Going against the flow—A review of non-additive means of drag reduction

The conventional mainstream method of polymer additives for reducing drag in pipes is disadvantageous from certain environmental perspectives. This paper aims to go against the flow by challenging the widespread preference for using polymeric and surfactant-based drag reducers and discusses the performance of five alternative methods that do not involve polymer additives: riblets, dimples, oscillating walls, compliant surfaces, and microbubbles. The hypothetical mechanisms involved, success of each approach, relevant stages of development, important findings, and possible avenue for future research are discussed. The results obtained in this paper have exposed the potentials of new drag reduction technologies.