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All Springer/NP/PCP Air Gun Discussion General > Engineering- Research & Development

Trying to add pressure sensors to LDC testing.

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OldCrow:

--- Quote from: WhatUPSbox? on March 28, 2024, 01:43:44 PM ---A while back I had pulled some frames from that video and overlayed a rough scale in pellet diameters. That bottom image would roughly fit inside the segments I'm using in my setup. Good seed for flow theorizing.  :D

--- End quote ---

Those diamonds look to be about 4/5 of one caliber.  For a .177 that works out to about 95.8 kHz (as an audio wave front).  It looks like the standing waves are disrupted by dispersion and turbulence at about 10 wave lengths from the muzzle.  That might suggest dimensions for baffling material.

I would lay odds they were caliber specific.

Just speculating:
At STP MACH 1 is 1130 fps (344,424mm/sec)
Works out to approximately 95.8 kHz for .177,  77.0 kHz for .22 and  67.8 kHz for .25.

Wave guide calculations:=>   uspas.fnal.gov/materials/10MIT/Lecture5.pdf

Good information!

WhatUPSbox?:
Playing the video at .25 speed in YouTube, it looks to me like you can see the stepping of the 15 khz frames. If that is true, then maybe 2 standing waves/frame or 30 khz. Even that may be hard to detect.

OldCrow:

--- Quote from: WhatUPSbox? on May 20, 2024, 02:36:36 AM ---Playing the video at .25 speed in YouTube, it looks to me like you can see the stepping of the 15 khz frames. If that is true, then maybe 2 standing waves/frame or 30 khz. Even that may be hard to detect.

--- End quote ---

Umm, If the waves are "standing" then you can establish the frequency from the wavelength without worrying yourself about frame rates, or am I wrong about that?  The diamonds are 1 lambda, aren't they?

If all of that is correct then I stand by my estimate, otherwise correct my thinking, please.

WhatUPSbox?:
I probably should not have used the term standing waves. I don't know what those features are and what the pellet speed is. My only observation was that two of them appear to emerge during one 15khz frame. This is not really part of what I hope to measure other than knowing that there are potential acoustic events up at those frequencies.

OldCrow:
I see what caused the confusion now.  Yes sir, Those are "shock diamonds" or "thrust diamonds" and are called by other names as well.  They are standing waves.
I am not allowed (dont have enough posts) to post external links so:  en.wikipedia.org/wiki/Shock_diamond <= cut and past into your browser.  Someone linked some good links earlier and the wikipedia article provides additional references which are helpful as well.  In those references is additional information regarding the mechanism whereby they are formed.

Anyway, thanks for providing the video link and the static captures.  Very interesting path to run down.  I had not realized that these devices were principally active at such high frequencies.  That's going to drive some of my simulations if I ever get back to working on designs.

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