Focused wave engine runs!

Moderator: Mike Everman

Re: Focused wave engine runs!

Postby Graham C. Williams » Sun Sep 26, 2004 6:11 pm

Dear Bill.
I agree with you. The motor is a quarter wave resonator. The induction pipe can be sized to allow 2 or 3 reflections between the highest and lowest pressures.
Graham.
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Re: Focused wave engine runs!

Postby hinote » Sun Sep 26, 2004 6:12 pm

steve wrote:

The short answer is that no kind of rig does that- it is almost entirely done by hand with a hammer and anvil. To start with I use a slip roll to begin to curl the metal but after that there is no machine to help me. I will squash part of it with the vice grip, then then hammer it smooth on the anvil and repeat this many times. I gradually work the two edges together untill they are close enough for me to tack weld.


Steve--you're getting top quality on your cones, IMHO. I'm going to have to increase my own standards on my parts to match you.

Have you had any problem with the lengthwise seam bulging outward--as a result of the heat being applied to the outside of the joint? (I have)

Interesting to note that the machine-rolled and welded megs from Burns Stainless look pretty round, but if you cut into them to shorten/adjust end diameters, they go out of round pretty badly. Internal stress. Maybe we should "cook" our parts in a hot oven to try and relieve stress.

Bill H.
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Re: Focused wave engine runs!

Postby hinote » Sun Sep 26, 2004 6:17 pm

Graham C. Williams wrote:Dear Bill.
I agree with you. The motor is a quarter wave resonator. The induction pipe can be sized to allow 2 or 3 reflections between the highest and lowest pressures.
Graham.


OOOH--I LOVE it when I'm right. I wish I could blame it on my expertise, but that ain't gonna happen soon.

It seems that switching to a longer intake (to catch the more primary harmonic) would result in a more powerful acoustic effect. Maybe that's what SNECMA had in mind when they used a long intake on the folded turbine combustor recently discussed here.

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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Sun Sep 26, 2004 6:27 pm

hinote wrote:I'm currently looking at it as a 1/4-wave (closed-end) resonator, with a half-wave intake riding along in synch.


Bill, the pressure scenario makes that very unlikely. It would require a more or less constant pressure at either end of the intake tract, at least in respect to the situation at the bottom of the combustion chamber, which is obviously a pressure antinode.

You just don't get that, either in a Chinese or in Larry's FWE.

Graham (and I think Hank) have said that the intake should be longer. Changes to its length would certainmly be very instructive. The first impulse is to make the outer part longer, but I would also like to see it extended inwards, towards the concave chamber bottom.
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Re: Focused wave engine runs!

Postby hinote » Sun Sep 26, 2004 6:33 pm

Bruno Ogorelec wrote:
Graham (and I think Hank) have said that the intake should be longer. Changes to its length would certainmly be very instructive. The first impulse is to make the outer part longer, but I would also like to see it extended inwards, towards the concave chamber bottom.


Placement of the inside end of the intake HAS to be critical, because it must coincide with the nodes of the primary acoustic system.

It's possible (probable?) that the inside end must be relocated when the intake length is changed. More research required.

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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Sun Sep 26, 2004 6:50 pm

hinote wrote:Placement of the inside end of the intake HAS to be critical, because it must coincide with the nodes of the primary acoustic system. It's possible (probable?) that the inside end must be relocated when the intake length is changed. More research required.


That is exactly my point. If the intake is a quarter wave resonator riding on a harmonic of the entire tube, the extension inwards should not matter in terms of resonance. It might change the pattern of gas flow to some extent, but not the resonance. Why? Because for the intake, the entire bottom area of the chamber is the pressure antinode. The resonant length is the distance between the geometric center of that volume and the outer end of the intake, or possibly between the bottom of the chamber and the end of the intake. In either case, the exact placement of the inner end should not matter.

However, if the intake is a half-wave (open tube) resonator, which I doubt, extending its length inwards will have a similar effect to extending it outwards, because it will change the acoustic length.

I think (or, better to say, I hope) that we have a clearcut alternative here, ideal for the testing of our assumptions. A rare opportunity. However, as frequently happens in pulsejets, other factors may be at work here, clouding the theoretically simple and clear case.
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Re: Focused wave engine runs!

Postby hinote » Sun Sep 26, 2004 7:04 pm

Bruno Ogorelec wrote:
That is exactly my point. If the intake is a quarter wave resonator riding on a harmonic of the entire tube, the extension inwards should not matter in terms of resonance. It might change the pattern of gas flow to some extent, but not the resonance. Why? Because for the intake, the entire bottom area of the chamber is the pressure antinode. The resonant length is the distance between the geometric center of that volume and the outer end of the intake, or possibly between the bottom of the chamber and the end of the intake. In either case, the exact placement of the inner end should not matter.

