Body shapes and flow | Physical laws in swimming

Body shapes and flow

Not the frontal area of a body as previously assumed, but the ratio of frontal area to body length plays the most important role in the resistance in water. This can be illustrated by the following example. If you pull a plate and a cylinder with the same front surface through water, the water resistance in front of the body is the same, but the turbulence in the wake is considerably different.

The term frontal resistance is therefore not quite correct, since the turbulence in the wake brakes the body more strongly. According to the latest findings, the spindle-shaped structures of the penguins have the least turbulence in the wake. Fish with these body shapes are among the fastest swimmers. An example of backflow: A person running through the water pulls a partner crouched on the water surface behind him by the resulting suction effect.

Propulsion in water

Propulsion in water can be achieved by changing the shape of the body (fin movement in fish) or by propulsion-generating constructions (propeller). In both methods, water is set in motion and thus acts back on the floating body. The reciprocal reaction is called abutment.

In the following, the three principles for locomotion in water are explained in more detail. 1. pressure paddle principle: e.g. duck feet: The feet of the ducks are moved perpendicular to the direction of movement (backwards). On the back side a negative pressure (dead water) is created, which slows down the floating body.

A lot of energy is necessary and the propulsion is low. 2. backward jet principle: Ex. squid: The squid collects water in its body and ejects it through a narrow channel.

This results in a drive to the body 3. Undulation principle: Example dolphin: Behind each body rotating water masses appear in the wake. These rotating water masses are in most cases however disordered and have a braking effect.

In the case of the dolphin, the water masses are ordered by a body wave and can therefore be useful for propulsion. These ordered water masses are called vortex. In swimming, however, it is very difficult to get the water masses to rotate in an orderly fashion by body movement. In the power range, however, it enables very high swimming speeds.