Boundary Layer Flow over Elastic Surfaces

Compliant Surfaces and Combined Methods for Marine Vessel Drag Reduction

Viktor V. Babenko, Ho-Hwan Chun, Inwon Lee

• Publisher: Butterworth-Heinemann
• Year: 2012
• ISBN:
  9780123948212
• Topic: Physical Sciences
• Subtopic: Mechanics

While other methods of drag reduction are well-known in marine R&D and ship design environments worldwide, compliant coating drag reduction remains less well-known and poorly understood. This important book presents cutting-edge techniques and findings from research sources not generally accessible by Western researchers and engineers, aiding the application and further development of this potentially important technology.

Beginning with an introduction to drag reduction that places the authors' work on elastic surfaces and combined techniques in context, the book moves on to provide a comprehensive study of drag reduction through elastic coating with both flow and material properties considered. Coverage includes:

• Experimental findings around coherent vortical structures (CVS) in turbulent boundary layers and methods of controlling them
• Static and dynamic mechanical characteristics of elastic composite coatings, as well as new techniques and devices developed for their measurement
• Combined methods of flow control and drag reduction, including the effect of injection of polymer solutions, elastic coatings and generated longitudinal vortical structures on hydrodynamic resistance

Intended as a reference for senior engineers and researchers concerned with the drag reduction and the dynamics of turbulent boundary layer flows, Boundary Layer Flow over Elastic Surfaces provides a unique source of information on compliant surface drag reduction and the experimental techniques around it that have shown measurable and repeatable improvements over recent years.

This compilation of research findings and new techniques developed for measurement will aid R&D engineers, naval architects and senior designers in their quest to achieve drag reductions that will deliver significant efficiency savings.

• Unique source of information on compliant surface drag reduction—an important area of technology with practical application to ships—from otherwise inaccessible research studies
• Updates the knowledge-base on boundary layer flow and surface friction reduction, critical topics in the global quest for increased ship efficiency and fuel economy
• Reveals new techniques and devices developed for measurement and provides a comprehensive study of drag reduction through elastic coating with both flow and material properties covered

Chapter 1

Interaction of the Free Stream with an Elastic Surface

1.1 Introductory Remarks

In the middle of the twentieth century the publication of three pioneering articles triggered a large worldwide trend in drag reduction research, both experimental and numerical. In 1948, Toms [528] was the first to report significant drag reduction at high Reynolds-number flow of a polymer solution, after whom this phenomenon is named. To date, drag reduction by polymer injection has remained an active topic from both an academic and practical viewpoint. In the 1960s, Kramer published a series of articles on achieving essential drag reduction for longitudinal streamlined cylinders with a surface elastic coating [311–318]. His designs of elastic coatings were inspired by the structure of the epidermis of dolphins. Since then, vast amounts of experimental and theoretical research on the flow over elastic surfaces has been done all around the world, either to reproduce or to understand the drag reduction effect. In the 1930s, after one decade of research, Gray [213] proposed the celebrated Gray’s Paradox. He calculated that a dolphin’s swimming speed is approximately 10 times the speed that could be developed by its muscular weight. This led to much biological research aimed at either substantiating or refuting Gray’s assertion.

The combination of these three ideas gave rise to a novel research direction named hydrobionics. It would be nearly impossible to describe the numerous achievements in research field. Therefore, only the major research trends obtained at the key institutions will be mentioned here. Regarding polymer injection drag reduction, extensive research has been carried out in the Applied Research Laboratory (ARL) at Pennsylvania State University, USA. In the former Soviet Union research into this field was performed by the Academy of Sciences of USSR in Novosibirsk, Moscow, St Petersburg (all now in Russia) and Kiev and Donetsk (now in Ukraine). The major findings pertaining to the drag reduction mechanism are: 1) the necessity of preliminary stretch of polymer macromolecules, 2) the drag reduction effect resulting exclusively from the polymer solution in the near-wall region and 3) damping of the vertical structures by the grid-like structure of polymer macromolecules settled in the near-wall region. With regard to the development of elastic coatings for drag reduction, which is the main topic of this monograph, a review of the extensive research will be given. For the third field described above, the biological research, studies have been conducted in various countries, such as the USA, Germany, Ukraine and China.

