For Chapter 2, here is part A of the new stories and also the updates to the items in the book, including many video links and journal citations. If you want all the video links (hundreds) and journal citations (thousands) for this chapter, go to

http://www.flyingcircusofphysics.com/pdf/Chapter2_Ref_Com.pdf

First, a list
2.1  Gurney flap in race car downforce
2.3  Aerodynamics of passing trains
2.4  Collapse of the old Tacoma Narrows Bridge
2.7  Scott Macarney crash in downhill ski racing
2.10  Cards as weapons
2.15  Golf ball dimples
2.26  Meandering rivers

2.35  Swimming in dead water
2.39  Dust devil core
2.39  Martian dust devils

Reference and difficulty dots
Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages


Now the stories:

2.1  Gurney flap in race car downforce
Jearl Walker www.flyingcircusofphysics.com
September 2006   A race car is held onto a track by the downforce due to the flow of air above and below the car's body and (on some types of cars) the front and back wings. Part of the downforce can be due to a gurney flap, which made no sense at all when it was invented by Dan Gurney in 1971. Faced with a race car that was running too slowly, Gurney seemingly whimsically decided to fit an upright, short flap along the full length of the trailing edge on the car's rear wing. That did not make sense because the flap stuck up into the airstream passing over the wing and thus should have obstructed the stream and added to the air drag on the car. After all, engineers go to great care to streamline a car in order to reduce obstructions and air drag.
    When driver Bobby Unser took the modified car around the track, the car's speed was no better than previously, but after Unser climbed out, he took Gurney off to a point of privacy and explained: The car was no faster simply because there was now so much downforce on the rear wing that the car was no longer "balanced" in a turn. All they had to do was increase the downforce on the front and the car would be able to take the turns very fast.
    So, how is the downforce increased by a gurney flap, as it came to be known after other racing engineers finally caught on to Gurney's secret design? (A modern version is shown in the image here, from The359. Do you see the short upright gray ridge that runs along the right-hand edge of the wing?) There are two reasons for the flap's increase in downforce: (1) The airflow along the top of the wing is deflected slightly upward by the flap and thus pushes downward on the wing. You feel a similar (but more simple) downward force if you have ever angled your hand in passing air, with the trailing edge of the hand somewhat upward. (2) The air flows over and under the wing form oppositely rotating vortices just behind the flap and extending a bit higher than the flap. The extra height causes extra deflection of the airstream passing over the flap and thus extra push down on the wing. Also, the presence of the vortices allows additional tilting of the wing without the airflow resulting in stall, in which the stream under the wing breaks away from the wing prematurely. Such breakaway would ruin the downforce.
Source: Howard, K., "Gurney flap," http://www.allamericanracers.com/gurney_flap.html 

www.f1journal.com/f1_teknik/tek_acv_030501a.html

· · ·  Mukherji, T., “Investigating the aerodynamic response of a NACA 0012 airfoil with gurney flap,” (2006) http://www.duke.edu/~tkm8/Aero.doc 
· · ·  Troolin, D. R., E. K. Longmire, and W. T. Lai, “Time resolved PIV analysis of flow over a NACA 0015 airfoil with Gurney flap,” Experiments in the Fluids, 41, 241-254 (2006)

Want more references? Use the link at the top of this page.

2.3  Aerodynamics of passing trains
Jearl Walker www.flyingcircusofphysics.com
Dec 2008     If you stand next to a rail track as a high-speed train passes you, you can be knocked down by the pressure wave shed by the front of the train. As the train plows through the air, it compresses the air. If you could see the compressed air, it would roughly form a cone with the apex located at the front of the train. As the train moves, the pressure cone is continuously being formed.

If you are near the track, the pressure cone will sweep past you, followed by a rapid decrease in air pressure as the air attempts to regain its initial pressure. This cone of high pressure and then low pressure is also shed by high speed airplanes and is sometimes made visible if water moisture condenses to form a fog of water drops in the low pressure portion of the wave. Below I give a link to photographs and videos where you can see conical fogs hugging high-speed airplanes.

The situation with a high-speed train, however, is different in that a train can be much longer than an airplane. Air tends to be entrained (pulled along) by the cars behind the front of the train, which causes a suction action on the air somewhat farther from the train.

Although we cannot see the high pressure cone or the entrainment of air directly, we can see the effects in the following video. You may want to watch it twice because the first part is horrifying --- two people dash across the track of an oncoming, very high speed train. The woman just barely misses being hit. Try to watch the man that is already on the platform; in particular, watch his stance and his clothing.

http://www.youtube.com/watch?v=Z5stmjA74-s&feature=related

Can you see when he is hit by the high-pressure wave and then when he is pulled toward the train? Can you also see airborne debris that is pulled along with the train?

