<p>This study employs large eddy simulation to systematically investigate the primary flow patterns and flow field characteristics in a cross-pipe configuration featuring various adjacent closed-cavity length ratios (<i>L</i>_<i>a</i><i>/L</i>_<i>b</i> = 1, 2, and 0.5) at a Reynolds number of 10,000. The spatial distribution of acoustic sources is qualitatively evaluated based on the extracted λ-vector and Lighthill’s acoustic analogy. Spectral proper orthogonal decomposition (SPOD) is employed to elucidate the correlation between flow structures and noise generation. The results highlight the impact of cavity geometry on flow behavior and flow-induced noise. For <i>L</i>_<i>a</i><i>/L</i>_<i>b</i> = 1, the flow is characterized by three unstable shear layer structures, which dominate the distribution of vorticity and Reynolds shear stress. The noise sources generated by fluid–wall interactions span the entire downstream region, the cross-junction, and the straight cavity (cavity A). For <i>L</i>_<i>a</i><i>/L</i>_<i>b</i> = 2, the extended cavity length enables full flow recirculation, which reduces vortex strength and weakens acoustic source intensity, while increasing its distribution range. However, the increased length of the side cavity (cavity B) enhances flow separation at the downstream trailing edge, which in turn generates additional acoustic sources near the outer downstream wall. SPOD analysis reveals that the dominant mode across all configurations is a large-scale wavepacket structure originating from flow separation at the downstream leading edge. An extended straight cavity enhances internal recirculation, reducing the main flow energy, thereby weakening the wavepacket structures and accelerating their downstream dissipation. For <i>L</i>_<i>a</i><i>/L</i>_<i>b</i> = 0.5, the wavepackets’ behavior below the cross-junction is intensified. At high frequencies, the downstream wavepackets become smaller and more concentrated, with a slower rate of dissipation. Therefore, valve installation in hydrogen pipelines should prioritize extending the straight cavity. This study provides valuable guidance for optimizing structural vibration and noise generation in hydrogen pipeline systems.</p> Graphical abstract <p>Contours of <b>a</b> the instantaneous and <b>b</b> the time averaged lambda magnitude for three configurations with <i>L</i>_<i>a</i> /<i>L</i>_<i>b</i> = 1, 2, and 0.5</p>