Flow visualization is perhaps the oldest investigative tool in fluid mechanics and hydraulics. Remarkable insights into fluid flow, such as those made by Leonardo da Vinci, were obtained over time using this straightforward method. The advent of the digital revolution in modern times transformed these essentially qualitative techniques to strong quantitative ones. Among the distinctive advantages of flow visualization over the other experimental approaches is that they are essentially nonintrusive. In recent years, twoor three-dimensional vector and associated scalar-flow quantities have become available through continued and sophisticated technological development. Currently, the modern flow visualization instruments gain continuously spatial and temporal capabilities such that they are increasingly complementing the data obtained by Computational Fluid Dynamics ~CFD!, another offspring of the digital revolution. The rapid, intensive, and extensive evolution of the flow visualization techniques in past decades, did not allow fast assimilation of the techniques in current laboratory practice. The need of evaluation, validation, and dissemination of the benefits of these techniques was continuously felt during these decades. From this respect, the publication of this book is valuable and timely. The book consists of twelve chapters covering a large range of flow visualization techniques including new methods not widely covered in the technical literature ~e.g., molecular tagging velocimetry, thermo-chromic liquid crystals, threeand four-dimensional ~inclusive of time! imaging!. The contributing authors are leading scientific authorities with extensive expertise in the presented techniques. Chapter 1 familiarizes the reader with the interpretation of flow patterns with a strong emphasis on the significance of the critical points. The relationships between tensor invariants and streamline patterns are explained in detail using comprehensive illustrations and theoretical derivations. It is also shown that a combination of invariants can be used to detect local vortices from the three-dimensional velocity information obtained by CFD. Chapter 2 discusses the well-known hydrogen bubble technique, initially developed for visualizing organized turbulent structures in turbulent boundary layers. The configuration of the bubble-generating systems, probes, lighting procedures, and the limitations of the method are briefly described. Chapter 3 describes another traditional experimental approach, namely visualization using dye and smoke. The use of the technique is separately described in accordance with the working fluid used in the experiments, i.e., water or air. Special attention is given to the capturing of the visualized information. Chapters 4 and 5 treat two newer techniques based on laser light interaction with atoms or molecules contained in the flows. Chapter 4, dealing with Molecular Tagging Velocimetry ~MTV!, first introduces the reader to the particularities of the photochromic materials. Similarly, Chapter 5 familiarizes the reader with the underlying principles of the planar-laser imaging. LaserInduced Fluorescence ~LIF! and Planar-Doppler Velocimetry ~PDV! are subsequently described in this chapter. Results obtained with the above mentioned methods are illustrated in the last parts of Chapters 4 and 5. Chapter 6 describes Digital Particle-Image Velocimetry ~DPIV!, one of the most innovative flow diagnostic technologies in contemporary fluid mechanics. Following a brief description of the measurement principle involved, post-processing methods, and a valuable section on the sources of errors associated with DPIV measurements are presented. Given the widespread use of the DPIV, even for largescale flows, such as Fig. 1 shows for a river cross section, the application section of this chapter could have been more generous. Chapters 7 and 8 are associated with surface measurement technologies. These methods have evolved considerably in recent years. Surface temperature sensing techniques using thermochromic Liquid Crystals ~LCs! are concisely described in Chapter 7. Examples of LC application to boundary layer heat transfer processes are illustrated at the end of the chapter. Surface pressure and surface shear stress measurements made with the aid of a special coating technology are detailed in Chapter 8. Pressuresensitive paint ~PSP!, shear-sensitive liquid crystal coating ~SSLCC!, and Fringe-Imaging Skin-Friction Interferometry ~FISF! are extensively documented. Practical means to achieve proper lighting, calibration, and data reduction methods are described for each of the methods. Well-illustrated examples support the importance of these methods to nonintrusively measure the pressure distribution and shear stresses in practical aerodynamics and experimental fluid mechanics. Chapter 9 introduces the reader to the visualization methods for compressible flows. A comprehensive introduction to basic optical concepts related to the subsequent described techniques makes this chapter selfcontained. Practical configurations and results obtained with Shadowgraph, Schlieren, Interferometers, and Holography are succinctly presented at the end of the chapter. Chapter 10 and 11 concern 3and 4D flow measurement methods. These quite complex techniques are the most recent developments of the experimental methods technologies and continue
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Jun 2, 2022
Syed Qasim Zaheer + 1
Open Access