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Topic: The sonic boom

More times than not when I’m talking to new whip enthusiast the question of what makes the loud cracking sound comes up... The short answer is a sonic boom. Then the next question is. What makes the sonic boom?  The answer I give is.

When a whip moves through the air, pressure waves are produced. When the whip exceeds the speed of sound the pressure waves compress and produce a shock wave.    The shock wave is heard as a crack. 

There is a controversy about this... There is no doubt as to the speed and the creation on the sonic boom rather the controversy lies with “when” the boom is produced.  See this link.

http://www.scientificamerican.com/artic … hips-crack

Professor Alain Goriely of the University of Arizona argues, the loop formed as the whip is thrown speeds as it runs the length of the whip and when the loop hits the speed of sound you hear the crack...  I’m not a physicist or mathematician so I can’t say if Professor Goriely’s calculations are correct.

Comments on the article by whip maker Benjamin Scott make sense to me as another whip maker with whip making and cracking experience.

But the practical experience of observation and the fact that whips will travel twice the speed of sound are at odds...

Here is my hypothesis: The loop is formed in the whip, as the loop runs down the whip, it’s accelerating the tip. The loop is dragging the tip along the whip. If the loop is dragging and accelerating the tip, the tip would reach the speed of sound before the loop. When the tip reaches the speed of sound the whip cracks, even though the whip has not been fully extended and continues to pick up speed.

Anyone else have an opinion on this?

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Re: The sonic boom

Skip,


I'm not a physicist either, but I do think you have a good point there Skip.

Believe it or not, I actually supplied a bullwhip to Professor Goriely when he was working on that project. Probably one of my greatest contributions to science!! LOL!  tongue

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Re: The sonic boom

rhettswhips wrote:

Skip,


I'm not a physicist either, but I do think you have a good point there Skip.

Believe it or not, I actually supplied a bullwhip to Professor Goriely when he was working on that project. Probably one of my greatest contributions to science!! LOL!  tongue

Really! wow that's cool.. To bad he didn't mention you in his research paper. smile

I still think he's wrong.. wink

I posted the same question on a science forum I go to. Maybe some the the crazy smart guys there can figure it out...

skip

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Re: The sonic boom

Skip wrote:

Really! wow that's cool.. To bad he didn't mention you in his research paper. smile

It keeps me humble. LOL!  wink

The Professor bought the whip in March 2003. I don't think it was the only whip he used in it, but it was cool to be a part of the project nonetheless.

A prof @ UNC bought a whip a couple of years after that, but I'm not sure what he did with it.

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Re: The sonic boom

Nice Article!!! But Goriely wrote a lot more, it´s very intresting!!!

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Re: The sonic boom

I would have to disagree with the loop causing the crack theory. If this were in fact the case, why would the size, length, material, or even absence of a cracker have any affect at all on the crack. It just doesn't make sense. We all know the difference between a nylon and a poly cracker. Why would one sound different than the other if the loop in the thong remains constant?

7 (edited by quattro 01.06.2010 11:08:32)

Re: The sonic boom

The only aspect they have calculated is the shape of the whip. With a small whip the loop is very close and a long whip has a bigger loop. The flexibility of the whip dictates the loop size too. You only talk about the end (the cracker) and not the whole whip. They studied the whip but not the end and so you talking about two different topics.

The whole whip is really difficult to explain. The thong and fall is studied enough by goriely(the effect of the shape and the motion). But the cracker and the sound of it is a misteria till today. But in the following text are my thoughts about it. Please do your own experiments with it.

Use some crackers with an very elastic material(like rubber use fine fibres), and another which has a low tension module (like kevlar). By the same size it will crack different. This is the factor tensile. The loop get´s longer and the crack is not so sharp. You can amplify the effect with a elastic fall (try it with a poly fall and then with a kevlar fall, but beware of the weight)

Use one like nylon with fine fibers and one like poly with bast fibers. The poly fibers are not so flexible like the nylons so the tuft will be not the same. This is factor fibres.

The next factor is the shape. It might seems to be unbeliveable, but not only round crackers were used. The Karbatschenknaller use 50mm ribbons which has the same weight than a 3mm round cracker. The tuft is also bigger, and so the energy of the whip is shared to more air. The cracker reaches not the speeds like a round cracker but it lasts for a sonic boom. The crack is deep and loud.

The last 2 factors are the temperature and the tensilestrength. If the material is to weak the tuft(or the cracker itself) will vanish to thin air. Is the material strong enough you can reach speeds over Mach 2,5 and there the temperature is high enough to melt the tips(only 0,1-1mm) of the tuft. So they are thicker and break by the next crack. The cracker gets fast shorter. The crack itself sounds high but not so loud, because more energy were converted in heat. For this problem you can use the aramids like vectran or kevlar which has a meltingpoint of over 400°C.

