My latest Thermojet

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Dave_G
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Re: Electrical vs Thermoacoustic

Post by Dave_G » Mon Jul 16, 2007 2:16 pm

larry cottrill wrote:I disagree that impedance is only drag in this case. Drag would be the "resistance" component. There is also force needed to accelerate the gas mass in the pipe, though in the case of the intake this seems pretty small. That inertial component is the "reactance" part of our total impedance. Like any good reactance, it will be highly frequency dependent. Would you not agree with this assessment?
An interesting discussion, reminescent of the golden days of yore. :-)

I do know that Hiller Corp. spent considerable man-hours to develop a complete electrical analog of the valvelss pulsejet for use in design and performance prediction. Alas, I've been tied up 40-50 hours per week for several months now in engine testing and consequent data collection, and haven't had time to read the report in any detail. Besides, I'm building a computational method-of-characteristics based approach to performance prediction and engine analysis, so following that particular methodology is not at the top of my priorities right now.

Both Lockwood and Kentfield knew that Acoustic Theory, despite being well-established, is nonetheless suitable only for small perturbation flow (such as the analysis of musical instruments).

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Re: Electrical vs Thermoacoustic

Post by larry cottrill » Mon Jul 16, 2007 4:27 pm

Bruno Ogorelec wrote:
larry cottrill wrote:Drag would be the "resistance" component. There is also force needed to accelerate the gas mass in the pipe, though in the case of the intake this seems pretty small. That inertial component is the "reactance" part of our total impedance. Like any good reactance, it will be highly frequency dependent. Would you not agree with this assessment?
Of course. But, I'd say that this part is straightforward. It's just mass whose inertia has to be overcome.
It has to be more complicated than that. I believe that air in an open-ended tube acts as an inductive reactor. It is what we would call a "filter choke" or "RF choke". Not only do I believe this is the correct model, I think I can explain why:

When a pressure pulse is applied from one end of the tube, the air begins to compress, then rebounds back to its original volume and density. If the pulse is brief OR if the mass is large, this can take place with little or no net motion of the air mass at the end of the process. If the pulse is long OR the mass is small, the compression and rebound still occur but the mass is displaced along the tube during the process. This amounts to a "low pass filter" - a long pressure pulse will result in a net flow, where a brief pressure pulse will not.

In most of our intakes, the air mass is easily displaced by the relatively long-acting pulse of combustion pressure - in fact, the contents of the intake are virtually blown clear - while the high-frequency harmonics are practically blocked. The Schubert intake is simply an attempt at making the intake content so massive that choking even occurs at the fundamental frequency of the engine! Of course, we think of this as having a particular acoustic length, but I think the internal mechanism is exactly as I've described. By this hypothesis, the Schubert pipe should be around twice the length of a properly designed "linear engine" intake of the same diameter, as measured from the explosion center to the "acoustic point' just in front of the intake flare. (It will not be exactly twice the conventional intake length, because it will run even cooler internally than a conventional intake.)
Thanks for the good wishes. I hope this finds you well.
You're welcome. Well enough under the circumstances, I guess. The stainless Lady Anne is almost finished, and will be done even though I am momentarily out of money to put into it. No problem, really - my fabrication contractor "knows where I live" ;-)

L Cottrill

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Post by multispool » Mon Jul 16, 2007 8:34 pm

Hee Hee!

Never though this would turn into a debate. Keep it up folks, its interesting!

Recalling electrical theory from memory is probably a bad move so I will stay in the background on this, other than to suggest that if the pipe is acting as an inductor then we should be talking of reluctance rather than reactance.

