Drag reduction is an area of research and development in fluid mechanics, where the energy efficiency of fluid transport systems can be improved by controlling fluid flows. For example, one can reduce the fuel consumption of aircraft, ships, trains and motorcars by applying different drag reduction technologies. Here, using less fuel does not only imply less emissions of harmful gases to environment, but also lead to a reduced noise level, contributing much to an enrichment of the quality of life. The Advisory Council for Aviation Research and Innovation in Europe (ACARE) has recently published the EU’s vision for future aviation, where CO2 and NOx emissions should be reduced by 75% and 90%, respectively by the year 2050, accompanied by a noise reduction of 65%. A new EU framework programme for research will be launched later this year with a budget of € 80 billion in order to achieve an economic growth of a sustainable society, where drag reduction technologies will play an important role in achieving breakthrough innovation.
Drag reduction can be obtained by controlling either turbulent or laminar flows. Turbulent drag reduction has been an active area of research, where turbulence can be supressed by flow surface modifications such as riblets, or drag-reducing additives such as long-chain polymers and surfactants. The latter is the drag reduction technique that the author of this book has extensively investigated over past years. The laminar drag reduction, on the other hand, is a relatively new areas of research, where the laminar flow can be controlled by microscopic surface modification, allowing the flow to slip over the wall. Here again, the author has contributed in establishing the mechanism of drag reduction by hydrophobic surface.
The majority of this book’s contents comes from the author’s own research. The first chapter gives an overview of drag reduction methodology and an introduction to the concept of fluid slip. This is followed by experimental and analytical results of flows through circular pipes and ducts with hydrophobic surface, where both Newtonian and non-Newtonian flows are discussed. Throughout these chapters, the mechanism of laminar drag reduction can be explained as a result of slip flows over microscopic patterns created by hydrophobic surface, where the mean velocity profile gradient is reduced. Flows between coaxial cylinders and over a rotating disk are discussed next, where changes in flow patterns over hydrophobic surface are described. The measured torque on rotating cylinders and disks confirmed that the laminar drag reduction can also be obtained over rotational components. The final two chapters of the book deal with flows over circular cylinders and spheres with hydrophobic surface, where experimental data are compared with numerical simulation results at low Reynolds numbers.
This book serves as a comprehensive guide to the latest information and understanding of laminar flow control using hydrophobic surface. It is highly recommended to postgraduate students, academics and researchers as well as to design and practicing engineers.
University of Nottingham
Hydraulic transportation systems with a pumping operation are frequency used in petroleum pipelines or the process lines of many industrial plants. Thus, approaches for reducing the pumping power promote energy savings. For example, we highlight the application of drag reduction phenomena as one such procedure that can reduce the transportation power.
As well known, in 1883 O. Reynolds described the flow behavior in a pipe, namely, the real fluid flow regimes which are classified as laminar or turbulent on the basis of the flow structure. Flow structure in the turbulent regime is characterized by random motions resulting from the turbulence, which is a dominant factor for the drag. Therefore, if the flow is turbulent, modification of the turbulence becomes the target for achieving a reduction of the drag. Since Toms’ effect was reported in 1943, many researchers have investigated the turbulent drag reduction that is achieved using numerous drag reducing additives, including high molecular weight polymers, micro fibers or particles, surfactants and bio-polymers etc. Currently, polymer or surfactant solutions are applied in many pipeline systems.
On the other hand, in the laminar regime, the flow structure is characterized by smooth motion in the layers with no turbulence. Thus, it is necessary to establish a new concept within the turbulence modification procedure in order to obtain laminar drag reduction. In general, there is no slip at the boundary for real fluids; the fluid in direct contact with the solid boundary has the same velocity as the boundary itself. This is an experimental fact based on numerous observations of fluid behavior. If fluid slip occurs at the solid boundary, the drag or loss will be reduced compared to that of the case of no slip; laminar drag reduction is achieved in this scenario. The development of a hydrophobic material makes it possible to cause a relatively large fluid slip at the solid wall. Although the fluid slip is not significant for achieving drag reduction in turbulent regime, using a fluid slip we can obtain drag reduction in a laminar flow.
This book was written to meet for a discussion of laminar drag reduction utilizing the fluid slip of Newtonian fluids at a highly water repellent wall. The author hopes that this book will serve the need they see and be useful researcher and engineering at universities and for the practical engineer on new drag reduction phenomena related the interaction between liquid and the hydrophobic wall.
Many sources for the experimental results in this book have been drawn from papers that were produced at the Fluid Engineering Laboratory in the Faculty of Engineering at Tokyo Metropolitan University. I am indebted to a great many teachers and students who have assisted me during my work at the university. This book would never have been written without their cooperation.
The author acknowledges gratefully the assistance and cooperation of Dr. S. Ogata, Associate Professor at Graduate School of Science and Engineering of Tokyo Metropolitan University.
CONFLICT OF INTEREST
The author confirms that this ebook contents have no conflict of interest.
Tokyo Metropolitan University
List of Contributors
Tokyo Metropolitan University