Fluid hydrodynamics plays a role in a swimmer's overall performance, because the forces produced in and by water can either increase or decrease a swimmer's velocity. By educating yourself on the behavior of liquids and their effects on objects, and the four principles of hydrodynamics, you can learn to swim more proficiently and optimize your results.
Forward Power
Swimmers who successfully master "lift" understand that using it helps them create a "sculling" effect, which in turn, produces more torque and power. Lift is the perpendicular force relative to the working arm and comes into play in freestyle propulsion, wherein a swimmer performs a constant windmill-like arm movement to propel himself forward. Sculling forces help swimmers conserve more energy that can be used at a later time.
Center of Buoyancy
You can learn how to use buoyancy to your advantage to sharpen your swimming exercises, or techniques. Focus on your "center of buoyancy" -- identify the region of your sternum that allows maximum balance while propelling yourself through water -- to swim more proficiently by pressing your upper chest into the water. In turn, this brings your legs closer together, establishes balance and reduces drag as you propel yourself forward. You can work with this concept to improve your breaststroke, backstroke and crawl.
Swimming coaches stress the importance of using lift and drag. Propulsion is the force generated by your arms and legs to propel yourself forward, and uses lift and drag to maximize velocity in freestyle swimming. Pull backward with your hands curved and at right angles to the pulling direction to maximize your distance per stroke. You should notice a sweeping motion as you propel forward, since propulsion utilizes as much drag and lift resistance as possible relative to the swimmer's hand positions.
Smoother Strokes
As you propel yourself through the water using both your arms and legs, you will also use larger muscle groups such as your chest, your upper and lower back muscles, and your shoulders, to generate more torque and power and significantly decrease the amount of resistance from drag. As you perform a pushing motion upon completing your arm movement, you will notice less restriction by the force of drag, allowing you to perform smoother stroke movements.
Forces acting on a floating body
In order to float in water an object must be less dense than the water. This difference in density causes the object to float. This happens by way of a buoyancy force that "lifts" the object. The principle behind this lift is called Archimedes' principle, which states that any object (regardless of its shape) that is suspended in a fluid (such as water), is acted upon by an upward buoyant force equal to the weight of the fluid that is displaced by the object.When an object is placed in a fluid such as water, and floats as a result, the part of the object that lies below the surface of the water displaces a water weight equal to the weight of the object. This weight is equal to the buoyant force pushing upward on the object. The amount that the object sinks below the water surface corresponds to the equilibrium point, in which the object weight is equal to the buoyant force pushing upwards on the object.
Furthermore, the buoyant force acts through the center of buoyancy, which is the centroid (center) of the immersed part of the object - this is the volume of water that is displaced by the object. For the object to maintain its orientation in the water (i.e. not rotate) this buoyant force must pass through the center of mass of the object. If this buoyant force does not pass through the center of mass, and is offset from it as shown in the figure below [1], then the object will rotate until the buoyant force passes through the center of mass. At this point rotational equilibrium is reached. In the figure below the buoyant force acts at a point that is to the right of the center of mass and as a result will cause the swimmer to rotate counterclockwise.
Forces acting on a swimmer moving through the water
A swimmer propelling him or herself through the water has four basic types of forces acting on him or her. These are illustrated in the figure below [1].
Just like for an airplane wing, the convention for showing the forces acting on the hand is to break them down into a drag force and a lift force. The drag force acts in the same direction as the (oncoming) flow velocity. The lift force acts perpendicular to the flow velocity and points in the direction of inclination of the hand (relative to the flow velocity). The amount of inclination is given by the angle of attack. The magnitude of the lift force and drag force is related to the angle of attack. The resultant force is the vector sum of the lift force and drag force. This is the actual force that acts on the hand (and is felt by the swimmer) as it moves through the water at velocity V.
Note that the swimmer can move his hands in any three-dimensional direction in the water, causing the lift and drag forces (and resultant force) to act in any direction also. The figure above shows a single case where the flow of the water is towards the palm of the hand at the shown orientation.Swimmers use a complicated hand motion to generate the maximum thrust possible, depending on the swimming style used. This is illustrated in the figure below for the front crawl [3]. Note that D is the drag force acting on the swimmer's hand, L is the lift force acting on the swimmer's hand, and R is the resultant force acting on the swimmer's hand.
For a competitive swimmer, the intent is to point the resultant thrust force in the desired direction of motion as much as possible. This is illustrated in the figure below. The part of the resultant force that is used for propelling the swimmer in the desired direction of travel is the component force, as shown. Through the use of proper technique the swimmer keeps the angle θ as small as possible, since this maximizes the amount of force that is used to propel the swimmer along.
The underwater dolphin kick
In competitive swimming the dolphin kick is used to maximize the speed at the start of the race and after the turns in all the swimming styles besides the Breaststroke. Swimming underwater using the dolphin kick is more efficient than swimming at the surface because there is one less source of drag, known as wave drag, which is generated when swimming at the surface. Elite swimmers can propel themselves at up to 2.5 m/s underwater using the dolphin kick. This is comparable to the top surface speeds reached by elite swimmers. (Note that these swim speeds can be easily determined by looking at the world records for given swimming distances).
References
1. R. Schneider. Institute of Physics, Ernst-Moritz-Arndt University, Germany. //www.mnf.uni-greifswald.de/fileadmin/physik/PhysikSport9.pdf2. V. Gourgoulis et al. The effect of leg kick on sprint front crawl swimming, Journal of Sports Sciences, 2013.
3. H. M. Toussaint, A. P. Hollander, C. Van Den Berg, and A. Vorontsov. From "Exercise and Sport Science", Chapter: Biomechanics of Swimming, Publisher: Philadelphia, Lippincott, Williams & Wilkins, Editors: Garret W.E., Kirkendall D.T., pp.639-660, 2000.4. T. M. Barbosa, D. A. Marinho, M. J. Costa, and A. J. Silva. Biomechanics of Competitive Swimming Strokes, Biomechanics in Applications, 367-388, 2011. //bibliotecadigital.ipb.pt/bitstream/10198/6200/1/InTech-Biomechanics_of_competitive_swimming_strokes.pdf
5. A. E. Minetti, G. Machtsiras, and J. C. Masters. The optimum finger spacing in human swimming, Journal of Biomechanics, 42: 2188–2190, 2009.
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