streamlining

(design engineering) The contouring of a body to reduce its resistance to motion through a fluid.

The contouring of a body to reduce its resistance (drag) to motion through a fluid.

For fluids with relatively low viscosity such as water and air, effects of viscous friction are confined to a thin layer of fluid on the surface termed the boundary layer. Under the influence of an increasing pressure, the flow within the boundary layer tends to reverse and flow in an upstream direction. Viscosity tends to cause the flow to separate from the body surface with consequent formation of a region of swirling or eddy flow (termed the body wake; illus. a). This eddy formation leads to a reduction in the downstream pressure on the body and hence gives rise to a force opposite to the body motion, known as pressure drag. See also Wake flow.

Flow about bodies in uniform subsonic flow. (a) Blunt body. (b) Streamlined body.

In general, streamlining in subsonic flow involves the contouring of the body in such a manner that the wake is reduced and hence the pressure drag is reduced. The contouring should provide for gradual deceleration to avoid flow separation, that is, reduced adverse pressure gradients. These considerations lead to the following general rules for subsonic streamlining: The forward portion of the body should be well rounded, and the body should curve back gradually from the forward section to a tapering aftersection with the avoidance of sharp corners along the body surface. These conditions are well illustrated by teardrop shapes ( illus. b).

At supersonic speeds the airflow can accommodate sudden changes in direction by being compressed or expanded. Where this change in direction occurs at the nose of the body, a compression wave is created, the strength of which depends upon the magnitude of the change in flow direction. Lowering the body-induced flow angle weakens this compression shock wave. When the flow changes direction again at the midpoint of the body, the air will expand to follow the shape of the body. This change in direction creates expansion waves. At the tail of the body the direction changes again, creating another compression or shock wave. At each of these shock waves, changes in pressure, density, and velocity occur, and in this process energy is lost. This loss results in a retarding force known as wave drag. See also Shock wave.

Bodies which are streamlined for supersonic speeds are characterized by a sharp nose and small flow deflection angles. Because the intensity of the shock wave and the drag level is dependent upon the magnitude of the change in flow direction, the width or thickness of the body should be minimal. See also Boundary-layer flow.

Most often associated with the aerodynamic forms of many products in the United States in the 1930s, streamlining reflected popular interest in speed records on land, sea, and in the air. However, although such forms were often equated with fast-moving objects, such as the Douglas DC-3 airliner of 1933, the Burlington Zephyr and Union Pacific M10,000 railway locomotives (See Chicago Century of Progress Exposition), and the Chrysler Airflow car (1933), they were also used to provide a symbolic link with technological progress through the appropriation of aerodynamic forms in static objects. These ranged from the flowing lines of Kem Weber's Airline chair (1934) and the rounded forms of Walter Dorwin Teague's Bantam enamelled metal camera (1936) for Eastman Kodak to the sculptural dynamism Raymond Loewy's Coldspot refrigerator (1935) for Sears Roebuck & Co. and Peter Müller-Munk's chrome-plated brass steamer funnel-like Normandie jug (1931). Such forms became the basis for countless vacuum cleaners, toasters, radios, fountain pens, and items of furniture. Indeed, many objects took on a number of explicit styling features that came to symbolize speed. ‘Speed whiskers’—thin horizontal strips, often of chrome, applied to the surface of objects—could be seen running in parallel lines on bona fide transportation designs such as Loewy's Silversides motor coach (1940) for Greyhound and his S-1 railway locomotive (1935) for the Pennsylvannia Railroad. Nonetheless they were applied to countless static situations, ranging from the entrance to the McGraw-Hill Building by Raymond Hood and J. André Fouilhoux in New York to the ubiquitous roadside diners and the domestic, as in the streamlined earthenware refrigerator jug (1940) by J. Paulin Thorley for the Westinghouse Electric Co. The public appetite for the symbolism of speed and contemporaneity had been whetted by the almost science fiction-like renderings of the technological utopianism of industrial designer Norman Bel Geddes in his book Horizons (1932), as well as science fiction itself which in the same period saw the creation of Flash Gordon, Buck Rogers, and Superman. It was also a dynamic feature of the New York World's Fair of 1939-40. Although a streamlined aesthetic made some impact in European products, it was most apparent in transportation developments with the design of new locomotives by engineers such as Nigel Gresley and also in a number of cars such as the Burney Streamliner in Britain, the Tatra in Czechoslovakia, and Porsche's designs for the ‘Volkswagen’ in Germany. Its impact was greater in the 1950s when the appeal of American products symbolizing the rapidly changing world of technology became more alluring with increased levels of affluence.

Aerodynamic design in the 1920s and 1930s in which curving walls, long strips of windows (often curved around elements of a building), and thin flat roofs with pronounced overhangs were used, often in factories to suggest cleanliness and modernity. It was a characteristic of Modernism, and was promoted by designers such as Loewy and Teague.

Making the overall shape of a body moving through a fluid streamlined so that it offers the minimum resistance to the fluid flowing around it. Streamlining, for example by cyclists, skaters, and skiers who assume a more crouched body position, reduces the amount of turbulence created, especially at the rear, and also reduces the area of the body orientated perpendicular to the fluid flow.