Flow-Acoustic Coupling in $T$-Junctions: Effect of $T$-Junction Geometry

The flow-acoustic coupling mechanism in a $T$-junction, which combines flows from two branches, forming the “cross-bar” of the $T$-junction, into one pipe, forming the “stem” of the $T$-junction, is investigated experimentally. The $T$-junction has a step pipe expansion at its inlets. The shear layer separating from this step expansion is found to excite intense acoustic resonances over multiple ranges of flow velocity. The excited acoustic mode is confined to the branch pipes and has an acoustic pressure node at the centerline of the $T$-junction. The length of the expansion section of the $T$-junction is found to control the frequency of the shear layer oscillation and therefore determines the ranges of flow velocity over which acoustic resonances are excited. Introducing asymmetry in the $T$-junction expansion length has shown little influence on the excitation of acoustic resonance. An additional $T$-junction arrangement made of rectangular cross-sectional ducts is also investigated to facilitate a flow visualization study of unsteady flow structures in the $T$-junction during acoustic resonance, and thereby improve understanding of the acoustic resonance mechanism and the nature of the aero-acoustic sources in the $T$-junction.