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Re: The sonic boom

Check out this YouTube video where Adam Winrich cracks a whip and the science guys capture the pressure waves of the sonic boom from the cracker of the whip:

"Sonic boom visualized by Schlieren photography"
http://www.youtube.com/watch?v=lbomsOPSSII

This doesn't show the "loop" creating the sonic boom, but the cracker...

Interesting...

9 (edited by Andree 21.06.2011 12:49:54)

Re: The sonic boom

thx for bringing this up again, i really like the topic.

We still are comparing apples and oranges.
And i am pretty sure that i also fail to put all together, but sometimes it is the fun wandering how it works.

In my opinion the profersor is right with his math's. A whip has a complex motion, there is the trow and what he call's the loop.
I prefere to call the last one a wave. The wave will accelerate and because of the decresing mass (and no energie lost) this will be the energie source that causes the actual crack. In the mean time the whip travels to where you have been steering it, carring the wave.

The video shows clearly that the cracker makes a quick movement at the moment that the wave can not be transmitted any longer by the whip (whip, fall and cracker must been seen as one and the same for carring the wave) The wave come's off the whip and cracks.

In sailing i learned that some parts in a (water)wave are travelling with twice the speed of the wave itself (motion in motion).

When the wave comes off the whip all remaining energie is transformed in speed. I am sure the proffesors math's will show a factor 2 compaired with the last speed of the wave.

for seeing the movement of the cracker i like the part with the firewhip at about 30 seconds of the vid.

quattro made a few good statements and when using elastic materials there is a great energie loss

in the cracker the energie is spread over all the fibres and creating lots of sonic booms.
The difference in sound is also complex, a large whip will have a slower wave and a deep boom and a small signal a higher sound???
quattro mentioned the size of the loop / wave, i have no answer.

I will do some private research this afternoon and enjoy myself smile smile

Andree

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Re: The sonic boom

well im just a tattoo artist and a whip maker/cracker rookie. i'm by no means a physisist (or a very good speller lol) but i believe the loop or wave, the fall and the cracker all could be to blame for the sonic boom. after all at 768 mph in 4 to 8ft. doesnt allow much room to figure which part is moving the fastest.

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Re: The sonic boom

I will try and find the rest of the article but, the end of the cracker is what causes the sonic boom, not the loop.  The loop is certainly an effective means by which momentum can be transfered to the cracker tip.

It is the handoff of momentum that make whips work.  Momentum is mass multiplied by velocity.

As you start to throw the whip the velocity is low because, the entire mass of the whip is participating.  As the whip start to straighten out in front of you, a loop is formed.  As the loop travels down the whip, the heavier back end of the whip is coming to a stop.  The loop continues to transfer momentum to the lighter front end of the whip.

This process continues until the only thing that is moving is the tip of the cracker.  Since momentum is conserved, the velocity must increase.  Because the end of the cracker is so much lighter than the entire whip, its velocity is greater than the speed of sound in order for momentum to be conserved.

There are loses due to internal friction in the plaiting, air drag and other phenomenon.  This is why a well made whip has a more powerful crack.  It is simply more efficient than a lesser whip, resulting in less loss of energy.

Chris

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Re: The sonic boom

Try and crack a whip without a cracker on it, or without a tassel. That right there proves that it's the cracker, and more efficiently the tassel that breaks the sound barrier. Also, when you have a brand new cracker, it doesn't crack too well until the tassel frays because it can push more air.

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Re: The sonic boom

Since I do have a rather extensive background in physics, shock wave production and supersonic fluid dynamics, I will try my best to explain how the whip crack occurs.

Summary of the production of the 'crack':
The initial velocity of the whip multiplied by its mass has initial momentum. As motion continues down the length of the whip momentum is conserved (minus some frictional losses). Near the tip the velocity is greatly increased to conserve momentum. The popper carries a packet of air with it due to the no-slip boundary condition. If the speed of the popper is sufficient it will create a 'whizz' sound which is the development of a Kármán vortex street. If the popper suddenly changes momentum, the packet of air that the popper carries with it will create a separation zone. If the change is fast enough, a separation bubble of low pressure occurs. When the air around this separation bubble pushes on the separation bubble, the bubble will collapse and the impact of air molecules will dissipate it's momentum in a wave.
If the separation velocity of the air packet, or inertial fluid drag pocket, and popper exceed the speed of sound, a cavitation bubble occurs between the two because the air molecules cannot move fast enough to fill in the space between them. When a cavitation bubble collapses a sharp 'boom' is heard because there will be a sharp increase in air pressure upon collapse (this pressure jump is a step function).  That step function in air pressure dissipates out wards and is the sound of the crack when it hits the ear drum. When the popper rotates while creating the cavitation bubble, as in looping cracks, this generates vorticity around the bubble which aids in its growth, and produces and even louder crack. Think about the air around the cavitation bubble spinning like on a merry-go-round and the centrifugal reaction carries the cavitation boundary outwards.