In England and other places, a danger line is marked on the platforms, near the outer edge --- you are to stand behind the line. One reason is so that you are not hit by any side projection from a train. The other reason is that if a train passes rapidly through the station (instead of slowing to a stop), you will not be knocked off balance by the high-pressure cone and then pulled onto the side of the train by the suction action of the entrainment of air. In this video, the cameraman is apparently knocked down by either a projection from the train or by the pressure cone.

http://www.youtube.com/watch?v=aXvQ_wdkSZ0&NR=1

You can also hear and see the pressure effects from inside a train if the train passes another, nearby train at high speed. I notice the effects on the trains running between Gatwick Airport and London whenever a train passes an oncoming train. Both trains are moving fairly rapidly but the relative speed between the two (the speed of either relative to the other) is the sum of the two speeds. Thus, the relative speed is high, which causes a significant decrease in the air pressure between the trains as they pass each other. I can feel the sudden drop in pressure. I can also hear the effect as the passenger doors facing the other train are yanked outward against their restraints by the decreased air pressure between the trains. In earlier days, when train speeds were first being increased and side-by-side tracks were being laid, train designers did not know about the low pressure that could develop between passing trains. One result was that the glass would be sucked out of the windows that faced one another.

In this video

http://www.youtube.com/watch?v=7a9BhoR02SY&feature=related

people leaving an apparently stalled train walk over an adjacent track as another train approaches at a fairly high speed. My first reaction to the video was to say, “Oh no, someone is going to be hit.” But then I noticed the several people caught in the narrow space between the stalled train and the moving train. They were caught in a low pressure region where the air was being entrained and pulled along by the moving train. They were very lucky they were not pulled onto the moving train.

http://www.youtube.com/watch?v=EqJAM8A8H-o Video of two fast trains passing each other in opposite directions. Note the uncontrollable motion of the camera.

Videos and photos of condensation cones created by high-speed aircraft: go to http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=39

and then scroll down to item 3.54 Photo of a shock wave shed by an aircraft.

References
Dots ·  through · · ·  indicate level of difficulty
Journal reference style: author, title, journal, volume, pages date)

· Price, B. T., “I. Social significance of airflow problems. Airflow problems related to surface transport systems,” Philosophical Transactions of the Royal Society of London A, 269, 327-333 (1971)
· ·  Camerlingo, C., and A. Varlamov, “An incident on the train,” Quantum, ??, 42-44 (November-December 1990)
·  Fuji, K., and T. Ogawa, “Aerodynamics of high speed trains passing by each other,” Computers & Fluids, 24, No. 8, 897-908 (1995)
· · ·  Howe, M. S., “Pressure transients generated when high-speed trains pass in a tunnel,” IMA Journal of Applied Mathematics, 65, 315-334 (2000)
· ·  Schetz, J. A., “Aerodynamics of high-speed trains,” Annual Review of Fluid Mechanics, 33, 371-414 (2001)
· · ·  Howe, M. S., “On the infrasound generated when a train enters a tunnel,” Journal of Fluids and Structures, 17, 629-642 (2003)


2.4  Collapse of the old Tacoma Narrows Bridge
Jearl Walker www.flyingcircusofphysics.com
September 2006   One of the most popular physics videos ever made shows the collapse of the Tacoma Narrows bridge, which dramatically began to oscillate in a moderate wind one morning soon after it was officially opened. The oscillations built up until the main span ruptured. As I describe in the book, the subsequent analysis of the bridge's collapse modified the role of aerodynamics in the construction of all large bridges thereafter.
   A recent paper by Green and Unruh clarifies the role of the vortices in bringing down the bridge and builds on earlier modeling by Larson. As the wind encountered the bridge's girder at the left side (as seen in the video), it generated a series of vortices, alternating just above and just below the left edge of the bridge, each being swept rightward across the bridge. If you watch the video, you can see one of the votices as dust (presumably from disintegrating pavement) swirls around in it.
   Each vortex was at a lower air pressure than the normal air pressure. Thus, when a vortex formed, say, below the left side of the bridge, the normal air pressure above that point tended to push the bridge downward. And when a vortex formed above the left side of the bridge, the normal air pressure below that point tended to push the bridge upward.
    However, whether this tendency of pushing feeds energy into the vertical oscillation of the bridge depends on the speed of the votices as they are swept rightward across the bridge's width. Below a certain critical speed, the vortices could not cross the width in the time of one bridge oscillation and the pushing actually opposed the oscillation, draining energy from it. (When part of the bridge was moving, say, upward, the pushing by the vortex there was downward.) At the critical speed, the crossing time matched the oscillation time, and the vortices did no net work on the oscillations (provided no change in energy).
    The important thing that happened the morning of the bridge's collapse is that the wind exceeded the critical speed and the vortices crossed the width in less time than a full bridge oscillation. In that case, the forces from the vortices fed energy into the oscillations because their pushes were in the direction of the bridge's motion. Finally, the oscillations were severe enough to rip apart the bridge.
Movies and newsreels
http://www.youtube.com/watch?v=j-zczJXSxnw
http://www.youtube.com/watch?v=HxTZ446tbzE Newsreel with narration and music
http://www.youtube.com/watch?v=P0Fi1VcbpAI

