Not Just Going with the Flow
Aquatic animals can modify their fluid environment to improve efficiency
Swimming fish and dolphins appear to move effortlessly through the water. Even when they glide, they don’t seem to lose any speed. However, rigid and fixed laws of physics and principles of hydrodynamics dictate how water flows around an animal. This flow determines the forces the animals must generate and the energy they must expend to move. Animals propelling themselves through water must contend not only with pushing back on the fluid but also with forcing their way through an incompressible medium. Despite the inflexibilities of these physical forces, underwater animals have developed highly effective means to control flow and move economically through the aquatic environment.
The observation of the interaction between an aquatic animal and the water around it, called its flow field, goes back to Leonardo da Vinci. Da Vinci recognized the advantages of the streamlined shape of a fish based on the flow surrounding it. He argued that the fish could move through the water with little resistance, or drag. The streamlined shape allowed the water to flow smoothly over the body. Later, in 1809, Sir George Cayley, the father of aerodynamics, examined the streamlined body shapes of a trout and a dolphin. He considered their similar shapes to be solids of a design that offered the least resistance to flow. More recently, Steven Vogel of Duke University introduced biologists to the interaction of organisms and flow with his 1981 book Life in Moving Fluids. His contribution resulted in a greater appreciation of the interaction between animals and the liquid medium.
Manipulation of flow is accomplished both passively and actively. Animals use passive mechanisms involving design of the body and texture of their surfaces, which alter flow conditions against the body surface in order to reduce drag. On the other hand, active control of flow involves mobile fins and paddles, which regulate water movements that are shed into the wake as vortices. Vorticity is the tendency of a fluid to rotate or spin. An extreme version of vorticity is a vortex. The vortex is a spinning, cyclonic mass of fluid, which can be observed in the rotation of water going down a drain, as well as in smoke rings, tornadoes and hurricanes.
Up until the last 20 years, the control of flow by animals has largely been conceptualized rather than visualized. Expectations of the interaction of animals and water flow were based on simple engineered systems. Streamlined forms, synthetic wings, wavy plates and polished surfaces were the standards to compare with animals. However, animals exhibit a wide diversity and complexity of shapes and movements, which can influence the flow dynamics in ways not previously envisioned. Animals change shape during motion, unlike human-designed systems, further complicating analyses of motion in a fluid environment.
Biologists have recently been employing the same techniques as engineers to visualize flow, and these methods have greatly aided biologists in describing and quantifying the flow fields around animals. One mechanism is to introduce dye into the water along the surface or in the wake of a swimming animal, which shows a continuous record of the trajectory of the fluid. Similarly, particle image velocimetry (PIV) can visualize the pathway of the flow by illuminating reflective particles introduced into the fluid. The particles are illuminated with a laser that has been projected into a wide “sheet” rather than a single beam. This optical method also can define the velocity of the flow in a two- or three-dimensional field around the animal. Tracking the particles in the flow requires high-speed cameras and sophisticated computer software. Compared to these experimental methods of flow visualization, computational fluid dynamics uses computers to simulate the flow from numerical solutions, which are based on theoretical equations governing fluid movement.
Research using these varied flow visualization techniques has given new insights into how animals manipulate flow. Long-standing ideas are being tested regarding the best designs and mechanics of movement with regard to enhanced propulsion, reduction in drag, and the coordination of feeding and locomotion. Understanding how animals can control flow has immense implications not only for understanding the evolution of aquatic species, but also for developing biologically inspired machines and even for elucidating global climate change.