Date: August 8, 2014, 11:16 AM
In sports fluid mechanics, understanding the dynamic effects of body movement is crucial. For instance, the "lift" generated by a freestyle swimmer's hand moves forward, while the "lift" from a rotating ball acts laterally, and the "lift" from a car's tail can even be downward. This is because racing wheels need to maintain sufficient contact with the track at high speeds to ensure continuous traction and power. Air resistance against the plate and the lift generated by the swirling motion of a ball create vortices and impulses, which are reactions from the fluid.
Although Bernoulli’s Law helps explain most of the propulsion forces in fluids for humans or sports equipment, there are some phenomena that are hard to fully account for using this principle alone. Examples include the force behind free kicks in soccer, butterfly strokes in swimming, and backstroke movements. There’s also the case of swimmers who use a fin-like ankle device, similar to a dolphin’s tail, to move through water without using their hands. By pushing with their waist and moving their legs, they achieve higher speeds than in freestyle swimming. The ankle’s up-and-down motion resembles the tail fins of fish and dolphins, creating more noticeable deformation during movement.
According to hydrodynamic studies of aquatic creatures, efficient propulsion methods—like those used by whales, dolphins, and fast-swimming fish such as tuna and swordfish—are similar to the aerodynamics on bird wings. Research in hydrodynamic tanks has shown that vortices form during these movements, much like the leg motions in freestyle, butterfly, and backstroke swimming. Take Qiyong, for example. When the ankle is at its highest point, it pushes downward, creating a counterclockwise vortex that gives the water behind a backward impulse, resulting in a forward reaction force on the body. At the lowest point, the ankle moves upward, generating a clockwise vortex that propels the body forward.
Additionally, when fluid flows around an object, under certain conditions, it can cause periodic resistance from Karman vortices. In contrast, the vortices from the tail in swimming are arranged oppositely, so they do not contribute to resistance but instead aid propulsion. Vorticity plays a dual role: on one hand, stronger vortices increase thrust, making them essential for overcoming resistance; on the other hand, excessive vortices lead to greater energy losses, proportional to the square of the vorticity. Therefore, a balance must be struck between generating enough vorticity for propulsion and minimizing unnecessary turbulence.
In summary, applying fluid mechanics in sports requires attention to several key factors. First, fluid resistance should include inertial resistance, as human and equipment movements in fluids are typically variable-speed, and inertial resistance can be significant. To reduce this, maintaining smooth, continuous motion and minimizing abrupt changes in speed is essential. Understanding these principles helps optimize performance in various sports involving fluid dynamics.
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