## Stainless FWE with small intake runs at -3 F !!!!!!!!!!!!!!!

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Ogge
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Eric wrote: HAVE YOU EVER EVEN TAKEN A PHYSICS CLASS? You claim to be a PDE engineer but you seem more like an Ivar type to me.

Speed = wavelenght * frequency
Frequency = speed / wavelength
wavelenght = speed / frequency
As a matter of fact, I attended Georgia Tech and Southern Polytech. I am a degreed electrical engineer with Engineering Mathmatics minor. I graduated with HIGH HONORS. Electrical engineering is a specialization in physics. I worked my way from junior engineer to Sr engineer with Lockheed Martin Aeronautical Systems Co. at the F-22 and C-130 plant in Marietta Ga.

The formulas posted above are for use in a constant medium environment for the behavior of waves and requires movement ie the term 'speed'. We have an delta speed which causes a delta wavelength of the molecular oscillations in the "traveling wave" due to the tempature environment of the medium of travel.

A "Standing wave" has properties like a wave but it is NOT a true wave, it is standing still, it has NO MOVEMENT ie no velocity, its just the net vector addition of the real waves that are moving.

Only the "traveling waves" that create the standing wave have movement down the axis of the tube, which corresponses to the appearance of a frequency change in the "standing wave".

This traveling wave is moving thru the medium which has different properties due to tempature and pressure fluxuations. The molecules traveling in the wave are the transport medium of that energy. When a molecule gains more energy, the Aplitude of its oscillation is increased, not its frequency of oscillation.

A "standing wave" only oscilates transversly from side to side setting up a vibration in the waveguide generating traveling waves towards the outside observer due to the vibration. All it is is a noise generator that is stealing energy from our cycle and turning it into sound.
Eric wrote:
FREQUENCY AT WHICH A CLOSED END PIPE WILL RESONATE ( FIRST HARMONIC) :
f=v/4L

f (frequency of sound (Hz)) = v (
velocity of sound in air (m/s)) / 4 L (lenght of tube in meters)

We are talking about 2 different things here. You are talking about external noise generating properties. Im talking about "traveling waves" that are making up the "standing wave". The velocity term in the above equation is the velocity of the "traveling waves" that created the "standing wave". A "standing wave" by definition has NO velocity ie standing still.

Of course the "standing wave" is changing its appearance of frequency but that does not imply that the "traveling wave" is changing frequency. The same frequency travels at different wavelengths based on the medium its traveling thru. A 1 khz sound under water has a different wavelength then 1 khz sound traveling thru air of the same tempature. Its the wavelenth (distance covered per unit time ie speed) of the oscilation that is altered by a change in the medium of propogation.
Eric wrote:THE SPEED OF SOUND IS AFFECTED BY TEMPERATURE!!!
Obviously. Since the wavelength (distastance per cycle) of the resonance chamber is fixed due to physical geometry it cant change and the tempature changed, the resonance frequency of the resulting "standing wave" has to change its the only variable that can change.

But conversely if there has been a frequency change in the resonance, there must have been a coresponding change in wavelength in the "traveling wave" to account for this.
Eric wrote: DID YOU GET YOUR ENGINEERING DEGREE BY MAIL? ...
There is no need to SHOUT or to be insulting obviously we misunderstood each others refrence to "frequency".

We were each refering to a different frequency one of "standing wave" vs "traveling wave". Any you are applying speed to a sudo wave that is standing still.

When I think of "frequency", I think of the "traveling wave" frequency as a key component and the standing wave as just the outward observable results of what is going on below the surface.

I guess thats my EE background, we dont deal with "standing waves" very often, but we deal with alot of traveling waves. We generate the "traveling waves" to drive a system (Radio for example).

A "standing wave" is generaly the result of screwing up our power transfer. For example, a mismatch of impedance from a transmitter to the antenna can cause a "standing wave" to reflect back into the transmitter. It can also cause heating of the waveguide or burn up the trasmitter as well as a loss of transmitted power. It is generally our goal to prevent "standing waves" as a useless waste of energy.

The "traveling wave" frequency can tell us about how the waves will reflect, refract, diffract in our environment therefore it is of critical importance to understanding pulsejets and their designs.

Appearently you are just interested in the outside observable effects of the noise generation. Granted the standing wave may have its uses too. It might be giving us some clue as to what physical geometry of the tube lengths are valid for self sustaining. It just appears that most people on this board tend to totally disregard the traveling waves or not fully understand their importance.

Part of why I even brought this up is to get a chance to try to pass on that importance. I have always been a big fan of understanding WHY something works the way it does instead of memorizing formulas without understanding. In order to do that you need to know the root causes as well as the effects they have.

