A. Introduction

Drag Reduction by the introduction of minute concentrations of polymeric materials was first discovered during the Second World War and was described, for the first time, by Toms (1948) [1]. Since this first publication, there has been an enormous amount of work on drag reduction, and in particular drag reduction by the addition of polymers.

The definition of the percentage drag reduction for pipe flow is as follows:

$$\label{eq1} \mbox{% drag reduction}=\frac{\Delta p_s-\Delta p_p}{\Delta p_s}\times 100\tag{1}$$