Journal of Fluids and Structures 25 (2009) 42–59 Flow-excited acoustic resonances of coaxial side-branches in an annular duct D. Arthurs, S. Ziada à Department of Mechanical Engineering, McMaster University, Hamilton, ON, Canada L8S 4L8 Received 14 December 2006; accepted 16 March 2008 Available online 22 May 2008 Abstract This paper investigates the aeroacoustic response of an annular duct with closed coaxial side-branches, and examines the effect of several passive countermeasures on the resonance intensity. The investigated geometry is inspired by the design of the Roll-Posts in the Rolls-Royce LiftSystem s engine, which is currently being developed for the Lockheed Martin Joint Strike Fighter (JSF s ) aircraft. The effects of design parameters, such as diameter ratio, branch length ratio and thickness of the annular flow on the frequency and resonance intensity of the first acoustic mode are studied experimentally. Numerical simulations of the acoustic mode shapes and frequencies are also performed. The annular flow has been found to excite several acoustic modes, the strongest in all cases being the first acoustic mode, which consists of a quarter wavelength along the length of each branch. The ratios of the branch length and diameter, with respect to the main duct diameter, have been found to have strong effects on the frequency of the acoustic modes. r 2008 Elsevier Ltd. All rights reserved. Keywords: Acoustic resonance; Coaxial side-branch; Annular duct 1. Introduction Problems involving acoustic resonance in piping systems with closed side-branches are common in industrial applications and have been reported by several authors such as Chen and Florjancic (1975), Chen and Stu¨ rchler (1977), Baldwin and Simmons (1986) and Gillessen and Roller (1989). Generally, the flow in the main pipe excites a standing sound wave consisting of an odd number of quarter wavelengths along the length of each branch in single and coaxial side-branch systems, as illustrated in Fig. 1. This phenomenon often generates large pressure oscillations in the side-branches and may result in severe noise and/or vibration problems and possibly fatigue failure of the piping system or its associated instrumentations. Previous investigations of this phenomenon by Bruggeman et al. (1991), Ziada and Bu¨hlmann (1992) and Ziada (1993), among others, have shown that the pulsation amplitude in the side-branches can be much higher than the dynamic head in the main pipe. This phenomenon can therefore pose serious limitations on the reliable operation of piping systems with closed side-branches. A recent example of the damaging effects of this phenomenon is the acoustic fatigue damage of the steam dryer of a boiling water reactor in Quad Cities nuclear plant. In this case, the pressure pulsations generated by ARTICLE IN PRESS www.elsevier.com/locate/jfs 0889-9746/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfluidstructs.2008.03.007 à Corresponding author. E-mail address: ziadas@mcmaster.ca (S. Ziada).