Still photos
http://www.ketchum.org/bridgecollapse.html

· · ·  Matsumoto, M., H. Shirato, T. Yagi, R. Shijo, A. Eguchi, and H. Tamaki, “Effects of aerodynamic interferences between heaving and torsional vibration of bridge decks: the case of Tacoma Narrows Bridge,” Journal of Wind Engineering and Industrial Aerodynamics, 91, 1547-1557 (2003)
· · ·  Ricciardelli, F., “On the wind loading mechanism of long-span bridge deck box sections,” Journal of Wind Engineering and Industrial Aerodynamics, 91, 1411-1430 (2003)
· · ·  Schmit, R. F., M. N. Glauser, and G. Ahmadi, “Flow and turbulence conditions in the wake of a H-section in cross flow,” Journal of Fluids and Structgures, 19, 193-207 (2004)
·  Ulam, A., “A bridge too far?” Discover, 25, 40-43 (May 2004)
·  Petroski, H., “Past and future failures,” American Scientist, 92, No. 6, 500-504 (November-December 2004)
· · ·  Green, D., and W. G. Unruh, “The failure of the Tacoma Bridge: a physical model,” American Journal of Physics, 74, No. 8, 706-716 (August 2006)
·  Cardno, C. A., “New Tacoma Narrows Bridge officially opens,” Civil Engineering, 77, 16-18 (September 2007)

Want more references? Use the link at the top of this page.

2.7  Scott Macarney crash in downhill ski racing
Jearl Walker www.flyingcircusofphysics.com
Feb 2008   In January a terribly frightening video raced across many media outlets. It shows the American skier Scott Macarney making the final jump in a downhill ski race at Kitzbuehel, Austria. Here is a link to one of many places running the video, but let me warn you that this is shocking. However, let me also reassure you that Macarney appears to be ok. Although he was airlifted from the race to Innsbruck and put into a forced coma to protect his brain, he was flown back home several days later and is predicted to make a full recovery.

http://www.youtube.com/watch?v=X-iWbr8rPfw Scott Macartney crash lands on the final jump in a ski race at Kitzbuehel
http://www.youtube.com/watch?v=he24trOJ2H4&NR=1  Same video
http://www.youtube.com/watch?v=P0lhMWaenCg&feature=related  News report with video

In the video you see Macarney leave the top slope of the jump at a measured 140 kilometers per hour. During flight to the lower slope, he loses control and then crashes on his side and back, breaking his helmet. Very dangerous.

Anyone can say what I just wrote: high speed, loss of control, thus bad crash. But with physics we can say more because we can say why he lost control.

Especially telling is the shot from behind Macarney as he leaves the upper slope. Notice that at the last instant, he leans to his right. He is shifting his weight to the right leg to make his upward jump as he comes off the edge. The leap upward from his right side put a slight spin on him around two axes. From the rear, we see him begin to rotate clockwise around a horizontal axis extending from us through him. If we could see him from above, we would also see him rotate clockwise around a vertical axis.

Because Macarney went into free flight with spin, he had a certain amount of angular momentum around the two spin axes. Only an outside torque can change that angular momentum; Macarney could not himself change it. Now, in some sports such as snowboarding, leaving the surface with angular momentum is desirable. The jumper knows how to manipulate the body’s orientation to alter the spin rate, so as to perform some fancy rotation during the flight. The angular momentum does not change, but the spin rate can. But that ability to manipulate the spin rate takes training.

When a skier makes a long jump, such as in a downhill race or in a ski jump, the focus is to avoid any spinning because the skis and the skier each must function as a wing. To do this, the skier holds the skis in front to “catch” the wind and also leans toward the skis so that the body also “catches” the wind. When the skier does all this, each wing provides a steady lift to carry the skier out over and then down onto the slower slope. Each wing also provides stability so that chance buffeting by the passing air does not ruin the proper orientation.

When Macarney began to rotate as he left the upper slope, he quickly moved out of the correct orientation. His body rotated to his right while his skis came up on his left. Whatever lift he then got on his skis made the situation worse because it pushed the skis up higher and thus rotated his body lower. By then he was straightening his body instead of leaning over to catch the wind, and so any upward push on his body by the air was insufficient to stop the rotation. When he reached the lower slope, he was rotated by enough that the impact was on his side and back, causing his head to hit so hard that the helmet broke.

Physics is everywhere, even in tragic events. Thankfully, Macarney’s crash was only frightening and threatening but not lethal.