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Re: Electrical vs Thermoacoustic

Post by Bruno Ogorelec » Mon Jul 16, 2007 9:10 pm

larry cottrill wrote:In most of our intakes, the air mass is easily displaced by the relatively long-acting pulse of combustion pressure - in fact, the contents of the intake are virtually blown clear - while the high-frequency harmonics are practically blocked. The Schubert intake is simply an attempt at making the intake content so massive that choking even occurs at the fundamental frequency of the engine! Of course, we think of this as having a particular acoustic length, but I think the internal mechanism is exactly as I've described. By this hypothesis, the Schubert pipe should be around twice the length of a properly designed "linear engine" intake of the same diameter, as measured from the explosion center to the "acoustic point' just in front of the intake flare. (It will not be exactly twice the conventional intake length, because it will run even cooler internally than a conventional intake.)
I wonder if it's all just different semantics. You say "an attempt at making the intake content so massive that choking even occurs at the fundamental frequency of the engine" and I say "drag so great that it equals peak internal pressure". It would be interesting to compare calculated mechanical drag predictions and acoustic predictions for total choking -- how much would they differ, I wonder. Foa's picture of the pulsejet cycle is rather interesting -- acoustic behavior superimposed over thermodynamic events. Very acute. (I hope I have not paraphrased him too badly.)

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acoustics and electrics

Post by Mike Everman » Tue Jul 17, 2007 4:25 am

Here's the end-all, be-all reference I've found for the similarities between electrical and acoustic circuits and the maths for each. This is a fabulous read, very well done:
http://www.du.edu/~jcalvert/waves/acoucirc.htm
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acoustics and electrics

Post by Mark » Tue Jul 17, 2007 1:08 pm

That's a neat link Mike. I just had time to skim it and found this about using a cold gauze which was interesting in placement and direction of flow. I love toying with my Rijke tube of Pyrex, or for that matter any fun ideas with resonance.

"A related effect that makes an even better demonstration is Rijke's tube, which I call his "Boomer." It was discovered in 1859, and is quite reliable in operation, not requiring the fiddling associated with singing flames. The gas flame is used to heat a doubled iron gauze placed in the lower part of the tube to bright red heat. When the flame is withdrawn, the tube breaks out into a very loud sound, in this case at around 100 Hz. This remarkable sound continues as long as the gauze is hot, about 10 seconds. In this case the excitation depends sensitively on the upward current of air. The hot gauze is placed where there are both pressure and velocity variations, and the net effect is to add heat in the proper phase. If the hot gauze is placed near the top of the tube, resonance cannot be excited. However, Rijke showed that if this gauze were cold instead, oscillation was again excited. The steady current in this case is downward."

"These singing tubes are excellent illustrations of the creation of oscillations by forcing a mechanical oscillator in the proper phase by forces depending on its own oscillation, which is a closed feedback loop of gain greater than unity. Heat is only one agency that may be active in these systems, and it acts rather strongly on gases. I have a saucepan that oscillates on a hot burner, reminiscent of Trevelyan's Rocker, an old demonstration."
http://www.du.edu/~jcalvert/waves/acoucirc.htm#Pipe
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Re: acoustics and electrics

Post by larry cottrill » Tue Jul 17, 2007 5:02 pm

Mike Everman wrote:Here's the end-all, be-all reference I've found for the similarities between electrical and acoustic circuits and the maths for each. This is a fabulous read, very well done:
http://www.du.edu/~jcalvert/waves/acoucirc.htm
Mike -

That one (your desription and the link) ought to be a "sticky", maybe in the Off Topics Forum or something. What do you think? For a while back in the "old forum" days, there was a LOT of interest in electrical pulsejet modeling.

This somehow reminds me of the old "electric piston" engines that used a sliding steel slug alternately pulled and pushed by a "voice coil" sleeve, with the current controlled by a commutator on the crankshaft flywheel. What fun stuff!

L Cottrill

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ohms law of Pulse Jets

Post by Jim Berquist » Tue Jul 17, 2007 8:01 pm

I can see the relation! If you look a the Z in Ohmes Law for AC it fits!

The tank circuit he shown is analysis of the same. You have the circular Cycle motion of electrons. In Micro Wave they use the same things. It Travels through a tube. Wave Guide. and acts the same. In the tank circuit, the electrons or flow is influenced by the capacitor and choke. This only allows the band bass to get through. In the chamber the same is in effect. The tail pipe is the Q, or tune. U can tune it via the tail pipe or chamber...

In Micro Wave they do not think of back pressure so much, but reflective which is the same NO?

Look at Micro Wave From radio to wave guide and there is not difference.

Light energy and sound start to look alike.

Did we do a turn around?? They look the same to me!

Jim
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Old forum acoustics group

Post by Viv » Tue Jul 17, 2007 9:09 pm

Hey guys!

Just before you get too excited if you look back in the old forum near the beginning you will see we went down the electrical theory acoustic theory analogue back then with an off forum email group to follow it up in detail.

It was fun with people like Don and Nicolas doing a lot of work but in the end it came down to the fact the maths could not describe a known engine with any real accuracy.

but we have moved on since then so maybe worth revisiting.

Viv
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Re: Old forum acoustics group

Post by larry cottrill » Wed Jul 18, 2007 7:21 pm

Bruno Ogogrelec wrote:I wonder if it's all just different semantics. You say "an attempt at making the intake content so massive that choking even occurs at the fundamental frequency of the engine" and I say "drag so great that it equals peak internal pressure". It would be interesting to compare calculated mechanical drag predictions and acoustic predictions for total choking -- how much would they differ, I wonder. Foa's picture of the pulsejet cycle is rather interesting -- acoustic behavior superimposed over thermodynamic events. Very acute. (I hope I have not paraphrased him too badly.)
No, I don't think this is just a semantic difference. Drag / resistance would be a DC phenomenon, while reactance is ONLY an AC phenomenon or property. You can look at the drag on gas motion at a particular station in a pulsejet AT A PARTICULAR INSTANT IN TIME and it will be exactly the same as it would be in a ramjet station of the same diameter and same gas condition (temperature, density and relative velocity). Similarly, changing only the DC resistance has no effect on the frequency or basic operation of a simple coil & capacitor tuned circuit. On the other hand, a mass of gas in a pipe WILL act as a "delay line" for a pressure pulse introduced at one end, just like the delay line in a color TV that delays the monochrome signal so the more heavily processed chroma signal can stay sync'd up with it. This delay line is basically just a coil of carefully calculated reactance. It literally "slips" the phase of the signal back a few degrees at a certain frequency while leaving its shape unaltered.
Viv wrote:Hey guys!

Just before you get too excited if you look back in the old forum near the beginning you will see we went down the electrical theory acoustic theory analogue back then with an off forum email group to follow it up in detail.

It was fun with people like Don and Nicolas doing a lot of work but in the end it came down to the fact the maths could not describe a known engine with any real accuracy.

but we have moved on since then so maybe worth revisiting.

Viv
It's important to understand the detail differences in the two phenomena and the ways in which they make it hard to simulate the pipe with electronics. In the fine article Mike pointed out to us, the acoustic devices (e.g. organ pipes) are very simple to describe because there is the same air condition (temp & density) throughout the whole pipe. But it is not so with our favorite toys - there is very rapid and wild variation in temperature over very short periods of time at every engine station! That affects wave propagation and fluid drag, causes reflection and refraction effects, and probably a lot I've never thought of. There is practically no "density" or "temperature" equivalent in an electrical path, unless maybe they could be introduced by finely tuning the behavior of semiconductors or vacuum tubes or some such. Total mass in a particular section might not be so hard, as it can be represented by the charge in a capacitor.

It should be noted (as mentioned in the article) that a transmission line is represented by units of capacitance-inductance "tank circuits", and MAYBE a pipe with different temperatures (and possibly, cross section areas) along its length could be simulated by a series of little tank circuits of varying size and Q value (the Q of a tuned circuit is easily calculated). That might do it for us, but could get to be a pretty complex model if reasonable granularity is desired. It also has to be remembered that the chamber(s) of an engine is(are) also characterized by significant inductance, not just pure capacitance. And of course, in all of this there would be simple resistance, too.

This also says nothing about the non-linearity of drag, which probably has no electrical equivalent in pure resistance (though, again, some semiconductor might comply) nor about actual two- or three-dimensional effects like shear and turbulence (however, our one-dimensional solvers like UFLOW1D and NUDiS don't give us anything for these, either, and we still use them).