The more fluffy the popper the bigger the cavitation bubble if the velocity is the same. A fluffier popper also causes more drag and slows down the end velocity.
Even if you do not break the sound barrier, a small separation bubble can collapse fast enough to sound really close to a crack.
The larger the relative separation velocity of the popper and the inertial fluid drag pocket the larger the cavitation bubble.

Why certain whip cracks are louder than others:
As we all know, certain cracks are louder than others. They can fit into 4 categories:
Termination of Linear Momentum - e.g. Flicks
Reversal of Linear Momentum - e.g. Towel cracks
Termination of Rotational Momentum - e.g. Cattlemans
Reversal of Rotation Momentum  - e.g. Overheads

For cracks using the same whip, this is the order from quiet cracks to loud cracks. This is because when a the momentum of the whip terminates, the cavitation bubble it carries separates at the same velocity as the end of the whip prior to separation. When we reverse the momentum (pull back on the whip) the inertia of the cavitation bubble (more accurately the mass of the air around the cavitation bubble) carries it foward while the cracker moves backwards, so the relative escape velocity of the cracker and cavitation bubble is increased. The same is true for when we have a loop in the crack, the main difference is that the bubble  developed is extend in a linear fashion or created an expanding development of vorticity with added angular momentum. Imagine the difference of a single gram of gun powder in a closed tube versus 2 grams (linear + angular) of gun powder not in a tube being set off.

Much more complicated mathematics is involved, but if you think about the cavition bubble moving away from the popper at with a certain amount of momentum, the following occurs:
Termination of Linear Momentum = > Linear Velocity of Cavitation Bubble
Reversal of Linear Momentum = > Linear Velocity of Cavitation Bubble + Linear Velocity of Retreating Popper
Termination of Rotational Momentum = > Linear Velocity of Cavitation Bubble + Angular Velocity of Cavitation Bubble
Reversal of Rotational Momentum = > Linear Velocity of Cavitation Bubble + Angular Velocity of Cavitation Bubble + Linear Velocity of Retreating Popper + Angular Velocity of Retreating Popper

Imagine if there are two spheres in water, if you pull them apart water will rush in between them. If fast enough, water can rush in fast enough and the liquid water will turn into water vapor. This is what happen if propellers in water spin too fast. If you only pull on one sphere the volume in between expands slower than if both moved. Now if you not only pull them apart, but you also increase their size and spin them, even more space develops between them as well as vorticity. This is what happens when a loop in the whip terminates, the inertial fluid drag pocket is now a torus (donut shaped) that is spinning. The torus is the motion envelope of the inertial fluid drag pocket and not just the pocket itself. Also the spinning action will cause a centrifugal reaction assisting the toroidal cavitaion bubble to increase in size. Imagine the air/water around it on a merry-go-round wanting to continue motion outward.

I hope this helps explain what happens when a whip cracks!

My basis: A Master's Degree in Multi-Phase Interaction in Computational Fluid Dynamics; Programmer for Institute of Hydraulic Research, Research with Eglin Air Force Base in High Speed Fluid Dynamics; Speaker at an American Institute of Aeronautics and Astronautics Conference on Multi-Phase, Multi-Scale, and Multi-Material Flow; and Discussion with Japanese Universities on Shockwave Production and Numerical Simulation of Shockwaves.

P.S. I have debated finding a University to acquire my Ph. D. in Fluid Dynamics with my dissertation on Whip Cracking Shock Wave Dynamics.

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Re: The sonic boom

Man, your speaking way above my understanding.  All I know is it does. 

If I swing a whip, suddenly change direction forming a loop, and as that loop travels down an ever decreasing diameter, it speeds up till the very end is breaking the sound barrier.  Yes, there is a lot more mathematics and physics and fluid dynamics involved, but your talking to a guy who had trouble with high school trigonometry and advanced physics.

I just like the sound that it makes.  Make it go BOOM!

Here's wishing you find time for the things you want to do, and for the things you need to do.
We see ourselves in our children and hope for a better future.

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Re: The sonic boom

Wow, that's certainly put the crack into context.