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Fluid Dynamics

Shape And Drag

Moving automobiles and airplanes experience a resistance or drag due to the force of air sticking to the surface. Another source of resistance is pressure drag, which is due to a phenomenon known as flow separation. This happens when there is an abrupt change in the shape of the moving object, and the fluid is unable to make a sudden change in flow direction and stays with the boundary. In this case, the boundary layer gets detached from the body, and a region of low pressure turbulence or wake is formed below it. This creates a drag on the vehicle due to the higher pressure in the front. That is why aerodynamically designed cars are shaped so that the boundary layer remains attached to the body longer, creating a smaller wake and, therefore, less drag. There are many examples in nature of shape modification for drag control. The sea anemone, for instance, continuously adjusts its form to the ocean currents in order to avoid being swept away while gathering food.

Resources

Books

Batchelor, G. K. An Introduction to Fluid Dynamics. Cambridge: Cambridge University Press, 2000.

Fox, Robert W., and Alan T. McDonald. Introduction to Fluid Mechanics. 5th ed. New York: John Wiley & Sons, 1998.

Ingram, Jay. The Science of Everyday Life. New York: Viking Penguin Inc., 1989.

The Japan Society of Mechanical Engineers, eds. Visualized Flow-Fluid Motion in Basic Engineering Situations Revealed by Flow Visualization. Oxford: Pergamon Press, 1988.

Kundu, Pijush K., and Ira M. Cohen. Fluid Mechanics. 2nd ed. San Diego: Academic Press, 2001.

Wegener, Peter P. What Makes Airplanes Fly? New York: Springer-Verlag, 1991.

Periodicals

Valenti, Michael. "Underwater Creatures Go With The Flow." Mechanical Engineering (May 1993): 130.


Sreela Datta

KEY TERMS

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Boundary layer

—The layer of fluid that sticks to the solid surface and in which the speed of the fluid decreases.

Compressibility

—The property that allows a fluid to be compressed into a smaller volume.

Laminar

—A mode of flow in which the fluid moves in layers along continuous well-defined lines known as streamlines.

Reynolds number

—A number that characterizes a flow situation and allows flows of different fluids in different situations to be compared.

Turbulent

—An irregular, disorderly mode of flow.

Viscosity

—The internal friction within a fluid that makes it resist flow.

Wake

—The area of low pressure turbulent flow behind a moving body that causes the body to experience resistance to its forward motion.

Additional topics

Science EncyclopediaScience & Philosophy: Ferroelectric materials to Form and matterFluid Dynamics - Factors That Influence Flow, Reynolds Number, Laminar And Turbulent Flow, Bernoulli's Principle