Eric wrote: You claim to be a PDE engineer but you seem more like an Ivar type to me.
BTW, what is an "Ivar type"?
Last edited by Ogge on Thu Jan 27, 2005 6:16 pm, edited 6 times in total.
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Mike Everman
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

There is a little frustration here that I have been feeling too. Your tone is lecturing and not the least bit humble. Perhaps just tone down a bit and we'll all have more fun.
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larry cottrill
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

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EDIT: I wrote the following over a long period of time while doing other work at my day job, and now that I see your response to others, I have a better idea where you're coming from. At the submicroscopic level that you're thinking about, I'm sure you're basically correct. I think it's often difficult to see it that way when you're used to thinking at the broader scale that we're used to. So, consider this what I was trying to say while I was misunderstanding what you were talking about ;-)
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While I cannot claim the educational credentials of these young scholars, I have to basically agree with them on this issue, from the standpoint of experience. I think the disagreement on this point comes from the fact that you are used to walking comfortably in a realm where it is often completely reasonable to view frequency as the independent variable from which everything else seems to be derived. In that world, for example, you can't pass a signal through anything that alters the signal frequency, but you can do just about anything you can imagine to the wave form. The same thing holds true for me in thinking about the behaviour of light - in the realm of electromagnetic propagation, frequency is king. This is mostly because the "medium" never varies [again, the value C always applies, in the absence of any evidence of refraction or alterations in the medium - this is why I was interested in whether radio waves are ever refracted].

But in vibrating pipes, alas, it is not so. Our pipes are more like a mechanical kind of VFO [variable frequency oscillator], where frequency of operation is a dependent variable - a function of physical constraints that can be adjusted. My Dynajet, for example, starting cold and running up to a fully heated state, definitely goes up in frequency by several notes of the musical scale. It is ALWAYS doing quarter-wave oscillation, but the frequency varies with the ever-increasing absolute temperature of gas in the pipe. Its internal wavelength is determined by the pipe length, and can't change much [a little bit, perhaps, due to the slight lengthening of the pipe with temperature].

Now, once the pressure wave is handed over to the outside world, we have something more like "normal" propagation, because we have a medium that doesn't usually change in any noticeable way. In such a "constant condition" scenario, frequency surely dominates. Even if we change the condition, there's still nothing we can do with frequency per se at that point.

I guess the rule is that anywhere we CREATE wave action, we can control frequency by altering physical attributes of one sort or another [in our case, physical geometry], but where we PROPAGATE waves through space, frequency becomes independent, and all we can do is filter, obstruct, reflect, re-shape, etc. And, I guess we can set up resonances somewhere to create harmonics to mix in, as well, if we like. It's all just filtering of one kind or another at that point, though.

We have to view our hot pipes as positive feedback oscillators, with some of the constraints capable of meaningful control and others beyond our direct influence. That's what makes it so difficult - not all the variables have convenient knobs on them for us to tweak as we observe. We cannot be a part of all the feedback loops, only a tiny few of them.

L Cottrill
Last edited by larry cottrill on Wed Jan 26, 2005 9:23 pm, edited 1 time in total.

Ogge
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

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pezman
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Lively! Just a few observations.

- Light can be scattered at a different frequency (Raman scattering). In fact, I wonder of detonation might be thought of as a cousin of Raman scattering (pump random frequencies in and get a clear tone out)

- Yes, sound waves and EM waves can undergo refraction. For example, one of the biggest challenges in underwater acoustics is determing the actual direction of a sound source because of the non-isotropic environment.

- Most E&M equations involve the solution of Poisson's equation -- a simplification of the Helmholtz equation that applies when there is no dispersion term. When you are talking about motion driven by heat in a compressible medium, you're in a different world -- dispersion matters. Trying to use wave-guide concepts and optical focusing ideas to predict what will happen in a combustion front prior to detonation seems misguided (but who really knows).

- All that said, the interesting thing about detonation is that the non-linearities result in a fairly reliable structure. Cell sizes in certain gas mixtures are pretty reliable. Throw in some Nitrogen and things get less organized (due to Nitrogen-Oxygen interaction?). Throw in something like N2O and cell structures get even less organized. Solving for boundary values to reinforce the transverse waves in a detonation front may well have some promise, but only in those cases where the cells are fairly regular.

- Containment geometry can have some interesting effects. Shoot a MAPP pop-gun into a parabolic shell and see what I mean.

There's a certain amount of talking past each other in all this. Everyone brings their own set of asumptions to the discussion. For example, most folks assume that the jet needs to be self-sustaining.

My guess is that the design that Adam proposed will suffer from the same effect that the Bruno BCVP does. In the Bruno BCVP, models suggest that the internal pressure builds up on successive cycles to the point where the engine stops aspirating and Bruno's description of the engine's operation suggests that this may actually be what happens in practice. There are potential solutions to this ...

No doubt all will be settled if someone gets something working.

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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Of course it does not change the frequency. The analogy is not a tuning fork, though. The point is that the time it takes for the information to travel from fork to mic and back again is shorter as the temp in the room goes up, and you must think of your tuning fork as being so damped that it must be plucked almost every cycle by the return wave. If the temp is wrong, the pluck comes back at the wrong time, and damps what little the fork had left in the way of energy.
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Ogge wrote:
There is a little frustration here that I have been feeling too. Your tone is lecturing and not the least bit humble. Perhaps just tone down a bit and we'll all have more fun.
Agreed!

