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Supersonic bathtub physics: What happens when discs are pushed through water?

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Sonic booms can occur in fairly routine settings: for example, it is a sonic boom you hear when a whip cracks. But in your bathtub? Apparently, whenever a hard object falls into a pool of water, a jet of air is produced that briefly reaches supersonic speeds. disc-shaped object in water

To study this, physicists at the University of Twente, in the Netherlands, and at the University of Seville set up an experiment in which they plunged a disc-shaped object flat down into water at the relatively leisurely speed of one meter per second. 

The disc displaced the water and created an air bubble in its wake as it sank.

air bubble created as disc sinks

As the water closed in to form the bubble, it pushed air up through a tighter and tighter neck, making it accelerate.

water coses in to form bubble"It’s like a little nozzle which closes," explains Twente’s Detlef Lohse, similar to what happens in a rocket engine. To track the air’s motion, the team filled it with glycerin droplets produced by a smoke machine of the type used in dance clubs.

Using a high-speed camera and with the aid of computer simulations, the researchers estimated that the jet reached a speed of 350 meters per second at its peak, or just above the speed of sound, the team reports in an upcoming issue of Physical Review Letters.

Although for objects of different shapes and sizes many details may change, "the physics is the same," Lohse says. "Dropping a stone into the water you create a supersonic jet."

The researchers now plan to study the sound waves produced by the air jets created in the plunging object experiment.

Credit: Detlef Lohse

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  1. 1. KevPat 11:39 am 12/26/2009

    This is old news?
    I’m sure this research started at a minimum 5-10 years ago.

    http://www.popsci.com/scitech/article/2004-06/supercavitating-torpedo

    Link to this
  2. 2. dbtinc 8:32 am 12/27/2009

    fascinating – I will rest well tonight. Anyone have any Schlieren data of flatulence discharge?

    Link to this
  3. 3. yzlv 9:25 am 12/27/2009

    very interesting! Many scientific progress is based on the finding in routine life.

    Link to this
  4. 4. yzlv 9:30 am 12/27/2009

    interesting! most of scientific progresses are based on the finding in routine life.

    Link to this
  5. 5. mistercat 12:04 pm 12/27/2009

    I wonder how big the disc would have to be for the sonic boom to be audible

    Link to this
  6. 6. Hotwir3 9:42 pm 12/27/2009

    The sonic boom of a whip cracking is a myth…

    Link to this
  7. 7. funnyfun 2:01 am 12/28/2009

    FYI vendicar9. unless you live in the Netherlands you are’nt paying a penny of it. another FYI, what defines any information as useless information? who is anyone to decide what information will ever be useful or not. This just might lead to jet powered elephants which can take you from point a to b for a penny a mile.

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  8. 8. Michael Hanlon 5:13 am 12/28/2009

    To think, all these years when a pretty girl batted her eyelashes at me, I thought the Boom Boom I heard was the beating of my heart.
    Turns out it was just her eyelashshes pushing aside the air supersonically!

    Link to this
  9. 9. binla 6:57 am 12/28/2009

    I was just wondering how I could set this up as a science experiment with middle schoolers minus the expensive equipment?

    Link to this
  10. 10. StarMonkey 9:22 am 12/28/2009

    this might explain my favorite swimming pool trick: i cup my hand slightly and position it palm-down a few inches from the surface of the water. i then quickly bring it straight down into the water. i call it a ‘depth charge’ because of the boom it makes. anyone with their head under water near me when i do it reports that they can actually feel the thud, even though i’m not really using that much force.

    Link to this
  11. 11. StarMonkey 9:23 am 12/28/2009

    this might explain my favorite swimming pool trick: i cup my hand slightly and position it palm-down a few inches from the surface of the water. i then quickly bring it straight down into the water. i call it a ‘depth charge’ because of the boom it makes. anyone with their head under water near me when i do it reports that they can actually feel the thud, even though i’m not really using that much force.

