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Fluid Dynamics of Swimming | |
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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 fastest swimmers use the fundamental (basic) principles of fluid dynamics the best. These same principles have been used by anyone who has ever treaded water (keeping your head above water by moving your arms and legs).
There are four primary forces acting on a swimmer:
The pressure drag comes from the front of the swimmer exposed to the water and the flow separation behind the swimmer. Since the flow is already turbulent (not laminar or smooth) a swimmer must streamline his body to reduce the flow separation even more (see the figures below). Frontal drag is reduced by thrusting and kicking, which brings the body up higher out of the water. Meanwhile, skin friction increases as the body becomes more streamlined. But, pressure drag is the main concern (worry) and if it is reduced then overall drag is reduced.
Arm Stroke 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 comes from pushing back on the water like a paddle wheel. For a while it was thought that moving the arms in shorter strokes, like a caterpillar paddle wheel, would increase speed. This 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 understand this quote, we should look at what is the most efficient arm stroke. The straight arm is brought around in a full circle, with the hand flat (see paddle wheel figure). 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.
Interestingly, the paddle wheel or propeller style movement creates lift and thrust. In this case lift adds to thrust. To visualize this better, think of the swimmer's arm like an airfoil (or wing). As the arm plows through the water lift is created which lifts the body closer to the surface of the water. This reduces drag. So, even though the swimmer is moving from right to left, his hand is moving from top to bottom and the lift force adds to the thrust (see figure). Therefore, the swimmers arm stroke becomes similar to an airplane's propeller. 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 (moves) the hands back and forth near the surface. The legs and feet are making similar movements. This results in the production of lift, which in turn keeps the swimmer's head above water. Kick The swimmer's kick has two purposes. First, to add thrust and the second to acquire (get) stability. It is important to keep the feet in the water. Each time the swimmer's feet leave and enter the water, air comes along, increasing drag and reducing thrust. 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 breast stroke kick. The breast stroke kick is most similar to the movements of the legs while treading water. Remember, while treading water, the legs are moved through the water in a similar fashion as the arms to produce a lift force. As the swimmer moves each arm around through the water he must be careful not to twist his body. This increases pressure drag as the body becomes less streamlined. A proper kick helps to keep the body streamlined, thereby reducing the drag (see figure).
In the freestyle (breast stroke), there 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 (short distance, fast pace), 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 is similar to the movement of a dolphin tail. The butterfly kick has a stabilizing effect when it is done in time with the stroke. Studies show a remarkable similarity between champion butterfly swimmers and captive dolphins. Summary 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 apply 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. Mark Spitz, one of the best swimmers in history, used all of the techniques shown here.
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