Introduction

Fluid drag or friction depends on the physical characteristics of the fluid and solid surface. It is very important to control fluid flow and achieve drag reduction. To this end, it is necessary to understand the phenomenon of drag reduction from the viewpoint of energy saving in hydraulic transportation systems. Studies on flow drag have greatly contributed to the development of hydraulics and fluid mechanics. Although we can compute the drag of a blunt body or pressure loss of a channel in various sorts of fluid flow by using the systematized knowledge gained thus far, still some unexplained phenomena have occurred in actual flow-field. Thus, it can be considered that there are two research directions; one concerns energy saving in practical applications; and the other involves the clarification of the phenomenon due to changes in the physical characteristics of fluids and surfaces during experiments. With regard to energy saving, it is possible to design and construct apparatus for simulating drag reduction by using experimental data. In this section, a drag reduction technique employing either drag reducing additives or drag reducing walls is explained. In addition, some existing experimental data on the characteristics of drag reducing walls used in laminar drag reduction are summarized.

Keywords: Drag reduction, passive control, drag reduction additives, drag reduction walls, fluid slip, Navier’s hypothesis, hydrophobic wall, highly water repellent wall, wettability, contact angle.