Flow dynamics and oscillation mechanisms in T-junction mixing flows using time-resolved particle image velocimetry and spectral proper orthogonal decomposition analysis

T-junction mixing flows are prevalent in industrial applications such as chemical processing, pharmaceutical manufacturing, water treatment, and nuclear power plant piping networks. Understanding the mixing and oscillation process between main and branch flows is critical for enhancing fluid mixing and suppressing oscillations. This study employs time-resolved two-dimensional particle image velocimetry to analyze velocity distribution and oscillation characteristics in T-junction water flows with branch-to-main flow velocity ratios (Vr) of 1.5, 2.0, 3.0, and 3.5, under 0 and 30 °C temperature differences. The experiments reveal that oscillations primarily occur downstream of the T-junction, within the range of X/D = 0.5–2.5, and exhibit broad-frequency characteristics. These oscillations are driven by the interaction between the recirculation zone and the mainstream, as well as shear layer instability caused by velocity gradients at the interface. Vortex shedding from the branch flow and energy redistribution in the flow field further contribute to the oscillation dynamics. Spectral proper orthogonal decomposition analysis identifies the dominant oscillation frequencies in the Strouhal number range of 0.13–0.36, with the most intense oscillations observed in the Y direction. The increase in the branch-to-main flow velocity ratio expanded the oscillation regions and amplified their amplitudes. In contrast, temperature differences enhanced fluid mixing, which reduced the oscillation regions but increased the amplitudes, primarily due to changes in fluid viscosity. The findings provide valuable insights into the flow dynamics and oscillation mechanisms in T-junction mixing flows, offering a foundation for optimizing mixing efficiency and mitigating oscillations in industrial applications.