L Cottrill

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Post by Jim Berquist » Wed Jul 18, 2007 8:01 pm

Acoustical Z: Ansers . Com

Acoustic impedance
At a given surface, the complex ratio of effective sound pressure averaged over the surface to the effective flux (volume velocity or particle velocity multiplied by the surface area) through it. The unit is the N · s/m5 (newton-second/meter5), or the mks acoustic ohm. In the cgs system the unit is the dyn · s/cm5 (dyne-second/centimeter5). See also Sound pressure.

Specific acoustic impedance is the complex ratio of the effective sound pressure at a point to the effective particle velocity at a point. The unit is the N · s/m3, or the mks rayl. In the cgs system the unit is the dyn · s/cm3, or the rayl. The difference between specific acoustic impedance and acoustic impedance is in the specification of impedance at a point, as compared to the average over a surface.

Characteristic acoustic impedance is the ratio of effective sound pressure at a point to the particle velocity at that point in a free, progressive wave. This ratio is equal to the product of the density of the medium times the speed of sound in the medium. The characteristic impedance of a sound wave is analogous to the characteristic electrical impedance of an infinitely long, dissipationless transmission line. It is common in acoustical analyses to represent specific acoustic impedances in terms of their ratio to the characteristic impedance of air.

Acoustic impedance, being a complex quantity, can have real and imaginary components analogous to those in an electrical impedance. In applying this analogy, the real part of the acoustic impedance is termed acoustic resistance, and the imaginary part is termed acoustic reactance. See also Electrical impedance.


Now I know some of our Troops out there can model this some how!

Jim

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Re: Old forum acoustics group

Post by Bruno Ogorelec » Wed Jul 18, 2007 8:18 pm

larry cottrill wrote:No, I don't think this is just a semantic difference.
I fell out of the carriage of our debate in a sharp turn -- at about the third sentence -- having nothing to hang on to. (I know next to nothing about electrical phenomena.) But, thank you for making the effort to debate. I wish I could thrust and parry; the urge is there, but alas not the means to do it with. Maybe next time, when we are closer to my turf.

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Re: Old forum acoustics group

Post by Viv » Wed Jul 18, 2007 8:53 pm

Bruno Ogorelec wrote:
larry cottrill wrote:No, I don't think this is just a semantic difference.
I fell out of the carriage of our debate in a sharp turn -- at about the third sentence -- having nothing to hang on to. (I know next to nothing about electrical phenomena.) But, thank you for making the effort to debate. I wish I could thrust and parry; the urge is there, but alas not the means to do it with. Maybe next time, when we are closer to my turf.
Maybe if we just stick to beer and pretty women Bruno can join in

Viv:-)
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Re: Old forum acoustics group

Post by Bruno Ogorelec » Wed Jul 18, 2007 9:02 pm

Viv wrote:Maybe if we just stick to beer and pretty women Bruno can join in

Viv:-)
I am cutting down on beer.

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Acoustics

Post by Jim Berquist » Wed Jul 18, 2007 9:28 pm

Bruno: Your the man on this stuff!

It just seems some of the Basic Electrical formulas my apply to the Acoustic world. Electron Flow, Radio Waves and Acoustics may have more common ground then some people think.. They all can work as Waves.

Wouldn't it be cool to Fine Tune your C.C. just by threading a bolt into the CC to a certain depth? Micro Wave Can! Organs Can! Micro Waves act the same as Acoustics in a chamber. The Wave Guide, ( Tail Pipe ) is tuned to the desired frequency........Cool!

It seems that the properties of : Delay, Wave Front, Impedance or Ohms or Resistance, and Harmonics can be treated as common ground.

You should be able to tune you P.J. to a single frequency + say three harmonics + and -............

Power could be focused on the Dominate Frequency your tuned to... No wasted Energy on the Sperius harmonics.

Instead of a engine sounding like a Raped Ban chi, It would produce a pure tone/sound and shake the world!

Just My Thoughts

Jim
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