Buoys – rafts – and aquatic devices – Swimming aid to increase stroke efficiency – Foot attached
Reexamination Certificate
2002-03-05
2002-12-24
Sotelo, Jesus D. (Department: 3617)
Buoys, rafts, and aquatic devices
Swimming aid to increase stroke efficiency
Foot attached
Reexamination Certificate
active
06497597
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
This invention relates to hydrofoils, specifically to such devices which are used to create directional movement relative to a fluid medium, and this invention also relates to swimming aids, specifically to such devices which attach to the feet of a swimmer and create propulsion from a kicking motion.
BACKGROUND—Description of Prior Art
One of the major disadvantages which plague prior fin designs is excessive drag. This causes painful muscle fatigue and cramps within the swimmer's feet, ankles, and legs. In the popular sports of snorkeling and SCUBA diving, this problem severely reduces stamina, potential swimming distances, and the ability to swim against strong currents. Leg cramps often occur suddenly and can become so painful that the swimmer is unable to kick, thereby rendering the swimmer immobile in the water. Even when leg cramps are not occurring, the energy used to combat high levels of drag accelerates air consumption and reduces overall dive time for SCUBA divers. In addition, higher levels of exertion have been shown to increase the risk of attaining decompression sickness for SCUBA divers. Excessive drag also increases the difficulty of kicking the swim fins in a fast manner to quickly accelerate away from a dangerous situation. Attempts to do so, place excessive levels of strain upon the ankles and legs, while only a small increase in speed is accomplished. This level of exertion is difficult to maintain for more than a short distance. For these reasons scuba divers use slow and long kicking stokes while using conventional scuba fins. This slow kicking motion combines with low levels of propulsion to create significantly slow forward progress.
Much of the drag created is due to the formation of turbulence around the blade portion of the fin. This turbulence occurs because prior fin designs do not adequately address the problem of flow separation and induced drag while lift attempting to generate lift. This destroys efficiency and severely reduces lift. On an airplane wing for instance, Bernoulli's principle explains that the air flowing over the convexly curved upper surface must travel over a greater distance than the air flowing underneath the lower surface of the wing. As a result, the air flowing over the upper surface must travel faster than the air flowing underneath the wing in order to make up for the increase in distance. Because of this, the air pressure along the upper surface of the wing decreases while the air pressure underneath the lower surface of the wing remains comparatively higher. This difference in pressure between the upper and lower surfaces of the wing causes “lift” to occur in the direction from the lower surface towards the upper surface. Because of this pressure difference, the lower surface on an airfoil is called the high pressure surface, while the upper surface is called the low pressure surface.
Another way of creating lift is to very the angle of attack. This is the relative angle that exists between the actual alignment of the oncoming flow and the lengthwise alignment of the foil (or chord line). When this angle is small, the foil is at a low angle of attack. When this angle is high, the foil is at a high angle of attack. As the angle of attack increases, the flow collides with the foil's high pressure surface (also called the attacking surface) at a greater angle. This increases fluid pressure against this surface. While this occurs, the fluid curves around the opposite surface, and therefore must flow over an increased distance. As a result, the fluid flows at an increased rate over this opposite surface in order to keep pace with the fluid flowing across the attacking surface. This lowers the fluid pressure over this opposite surface while the fluid pressure along the attacking surface is comparatively higher. Because of this pressure difference, the attacking surface is the high pressure surface and the opposite surface is called the low pressure surface or lee surface.
The increase in pressure along the high pressure surface combines with the decrease in pressure along the low pressure surface to create a lifting force upon the foil. This lifting force is substantially directed from the high pressure surface towards the low pressure surface. Varying the foil's angle of attack in this manner is important in swim fin designs because it enables lift to be generated on both the upstroke and the down stroke of the kicking cycle.
Although this method of generating lift is commonly used on prior swim fin designs, many problems occur that significantly reduce performance. One problem is that prior designs place the propulsion foil at excessively high angles of attack. In this situation, the flow begins to separate, or detach itself from the low pressure surface of the foil. When this occurs, the foil begins to stall. The separated flow forms an eddy which rotates around a substantially transverse axis above the low pressure surface. This eddy causes the fluid just above the low pressure surface to flow in a backward direction from the trailing edge towards the leading edge. This separation decreases lift since it reduces the amount of smooth flow occurring over the low pressure surface. This is a serious problem because smooth flow must exist in order for lift to be generated efficiently.
When the angle of attack becomes too high, the foil stalls completely and the flow along the low pressure surface separates into chaotic turbulence. This destroys lift by preventing a strong low pressure zone from forming over the low pressure surface, or lee surface. As a result, only a small difference in pressure exists between the opposing surfaces of the foil. Many prior fin designs suffer from this problem because they employ a horizontally aligned blade which is kicked vertically through the water. In this situation, the angle of attack is substantially close to 90 degrees, and therefore the blade is completely stalled out. This causes the blade to act more like an oar blade or paddle blade rather than a wing.
As well as destroying lift, stall conditions also cause high levels of drag. When areas of laminar flow (a flow condition where fluid passes over an object in a series of undisturbed layers) are abruptly converted into chaotic turbulent flow, a high drag condition known as transitional flow occurs. Because prior swim fin designs create stall conditions and chaotic turbulence along their low pressure surfaces, they generate high levels of drag from transitional flow.
Another problem that occurs at higher angles of attack is the formation of vortices along the outer side edges of the blade which cause induced drag. The difference in pressure existing between the attacking surface and the low pressure surface causes the fluid existing along the blade's attacking surface to flow outward toward the side edges of the blade, and then curl around the outer side edges toward the low pressure surface. As this happens, the swirling motion creates a streamwise tornado-like vortex along each side edge of the blade just above the blade's low pressure surface. As the water curls around the side edges of the blade, these vortices carry the water in an inward direction along the low pressure surface. After this happens, the vortices curl the water in a downward direction against the blade's low pressure surface. As this water is forced downward against the low pressure surface, it is moving in the opposite direction of desired lift thereby further reducing lift. This downward moving flow deflects the fluid leaving the trailing edge at an undesirable angle that is oppositely directed to the direction of desired lift. Because the direction of lift is perpendicular to the direction of flow, this downward deflected flow (called downwash) causes the direction of lift to tilt in a backward direction. Consequently, a significant component of this lifting force is pulling backward upon the blade in the opposite direction of blade's movement through the water. This backward force is call
Knobbe Martens Olson & Bear LLP
Sotelo Jesus D.
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