2.10 Cards as weapons
Jearl Walker www.flyingcircusofphysics.com
June 2010  If you throw a common playing card haphazardly in the air, it simply tumbles and then flops onto the floor. Why does spinning the card as you launch it greatly increase the distance it will go? The current Guinness record is about 65.9 meters (216 feet and 4 inches), set by Rick Smith, Jr. When Rick was a student here at Cleveland State University, he was a pitcher for the school’s baseball team. In fact, when he now throws a card, he uses almost the same throw as he used for a curve ball when he was on the team. Here is a video of him on the Wayne Brady television show, where he throws cards directly through a banana, slicing it in two places:

http://www.youtube.com/watch?v=E9MnNcwVQAU&feature=related

Just as remarkable is this video in which business cards are thrown with great accuracy through hoops, flames, and balloons.

http://www.todaysbigthing.com/2010/03/31

And here is the legendary Ricky Jay breaking a pencil with a thrown card.

http://www.youtube.com/watch?v=SZdBTKN4FyQ

In the several more video links gathered below, you can see cards thrown into various fruits (even through the thick skin of a watermelon) and into a common wall.

My acquaintance with throwing cards began when I read the now famous book Cards as Weapons by Ricky Jay, who explains how cards really can be used in self defense. (Well, I am not sure that you could actually stop a charging ox or fend off a giant squid, as implied by the cover.)

However, you can seriously hurt someone, especially if an eye is hit. Indeed metal cards for throwing (shuriken) have long been associated with the fighting skills of ninja.

Launching the card

There are various styles of holding a card. Here are a few:

They all have one common feature: The card must easily slip from your grip after you have snapped your wrist in the launch. In that way, you the card spins rapidly around its central axis (the axis that is perpendicular to its face and through its center point).

You can launch the card with its plane at any angle to the horizontal but the card will tend to stay in the air longest (and travel farthest) if the plane is approximately horizontal. Then the card receives the greatest lift on its bottom face due to its continuous collision with the air in its path.

A spinning top

The primary purpose of the rotation around the central axis is to stabilize the card’s orientation, much like spin stabilizes a top. Here is a side view of a spinning top consisting of a disk (seen on edge) and a vertical central spindle.

If the top were not spinning, then it would be unstable and pulled over by the gravitational force after any slight chance disturbance. However, with it spinning, it is stabilized by its angular momentum. That quantity is the product of its angular speed and a property called its rotational inertia, which depends on the mass and how the mass is distributed relative to the central axis. That description may sound terribly abstract, but angular momentum is the quantity that is responsible for the magic of any spinning top.

In the figure, the top is rotating clockwise as seen from above, which means that the associated angular momentum is a vector that points down along the central axis. When the top is upright as shown, the angular momentum is fully vertical.

That vertical angular momentum can be changed only if a force acts in the plane of the disk, to increase or decrease the top’s angular speed. If, instead, the force leans the central axis off to one side, it cannot change the vertical angular momentum. The top still rotates around its central axis with the same angular momentum but now only a component (a portion) of it is vertical. Because the vertical angular momentum cannot change, the central axis itself must now move around the vertical, to make up the difference. This motion of the central axis is precession.

However, there is a hitch to this process. Is there enough energy for the precession? If there is not, then the top simply cannot lean over and must remain upright, a condition known in both street play and physics textbooks as the sleeping top.

If the force is slight and brief, the only source of the energy for the precession must be from the fall of the center of the top as it leans over. That fall can transfer energy from the gravitational potential energy of the top to the kinetic energy of the precession. However, the required amount of kinetic energy depends on the rate at which the top is spinning. If the top is spinning slowly, then the procession needed to maintain the vertical angular momentum is also slow and only a small energy transfer is required. In that case, the fall can provide enough energy. But if the top is spinning rapidly, then the procession must also be rapid and the required energy is more than can be supplied by the fall. In that case, the top cannot lean over. Thus, the spin stabilizes the top.

The spinning card

Essentially the same physics is behind the stability of a spinning playing card as it flies through the air. When launched from the right hand, the card spins clockwise as seen from overhead and its angular momentum vector points downward.

If the card is spinning rapidly, its orientation is stable in spite of the irregular buffeting by the air. Once air drag slows the spinning, the card becomes increasingly unstable and eventually flops onto the ground.

Loss of lift

I have trouble putting a large spin on a card and find that soon after I launch the card (with my right hand), the plane of the card rotates from being horizontal to being vertical. In my view, the plane of the card rotates counterclockwise. Of course, as the plane becomes more vertical, there is progressively less lift and the card then slides down to the ground (while still spinning).

This rotation of the plane of the card is due to the uneven lift on the card’s bottom surface --- there is more lift on the underside of the front of the card (whatever part is foremost as the card rotates around its central axis), as shown in this figure from a side view.

Because that lift is forward of the card’s center, it creates a torque on the card (toward your right in your view of the card), which rotates the angular momentum vector (and thus the plane of the card) counterclockwise.