However, if the intake is a half-wave (open tube) resonator, which I doubt, extending its length inwards will have a similar effect to extending it outwards, because it will change the acoustic length.

I think (or, better to say, I hope) that we have a clearcut alternative here, ideal for the testing of our assumptions. A rare opportunity. However, as frequently happens in pulsejets, other factors may be at work here, clouding the theoretically simple and clear case.


I'm going to respectfully disagree, and stick with my half-wave theory on the intake. The Uflow sims of the L-H we've been running clearly show the intake tube on that type resonating as a separate, but coordinated acoustic entity. I see no reason why the rearward-facing intake would respond differently in this regard.

I also think the inside location of the intake is critical--you're going to damp-out the resonation of either (or both!) the intake or the primary resonator if you don't carefully coordinate the location, and the length/frequency relationship.

Interesting discussion, don't you think? Somebody needs to take an operating engine and modify it to confirm some of the theories, and bury others as incorrrect.

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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Sun Sep 26, 2004 7:27 pm

hinote wrote:Interesting discussion, don't you think? Somebody needs to take an operating engine and modify it to confirm some of the theories, and bury others as incorrrect.


Oh, yes, great stuff! I agree that this is a good opportunity to test assumptions. The only problem that I can see is the difficulty of filtering out factors other than those we really want to observe. It is very difficult in a situation in which the factors are mutually dependent.
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Re: Focused wave engine runs!

Postby Mike Everman » Sun Sep 26, 2004 10:00 pm

Yeah, good stuff. You can see what Bill's talking about on UFLOW models of LH. The intake tube from opening to transition cone oscillates like an open tube at F1, with nodes at each end, but the inner node is dragged up and down by the cc pressure swings; so as Bill says, it plays it's part as part of the whole duct, as well as resonating locally.

I've noticed that the phasing of this local mode with the fundamental operating frequency changes with cycle number in UFLOW (as it rings down). Not faascinating in itself, but brings to mind again the question of what phasing is the best, and it's not certain that it should be in phase. For example, being out of phase with the duct pressure peak will not rob substantially from the peak, and has the benefit of making this peak "dwell" at the top, and also dwell at the bottom, promoting injestion.
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Re: Focused wave engine runs!

Postby Bruno Ogorelec » Sun Sep 26, 2004 10:44 pm

All water to my mill. I've been preaching my conviction that in pulsejets, acoustics are only superimposed over thermodynamic events, not inherent. You can see that here.

You can see a thermodynamic event on which a resonating vessel is trying to impose its acoustic rules -- and succeeding pretty well but still only partially. Duct geometry pushes it in one direction and the distribution of pressure in another. It dithers, trying to do two things at the same time.

Not that this perception changes anything in reality, of course, but it's nice to know.
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Re: Focused wave engine runs!

Postby Hank » Mon Sep 27, 2004 4:44 am

Hello- I've had a day or so to digest this thread, the damnable hurricane has passed and I would like to build on my original input to this topic.
This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should. I'd start lopping off incremental bits of the exaust and start watching for heat coloration to start moving down the pipe while leaving the intake the length it is now.
A more substantial set of motor mounts is called for.
The Potatoid Criteria are obviously not being met here, or something.
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Re: Focused wave engine runs!

Postby resosys » Mon Sep 27, 2004 6:39 am

This frequency/acoustics/pressure talk is the stuff I like to see. That has been my main focus for past few months while I've been working on the house and life. I relax by reading physics books and looking at papers on the internet....geeky, I know.

Anyhow, I'm getting up to speed on uflow and the science behind the waves, so keep up the good conversation.

Maybe the physics folks at work (UC Davis) would be interested in lending us some software and knowledge? I'll have to bug them.

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Re: Focused wave engine runs!

Postby larry cottrill » Mon Sep 27, 2004 12:13 pm

Hank wrote: This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should. I'd start lopping off incremental bits of the exaust and start watching for heat coloration to start moving down the pipe while leaving the intake the length it is now.

Strangely enough, I can easily concede that Hank may be onto something here.

Since starting to play around with the Elektra I, I have had the feeling that one of the possible glitches in these designs is the very low front-end impedance. Note, for example, that the behavior has been described as similar to the Lockwood, which is not what is intended. Remember my observation that on the Elektra I, the frequency was being determined independently of the tailpipe length? Well, even when my Elektra I is sustaining, the delivery from the tailpipe is pretty unimpressive, as judged by placing my hand about a foot behind it. Not what was expected or desired.

I think what's happening in these engines is basically that the intake stack is carrying the main load, trying to synchronize its half-wave and quarter-wave capabilities, as Bill and/or Mike E argued a few posts ago. The tailpipe looks like a low impedance at first glance, but really isn't because of the cold mass in the pipe. The only reason the pipe is critical is because it is acting like a big Schubert intake, and its "spring action" tends to damp the pressure flux in the chamber until you get it the right length. This could readily explain why the tailpipe length had to be so large in the Elektra I -- it's acting mostly in a half-wave mode! Once it is brought to resonance, then there is some output there, i.e. the 'Schubert' action is no longer dominant in the pipe.

