Pulse jet noise

[Viv]

Noise abatement is so baffling.

Mark, you don't even know how right you are.

Isn't that basically just because none of us has much experience in that area? Do you really think, say, an industrial noise expert or an acoustic designer would consider this some kind of insurmountable problem?

If this were a piece of equipment leaking light waves rather than sound waves, I could design a lot of good solutions. This is just hard because there is such a wide range of possible frequency components, and because we want/need to impose severe constraints on physical size, mass and expense.

It's an interesting psychological problem. Everybody knows that you can't do anything about it that's worth doing, so nobody ever tries anything - except Eric, who got tired of getting punched in the gut with the noise of his big jets and made a plywood shield that does the job. Of course, that only solves part of the problem, but it shows how effective simple solutions can be. The rest of us keep talking ourselves out of dealing with the problem.

How about a conical reflector around the tail end - something that would just redirect the wave rearward instead of outward? You might have to compensate for its presence, but if it worked, why would you care? Of course, I'm obviously thinking about noise from experimental static running - noise abatement in flight is inherently more difficult because the constraints are so much more severe.

L Cottrill

I had to chuckle over your first line Larry, after all Conception GLC inc. is a pulse jet company with an industrial designer and an acoustics expert, and no we did not think it was an insurmountable problem but just a problem to solve and patent the solution for our products.

The simple conical reflector won't be an answer as it will just move the wave expansion point from the tailpipe end to the lip of the cone were it will then expand in a bubble the same way as it did from the tailpipe lip.

It would boost static thrust by increasing suck back and working mass in the tailpipe but will not lower noise amplitude.

Noise expands in all directions.

Viv

[Viv]

I think that the pictures confirm my view.

First you get the shockwave. It spends some of the energy that the engine has just generated on broadcasting huge noise all over the countryside. Then you get mass flow, which generates thrust (a) because it propagates (mostly) in one direction, and (b) because it actually moves mass in that one direction.

I expected you to see what these pictures had to say Bruno but I was hoping for a few others as well;-)

Viv

[hinote]

Noise expands in all directions.

--But things aren't that simple in a pulsejet (as always).

I believe Mike E. can back me up on the following observation:

I've noticed that the loudest zone is to the side of the exhaust of a valveless PJ, and there's a distinct reduction in sound when observed directly from the rear.

I have a theory about this--and I would like to hear others comment about the effect. My theory is that the engine is putting out alternating hotter and colder mass-flow pulses, with the colder one being the result of the secondary inflow occurring late in the cycle and overlapping the next. This effect has been noted before, and a graph of the phenom is posted in the "M1E Modification Project" thread, on this Forum.

What I believe is occurring is that the hotter pulse is overrunning the colder, due to the velocity differences. This creates an effective "active noise cancellation" within the mass flow.

It would be interesting to move closer and farther during this observation, to see if the effect changes with distance.

Any other opinions?

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Mark]

I wonder if the horn shaped pulsejets are louder?
Mark

[hinote]

Hi Bruno,

one of the reasons a pulsejet will generate greater thrust in flight than stationary ...

I find this first sentence very contravercial when it is well know that pulse jets, especially the valved ones, deteriorate their thrust as forward speed increase.

Well, maybe my statement applys only to the valved pulse jets, but they do generate more thrust when static, for forward speed and dynamic air inlet RAM air deteriorate the valve's performance.

As for the rest, I will leave that to my acoustic expert.

Here's a graph from a tech paper written by a real expert on the subject, JAC Kentfield. It shows the effect of increasing velocity on the thrust and TSFC of a valveless pulsejet. The tech paper is AIAA #98-3879, page 7.


As you can see, the graph shows a thrust increase of more than 3X with increasing speed, and a corresponding TSFC decrease of about 40%.

I can hardly wait to get a valveless pulsejet into the air, and see the effects of forward velocity.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Viv]

Noise expands in all directions.

--But things aren't that simple in a pulsejet (as always).

I believe Mike E. can back me up on the following observation:

I've noticed that the loudest zone is to the side of the exhaust of a valveless PJ, and there's a distinct reduction in sound when observed directly from the rear.