Boomeranging cards

If you throw a rapidly spinning card upward at about 45 degrees from the horizontal, it will move up along that path until it reaches a highest point, and then it will move back down to you along almost the same path.

http://www.youtube.com/watch?v=yVzdP9XoJG4

As previously, the rapid spinning stabilizes the orientation of the card and its angular momentum. As the card moves upward, the increase in its gravitational potential energy comes from the decrease in the kinetic energy associated with translational motion of the card (that is, the motion of the center of mass of the card). The kinetic energy associated with the rotation does not change.

The card climbs the path until the translational kinetic energy is exhausted, just as a ball thrown straight up moves up to the point where its translational kinetic energy has been fully transferred to potential energy. Then the card (or the ball) reveres the transfer by moving downward.

Other objects

You could try throwing other flat objects, such as a CD or even a CD case. (Being a rock fan, I take great pleasure in throwing CDs of country music. Actually, I think the resulting scratches on the CD improves the music.) The physics is the same as with a card unless the object is too blunt or thick. Then the air drag will quickly bring it down. One last note: Be safe. You could really hurt and blind someone if you are not careful.

http://www.youtube.com/watch?v=W3tS2_p4hyE&feature=related quick tutorial

http://www.youtube.com/watch?v=KVZTwunYl9E Master of Champions with Rick Smith Jr

http://www.youtube.com/watch?v=A4TDeC3DyyM Ricky Jay

http://www.youtube.com/watch?v=rUILPeaokEI&feature=related card slices into apple

http://www.youtube.com/watch?v=1zJn7NOKhW4&feature=related slices bananas

http://www.youtube.com/watch?v=_8G5Ck1krjE&feature=related quick lesson

http://www.youtube.com/watch?v=tNP7RrG3lF0&feature=related card stuck in wall

http://www.youtube.com/watch?v=MMggSredtJk&feature=related Rick Smith Jr on Attack of the Show

http://www.youtube.com/watch?v=s4ga9Ymg9tE&feature=related Rick Smith Jr with Steve Harvey

http://www.youtube.com/watch?v=ONiPcFHeIX0&feature=related lots of tricks by Jav Jarquin

http://www.knifethrowing.info/throwing_cards.html instructions on throwing cards

References
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Jay, R., Cards as Weapons, Warner Books, 1977. WARNING: mature content
··· Lugt, H. J., “Autorotation,” Annual Review of Fluid Mechanics, 15, 123-147 (1983)
··· Jones, M. A., and M. J. Shelley, “Falling cards,” Journal of Fluid Mechanics, 540, 393-425 (2005)
x
2.15  Golf ball dimples
Jearl Walker www.flyingcircusofphysics.com
December 2006    Golfers have long realized that a dimpled golf ball will fly much farther than a smooth ball because the dimples somehow reduce the air drag on the ball. It is that drag force that opposes the ball's motion and drains energy from it. Understanding how the dimples decrease the drag force has been very challenging because the experiments with air flow past a ball are difficult to see or measure. Up until now, that is. 
    The air drag is primarily due to a difference in the air pressure between the front and rear of the ball. Let's take the perspective of a smooth ball, as if we rode along with the ball and felt the air streaming past us. As the stream moves around the surface of the ball, the air layer rubbing against the surface slows until it reaches a stagnation point, and then the stream breaks free of the surface. On a smooth ball, the break-away point occurs well before the air reaches the point on the rear that is opposite the impact point on the front.
    This break-away of the air stream creates a vortex-filled wake behind the ball. Because the air pressure in a vortex is low, this condition means that the ball has high pressure along its front surface and low pressure along its rear surface. The difference in the pressures on front and rear is the air drag that slows the ball.
    Dimples change the picture dramatically because somehow they delay the stagnation of the layer of air sliding past the surface of the ball. So, the layer clings to the ball until it reaches the point almost directly behind the front impact point. The break-away point (or the stagnation point) is said to be delayed because it occurs farther back on the rear surface of the ball.
    The result is that the vortex wake is much narrower and so the pressure across the rear surface of the ball is not so low. That means that the pressure difference between the front and rear is lower than with a smooth ball, perhaps 50% lower, and so the drag force is less by that same amount. What matters to the golfer is that a long drive goes much farther toward the hole.
    I've known all this since I wrote the first edition of The Flying Circus of Physics. For all those years the nagging question has been: "Yes, but why do the dimples delay the break-away point?"
    Jin Choi, Woo-Pyung Jeon, and Haecheon Choi of Seoul National University in Seoul, Korea, have now published an answer based on experiment because they figured out a way to measure the speeds down within and just above the dimples on a golf ball. A dimple causes turbulence in the air flow next to the ball's surface. Bringing faster air down next to the surface prevents the air next to the surface from slowing, stagnating, and then breaking free of the surface. Thus, we have the seemingly contradictory statement that the dimples lower the air drag on a golf ball by creating turbulence in the air flowing past the ball. For a golfer, then, turbulence is a good thing. 