But please think about this. If a tuning fork at say 300 Hz is hit in a room and you have a mike across the room does the tempature of the room effect the tone heard or just the travel time to get there?

Now take that same tuning fork and hold it to a resonance chamber (pipe organ tube) and measure the percieved resonance. Measure the difference in resonance tone as tempature changes.

Resonance is a function of wavelenght. Its the wavelength changes due to tempature that effect standing waves. The tube is not varying in size, neither is the frequency of the tuning fork.
An interesting analogy but to be my normal self:-) what happens now if we use the same room analogy but make the heating system of the room oscilate at say 150hz with a swing of say 200c to 900c

Oh and just to make it a bit more interesting lets have the heat applied as a wave traveling at 30 metres a second from the tuning fork to the mike.

And if you have a spare moment factor in far field heating from infra red:-)

AM?

Viv
PS points for the varible oscilator analogy but its more of a schotky avalanche effect as well with chaotic positive feedback:-)
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Ogge
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

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Ogge
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Eric,
I would like to appologize. In essence we were both right, but arguing about different things.

I was talking about the traveling waves, while you was talking about the resonant standing waves.

In a resonance tube, the wavelength of resonance is due to the fact that the geometry of the tube is fixed so any change in tempature requires a percieved change in frequency to the resonant standing waves. Therefore the equation frequency = speed/wavelength is the primary concern and since the speed IS changing the frequency of resonance MUST change because the wavelength (of the tube) cannot. If the standing wave changed, this implies that a change has also occured in the propogation of the traveling waves that make up the standing wave.

The source of the initial traveling waves are created during combustion and have a fixed frequency. The speed is changing due to tempature forcing a change in wavelength of the traveling waves. This change in wavelength alters the standing wave behavior due to the fact that fewer oscilations occur during the travel time down the tube thus causing the change in frequency of resonance of the standing wave. In this case the formula: Wavelength = speed/frequency is the primary concern.

So as long as we are talking about the same frequencies it will clear up any future misunderstandings.

Can we refer to the standing waves as tube resonance, engine resonance frequency or resonance waves and the traveling waves as the chemical resonance, pressure waves or Energy waves as I like to refer to them?
Last edited by Ogge on Fri Jan 28, 2005 11:48 pm, edited 5 times in total.
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

All said and done, it's about the standing wave that is the fundamental resonant frequency of the duct, and the travelling waves and their sex changes at the openings that make it all possible.

The fact that the medium is accelerating, decelerating, reversing and expanding as energy dumps in and temperature gradients swing radically during every cycle is an engineering riddle that has me by the balls.
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Mark
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Who's paper was it that had the sex change anology in it? Was it Franco Marcenaro, I can't remember the spelling, but it all sounds so gay. I don't want my pulsejet undergoing any sex changes or emasculation, especially the dilo-dyne I mentioned earlier.
I don't want to confuse anyone further, but since we are talking sex change, the grouper (a saltwater fish) undergoes a sex change too.
http://www.flmnh.ufl.edu/fish/Gallery/D ... ouper.html
In the 3rd or 4th paragraph you can read about the sex change, this gag grouper is the kind I catch here in Pensacola.
http://oceanexplorer.noaa.gov/explorati ... a/gag.html
Mark
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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

Mark you never cease to make me laugh! Sex change does seem a little drastic of a description. I think it should be reffered to as something more like mood swings, I would much rather my pulsejet be considered bipolar than to have questions of its sexuality.

Eric

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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

pezman wrote:In the Bruno BCVP, models suggest that the internal pressure builds up on successive cycles to the point where the engine stops aspirating and Bruno's description of the engine's operation suggests that this may actually be what happens in practice. There are potential solutions to this ...
Yes. Some of them as old as the pulsejet idea. I am sorry I overlooked those when I was developing the concept. It would probably have saved me (and my dear collaborators) much time and frustration. But, tomorrow is another day...

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### Re: Stainless FWE with small intake runs at -3 F !!!!!!!!!!!

The temperature varys a lot by fuel, im not really sure with propane. Propane is realitivley low btu compared to a gas / meth/ nitro mix. If i had to take a guess at the average temp... 3500 max temp and partial consumption of ambient air mixing it all down to around 2250 ish plus or minus a million degrees just to be safe.

The nature of harmonic resonance causes the pipe to resonate at many frequencies, the first being the strongest.

The frequency is inversely related to the lenght, you can try my calculator program and plug in different lenghts to give you a high / low frequency corresponding to the engine running at static / flight conditions and taking into consideration cooler engine temp/ more ram air pressure and thus not actually having to do any work yourself.

I could suggest some good reading material for harmonic resonance of open and closed end pipes/strings/stuff (my highschool physics book did a wonderful job of explaining it all with really good pictures). When a pipe goes into resonance it will be producing a 1st harmonic, 1st harmonic overtone, 2nd overtone etc. You cant change from 1st to 2nd because they are all dependant on eachother. You cant just skip 1 and go to 2, even if you could you wouldnt want to. Some choose to call them harmonics, others harmonic overtones, there is a difference in numbering thought, but thats what reading is for.

Eric

Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases