|Fluid Dynamics of Swimming||page 1|
Most people enjoy swimming, especially on a hot summer day. However, the level of swimming skill varies greatly between recreational and competitive swimmers. Even at the competitive level, some swimmers are faster than others. The difference lies in the ability to use the fundamental principles of fluid dynamics to one's advantage. The same principals which makes the best swimmers so fast are employed by anyone who has ever treaded water.
In order to understand how these principals apply, we must first understand the forces acting on a swimmer. There are four primary forces acting on a swimmer. These forces are similar to the forces acting on an airplane and are presented in the figure below. In the vertical plane, the weight of the swimmer is offset by the buoyancy. However, since people have varying natural ability to float, another means of overcoming the swimmers weight must be employed. This is accomplished from the arm stroke and kick. By pressing down on the water, an equal an opposite reaction occurs which lifts the swimmer higher in the water. The other primary forces are the thrust or propulsive force and the drag.
The drag can be divided into two components: pressure drag and skin friction drag. The pressure drag comes from the frontal area exposed to the water and the separation that occurs behind the swimmer. This is similar to the pressure drag of the smooth golf ball. In order to reduce the drag on the golf ball, dimples were introduced which changed the nature of the flow from laminar to turbulent. The turbulent flow delays separation and therefore reduces the pressure drag. However, the flow around a swimmer is already turbulent. Therefore, a swimmer must streamline his body to reduce the amount of separation as shown in the figure. The drag from the skin friction, on the other hand, increases when the swimmer becomes more streamlined since more surface area is exposed to the water. This is not really a concern, as the pressure drag is dominant and, therefore, the overall drag decreases.
Generally speaking, the arm stroke produces the majority of the thrust. The difference between swimmers is how the arm stroke is used to produce thrust. The best swimmers not only achieve thrust by pushing back on the water, but also by moving their hands and arms like a propeller.
The most obvious production of thrust does come from pushing back on the water like a paddle wheel or rowing a boat. In fact, the straight arm pull was originally thought to be most efficient. Recently, some people have advocated a straight-back pull stroke. This concept was tested using a riverboat equipped with a caterpillar paddle wheel (see figure). Unfortunately for the inventor, the boat practically stood still. The explanation for this can be found in the following quote:
Maximum efficiency in water is achieved by pushing a large amount of water a short distance rather than by pushing a small amount of water a large distance.To fully appreciate the significance of this quote, we should look at the most efficient arm stroke. This stroke involves moving the arm along a curvilinear path. This way the swimmer is always pushing back on still water. The advantage here is that the still water offers more resistance than the water that is already moving back.
As mentioned above, the fastest swimmers also generate thrust by moving their arms and hands like a propeller to generate a lift type force. To visualize lift as a propulsive force, think of an airfoil flying from right to left. Notice that the lift is acting up in the positive vertical direction. Now, rotate the airfoil 90 degrees so that it is moving from the top of the page to the bottom. Notice that the lift force is now directed to the left which is the direction that the swimmer is moving. So, even though the swimmer is moving from right to left, his hand is moving from top to bottom and the lift force contributes to the thrust. Therefore, the swimmers arm stroke becomes similar to an airplane's propeller. It should also be noted that the hand must continue to move in the directions perpendicular to the direction of forward movement in order for this lift force to be produced. Studies of Olympic caliber swimmers show that this is indeed the case.
This same lift force is generated while treading water. While treading water, one does not push down on the water. Instead, one sculls the hands back and forth. This results in the production of lift which in turn keep the swimmers head above water. Champion swimmers have simply learned how to apply this same technique to their arm stroke to produce an increased thrust which propels them through the water faster than their competitors.
As can be seen in the figure, the drag acts in the vertical direction. Therefore, the normally detrimental drag force is now applied to raising the body higher in the water to counteract the swimmers weight. To further increase the propulsive force, the hands pitch can be altered to take advantage of the greater resultant force as shown in the diagram below.
The kick provides a stabilizing effect in addition to the propulsive force. Most swimmers only get a small amount of propulsive force from their kick. The first way to improve the kick is to keep the feet in the water. When a swimmers feet enter the water, a significant amount of air enters as well. The air increases the drag as well as reducing the propulsive effect. The best swimmers go further by moving their feet during the kick to produce the same lift force achieved by their hands. This is much harder to visualize unless you look at the breaststroke kick. The breaststroke kick is most similar to the movements of the legs while treading water.
Again, while treading water, the legs are moved through the water in a similar fashion as the arms to produce a lift force. In the breaststroke kick, the legs and feet are moved in a similar manner to produce a significant amount of thrust. The same mechanism is employed in the kick of the other strokes to a smaller extent.
Due to the asymmetrical armstroke of the freestyle and backstroke, the kick also acts as a stabilizer. In these two strokes, as one arm is recovering out of the water, the other is producing a propulsive force. Since this force acts to the side of the swimmers center of gravity, a moment is applied to the body which causes the swimmer to twist in the water. This twist increases the pressure drag as the body becomes less streamlined. A proper kick helps to keep the body streamlined, thereby reducing the drag.
In the freestyle, their are three types of kicks: The six-beat kick, the two-beat kick, and the two-beat crossover kick. The six beat kick is favored by sprinters while the variations of the two beat kick are favored by distance swimmers. However there is no proof that one kick is better than another. However, the six beat kick might help lift the sprinter higher in the water, while the two beat kick retains the stabilizing effect while conserving the energy of the long distance swimmer.
The butterfly kick is also known as the dolphin kick since it simulates the movement of a dolphin tail. The thrust, as mentioned previously, is derived from the sculling motion of the feet through the water. The butterfly kick also has a stabilizing effect by aiding in the timing of the stroke. Most people find the butterfly the most difficult to perform. However, with the proper stroke mechanics and timing, one should find that the butterfly is a very simple stroke to perform.
Since the butterfly kick mimics the movements of a dolphins tail, comparisons between man and dolphin have been made. The studies show a remarkable similarity between champion butterfly swimmers and captive dolphins. The differences lie in the duration of the up and down strokes of the kick. The dolphin has a much faster up-stroke as the muscles have developed for this feat. The down-stroke is faster in man and so the overall duration of the kick is very similar.
So, as one can see, the gap between champion swimmers and recreational swimmers is the ability to use the basic principles of fluid dynamics to ones advantage. This shouldn't be too difficult, after all, anyone who has ever treaded water knows how to app ly these concepts. However, applying them while swimming is another issue. The fastest swimmers don't think about how their hands move through the water, but rather have a feel for the water. In fact, Mark Spitz used to say that he pulled his hand straight back down the middle of his body. Of coarse, this would be terribly inefficient as we discussed. In reality, Mark used all of the techniques shown here to make him one of the best swimmers in history.
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