http://www.cookeassociates.com/seesite/BALLS/balls_students_background.htm  golf ball (go down to the photos showing smoke tracers moving past a tennis ball)

·  Aoki, K., A. Ohike, K. Yamaguchi, and Y. Nakayama, “Flying characteristics and flow pattern of a sphere with dimples,” Journal of Visualization, 6, No. 1, 67-76 (2003)
· · ·  Penner, A. R., “The physics of golf,” Report on Progress in Physics, 66, 131-171 (2003)
· Won, S. Youl, Q. Zhang, and P. M. Ligrani, “Comparisons of flow structure above dimpled surfaces with different dimple depths in a channel,” Physics of Fluids, 17, article # 045105 (2005)
·  Choi, J., W.-P. Jeon, and H. Choi, “Mechanism of drag reduction by dimples on a sphere,” Physics of Fluids, 18, article # 041702 (4 pages) (2006)
· ·  Libii, J. N., “Dimples and drag: Experimental demonstration of the aerodynamics of golf balls,” American Journal of Physics, 75, No. 8, 764-767 (August 2007)

Want more references? Use the link at the top of this page.


2.26  Meandering rivers
Jearl Walker www.flyingcircusofphysics.com
November 2006    Nearly all rivers meander, that is, curve to one side and then the other as the water generally moves downhill, as you can see in this photo by "Sachin - A matter of life and death." In some cases, the meander is so extreme that water even moves (briefly) uphill along a loop. This tendency of meandering and looping has long fascinated hydraulic engineers, environmentalists, and physicists. After all, except when meeting a solid obstacle, water should flow directly downhill because of the gravitational pull on it. The next time you fly over land, examine the rivers that you pass---they all meander, even the ones that do not have solid obstacles, such as rock outcroppings, deflecting them.
    Early investigations tended to concentrate on the mathematics behind a meandering shape because other things tend to buckle in similar shapes. For example, if you hold a thin metal strip (such as a metal ruler) between your hands and then compress it by moving your hands toward each other, the strip's sideways buckle resembles a typical loop in a river meander. The buckled shape has to do with the energy associated with the compressed parts of the rod. That energy is reduced to a minimum if the rod takes on the buckled shape instead of remaining straight. Could a similar energy reduction be attributed to river meander?
   Recently Brian Hayes examined that question's history in an article in American Scientist (vol. 94, no. 6, pages 490-494, November-December 2006). In particular, he explored the research of Luna Leopold who coauthored a delightful article in Scientific American in June 1966. Both Hayes and I were captured by that article, and I eagerly included the subject of river meandering when I began writing the original Flying Circus of Physics material.
   Although an explanation of river meander due to a mathematical reduction in energy is very tempting, the situation in an actual water flow is far too complex for the explanation. Instead, we must consider how, once chance has diverted the flow slightly to one side, the deflection is enhanced when water moves into the deflection. The deflected water's path is (roughly) spiral, with downward motion along the outer bank in the deflection. That downward portion tends to carve away the outer bank, making the deflection even more pronounced. Given enough time, the deflection forms a loop. Further erosion can even cut off a loop, leaving it isolated from the rest of the river. The loop is then usually called an oxbow. 

http://www.stacey.peak-media.co.uk/Year7/7-7Rivers/7-7Meanders/7-7Meanders.htm River meander images  

· · ·  Edwards, B. F., and D. H. Smith, “River meandering dynamics,” Physical Review E, 65, article # 046303 (12 pages) (2002)
·  Hooke, J., “River meander behaviour and instability: a framework for analysis,” Transactions of the Institute of British Geographers, 28, No. 2, 238-253 (2003)
·  Hooke, J. M., “Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river,” Geomorphology, 61, 225-238 (2004)
· · ·  Camporeale, C., and L. Ridolfi, “Convective nature of the planimetric instability in meandering river dynamics,” Physical Review E, 73, article # 026311 (7 pages) (2006)
·  Hayes, B., “Up a lazy river,” American Scientist, 94, No. 6, 490-494 (November-December 2006)
· · ·  Seminara, G., “Meanders,” Journal of Fluid Mechanics, 554, 271-297 (2006)
· · · Constantine, J. A., and T. Dunne, “Meander cutoff and the controls on the production of oxbow lakes,” Geology, 36, No. 1, 23-26 (January 2008)

 
Want more references? Use the link at the top of this page.

2.35  Swimming in dead water
Jearl Walker www.flyingcircusofphysics.com
July 2009    Dead water refers to a peculiar sea condition in which a ship is hardly able to move, even at full power. In The Flying Circus of Physics book I describe one of the earliest accounts of dead water. While on a polar expedition in August 1893, the ship Fram encountered dead water on the northern coast of Siberia. The ship was capable of traveling 6 or 7 knots but in the dead water it could manage only 1.5 knots, even though both the water and the weather were calm. Moreover, control of the ship was marginal; in fact, the captain was forced to travel in loops to escape the dead-water region. The water was not visibly different from any other stretch of ocean water.

