Modal dynamics governing tonal noise in high-speed rectangular dual-impinging jets

An experimental investigation is conducted to examine aeroacoustic instabilities in high-speed rectangular dual-impinging jets (DIJs), with a focus on the modal dynamics responsible for tonal noise generation. Comparisons are made with a circular baseline over expanded Mach numbers M = 1.0–1.2, impinging distances h / D = 2–3 and a fixed centre-to-centre spacing of S / D = 3.5. Time-resolved wall-pressure fields on the impingement plate and far-field sound are measured synchronously using fast-response pressure-sensitive paint and microphones. Dominant impingement tones and their associated surface-pressure patterns are identified and linked using cyclostationary spectral proper orthogonal decomposition and coherent output power analysis. Both circular and rectangular DIJs exhibit an annularly symmetric impingement mode whose tonal frequency is broadly consistent with classical single-loop jet–plate feedback scaling. In the rectangular configuration, this mode develops an elliptical footprint and a reduced dual-jet interference region, leading to attenuation of the corresponding tone relative to the circular case. In addition, rectangular DIJs support a flapping antisymmetric plate-pressure mode characterised by alternating pressure-wave orientations and a four-stage evolution over the oscillation cycle. This mode is associated with a prominent tone at Strouhal number St = 0.42, which departs from the integer harmonics of the baseline feedback loop. Finally, a co-resonance framework is employed to interpret why the geometry-enabled tone is strongly amplified under the present dual-jet conditions. Overall, these results demonstrate that transitioning from circular to rectangular nozzles redistributes modal dominance in high-speed DIJs and reorganises the balance between attenuated harmonic peaks and an additional geometry-enabled tone.