Aquatic Creatures
Water covers over two-thirds of the surface area of the Earth. Nearly all of that is saltwater, but salt or fresh, the water is home to an amazing array of creatures that are as expert at moving through water as flying animals are at moving through air. T
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Aquatic Creatures
Look deep into nature, and then you will understand everything better. Albert Einstein
Water covers over two-thirds of the surface area of the Earth. Nearly all of that is saltwater, but salt or fresh, the water is home to an amazing array of creatures that are as expert at moving through water as flying animals are at moving through air. These aquatic creatures and marine mammals interact with hydrodynamic forces on a constant basis. An examination of the effect of hydrodynamic forces on these animals provides an understanding of fluid dynamic principles in action. For a fish or other animal to move through the water, it must generate sufficient lift to equal or exceed the force of gravity. It must also be capable of generating sufficient thrust to overcome resistance and provide forward motion. This can be an exhausting process. As a result, the most economical production of these four forces—lift, gravity, thrust, and resistance—is fundamental to an animal’s ability to thrive in an aquatic environment.
Lift Lift is the upward force that counteracts the downward force of gravity. It is an essential force for anything wishing to float above the bottom. Without sufficient lift, a fish will sink. As it sinks, the pressure of the water in the column above the fish will increase, causing the fish to sink at an increasing rate. At some point, the fish will be unable to generate sufficient lift to overcome the pressure of the water column above it. It will not be able to rise again. Fortunately, aquatic animals have several means of creating lift. The simplest and most energy efficient source of lift is buoyancy. It is the force Archimedes discovered while in the baths in 212 BC. Buoyancy is the amount of upward force exerted by a fluid on a body immersed in it (Fig. 3.1). It is equal to the G. Hagler, Modeling Ships and Space Craft: The Science and Art of Mastering the Oceans and Sky, DOI 10.1007/978-1-4614-4596-8_3, © Springer Science+Business Media, LLC 2013
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Aquatic Creatures
Fig. 3.1 The buoyancy of an object is equal to the weight of the fluid displaced by that object
weight of the fluid displaced by the object. When an object’s mass equals the mass of the fluid it displaces, an object is said to be neutrally buoyant. As a result, in calm conditions, it will remain at a uniform level. If an object is denser than the fluid supporting it, the object will sink. When an object is sinking, it is said to be negatively buoyant. If rising, it is said to be positively buoyant. Buoyancy in aquatic animals comes from a variety of sources. Most require an action on the part of the animal, but for marine mammals, blubber is one source that does not.
Blubber Blubber acts as a passive mechanism for buoyancy in marine mammals. This dense vascularized layer of fat beneath the skin is one of their most widely recognized and universal characteristics.1 The buoyancy resulting from the presence of blubber requires no energy expenditure on the part of the mammal. “In most marine mammals (except the s
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