Unsteady flow behaviors and flow-induced noise characteristics in a closed branch T-junction

Abstract

In the present study, dynamic delayed detached eddy simulation is utilized to explore turbulent flow in T-junctions at a Reynolds number of ReD = 2.0 × 104. Three systems with varying corner cavity depth-to-diameter ratios (Ld/D = 1, 2, and 4) are examined to elucidate the interplay between unsteady flow and flow-induced noise. The analysis employs Lighthill's acoustic analogy to scrutinize surface dipole acoustic sources and their noise propagation characteristics. Coherent flow structures, characterized as wavepackets, are identified through spectral proper orthogonal decomposition, demonstrating consistent dominance in modes and dipole distributions across the systems. In the system with Ld/D = 1, wavepackets originating from the downstream region of the junction exhibit a pronounced flapping behavior attributed to the Kelvin–Helmholtz instability. Most dissipate with the mainstream flow, whereas a portion interacts with the wall, forming dipole acoustic sources. For systems with Ld/D = 2 and 4, the dominant mode transitions to the junction adjacent to the corner cavity, expanding continuously after separation until obliquely colliding with the wall, resulting in expanded dipole distributions. Mechanisms underlying flow-induced noise generation are unveiled by extracting transient vorticity fields within oscillation cycles. For shallow corner cavity depths (Ld/D = 1), periodic oscillatory vorticity shedding from the junction's sidewall significantly contributes to far-field sound pressure. As the cavity is deep enough to support one or more full recirculations of the fluid (Ld/D = 2 and 4), periodic vorticity shedding from the trailing edge directly impacts the wall above the junction, simultaneously suppressing flapping behavior at the leading edge and weakening overall dipole acoustic source intensity.