Sports fluid mechanics matters needing attention

Sports fluid mechanics should pay attention to the dynamic effects of body movement. For instance, the "lift" generated by a freestyle palm is directed forward, while the "lift" from a rotating ball is lateral, and the "lift" from a car's tail can even be downward. This occurs because racing wheels need to maintain sufficient contact with the track at high speeds to ensure continuous traction and driving force. The interaction between air flow and surfaces, as well as the turbulence caused by swirling balls, creates vortices and impulses that result in reaction forces from the fluid. Although Bernoulli’s principle helps explain many aspects of propulsion in fluids for humans and sports equipment, some phenomena remain challenging to fully understand using this theory alone. For example, the propulsion from free kicks, butterfly strokes, and backstrokes often involves complex fluid dynamics beyond simple lift. There is also the case of swift swimming, where athletes wear ankle fins resembling a dolphin’s tail and move without using their hands. Instead, they use their waist to push their legs and swing their heels up and down, achieving greater speed than traditional freestyle. During the up-and-down motion of the ankle, similar to the tail fins of fish and dolphins, the deformation is more pronounced. Hydrodynamic studies on aquatic animals show that efficient propulsion methods—such as those used by whales, dolphins, and fast-swimming fish like tuna and swordfish—are analogous to the aerodynamics of bird wings. Research in hydrodynamic tanks has shown that vortices are created during these movements, much like the leg motions in freestyle, butterfly, and backstroke swimming. Take Qiyong as an example. When the ankle reaches its highest point, it pushes downward, creating a counterclockwise vortex that imparts a backward force on the water, resulting in a forward reaction on the body. When the ankle is at its lowest point, it pushes upward, generating a clockwise vortex that gives a backward momentum to the water above, again producing a forward thrust. This is the key propulsive force in this unique swimming style. Additionally, when fluid flows around an object, under certain conditions, it can produce periodic Karman vortices, which create resistance. In contrast, the vortices generated by the tail motion are arranged oppositely, making them more beneficial for propulsion rather than resistance. From a dialectical perspective, vorticity plays a dual role: on one hand, stronger vortices contribute to greater thrust, so maintaining a certain level of vorticity is essential to overcome resistance. On the other hand, excessive vorticity leads to higher energy losses, proportional to the square of the vorticity, so it’s important to manage it effectively. In summary, based on the analysis of fluid properties, resistance, and propulsion, several key points should be considered in applying fluid mechanics in sports: first, the resistance must include inertial resistance, not just viscous drag. This is due to the fact that human movement and sports equipment often involve variable-speed motion, and inertial resistance can be significant. To reduce this, swimmers and athletes should aim to maintain smooth, continuous motion and minimize abrupt changes in velocity.

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