Aeroacoustic Response of an Annular Duct with Coaxial Closed Side Branches

Abstract

Ducts with closed side-branches are known to be very liable to flow excited acoustic resonance of the branches. This is particularly true when the duct contains multiple branches in close proximity, as in the case of coaxial branches. This paper focuses on flow-excited acoustic resonance of an annular duct with coaxial sidebranches. This geometry is encountered in practical applications such as the roll posts of the vertical lift system on the STOVL version of the Joint Strike Fighter (F35B JSF). Flow tests including measurements of the acoustic pressure and complemented with numerical simulation of the acoustic modes have shown that the first acoustic mode of the side-branches is strongly excited at low Mach number flow. The higher modes are also excited, but at higher velocities. Strong acoustic resonances are generated over wide ranges of lock-in such that it is hardly possible to operate the system at any Mach number below ≈ 0.4 without exciting an acoustic mode. The simulated acoustic modes agree well with those observed in the experiments. Countermeasures to eliminate the resonance of the lowest modes are currently being investigated. I. Introduction Acoustic standing waves in closed side-branches are often excited by the flow in the main duct. The modes excited are odd multiples of quarter wavelengths along the branches. When such resonances occur, the acoustic pressure pulsations can be higher than the dynamic head in the main pipe and therefore sidebranches can generate serious vibration and/or noise problems. Acoustic resonances of side-branches are caused by a feedback excitation mechanism referred to in the literature as “fluid-resonant mechanism”, see for example Rockwell and Naudascher. 5 This mechanism is self-sustained by the coupling between the unstable shear layer formed at the mouth of the branch and the acoustic particle velocity of the resonant standing wave in the side-branch. The majority of recent theoretical models of the fluid-resonant excitation mechanism is based on the acoustic analogy developed by Howe 3,4 . According to this analogy, the instantaneous acoustic power, P , generated by the convection of vorticity field, ω , within a sound field is given by