系統系專題演講公告

演講題目1：Variational Solutions: The Contributions of Ritz and Rayleigh, and What They Never Told Us

主 講 人：Prof. Jerry H.Ginsberg
Ｇ.W.Woodruff Chair
School of Mechanical Engineering
Georgia Institute of Technology　　

演講日期：3月16日(星期二)上午10:10~12:00

演講地點：系統系館一樓視廳教室

摘要：
Lord Rayleigh and Walther Ritz were contemporaries who made important contributions to the use of variational principles in vibrations. Rayleigh is widely recognized for his contributions, but Ritz' work is often thought to be an adjunct to Rayleigh's. The seminar will address this misconception. It will begin with a biographical perspective contrasting the longevity and breadth of Rayleigh to the brilliant but brief life of Ritz. It then will examine some of the ways in which variational principles have been employed to study vibratory and acoustic phenomena. The differences between the contributions of these pioneers, and the rivalry that was engendered by the similarities, will be highlighted. This will lead to examination of some analytical and computational insights regarding the use of variational principles and the properties of their solutions that have emerged in the last quarter century, accompanied by some personal observations on efficient application of those principles. An item of special interest is the implementation of variational solutions in conjunction with mathematical software, which makes these techniques a powerful, yet readily implemented, tool. The seminar will close with an examination of some situations where variational solutions should not be attempted.


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演講題目2：Analytical models for interior cavity resonance phenomena with application to SOFIA

演講日期：3月19日(星期五)下午1:10~3:00

演講地點：系統系館一樓視廳教室

摘要：
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint effort between NASA and the German Space Agency that has installed a 20 000 kg telescope in a modified 747-SP aircraft. The modifications entailed constructing bulkheads, one of which is used to provide the active mount for the telescope, and a door that rotates to open as much as 90° of the fuselage circumference to the atmosphere. This configuration creates a large compartment that can exhibit acoustic resonances at low frequencies. Concern arose that a Rossiter mode, which is an aerodynamic resonance in which turbulence vortices shed from the leading edge of a gap form a coherent standing pattern at certain speeds, would create a strong acoustic source for acoustic cavity and structural resonances, which would be catastrophic if all three coincided. In order to explore this concern in a timely manner one-dimensional and two-dimensional models of a fluid cavity bounded by rigid walls adjacent to elastic structures and excited by a point source were created. Unlike conventional analyses of interior cabin noise, in which vibrating walls are the acoustic source, the analyses show that the structure represents a compliant boundary that couples with the acoustic modes. This leads to some surprising conclusions regarding the triple resonance phenomenon, as well as some general insights to the role of structural compliance for cavities that are excited by turbulent external flows. The presentation concludes with a description of the flight test program now underway.




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