1. [1-61 provide a summary of all these approaches to drag reduction. A brief discussion of drag reduction techniques and mechanisms is given below. (a) Polymer addition
2. Research on the addition of a small quantity of polymer to a solvent like water to reduce its drag due to turbulent motion (below that of the solvent alone) owes its origin to B. A. Toms [Z, 71. In 1946 he discovered that, in the turbulent regime and at constant pressure, the rate of flow through a pipe increased with an increase in polymer concentration. The result indicated a lowering of the wall shear stress. After this discovery, more than 1000 papers have been written on this topic. Very large drag reductions (up to 80%) have been achieved. The progress has been reviewed in Refs. 18-13]. (b) Compliant wall
3. There has been a limited research effort on the use of microbubbles to reduce viscous drag in water flows 122-271. Although the number of studies made is far less than for devices like the compliant wall, the flow description emerging is clearer. The microbubble technique involve. the injection of bubbles of up to 50 imin diameter through a porous wall and into the boundary layer. The bubbles must be -11 to
4. Schlichting wave and thereby delay transition . 128-311. In the other approach, transition is
5. Out of all the techniques studied most recently, the flew over longitudinal surface riblets and boundary layers altered by an outer layer device (OLD) have, by far, the immediate potential of being applied in practice. Additionally, they could be retrofitted [61. Ualsh and Lindemann 136-381 have made direct measurements of drag on a 28-cm-wide and 90-cmlong flat plate covered with longitudinal eurface riblets whose height and @pacing were of the order of the thickness of the viscous sublayer. The experiments were performed in air. Various riblet geometries were tented. A maximum drag reduction of 8%, compared with that on a smooth flat plate, was observed in two riblet geometries in a zero-pressure gradifnt boundary layer-a symetric rgroove with h - s+ - 12and one