I have a theory about this--and I would like to hear others comment about the effect. My theory is that the engine is putting out alternating hotter and colder mass-flow pulses, with the colder one being the result of the secondary inflow occurring late in the cycle and overlapping the next. This effect has been noted before, and a graph of the phenom is posted in the "M1E Modification Project" thread, on this Forum.

What I believe is occurring is that the hotter pulse is overrunning the colder, due to the velocity differences. This creates an effective "active noise cancellation" within the mass flow.

It would be interesting to move closer and farther during this observation, to see if the effect changes with distance.

Any other opinions?

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

Ok you got me! I oversimplified a statment, damn! note to self "do not make simple statements':-)

Yes you are perfectly correct the loudest point is normally (sometimes, mostly) at 45 degrees to the tailpipe with a corrosponding decrease at zero degrees, ie behind the engine.

Noise cancelation due to local speed of sound variations? not noise cancelation no, but maybe absorbation of energy would be a better term, faster masses losing energy to slower masses due to energy exchange.

This would also lead to increased vortex generation.

Viv

[leo]

For Hinote, when you have the graph on your screen push the print screen button and open a graphic program, then paste and cut what you need.

Sorry. you already post it.

[hinote]

As you can see, the graph shows a thrust increase of more than 3X with increasing speed, and a corresponding TSFC decrease of about 40%.

I can hardly wait to get a valveless pulsejet into the air, and see the effects of forward velocity.

Actually, the effect can be utilized at much lower airpseeds than graphed above.

The use of a well designed pressure-recovery duct at the intake would amplify the effect at lower speeds.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[steve]

here's a thought-
it is based on the same principle that allows silencers for firearms to function.

the biggest problem would be to get your engine to run with all that crap on the end of the tailpipe, but I bet the noise reduction would be significant.

[Viv]

here's a thought-
it is based on the same principle that allows silencers for firearms to function.

the biggest problem would be to get your engine to run with all that crap on the end of the tailpipe, but I bet the noise reduction would be significant.

Look at the fish tail cut on a traditional Glugareff pressure jet Steve, yours is a multi fishtail cut, on the Gluhareff this feature does not reduce the noise output, the only thing it does is to brauden the resonance point to make the engine easier to tune by bluring the actual end of the tube.

Viv

[Bruno Ogorelec]

It is the wave form of the soundwaves that need to be inverted. This is a much more complex pattern than the "simple" work cycle of the jet, and definately much harder to cancel out with another yet.

Also, note that it needs to be inverted. It is not a question of creating noise when it is silent and vice versa, it is a question of creating noise with an iverted polarity when it is noisy in such a way that they cancel each other out where the listener is. Otherwise you will not lower the soundlevel, just make it more consistent (and perhaps less annoying).

Damn. I find myself nodding in approval quite against my wish. Very grudgingly, I have to agree. Damn.

[Bruno Ogorelec]

I would never have expected the noise abatement topic to generate one of the most interesting recent debates in the forum. I love this! Perks up the interest in me just when it was slowly dying of frustration.

[Viv]

I would never have expected the noise abatement topic to generate one of the most interesting recent debates in the forum. I love this! Perks up the interest in me just when it was slowly dying of frustration.

Nice to have some one to talk too after two years working on my own:-)

Viv

[Mike Everman]

Larry made a beautiful illustration of why a PJ valveless would have the greatest noise at that 45 degree area. I did not notice the same effect on my u-bent motors, where the exits are nearly on a plane; stands to reason.

As to Anders' comments, he's absolutely right that the antiphasing of two motors isn't perfect, but it is roughly sinusoidal. Problem there is we'd be attacking the wrong thing; the fundamental pressure waves aren't what you want to mess with, but the shocks are, and as I think he is implying, they are not blendable for noise reduction, of course.

The secondary noise that may also be beyond us is the wildly varying mismatch of velocities between exhaust and ambient, which a turbojet has, but is constant, but still quite noisy, correct?

Once again, if we could move much more air much closer to ambient velocity, the quieter and more efficient our thrust will be!

[Bruno Ogorelec]

Once again, if we could move much more air much closer to ambient velocity, the quieter and more efficient our thrust will be!

Yes! Yes!