Science of Swimming Faster - Text Version | SlideHTML5
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This error is perhapsthe greatest contributor to incorrect analysis and understanding of the fluiddynamics associated with swimming. Pressure Drag and Viscous DragIn an ideal fluid, figure 1. Note that inthis ideal case, the flow is symmetric front to back.
From the Bernoulli equa-tion, we would then expect that the pressure on the back of your body wouldexactly return to the pressure on the front of your body. Fluid dynamicists refer to this situation as complete pressure recovery.
A great deal of turbulence lots of interacting vortices is generated behind your body. Rather, what is called a wake region is found behind your body, shown in figure 1. Now a pressure difference is seen between the front and back of your body; the higher pressure is in front, and the lower pressure is in back.
That frontal area would naturally be the same whether you were standing facing the wall or standing with your back to the wall. Because pfront is larger than pback, you feel the wind pushing you backward as you try to move forward into it. This phenomenon is called pressure drag or bluff body drag.
If the body shape does not allow the flow to travel smoothly from front to back, then pressure drag will be significant. If, however, the body shape permits smooth flow all the way around, if the body is streamlined like a wing, then pressure drag will be minimal or nonexistent.
That does not mean that the drag will be zero. All real fluids are viscous, which means there is always a frictional resistance to motion through or by a fluid. To visualize this, think about a brand new deck of cards. Imagine that you set the deck on a flat horizontal table and place the palm of your hand flat on top of the deck. Now imagine sliding your hand horizontally to the right, as shown in figure 1. If the cards were perfectly frictionless, they would not sense the motion of your hand and the entire deck would stay put.
The deck after the application of the sliding force, F, is shown as grey rectangles. Clearly, if no friction were present, the before and after pictures would be identi- cal and look like figure 1.
In reality, some amount of friction is present between your hand and the top card, between the table and the bottom card, and between the individual adjacent cards in the deck. In that case, when your hand moves parallel to the table top, the top card moves with your hand, the bottom card sticks to the table, and all the cards in between move by varying amounts depending on how close they are to either the table or your hand.
This concept is illustrated in figure 1. The closer the cards are to your hand, the more they will move. The closer they are to the table, the less they will move. The card closest to the table is held in place by the frictional force between the table and that card.
The top card experiences a frictional force imparted by the movement of your hand, which drags it forward at the same speed as your hand. The rectangles represent different layers of fluid, or laminates. The arrows labeledF indicate a sliding force applied to the fluid layers. In the end,the entire deck starts to lean.
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Think of viscous or frictional drag in a similar fashion. Instead of playingcards, think of the rectangles in figure 1. Think of the tabletop as a streamlined body, say a wing on an airplane. Inthis analogy, we would be sitting inside the airplane looking out the window. The wing looks as if it is stationary, and the air is flowing over the wing from leftto right.
If the flow were frictionless, as shown in figure 1. The picture for viscous, or frictional flow, figure 1. The layer ofair closest to the wing would stick to the wing and move with the wing. From ourvantage point looking out the window, it would appear as if it were stationary.
Far away from the wing, the air would simply blow on by. In the region referredto as the boundary layer the air speed would vary from zero at the boundary outto the speed at which the plane is flying. This boundary layer region is typicallyvery thin. Over the wing of a large commercial airliner, it could be only a fewinches 5 to 10 cm thick before the air passed over the trailing edge of the wing.
Probably the simplest way to gain a conceptual understanding is to remember that as a body moves through any fluid, a bow wave forms in front of the body. For a boat or a swimmer, this wave is clearly visible.
Below a certain speed, the bow wave sits in front of the swimmer and the swimmer is essentially swimming uphill, trying to get on top of the wave but never getting there.
This phenomenon is known as wave drag. For every object, there is a critical speed at which it is possible for the object to climb up on top of the bow wave. This speed is known as the hull speed. When Vhull is achieved or exceeded, the swimmer is in a sense surfing her own bow wave. When this happens, the wave drag drops dramatically and the speed can increase equally dramatically.
To visualize this, think of a motorboat when you first try to get it to go as fast as it can and then when it later gets to high speed. When you first push the throttle forward, the hull is deep in the water. The bow rides very high, the back of the boat is very low, and the boat essen- tially plows through the water. When the boat speed exceeds its hull speed, the boat rises out of the water and can accelerate to high speeds by planing across the top of the water.
The reason that wave drag is so interesting is that the hull speed of a human swimmer turns out to be right around the same speed as world-class swimming speeds. As a rough estimate, suppose a swimmer is about 2 meters tall; his hull speed would be 1.
This speed translates to a We are, of course, neglecting the start and turn, but you will notice that only a relatively few swimmers can swim faster than this. So the next time you are watching an elite-level race, watch how high or how low the swimmers are in the water.
If you are watching the preliminary heats, you will probably notice that on average the faster swimmers ride higher in the water whereas the slower swimmers are lower in the water. Of course, other variables such as percentage and distribution of body fat will cause certain swimmers to float higher or lower. The polyurethane and neoprene body suits that were used in through were extremely helpful to many swimmers in terms of buoyancy.
They automatically sat higher in the water and gained much of the benefit of overcoming wave drag without having to generate the speed necessary to overcome it. As a quick summary up to this point, the three forms of drag experienced by a swimmer are pressure or bluff body drag, viscous or friction drag, and wave drag. Pressure drag and wave drag are the dominant forms of drag in swimming.
Friction drag dominates only for very streamlined bodies such as airplanes or Fluid Dynamics, Propulsion, and Drag 11animals that depend on high-speed swimming or flight for survival. In thosecases, friction becomes dominant not because frictional drag increases butbecause the pressure drag is extremely small, by virtue of body shape.
Note thatfish and birds typically do not move at the air or water interface like competi-tive swimmers do. As such, their wave drag is zero. In any event, swimmers cando little to eliminate viscous drag. Their focus needs to be on optimizing bodyposition to reduce pressure drag and optimizing stroke production to maximizethrust. Swimmers need to be constantly winning the battle between thrust anddrag by both maximizing thrust and minimizing drag.
The underlying question in this discussion is, What must a swimmerdo to make the water move in such a way as to generate as much thrust i. Two thrust generation mechanisms are dominant in competitive swimming: A third mechanism derived from wing theorygave rise to the S stroke popular in the s and s that continues at theclub level today. Because of the legacy surrounding the S stroke, we will examinethe underlying aerodynamic theory to provide a framework for comparing thestraight pull with the S stroke.
Actually, a variety of physically accurate ways can be used to explain each ofthe thrust production mechanisms. For ease of comparison, the best approachis to use a single physical concept and relate each of the mechanisms to thatconcept. Consider the example of an ice skater standing on a smooth sheet of iceholding a ball of significant mass.
At some instant in time, the skater brings theball to his chest and throws it by pushing it away from his chest. You have likelyheard of the law of equal and opposite reaction or possibly the conservation oflinear momentum. In this example, when the skater throws the ball forward,he ends up being propelled backward. How much or how fast each one movesdepends on their relative weights or to be precise, their relative masses.
But if the ball were the same weightas the skater, each would move an equal distance in their respective directions. The takeaway from this example is that to propel himself forward, the swim-mer must somehow take as much water as possible and throw it along the axisof his body past his feet.
This action will propel him forward in the water. Those balls of water arein fact the same stuff that the swimmer is trying to push his way through.posavski-obzor.info LIVE: Backyard Meet of the Century Bench Flights 3 and 4
How he does this is through technique. One way to propel water along the body and past the feet is to set up a system of counter-rotating vortices that pump fluid between them in a particular direc- tion.
An easy way to envision this is to think of a baseball pitching machine consisting of two rubber wheels that are very close together and spinning in opposite directions. When a baseball is dropped between the two wheels, it is squeezed through the gap and shot out by the spinning action of the wheels. The direction of the ball will be perpendicular to the line connecting the centers of the two spinning wheels.
This example illustrates the principle of vortex-induced thrust. Another example of this phenomenon is found in a swimming eel, shown in figure 1. As the eel undulates along, it generates vortices on either side of its body. If you think of the body undulations as a wave, the vortices form in the peaks and valleys of the waves as shown in the figure.
How those vortices are created is beyond the scope of this discussion. What is important is the orientation and interaction of the system of vortices. Observe that the two vortices on the top of the figure i. That is, they are two vortices that rotate in opposite directions like the two spinning rubber wheels on the baseball pitching machine. The entire vortex system acts to pump water along the eel from the head to the tail.
If the eel wants to accelerate for anyreason, it snaps its body to create a high-energy wave, producing a system ofmuch stronger vortices that pump more water, faster, along the axis of its body. This mechanism is how the dolphin kick works. The second dominant thrust production mechanism in competitive swim-ming comes from the arm motions in each stroke.
Keeping with our concept ofthrowing water backward, the arms and hands push as much water as possibletoward the feet. The arms must move in a manner to maximize both the amountand the speed of the water being pushed backward.
There is an alternative way to think about this. Recall that we talked about pressure differencesbetween the front of the bluff body and the rear.
We discussed that in real flowspast bluff bodies, the flow separates from the body, creating a wake region; thepressure does not recover behind the body, resulting in higher pressure in frontand lower pressure behind. For a body moving forward, for instance, if you stickyour arm out the window of a moving car with your palm facing forward, thenet action of the pressure difference multiplied by the frontal area of the armand hand results in a drag force opposing the forward motion.
In the case of the arm stroke, however, the arm moves in the direction oppo-site the swimming direction.
If we think of the hand and arm combination asa single bluff body with the palm of the hand typically facing toward the feet,then the pressure on the palm side of the hand and arm will be higher we hopesignificantly higher than the pressure on the back side. In this case, the dragexperienced by the arm pushing backward through the water is actually a thrustfor the swimmer.
In this context, thrust produced by the arm is drag backward. This idea is important in thecontext of swimming because it is the underlying phenomenon that gave riseto the S-shape pull pattern that misdirected many thoughts on ideal swimmingbiomechanics. The concept is that the swimmer can generate thrust by sweeping the handoutward initially and then inward toward the centerline of the body during thefront half of the pull; this is the top of the S.
The swimmer then moves the handoutward again during the back half of the pull. In so doing, the hand acts likea wing, particularly in the inward sweep. We will examine this in detail in thediscussion of the pull. At this point, we will focus on the underlying theory ofwings. Camber means the wing isnot symmetric top to bottom. In this case, the upper surface is curved whereasthe bottom surface is essentially flat.
The direction of the incoming wind isshown with an arrow labeled with the speed, V. The wing is said to have a chord length, b, anda span, S. This is the projected area,or the area of the silhouette of the wing from a viewpoint looking straight downon the wing. As air flows around the wing, the airspeed on top of the wing will be fasterthan the speed under the wing, simply because air must travel a greater distanceover the top of the wing than under the bottom. We refer to the upper and lower surfaces of the wing as the suction andpressure surfaces, respectively.
The important features of figure 1. The steepdrop-off in lift is because of stall, when flow no longer smoothly follows theupper wing surface but instead separates much in the manner of a bluff body. Modern commercial airliners use complex systems of flapsto increase CLmax to as high as 3. These planes are also designed to fly at angles of attack much lowerthan maximum lift to avoid the disastrous consequences of small changes inangle of attack leading to stall.
When applied to swimming, the hand is presumably acting as the wing. Thelift on the wing becomes a thrust force when the hand sweeps inward towardthe centerline of the body in the S stroke.
You can calculate the thrust that couldbe generated by such a motion in which the wing planform area, Aplan, would beroughly the projected area of the hand; the flight speed, V, would be roughly thespeed at which the swimmer sweeps her hand across her body in the S stroke.
Note that this is distinctly different from the speed at which the swimmer isswimming or the speed at which her hand moves in the direction from headto feet. We will use some representative numbers when we discuss the fluiddynamics of the arm pull. Force and Flow MeasurementUp to this point, we have been discussing in a somewhat theoretical way forcesthat swimmers apply to the water. You have likely gained some practical insightsinto fluid mechanics that will be helpful with respect to swimming.
In thissection, we provide a description of some of the state-of-the-art measurementsystems used in fluid mechanics. Ultimately, the challenge to understanding the fluid dynamics of swimmingis knowing exactly what the water is doing as the swimmer swims through itand what resulting forces the swimmer generates. Two key advances in flow andforce measurement techniques have made it possible to provide time-resolveddata of both flow and force. The technique employs a high-resolution digital video camera to record flow seeded with very small reflective particles.
Every individual video image in the recording is then captured and stored in a computer as raw data. A DPIV processing program takes pairs of successive video frames in the recording known as DPIV image pairs and computes how much the seed par- ticles have moved from one frame to the next. An example of how this process works appears in figure 1.
Two successive video frames taken from flow past a cylinder are shown in figures 1. Note that the images shown are only about 10 percent of the full frames. You can see the cylinder on the left side of both images and many little white dots that are reflections of laser light off small silver-coated glass spheres.
In the lower right of figure 1.
Science of Swimming Faster
In the second frame, figure 1. The previous location of the particles is shown as a square with a dotted white line.
The new location of the particles is indicated by a square with a solid white line. Assuming that the particles move exactly with the flow, the flow velocity associated with the water in the square region shown is then simply the particle displacement divided by the time between images. In practice, the computer is programmed to subdivide the two frames into identical grids of small squares like the one shown in figure 1. By repeat- ing the velocity calculation for the entire video frame and for every image pair in the video record, we can generate movies of the fluid velocity everywhere in the camera field of view.
Examples of DPIV measurements of flow around swimmers are shown in figures 1.