena. One is the process of combustion in a vessel of quasi-constant volume. This generates the second phenomenon -- the cycle of heating-expansion-cooling-condensation of gas in a duct. This, in turn, generates the third â€“ the establishment of resonance. The latter is amazingly similar to sound-wave resonance, only involving much greater pressure amplitudes than normally associated with sound and taking place under great changes in temperature.
Now, those three phenomena (and no doubt a few others besides) are bundled tightly together in a process in which they are not only coexisting but also interdependent.
This takes us back to the thing you said before, which made me respond vehemently in the first place. You simply cannot say that one of those aspects is a byproduct of another. It is like saying that you just need muscle to lift weights. No, you also need the bones to lend muscles support and you need joints to allow articulation and you need cartilage to keep them all together. Neglect one of those elements and the whole structure will start creaking and will soon collapse.
In other words, dampen the vibration, as you suggest, and you will dampen the pulsejet cycle. Do it too well and the cycle will collapse. The oscillation that you find superfluous is the very reason the thing works at all.
We may be talking at cross purposes here. Please note that I am not talking of the resonance of the duct itself, but of the resonance of the gas column in the duct. The standing wave I am talking about would be established even in a duct carved in a block of something completely non-resonant. It has nothing to do with the properties of the duct material.Ogge wrote:Standing waves cause the entire length of the tube to vibrate this is resonance. Resonance is the vibration of a waveguide. Its a mechanical force. These oscillation travel radially out from the axis of the tube. A pipe organ or tuning fork does not emit sound from the end of the tube, it is radiated outward down the entire length.
Maybe one can talk of dampening the vibration of the duct without affecting the vibration of gas. Maybe that would reduce the noise without reducing the gas pressure swings. If that can be done, Iâ€™d certainly welcome it. I donâ€™t know enough about the problem to say anything. Maybe you know more.
Noise is certainly an unfortunate by-product. It is more intense here than in many other jet engines, precisely because of the nature of the pulsating combustion. I agree with any effort to dampen noise, but dampening its origin -- the very oscillation that makes the pulsejet function -- is a bit counterproductive, donâ€™t you think?
BTW â€“ I am far from certain that what you say of organ pipes is true. I vaguely remember hearing somewhere that the material used for organ pipes is picked precisely for being non-resonant, so that the resonance of the pipe does not interfere with the resonance of the air inside it. But, as I said, I know next to nothing about the resonance of solids.