Gentoo penguins are the world’s fastest swimming birds, with maximum underwater speeds of up to 36 km/h (about 22 mph). That’s because their wings have evolved into fins that are ideal for moving through water (albeit virtually useless for flying in the air). Physicists have now used computational modeling of the hydrodynamics of penguin wings to gain additional insight into the forces and currents those wings create underwater. They concluded that the penguin’s ability to change the angle of its wings while swimming is the most important variable for generating thrust, according to a recent paper published in the journal Physics of Fluids.
“The superior swimming ability of penguins to start/brake, accelerate/decelerate, and turn quickly is due to their free-swinging wings,” said study co-author Prasert Prapamonthon of King Mongkut’s Institute of Technology Ladkrabang in Bangkok, Thailand. “They allow penguins to swim, propel and maneuver in the water and maintain balance on land. Our research team is always curious about sophisticated creatures in nature that can benefit humanity.”
Scientists have long been interested in the study of aquatic animals. Such research could lead to new designs that reduce the drag of aircraft or helicopters. Or it could help build more efficient bio-inspired robots for exploring and monitoring underwater environments, such as RoboKrill, a small, one-legged, 3D-printed robot designed to mimic krill’s leg movement so it can move smoothly in underwater environments.
Aquatic species have evolved in several ways to optimize their efficiency as they move through water. Mako sharks, for example, can swim as fast as 70 to 80 mph, earning them the nickname “cheetahs of the ocean.” In 2019, scientists showed that an important factor in how mako sharks can move so fast is the unique structure of their skin. They have small translucent scales, about 0.2 millimeters in size, called “denticles” all over the body, especially concentrated in the animal’s flanks and fins. The scales are much more flexible in those areas compared to other regions such as the nose.
That has a profound effect on the amount of pressure the mako shark encounters while swimming. Pressure drag results from flow separation around an object, such as an aircraft or the body of a mako shark as it moves through water. It’s what happens when the fluid stream separates from an object’s surface, creating eddies and vortices that impede the object’s motion. The denticles in sharkskin can bend at an angle of more than 40 degrees relative to the body, but only in the direction of the reverse flow (i.e. from tail to nose). This controls the degree of flow separation, similar to the dimples in a golf ball. The dimples, or scales in the mako shark’s case, help maintain the attached current around the body, reducing the size of the wake.
Swamp grass shrimp maximizes forward thrust due to the stiffness and increased surface area of its leg. They also have two drag-reducing mechanisms: the legs are about twice as flexible during the recovery stroke and bend heavily, resulting in less direct interaction with the water and a reduced wake (smaller vortices); and instead of three legs moving separately, their legs essentially move as one, greatly reducing drag.
There have also been numerous studies on the biomechanics, kinematics and fin shape of penguins, among other things. Prapamonthon et al. specifically wanted to dig deeper into the hydrodynamics of how the flapping wing generates forward thrust. According to the authors, aquatic animals typically use two primary mechanisms to generate thrust in the water. One is based on resistance, like rowing, and is very suitable for moving at lower speeds. For higher speeds, they use a lift-based mechanism, clapping, which has been shown to be more efficient at generating propulsion.