Preface
As its name suggests, aeroacoustics is a subject that connects the fields of aerodynamics and acoustics. It is relatively young for a mechanical science and of an age where many of us were taught by or have worked with the pioneers in this field. Part of the appeal of aeroacoustics is in the creativity and generous spirit of cooperation unleashed by this cross-disciplinary collaboration of experts, where extraordinary accomplishments have been made possible by fusion of fields and expertise. Aeroacoustics is a field where applied mathematics, science, and engineering play a pivotal role; a field where experiments in their classical role of testing hypotheses are very much part of current progress. Consider, for example, the sound radiated by an airfoil in turbulence. The science of turbulence is hard enough, and putting an airfoil into it feels likely to be an unmanageable complication. At low Mach number the sound radiated by the interaction is perhaps one millionth part of the pressure field generated. It is hard to imagine a problem that appears more badly posed. Remarkably, it has been rendered not only tractable but, with some simplification, solvable by entirely analytical methods with results that pass near, if not through, experimental measurements. It is very satisfying to be working in a field that regularly produces extraordinary achievements of this type. A number of these achievements are detailed in this book.
Stewart Glegg obtained his BSc degree in engineering science from Southampton University and went on to also obtain his Masters and PhD at Southampton studying under the direction of Mike Fisher at the ISVR. At Southampton he was fortunate to be introduced to the fields of fluid dynamics and acoustics by some of the pioneers in aeroacoustics, notably Geoff Lilley, Chris Morfey, Peter Davies, Phil Doak, and of course Mike. After graduating with his PhD he worked at Westland Helicopters under the direction of Dave Hawkings who introduced him to the field of rotor noise. In 1979 he returned to the ISVR as a faculty member, and in 1985 he moved to Florida Atlantic University in search of a warmer climate and new challenges. It was not long before he was conscripted by Feri Farassat to work on helicopter rotor noise under NASA sponsorship, and since that time he has been actively involved in aeroacoustics in the United States.
William Devenport also graduated in engineering science in the United Kingdom, at the University of Exeter, and went on to do a PhD at the University of Cambridge. His PhD research was in the experimental and computational study of turbulent separated flows under the capable guidance of Peter Sutton. While it was not his focus then, he was fortunate enough to be able to attend lectures on aeroacoustics given by Shôn Ffowcs Williams, to interact with Ann Dowling, and to share an office with some of their graduate students. His experimental work continued when he joined Virginia Tech as a postdoc, under the guidance of Roger Simpson. His interest in aeroacoustics truly began just after he was appointed as assistant professor in 1989. Ever supportive, Roger Simpson introduced him to Tom Brooks at NASA Langley. Tom had already been sponsoring Stewart to do predictions of broadband noise generated by blade wake interactions in helicopter rotors, and had decided that some experiments were needed to define the turbulence structure. Stewart visited Virginia Tech in the Spring of 1990 initiating a collaboration that continues today. Perhaps some of the longevity of our collaboration and interest derives from our different technical backgrounds and areas of expertise.
This book began its life on paper in 2005 when the authors jointly organized a course on aeroacoustics and hydroacoustics taught from Florida Atlantic University to graduate students there and at Virginia Tech. The initial core of the text (written by S.G.) was drafted as notes for that course with the intent of bringing together the fundamentals of aeroacoustics in a form and at a level that would be appropriate to graduate students, and that would give them the background needed to understand most modern papers and developments in the field. Over 11 years later this remains the goal of this book.
We have many to thank for their help in making this book possible. In terms of those who have enabled the long-term study of aeroacoustics of which this book is a part, we first thank the Department of Ocean and Mechanical Engineering at Florida Atlantic University and the Kevin T. Crofton Department of Aerospace and Ocean Engineering at Virginia Tech for providing us with secure and supportive research and teaching environments, over many years. Without their support it would not have been possible to develop the material for this text or to write it up. We also acknowledge the invaluable support and encouragement of our colleagues at these institutions including the members of the Center for Acoustics and Vibration at FAU, and Eric Paterson, Aurélien Borgoltz, and the members of ATFRG and CREATe at VT. We owe a debt of gratitude to the many graduate students we have had the privilege to work with. Some (mentioned below) directly contributed to this book, but all have contributed indirectly through the hard work and inspiration they dedicated to the mutual advance of understanding that underpins the relationship between student and advisor. Our research sponsors have in many ways enabled this book, and we are grateful to all. Among them are Tom Brooks, who along with his colleagues at NASA Mike Marcolini and Casey Burley, found a way to sponsor us continuously for the first 10 years of our collaboration; sponsorship that was fundamental in initiating, building, and linking our research programs. We have been fortunate enough also to receive long-term support from the Office of Naval Research, in particular through the programs in hydroacoustics and turbulence and wakes managed by Ki-Han Kim and Ron Joslin. We are very grateful not only for the sustained funding, but also for the motivation they have provided to move into new and exciting technical areas, and to take on challenges outside our comfort zone. We have also benefited greatly from collaborations and support associated with the Virginia Tech Stability Tunnel and its evolution as an aeroacoustic facility. In particular, we owe debts here to Ricardo Burdisso who played a central role in its transformation and who, along with his company AVEC Inc., have been inspirational partners in its further development and advancement. We also thank Wing Ng, and his company Techsburg Inc., without whom this upgrade would never have been possible. We owe a debt of gratitude to ONR, and particularly Ron Joslin, for his courage to be the first to support this venture when it must have appeared both unconventional and risky, and to ONR and General Electric for their encouragement and support in maturing many of its state of the art capabilities.
In terms of those whose efforts have directly contributed to this book we would first like to thank those who encouraged or inspired the book, in particular, Phil Joseph, Chris Morfey, Bill Blake, and Nigel Peake. We are particularly grateful to Chris who also carefully reviewed and commented on a number of chapters, and whose comments often challenged us to maintain the technical rigor of the book. A number of people provided material for or read and commented on specific sections or components of the book and we thank them all. They include Nathan Alexander, Jason Anderson, Manuj Awasthi, Neehar Balantrapu, Andreas Bergmann, Ken Brown, Ricardo Burdisso, Lou Cattafesta, Ian Clark, Dan Cadel, Alexandra Devenport, Mitchell Devenport, Mike Doty, Marty Gerold, Christopher Hickling, Florence Hutcheson, Remy Johnson, Liselle Joseph, Phil Joseph, Emilia Kawashima, Jon Larssen, Todd Lowe, Lin Ma, Henry Murray, Mike Marcolini, Patricio Ravetta, Michel Roger, David Stephens, Ian Smith, and Hiroki Ura.
Last but not least, above all we thank our spouses Inger Hansen (known to family and friends as Lisa) and Anne Devenport for their constant support of us in our careers, in this endeavor, and for their dedicated efforts in reviewing and correcting large fractions of the book.