International Journal of Aeroacoustics Honoring Professor Christopher Tam
The International Journal of Aeroacoustics has set aside a special issue, volume 23, June issue 2024, to honor Professor Christopher Tam, Robert O. Lawton Distinguished Professor of Mathematics at the Florida State University. The reasons for the special issue can be found in the Editorial of the issue. It states that Professor Christopher Tam's scientific and technical accomplishments are outstanding and encompass a wide range of topics in fluid mechanics. Chris has made stellar contributions in jet noise and pioneered the field of Computational Aeroacoustics, for which he is well renowned. Two of Chris's original findings have revolutionized our understanding of jet noise. He was the first to clearly establish a direct link between the largescale turbulence structures/instability waves in the shear layers close to the jet exit and the radiated turbulent mixing noise. He demonstrated that these structures are efficient generators of noise and constitute the dominant noise sources, especially in the downstream direction for highspeed jets. Chris was the first to emphasize the distinctly different requirements of Computational Aeroacoustics (CAA) as opposed to the goals of Computational Fluid Dynamics (CFD) and developed the dispersion-relation-preserving (DRP) scheme. In contrast to the conventional numerical schemes that were designed to achieve the highest order of approximation possible, the DRP scheme sought instead to minimize the error in the Fourier space, thereby preserving as much as possible the dispersion relation that governs the propagation of acoustic waves. In the article following the Editorial, the two guest editors further wrote "Professor Tam has maintained extremely high standards of research and an enviable track record for productivity, spanning more than five decades. The depth and breadth of his technical output are truly breathtaking, and he has inspired scores of researchers and graduate students".
"Professor Tam has maintained extremely high standards of research and an enviable track record for productivity, spanning more than five decades. The depth and breadth of his technical output are truly breathtaking, and he has inspired scores of researchers and graduate students". International Journal of Aeroacoustics, Vol 23, 2024
What is Aeroacoustics? Aeroacoustics comes from the combination of the two words "aero" and "acoustics". Aeroacoustics, as a research discipline, investigates the generation mechanisms and the propagation of aircraft noise, transportation noise and various types of flow generated noise and their suppression. Everyone who travels by air and communities located near airports, air force bases and naval air stations, would, undoubtedly, recognize the disturbing effects of aircraft noise. Military jets are more powerful than commercial jets. Their noise is not only louder but also more harmful to human health. On the deck of an aircraft carrier, young naval personnel, directing naval jets at takeoffs, are stationed close to the jet plume (see the movie Top Gun). As a result, many of them suffer severe hearing loss. In addition, military jets emit high levels of infra-sound (with frequency less than 20 Hz, the threshold of human hearing). The sound frequencies often match human body cavity resonance frequencies. This low frequency sound is a threat to human health.
In the US, aircraft noise is regulated by the FAA. Internationally, it is regulated by ICAO. However, each individual airport may impose its own noise rules. In the US, Ronald Reagan National Airport at Washington DC and John Wayne Airport at Orange County, California have the most stringent noise requirements. They have evening and night curfews for noisy airplanes. Overseas, London Heathrow Airport and Skripal Airport Amsterdam have the most restrictive noise rules.
Dr. Tam is a fellow of the American Institute of Aeronautics and Astronautics (AIAA), the Acoustical Society of America (ASA), the American Physical Society (APS) and a member of SIAM. Dr. Tam has received a number of awards. They include the AIAA Aeroacoustics Award, The AIAA Pendray Aerospace Sciences Literature Award. These two awards are on the list of "prestigious" physical sciences awards of the National Research Council. In addition, he was the recipient of the NASA Technology Utilization and Application Office Award, NASA TGIR Award (Turning Goals into Reality), Arnold Engineering and Development Center Annual Technology Achievement Award, AIAA Space Shuttle Flag Plaque Award and others. He has published over 140 journal articles. He has written a book entitled "Computational Aeroacoustics: a wave number approach" published by Cambridge University Press.
Over the years, Dr. Tam has done extensive consulting work. He has consulted for General Electric Aircraft Engine Division, Lockheed Aircraft Company, Georgia Tech Research Institute, Arnold Engineering and Development Center, AREVA (nuclear power company), Cessna Aircraft and others. Mathematicians, by and large, work on the theoretical side of problems. It is Dr. Tam's view that consulting is when mathematicians meet the real world. In the article of the special issue written by the two guest editors, they describe one of Dr. Tam's consulting episode with the title "A story with Cessna Aircraft". The following is what they wrote. "Most of us are familiar with Chris's erudite and authoritative technical presentations at conferences. It is fair to say that many are astounded and overwhelmed by the deep mathematical developments that underpin his theoretical expositions. Perhaps less well known is his foray as a consultant into designing a mechanical component for an airplane problem at Cessna Aircraft, a company renowned for executive jets. Cessna faced a predicament with their CJ4-Proto, an executive jet that gained popularity but was plagued by an annoying tone issue during high-altitude flights. Initially considered a mechanical vibration problem, the engineers at Cessna struggled for 6 months to identify the root cause, deploying various suppression devices without success. Desperation set in as management grew increasingly concerned about the potential impact on Cessna's reputation. A breakthrough occurred when an engineer stumbled upon Chris's papers on jet screech tones, a phenomenon not previously associated with full-size jet aircraft. Using frequency formulas from these papers, the engineer identified a match between theoretical predictions and flight measurements, transforming the problem from a mechanical issue to an aeroacoustic challenge. The chief acoustics engineer at Cessna contacted Chris, extending a consulting agreement to address the tone problem. Chris recognized that the tone originated from the interaction of instability waves and shock cells in the jet flow and proposed a solution involving a faceted wedge mounted on the engine pylon. This device aimed to disrupt the instability waves and, subsequently, the screech tones. After inspecting the CJ4's tail in Wichita, Chris designed and submitted the faceted wedge to Cessna. Subsequent flight tests confirmed the wedge's effectiveness in significantly reducing the helical mode screech tone, marking a breakthrough in addressing the issue. Ultimately, the modified faceted wedge proved to be a successful noise suppression device, reducing the tone levels to an acceptable standard and resolving Cessna's critical issue. This example illustrates well a straightforward solution to a practical real-world problem."
Incidentally, it seems worthwhile to mention that ScholarGPS (ScholarGPS.com) is an organization specialized in world-wide ranking of researchers, research institutions, universities and academic programs across all areas of scholarly endeavor. Their most recent ranking places Dr. Tam as number one in the area of Aeroacoustics.
