Abstract: This study investigates the evolution of flow structures and the acoustic standing-wave modes associated with flow-induced acoustic resonance in a dry ultrasonic cleaning head. Numerical simulations were performed using a hybrid RANS–LES approach to resolve the resonant flow arising from the coupling between high-frequency vortical structures and the first-order acoustic standing wave in a small coaxial cavity (L/D = 1, D = 10 mm). The standing-wave mode was identified, and the degree of flow–acoustic coupling was classified, through analyses of wall-pressure pulsations in the cavity and acoustic standing-wave modal characteristics—enabling a comparative investigation of the flow–acoustic coupling behavior. The results show that, as the coupling intensity between the vortical structures and the first-order acoustic standing wave increases, the resonant flow exhibits a characteristic double-swaying behavior. Meanwhile, the maximum incompressible momentum thickness of the shear layer decreases, accompanied by reduced momentum loss. In addition, vortex intensity is approximately orthogonal in phase to the amplitude of the streamwise velocity, whereas the vortex position exhibits a highly consistent phase relationship with the amplitude of the incoming-flow velocity. These findings provide theoretical guidance for acoustic source regulation and structural optimization of dry ultrasonic cleaning heads.