An Experimental PIV/POD Study on the Sequentially Coupled Flow Field of Swirl and Multiple Jets

The stepwise flow reconstruction induced by sequential coupling of swirl, primary jets, and dilution jets was experimentally investigated in a model combustor using high‐speed PIV and proper orthogonal decomposition (POD). Three nonreacting configurations were compared: pure swirl, swirl plus primary jets, and swirl with both primary and dilution jets. The primary jets intensified the upstream central recirculation zone, increasing the peak reverse velocity at X = 30 mm by approximately 100% (from U / U b = −0.50 to −1.0), with the subsequent dilution jets further amplifying it to −1.25. The primary jets also axially stretched the recirculation zone and weakened its downstream core velocity by approximately 50% (to U / U b = −0.25). The dilution jets dominated the far downstream region, reversing the centerline velocity from negative to a positive value of U / U b = +0.38 at X = 90 mm and eliminating reverse flow, thereby reshaping the flow into a quasiunidirectional pattern. POD analysis revealed that multijet coupling shifted the dominant dynamics from a low‐frequency quasiperiodic mode (1.65 Hz) under pure swirl to a higher frequency instability (2.4 Hz) excited by the primary jets. The pure swirl field already exhibited multifrequency behavior, but the primary jets systematically increased the characteristic frequency of the most energetic coherent structure. Spectral energy accumulation was noted near the Nyquist limit (7.5 Hz); however, given the 15 Hz sampling rate, these features are subject to aliasing and are interpreted only as qualitative indicators of unresolved higher frequency activity rather than as definitively identified modal dynamics. The primary jets concentrated turbulent energy in large‐scale structures (Mode 1 energy fraction reached 12.84%), whereas the dilution jets promoted energy redistribution across a broader range of scales, facilitating the transition toward a more fully developed turbulent state with a more uniform spectral energy distribution.