Woodruff School of Mechanical Engineering
Reintroducing Nonlinearity To Acoustics To Reveal Hidden Information
Prof. David Dowling
University of Michigan, Mechanical Engineering, Ann Arbor, MI
Tuesday, October 2, 2018 at 11:00:00 AM
Love Building, Room 109
Dr. Devesh Ranjan
Acoustic waves are omnipresent in modern life and are well described by the linearized equations of fluid dynamics. Once generated, acoustic waves carry and collect information about their source and the environment through which they propagate, and this information may be retrieved by analyzing recordings of these waves. Because of this, acoustics is the primary means for imaging and remote sensing in otherwise opaque environments, such as the Earth's oceans and crust, and the interior of the human body. For these information-retrieval tasks, acoustic fields are nearly always interrogated within their recorded frequency range or bandwidth. However, this frequency-range restriction is not general; acoustic fields may also carry hidden information at frequencies outside their bandwidth that can be revealed by reintroducing a marquee trait of fluid dynamics: quadratic nonlinearity. In particular, the wave-triad interactions considered in turbulence motivate the use of a quadratic product of complex acoustic-field amplitudes at two different frequencies to obtain acoustic-field information at the difference and sum of these two frequencies. Despite some fundamental limitations, this reintroduction of nonlinearity to acoustics enables a variety of remote sensing applications that were long thought to be impossible. In particular, it allows the detrimental effects of sparse-array recordings, random scattering, and many other unknown source-to-receiver propagation effects to be suppressed when the recorded acoustic field has sufficient bandwidth. Examples and applications based on simulations, laboratory experiments, and ocean propagation measurements are provided that involve frequencies from a few Hertz to more than 100 kHz, and propagation distances from tens of centimeters to more than 200 kilometers. [Sponsored by ONR, NAVSEA, and NSF].
Prof. Dowling is a professor of mechanical engineering and applied mechanics (primary appointment), and naval architecture and marine engineering at the University of Michigan. He earned his Ph.D. at the California Institute of Technology in 1988 in Aeronautics. After employment at Boeing Aerospace and the Applied Physics Laboratory of the University of Washington, he came to the University of Michigan in 1992. Since then he has taught and conducted research in fluid mechanics and acoustics. He has authored & co-authored more than 200 conference presentations and more than 90 journal articles, and has supervised or co-supervised 19 doctoral students. He is the primary author of: Fluid Mechanics, 6th Ed. (Academic Press, 2016), a senior or 1st-year-graduate level textbook. He served as an Associate Chair and Undergraduate Program Director for the Dept. of Mechanical Engineering at the University of Michigan from 2007 through 2009. He was an associate editor of the Journal of the Acoustical Society of America from 2003 to 2012. He is a fellow of the Acoustical Society of America, the American Society of Mechanical Engineers, and the American Physical Society – Division of Fluid Dynamics. He received the Student Council Mentoring Award of the Acoustical Society of America in 2007, the University of Michigan College of Engineering John R. Ullrich Education Excellence Award in 2009, and the Outstanding Professor Award from the University of Michigan Chapter of the American Society for Engineering Education in 2009.
Refreshments will be served.