Hydrobionics, the combined research of all three fields, was initiated in the Department of Boundary Layer Control and Hydrobionics at the Institute of Hydromechanics of the Academy of Sciences of Ukraine in 1965. Under the direction of Professor L.F. Kozlov a program of research has been conducted jointly with the Institute of Biology and Institute of Zoology of the Academy of Sciences of Ukraine and other organizations in the former Soviet Union. The results of the biological and hydrodynamic research led to the finding that the characteristics of dolphin skin are automatically regulated to reduce hydrodynamic drag. The results of this joint research between hydromechanical engineers, biologists and zoologists was recognized in 1982 as discovery N 265 [49] by the Academy of Sciences of the USSR.

Morphological studies on dolphin skin have led to the finding of specific structures and features of this skin. Various hypotheses concerning not only the epidermal structure but also other body organs have been put forward. One is that the microstructure of the external layer of the skin generates two-dimensional (2D) and three-dimensional (3D) disturbances in the boundary layer flow during the oscillatory swimming movement of body. Interaction between the disturbance and the boundary layer instability could generate a pressure fluctuation field in the near-wall region leading to a change in the boundary layer structure and causing drag reduction. Stimulated by this finding, the Institute of Hydromechanics carried out extensive experimental research on the interaction of boundary layer flow with various kinds of elastic plates simulating dolphin skin. The research topics were: the development of a natural laminar boundary layer, hydrodynamic stability, Goertler stability on curvilinear rigid and elastic plates, the turbulent boundary layer and the susceptibility of a boundary layer under various disturbances. These features were modeled as a susceptibility of various flows over elastic surfaces. The problem of susceptibility was explored in the work of Morkovin and Reshotko [366]. In the experience of Schilz [444], T–S waves were found to interact rapidly with the flat waves generated by a flexible wall.

Research on the drag reduction mechanisms of dolphins led to the development of the so-called “combined method of drag reduction.” This involves the damping of the organized vortical structure of the boundary layer using an elastic coating [25, 29, 33, 34]. The body structure and organs have also been investigated for other fast-swimming hydrobionts, such as shark, tuna, swordfish, squid, amongst others. Morphological research of these animals has confirmed that they have evolved specific organ features that leads to drag reduction. Other kinds of “combined methods of drag reduction” have been proposed based on these other fast-swimming hydrobionts. For example, Babenko [34] has proposed a design with an elastic damping coating through which the polymer solutions are injected for drag reduction.

The purpose of the first chapter of this monograph is to review the major features of such combined methods of drag reduction with in-depth analyses on the flow–elastic-surface interaction; hydrobionic drag reduction mechanisms are described in the following chapters. Generally speaking, hydrobionic research has suggested a drag reduction mechanism caused by the attenuation of coherent vertical structures (CVSs). Development of flow visualization techniques and computational fluid dynamics (CFD) have revealed the existence and significance of CVSs in the generation and maintenance mechanism of turbulent boundary layer flows. The research on turbulent boundary layer flows from the latter half of the twentieth century has been mainly focused on the identification and control of CVSs. Therefore, it is natural to begin this chapter by introducing various types of CVS encountered in the flow around a body and turbulent boundary layer flows (see Sections 1.2 and 1.3). After that the fundamental findings on the flow over elastic surfaces are described in Sections 1.4 and 1.5. Sections 1.6 to 1.8 deal with the issues of laminar–turbulent transition of boundary layer flow over a rigid plate. Discussions on the interaction between flow and elastic surfaces, such as the hydrobionic principles of drag reduction, boundary layer receptivity and CVS control are given in Sections 1.9 to 1.15.

1.2 Basic Types of Coherent Vortical Structures arising in the Flow about a Body, and Methods of their Control

The formation and interaction of vortices of various scales is observed everywhere, in the atmosphere and the ocean, in airflows around buildings, in various vegetation canopies on the ground and in water, in flows over various deepenings and barriers. Significant experience from experimental and theoretical research of such problems has been collected. One of the basic problems is the lack of understanding of the reasons and laws of appearance of various sorts of vortical structures. Rockwell systematized and analyzed some types of vortical structures [437]. Below the basic types of CVS and methods of their control are listed.

1.2.1 CVS Types

CVSs in a Boundary Layer

Based on much experimental research, the results of which are presented in articles and proceedings of conferences, etc. [4, 79, 173, 174, 178, 185, 277, 337, 388...