Dead water occurs when a layer of relatively fresh water overlies salt water, which can happen when a river empties onto ocean water or where an iceberg melts onto ocean water. Two interfaces play a role: the air–fresh water interface and the fresh water–salt water interface. Normally, much of the energy of a ship engine creates waves along the first of those interfaces—think of the wave production as a form of drag on the ship. In dead water, however, the ship produces two sets of waves, one along each interface, and so the drag is significantly more. The faster the ship tries to go, the faster its energy drains to the internal waves, as they are called, on the fresh water–salt water interface.

The ship’s bow is located above the first crest in the internal wave. The water just below that crest moves in the direction opposite the ship, opposing the ship’s motion. The Fram’s length was such that the rudder was also above a crest of the internal wave, and so the rudder was of little use in maneuvering the ship.

This video shows a toy boat moving over dead water, with the bottom salty layer dyed so that you can visibly distinguish the two layers. The boat is too small for the alignment of bow or rudder with the internal wave but you can see how the internal wave is created. Part of the boat’s energy goes into producing that wave.

http://www.youtube.com/watch?v=PCOL8kUtufg video from New Scientist

Recently a group of researchers (Sander P. M. Ganzevles, Fons S. W. van Nuland, Leo R. M. Maas, and Huub M. Toussaint) from The Netherlands investigated whether dead water can interfere with swimming. That is, if you swim through a region of dead water, is your progress slowed?

To see, a “swimmer” lay on a carriage that was gradually pulled over the top of a tank of water while he reached down into the water and pulled backward with a common swimming stroke. This was done several times first with the tank containing only fresh water and then the tank containing a layer of fresh water overlying very salty water. During the stroke, the swimmer’s hands would come near the interface between the two layers of water.

With both arrangements, the researchers monitored the hand and body motions and measured the speed of the carriage. They found that in the dead-water arrangement, propulsion of the carriage was significantly less, suggesting that significant energy from the stroke was being diverted to internal waves along the interface between the two layers of water.

The researchers also measured the speed of swimmers for a front-crawl swim of 5 meters, both in the fresh-water and dead-water arrangements. They reported that the average speed was about 15% less in the dead-water arrangement.

Less pronounced layering of water can occur in a fresh water lake due to temperature variations with depth (rather than salinity), especially in calm conditions when there not much overturning of the water. The researchers speculate that you might sense something strange about swimming in such lake if your strokes carry your hands down to an interface that separates significantly cooler water from overlaying water that has been warmed by sunshine. If your energy is diverted to internal waves along such an interface, you may grow tired surprisingly early in your swim.

http://www.newscientist.com/article/dn15003-mysterious-dead-water-effect-caught-on-film.html?feedId=online-news_rss20

References
Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Bascom, W., Waves and Beaches, Anchor Books, 1980, pages 139-140
· Jelley, J. V., “Sea waves: their nature, behaviour, and practical importance,” Endeavour, 13, No. 4, 148-156 (1989)
· Walker, J. M., “Farthest north, dead water and the Ekman spiral. Part 2: Invisible waves and a new direction in current theory,” Weather, 46, 158-164 (1991 )
·· Harleman, D. R. F., “Keulegan legacy: saline wedges,” Journal of Hydraulic Engineering, ASCE, 117, No. 12, 1616-1625 (December 1991)
··· Motygin, O. V., and N. G. Kuznetsov, “The wave resistance of a two-dimensional body moving forward in a two-layer fluid,” Journal of Engineering Mathematics, 32, 53-72 (1997)
· Ganzevles, S. P. M., G. S. W. van Nuland, L. R. M. Maas, and H. M. Toussaint, “Swimming obstructed by dead-water,” Naturwissenschaften, 96, 449-456 (2009)

2.39  Dust devil core
Jearl Walker www.flyingcircusofphysics.com
October 2006    The core of a dust devil can be defined in terms of the speed of the air around the center of the vortex. According to both theoretical models and measurements in natural dust devils, the speed is zero at the center and increases with distance from the center. At the edge of a core, the speed is maximum. The speed then decreases with greater distance from the center. So, were a dust devil to sweep over you (which is definitely not a good idea because of all the blown debris), you would feel the maximum wind speed as the near edge of the core passed you, almost no wind speed as the center of the core passed you, and then maximum wind speed again as the far edge passed you.
    Although small children and animals have, on rare occasion, been picked up by especially large dust devils, chances are that you would be just pelted by the debris. Be thankful that you are not on Mars. There dust devils are huge, large enough to show up on satellite imagery. There you might go flying.
http://www.youtube.com/watch?v=CQLCJFbABgg&feature=related Huge dust devil, with people on bikes riding through it and the camera operator walking through it

http://www.youtube.com/watch?v=u2gT9GRirN8&NR=1 Wal-Mart dust devil

http://www.youtube.com/watch?v=2rK-ctpFBz8&NR=1 Big dust devil develops at a baseball game

