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.
