Fish Tail

[PyroJoe]

Vortex rings are a typical phenomenon of pulse jet operation. In the past months I have questioned if throwing vortex rings is a good practice. In turbines, ram jets and rockets there is a consistent column of heat which can facilitate mass flow to the exterior of the engine.

If I understand this correctly it is easier to blow heated exhaust mass into a heated column of mass than into a ambient higher density volume. The velocity of the ejected gas should be higher when the column is there?

The vortex formation results in less heated column formation, and at times nearly renders it non-existant. When I look at the fish mouth tail, I wonder if this wasn't a past solution to avoid vortex formation. Any thoughts?

[metiz]

I thought fishmouths, on turbojets at least, were there to reduce noise and heat signature...

[Mr. Yuk]

My understanding of the fishmouth is to "trick" the engine into believing it's at a certain wavelength and give some tolerance to the construction of the engine/other variables. Somewhat of a patch you might say.

[luc]

I thought fishmouths, on turbojets at least, were there to reduce noise and heat signature...

Somehow correct in your description and probably right assuming it does some sort of "Noise reduction".

But, I will keep my answer in regards of what it does for "Pressurejets" for this is where my field is, and Mr. Yuk is correct, although a bit limited in the description.

Fishmouth on a pressurejet broadens "Pointy" soundwave amplitude to a bit wider and round amplitude, and the main reason for this is to ease the engine's tuning and make it more forgiving to many aspect (i.e : Tuning, fabrication, tolerances ...ect). And of coarse, if you have a "less sharpe" sound wave, but more a bit round, the price you pay for this is "Amplitude" and we all know the results of this.

So both of you are correct ... Yes in some way it reduces noise and yes it fools the engine ... That, talking for pressurejets of coarse.

Good day,

[roland2001]

i think one of these is close to what Luc is saying ---but i've heard of two different reasons 1st ) makes the engine much easier to "throttle" --as the "fishtail" is of varying lenghts depending upon where one might measures it at any given throttle setting 2) the "fishtail" helps to elliminate the "standing 1/4 wave" consequently reducing "back pressure" thus reducing the nearly ever present problem of reflux / regurgitation of high mass/heat combustion gases at/between the 2nd and 3rd conjunction of the intake / compressor stack------i use a "crown cut at the "tail" of the engine : 4 points as opposed to two points exhaust nozzle ---a thrust augmenter elliminates any and all regurg problems with this engine + makes for a significant increase in thrust , roland2001 PS: Hi Luc, kevin

[luc]

i think one of these is close to what Luc is saying ---but i've heard of two different reasons 1st ) makes the engine much easier to "throttle" --as the "fishtail" is of varying lenghts depending upon where one might measures it at any given throttle setting 2) the "fishtail" helps to elliminate the "standing 1/4 wave" consequently reducing "back pressure" thus reducing the nearly ever present problem of reflux / regurgitation of high mass/heat combustion gases at/between the 2nd and 3rd conjunction of the intake / compressor stack------i use a "crown cut at the "tail" of the engine : 4 points as opposed to two points exhaust nozzle ---a thrust augmenter elliminates any and all regurg problems with this engine + makes for a significant increase in thrust , roland2001 PS: Hi Luc, kevin

Hi Roland,

Well ... Of coarse if we go indept finding or explaining "ALL the reasons for and results of a fishmouth", probably aft this forum could have some thing to say, but raughly speaking, all the above are correct including your inputs Roland.

Now considering the above plus what you added, especially about throttle capabilities, I still beleive we could all agree that a "Fishmouth" makes an engine more "Forgivness" or "tolerant" to alot of stuff (i.e : Throttle, tunning, mistakes ... ect ect ect) ... Don't you agree?

Good day,

[roland2001]

Hey Bro Luc , i agree absolutely with your comment---nice to see you still on board with the site and pressurejet project(s) take care, kevin aka roland2001

[luc]

Hey Bro Luc , i agree absolutely with your comment---nice to see you still on board with the site and pressurejet project(s) take care, kevin aka roland2001

Hi Bud,

Indeed, I'm still on board, although very quiet I would say, and not that much with pressurejets since I'm done with these. Right now I'm kind of concentratiing on my job, my flight training and house renovations. But I like to keep in touch and participate when my inputs seems to be useful.

Anyway, thanks for the nice comments and glad to see you too.

Regards,

[PyroJoe]

Thanks for all the responses.
I think my concept was lost though. In a perpenducular cut tail pipe there is typically a well distributed boundary that creates a vortex due to the exhaust gas exiting the port.
My thoughts on the fish tail is that it allows some exhaust gas to exit early while some stays partially contained. This should disrupt the boundry required for vortex formation, and allow ambient air to more quickly back fill behind the positive exhast wave, to allow it to shear away from the port without vortex formation.

Mental Picture:(Instead of the "donuts on a rope" contrails you should end up with broken ropes. maybe even unbroken rope if the frequency is high enough)

[luc]

Thanks for all the responses.
I think my concept was lost though. In a perpenducular cut tail pipe there is typically a well distributed boundary that creates a vortex due to the exhaust gas exiting the port.
My thoughts on the fish tail is that it allows some exhaust gas to exit early while some stays partially contained. This should disrupt the boundry required for vortex formation, and allow ambient air to more quickly back fill behind the positive exhast wave, to allow it to shear away from the port without vortex formation.

Mental Picture:(Instead of the "donuts on a rope" contrails you should end up with broken ropes. maybe even unbroken rope if the frequency is high enough)

Hi,

If not done already, go see the movie on this poste viewtopic.php?f=5&t=4874#p57400 and let me confort you in saying that both configurations (Fishmouth or straight cut) produce that big vortex (I tried both). Pressurejets (Or any thermoacoustic engines) are simply not "Gas turbine". So, looking for a nice perpendicular exhaust jet stream with thermoacoustic engines is just futile.

You're also right about "Fishmouth allowing greater fresh air sucking", since the opening area is greater, but I also built an engine with a straight cut tail pipe that was delivering more thrust then an original Gluhareff built engine. The only significant difference I personally saw, is the tuning being a pain with the straight cut engine compared to the fishmouted one, and for the reasons I explained above.

I will try finding a movie with a straigh cut engine at night, for you ...

Good day,

[PyroJoe]

Yeah I saw that one, thanks for posting it,
I would guess longer tails and not running static would have a different effect. Maybe with longer pulses and higher containment. This one is valved, but is very near the broken rope form as seen at time 1:33

http://www.youtube.com/watch?v=dBNsxTR2640

With a small bit of vortex suppression I think broken rope could be an option.