    Link to this
  12. 12. lowndesw 9:31 am 12/28/2009

    This is NEW?? This is the same phenomena seen in the venus contractus of water entering a submerged inlet such as a culvert pipe, or the principal used by the high velocity torpedoes of the type suspected of sinking the Kursk.

    I wish someone would pay me to research something already well known and thoroughly understood.

    Please tell us the possible practical application of this , if there is one.

    Link to this
  13. 13. alelser 9:43 am 12/28/2009

    Hotwir3, perhaps you missed this paticular article discussing the sonic boom in this very publication: http://www.scientificamerican.com/article.cfm?id=true-cause-of-whips-crack

    Link to this
  14. 14. Chris F-E 6:42 pm 12/28/2009

    Hotwir3, what makes you believe that the sonic boom idea is a myth? I googled ‘What makes a whip crack?’ and every link confirmed the idea that it is a sonic boom. Apparently, there is evidence that it isn’t the tip of the whip causing the sonic boom, rather it appears to be ‘the loop that travels along the length of the whip that breaks the sound barrier and makes the cracking noise’. Click on some of the links available at: http://www.google.com.au/searchsourceid=navclient&ie=UTF8&rlz=1T4GGLR_enAU319AU322&q=What+makes+a+whip+crack%3f

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  15. 15. gar37bic 9:35 pm 12/28/2009

    hotwir,
    You are mistaken. The sonic boom associated with the cracking of certain long whips has been well documented since at least 1905: (http://hypography.com/news/physical-sciences/32479.html) It is considered the first man-made object to break the speed of sound. Recent work has shown that at certain points in the process, parts of the whip are travelling at _twice_ the speed of sound, however the boom is generated by a loop that is travelling at just over Mach 1. The tip can also experience forces as high as 50,000 G.

    Link to this
  16. 16. Michael Hanlon 1:37 am 12/29/2009

    Thinking cap time:::
    If the platter being slammed into the water had intelligence vibrating on it, would the air pick up that intelligence and transmit it into the air above the water? Wouldn’t that be a new type of audio amplifier?
    How deep does the platter have to plunge to realize the effect? Is it a ratio of diameter to depth? How important is viscosity of the liquid to the equation? Does the energy equation work out when you add up all the noise in the water with the noise in the air versus the energy to plunge the platter?
    To binla: If you have an oscilloscope and a couple of microphones you can measure the time interval of the sound at the plate to how long it takes the sound to get to the surface of the water. Then calculate how fast the sound had to travel to give those results. You could also do an Edgerton stop motion photo sequence and measure second hand (as they seem to do with the photos included here) the times and distances of displacements. Again calculating the maximum velocity of the air.

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  17. 17. Michael Hanlon 1:32 am 01/1/2010

    And either the title verb is wrong or the researchers used the wrong description to the press.
    The plate is not being "pushed" into the water. From the illustration it is being "pulled" into the water. This adds the effect of the pole under the plate dragging h2o molecules which are molecularly attached to the pole along with it when it moves. This starts an ordered motion of the other supposedly ‘chaotic molecules in the tank. Flaw? I think so.
    Try it the way it’s presented:: push the plate in and then get back to us with better data.

    Link to this
  18. 18. Michael Hanlon 1:26 am 01/17/2010

    Another possible flaw in the expeiment:::
    As the plate is plunged into the water and pushes said water aside, it has actually created a larger volume of space for the ‘local’ air to fill. That reduces the density of the ‘local’ air. The lower the density of a material, the higher the needed velocity to break the sound barrier.
    I do not see the calculation of the air density during the dynamic portion of the test calculated. So, the result is definitely in question and inferences doubted.

    Link to this
  19. 19. roomunik 4:05 am 01/10/2011

    The disc displaced the water and created an air bubble in its wake as it sank http://roomunik.blogspot.com

    Link to this
  20. 20. roomunik 4:07 am 01/10/2011

    The disc displaced the water and created an air bubble in its wake as it sank roomunik.blogspot.com

    Link to this

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