I disagree with Hank's method for testing this, though -- what I would do is start experimenting with the impedance of the intake, trying to get it up to a "just right" value without somehow ruining its acoustic properties in the process. Maybe some calibrated sleeve inserts or something [this would be fairly easy if Bruno's open-end fuel feed were used].

The one observation that I can see that argues against Hank's thesis is that the intake pipe is staying cool. I don't see how that could be, if that pipe is really acting as the main exhaust duct. On the other hand, there's a LOT more flame there than I'd like to see. Something that might be useful with the present configuration might be to somehow get a relative thrust comparison between the intake and tailpipe, using little thrust plates or some such. I would think that would be pretty much a 'sudden death test' for Hank's hypothesis.

I think all along we've had to admit that there's something about the present mode of operation that just doesn't appear to make sense in terms of how the main pipe of this engine is supposed to work. I agree with the earlier comment that we might gain some understanding by building one without the tail flare, and I intend to have mine start out life in that configuration, and add the flare later, as was suggested. I'm putting a check in the mail to Steve this morning, so I should have one of my own put together before long.

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Re: Focused wave engine runs!

Postby Mark » Mon Sep 27, 2004 12:52 pm

Perhaps with some pulsejet designs like the M.E.W. cone combustion chamber and straight pipe exhaust, the flare on the tail end isn't needed because it is getting enough reverse flow. If you added a flare, it might allow for too much reverse flow. I can't recall, but it seems the M.E.W. has a long straight tail, longer than the Dynajet, but maybe I am wrong. There also could be something about a cone combustion chamber that is inherently different than a combustion chamber with necking.

There are so many factors I think, when you start to toy with one, some other variable/dimension might need tweaking. I think with my tiny Logan for example, if I shorten the tail pipe, it doens't like that, even though it is spewing most of its exhaust and thrust out the side port. I think a larger and/or longer side port would be the way to go. Sorting it all out is like a can of worms in uncharted territory. To say something is one thing and not contingent on the other several segments is where the confusion begins. To me, it's as if every feature is a subset of some other greater whole, and the whole keeps moving around.
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Re: Focused wave engine runs!

Postby larry cottrill » Mon Sep 27, 2004 2:45 pm

Hank wrote:This engines pushing the reversion wave out the intake and the fact that all of the exaust duct appears to be cool to the combustion chamber suggests something amiss with tune. I wish to retract my original suggestion of increasing the length of the intake tract and suggest that some one-third of the length of the exaust be cut off. Perusal of the photos suggests that this engine is, in fact, running backwards. Too great a flow inwards from the exaust is creating a back-pressure that is not allowing thrust to flow the course it should.


Hank -

How would you square this hypothesis with the observation that there is, in fact, a small but visible tail flame? If the tailpipe isn't handling the primary wave, wouldn't the large cold mass keep any of the blast mass from ever getting out? If there is in fact "something amiss with tune", how can the engine be as easy to start as Steve asserts? [Of course, I realize that 'tuning' can itself mean a couple of different things. You could, for example, have good acoustic tuning but poor division of mass flows, theoretically.]

Hank, Graham et al -

What about the possibility of choking? Could it be that the "long nozzle" gets us to the point where some choking occurs? But, I would think that would show up under UFLOW - also, wouldn't it take an unusually high pressure to make it happen?

Mark wrote:There are so many factors I think, when you start to toy with one, some other variable/dimension might need tweaking. I think with my tiny Logan for example, if I shorten the tail pipe, it doens't like that, even though it is spewing most of its exhaust and thrust out the side port. I think a larger and/or longer side port would be the way to go. Sorting it all out is like a can of worms in uncharted territory. To say something is one thing and not contingent on the other several segments is where the confusion begins. To me, it's as if every feature is a subset of some other greater whole, and the whole keeps moving around.


Yes, I think that really comes into play here. Just when you think you can grab onto a good contender, you realize that it won't explain everything observed, at least in any obvious way. UFLOW seems hopeless to resolve the issue, because the engine as a whole doesn't cook down to a 1D schema and because analyzing the parts independently can't take into account the probable complex flow shifts and for that matter might not even allow complete acoustic analysis as far as the intake is concerned. We just don't have the analytical tools needed to 'bring it all together'.

How deeply gratifying to have brought forth such an amusing problem for philsophical analysis ...

And, remember: We still have no idea whether any usable thrust is actually being generated; this could be little more than a noisy gas hog of an annealing oven.

Steve - Did you ever observe any forward flexing of your two-point engine mount system? Anything that would actually constitute an observation of thrust force being in action while she runs?

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