Title：Optimal damping in ducts - The Cremer impedance
Time：9:30 to 10:30, May.21, 2019
Place：F210, School of Mechanical Engineering
Host：YANG Cheng, Associate Professor (Institute of Vibration, Shock & Noise)

Abstract
The Cremer impedance, first proposed by Cremer (1953) and then extended by Tester (1973), is supposed to give the maximum propagation damping in an infinitely long waveguide. Previous works including a uniform grazing flow have shown negative resistance in the low frequency range for both circular and 2-D rectangular waveguides, i.e., implying an active boundary. In order to further analyze the low frequency behavior of the Cremer impedance, especially the negative resistance, two investigations are conducted in the current work. First, the previously used Ingard-Myers boundary condition is replaced by the Brambley boundary condition with the introduction of a thin inviscid boundary layer, and results obtained with the two boundary conditions are compared to see the effect of a sheared flow. The frequency range where the two boundary conditions can be applied is also analyzed. Second, discussions regarding the validity of the low frequency result in both the up- and downstream directions from the perspective of mode merging are presented. This analysis is further extended from the fundamental mode to higher order modes in the frequency range where they are ‘just cut-on’.

Biography
Professor Mats Åbom is currently a professor in fluid acoustics at KTH-The Royal Institute of Technology. He obtained his Master and PhD degrees in 1978 and 1989, respectively, at KTH. After graduation, he worked as an assistant professor and then a senior researcher at KTH. Then, he spent 4 years in the industry where he was an acoustic specialist in ABB corporate research. He became a full professor at KTH since 2000. Professor Åbom was the head of the Marcus Wallenberg Laboratory for Sound and Vibration Research, vice-president of the European Acoustical Association and the director of the center for sustainable Aviation. His research interests include experimental methods and modelling of sound propagation and sources in particular related to flow ducts.