http://video.google.com/videoplay?docid=899964669942411501&q=dust+devils&hl=en

http://www.youtube.com/watch?v=YFwzNNEuOSY&mode=related&search= Dust devil (vortex) produced by a fire

http://www.youtube.com/watch?v=2SWTzZXc0sg Driving through a dust devil
http://www.youtube.com/watch?v=Kwa0ivfrcvE Whirlwind coming off a bonfire

http://www.youtube.com/watch?v=GtiDTT8JQsY More bonfire vortices, really good

http://www.youtube.com/watch?v=H37oeNVJUDM More of the bonfire vortices

http://www.youtube.com/watch?v=VDcRe1_bHjY Big dust devil at camping ground

http://www.youtube.com/watch?v=2iBjqFJsraM Paragliders picked up a whirlwind

http://www.youtube.com/watch?v=5Fw1qiAld2U&NR=1 Whirlwinds from a brush fire

The photo here is by ninevoltheart.

· · ·  Kurgansky, M. V., “Steady-state properties and statistical distribution of atmospheric dust devils,” Geophysical Research Letters, 33, article # L19S06 (4 pages) (2006)
· · ·  Balme, M., and A. Hagermann, “Particle lifting at the soil-air interface by atmospheric pressure excursions in dust devils,” Geophysical Research Letters, 33, article #L19S01 (5 pages) (2006)
· · ·  Gu, Z., Y. Zhao, Y. Li, Y. Yu, and X. Feng, “Numberical simulation of dust lifting within dust devils---simulation of an intense vortex,” Journal of the Atmospheric Sciences, 63, 2630-2641 (October 2006)
·  Oke, A. M. C., D. Dunkerley, and N. J. Tapper, “Willy-willies in the Australian landscape: The role of key meteorological variables and surface conditions in defining frequency and spatial characteristics,” Journal of Arid Environments, 71, 201-215 (2007)
· Oke, A. M. C., D. Dunkerley, and N. J. Tapper, “Willy-willies in the Australian landscape: Sediment transport characteristics,” Journal of Arid Environments, 71, 216-228 (2007)

Want more references? Use the link at the top of this page.

2.39  Martian dust devils
Jearl Walker www.flyingcircusofphysics.com
April 2007     Some of the Martian dust devils (whirlwinds) are so tall that they can be seen from a satellite orbiting Mars. In fact, the satellite can even see their shadows on the Martian surface. In addition, the Martian rovers have captured images and even “movies” of Martian dust devils from the ground level. Check out the URLs listed below.
     Dust devils on Earth are often charged because when dust, sand, and other debris are airborne, they tend to exchange electrons as they collide with another and the ground. As result, a dust devil can flash and pop as pockets of negative charge suddenly spark to pockets of positive charge. The Martian dust devils are likely to be highly charged, and because the air pressure there is much lower than on Earth, the sparking could be widespread.
     Just image that you are one of the first explorers put down on the Martian surface and have the bad luck of being in the path of a large dust devil. It would not be like the playful dust devils found in the American Southwest. Rather it would be a kilometer high and several hundred meters wide. And the winds may be up to 30 meters per second, more than enough to scour your viewing visor and the rest of your spacesuit with a continuous flow of airborne sand. As you wait out the passage, you are surrounded by a continuous play of sparks, as if you are at a celebration and somehow ended up having the celebration fireworks go off on your body. As your visor is scrubbed until it is opaque and sand is driven into every crevice of your spacesuit, you suddenly worry about what all that sparking might do to the microelectronics of your communications gear and the thermal control of your spacesuit. Oh, yes, there is also the oxygen control.
    Martian dust devils will not be playful to explorers. The rovers already there have been lucky to have avoided the big ones.
URLs: Movies and other images of Martian dust devils
http://science.nasa.gov/headlines/y2005/14jul_dustdevils.htm
http://antwrp.gsfc.nasa.gov/apod/ap050426.html   Several photos run as a video.
http://www.msss.com/mars_images/moc/7_1_99_devils/
http://mars.jpl.nasa.gov/gallery/duststorms/
http://www.msss.com/mars_images/moc/lpsc2000/3_00_dustdevil/
http://www.lpl.arizona.edu/~lemmon/mer_dd.html

·  Stanzel, C., M. Patzold, R. Greeley, E. Hauber, and G. Neukum, “Dust devil on Mars observed by the High Resolution Stereo Camera,” Geophysical Research Letters, 33, article # L11202 (5 pages) (10 June 2006)
· Drake, N. B., L. K. Tamppari, R. D. Baker, B. A. Cantor, and A. S. Hale, “Dust devil tracks and wind streaks in the North Polar Region of Mars: A study of the 2007 Phoenix Mars Lander Sites,” Geophysical Research Letters, 33, article #L19S02 (4 pages) (2006)

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