Samir Ziada
McMaster University, Canada
Title: Flow-sound interaction mechanisms during the resonance of trapped diametral modes in shallow cavities
Biography
Biography: Samir Ziada
Abstract
The excitation of trapped acoustic modes within ducted cavities is often encountered in various components of nuclear and conventional power plants, jet engines, turbo-compressors and other engineering equipment. This phenomenon is investigated for a square shallow cavity during the resonance of its lowest four acoustic modes (Figure 1). The unsteady flow structure and the acoustic mode shapes during these resonances are detailed by means of pressure measurements at various azimuthal locations, time resolved particle image velocimetry technique, and numerical simulation of the resonant modes using finite element analysis. Three different interaction patterns are analyzed, corresponding to the resonance of a single stationary acoustic mode, simultaneous excitation of two different stationary acoustic modes, and a special case of spinning mode resonance due to the excitation of two degenerate acoustic modes. After reviewing the general resonance response of the four acoustic modes, attention will be focused on the dual resonance case (i.e., simultaneous excitation of modes 3 & 4 which have different frequencies and mode shapes). In this case, the circumference of the cavity shear layer is found to be divided into 8 segments, each of which is acoustically excited at a frequency which is different from those exciting the adjacent segments. The excitation level is also not uniform over each segment. Despite this rather complex pattern of acoustic excitation along the shear layer circumference at the upstream cavity corner, coherent vortices of different frequencies and phase do form over various segments of the shear layer circumference and are found to retain their individual coherence as they travel along the cavity mouth up to the downstream corner. The results of this study substantially improve our understanding of this complex excitation mechanism and the acquired flow visualization images constitute a challenging benchmark case for the validation of Computation Aero-Acoustic (CAA) codes.
Figure 1: Acoustic pressure distributions of the first four trapped acoustic modes obtained by means of numerical simulation
Recent Publications
1. Aly K and Ziada S (2010) Flow-excited resonance of trapped modes of ducted shallow cavities. Journal of Fluids & Structures 26:92-120.
2.Aly K and Ziada S (2011) Azimuthal behavior of flow-excited diametral modes of internal shallow cavities. Journal of Sound & Vibrations 330:3666-3683.
3. Aly K and Ziada S (2016) Review of flow-excited resonance of acoustic trapped modes in ducted shallow cavities. Journal of Pressure Vessel technology 138:040803.
4. Hein S and Koch W (2008) Acoustic resonances and trapped modes in pipes and tunnels. Journal of Fluid Mechanics 605:401-428.
5.Tonon D, Hirschberg A, Golliard J and Ziada S (2011) Aeroacoustics of pipe systems with closed branches. International Journal of Aeroacoustics 10 (2):201-276.