A. Background

A considerable amount of energy is used in the pumping of fluids in turbulent flow through pipeline systems. Clearly, there is a potential benefit in such systems if the drag (i.e. the pressure drop) could be reduced below the value dictated by the normal friction factor relationships. Drag reduction is also important in the motion of objects (such as ships or submarines) through fluids. The search for means of reducing drag has been pursued actively for many decades. Drag reduction can be achieved by adding materials (polymers, surfactants, bubbles) to the fluids or by modifying the surface of the solid with which the fluid is in contact. The objective of this introductory section is to briefly review the various means of drag reduction. More detailed information on the more important methodologies is given in the succeeding sections.

There have been extensive publications on the subject of drag reduction and the literature on drag reduction probably now extends to several thousand papers and the magnitude of the task of considering every source will be appreciated. In this Section and the succeeding ones, the objective has been to consider a sufficient number of sources to pick out the key phenomena and prediction methods. Reflecting the large size of the literature on the subject, a number of review articles have been written and have been studied as part of the current exercise. These include the reviews by Lumley (1969), Virk (1975), Berman (1978), Hoyt (1989), and Pazwash (1984). In a report from the British Hydrodynamics Research Association (BHRA), White (1975) lists 1,009 publications on drag reduction, though these include a (small) number of papers on drag reduction methods such as compliant surfaces. Most papers have been concerned with polymers and surfactants as drag reduction promoters but it should be stressed that suspended particles can also act to reduce drag (Kane, 1989). It should also be noted that drag reduction with high molecular weight substances also occurs in nature; fish slimes, which produce drag reduction for swimming fish, contain such substances.

The main emphasis in this and the succeeding sections is on the use of drag reduction technologies to reduce the pressure drop in flow in pipes. The percentage drag reduction for pipe flow is defined as: