Experimental characterization of multimodal flow features in cavity-induced flapping jets

In this study, the flow features of flapping jets previously proposed are systematically characterized. Instantaneous pressure fields across cavities were measured using fast-response pressure-sensitive paint and denoised. The jet oscillations were primarily induced by the instability of the shear layer and the propagation of compression waves generated by vortex impact on the trailing edge. A spectral proper orthogonal decomposition (SPOD) analysis was also performed, the dominant spatial modes at spectral peaks were extracted. SPOD revealed that the dominant frequency was generated by the feedback flapping motion. The first-mode frequency transitioned from the first feedback flapping mode to the transverse acoustic resonance mode with increasing cavity length. The harmonics associated with the first mode of feedback flapping motion were generated by large-scale axisymmetric vortex shedding. Subharmonic modes at approximately half the first-mode frequency were observed for all cavity lengths, exhibiting symmetric flow structures. The pressure oscillations generated by three flapping jet devices with different scales were measured, and the dimensionless spectra were derived for various normalized cavity lengths. A modified prediction model reproduced the spectral peaks at various pressure ratios with higher accuracy than the previous model. The spectral peaks corresponding to the first and second modes of feedback flapping motion and the transverse acoustic resonance modes were identified. The effect of the Reynolds number and the pressure ratio on the oscillation frequency was investigated. The Reynolds number and pressure ratio affect high-order mode energy, oscillation thresholds, and mode transitions but have only a slight effect on dimensionless peak frequencies.