Fully turbulent inflow past a shallow cavity is investigated for the configuration of an axisymmetric cavity mounted in a pipe. Emphasis is on conditions giving rise to coherent oscillations, which can lead to locked-on states of flow tones in the pipeline-cavity system. Unsteady surface pressure measurements are interpreted using threedimensional representations of amplitude-frequency-inflow velocity or cavity length;​ these representations are constructed for a range of cavity depth.

For cases where locked-on flow tones occur, due to coupling of the unsteady shear layer with the pipe resonator, criteria for lock-on based on a quality (Q)-factor and dimensionless pressure amplitude are proposed and assessed. These criteria lead to the definition of threshold velocities for the onset of flow tones.

A technique of high-image-density particle image velocimetry is employed in conjunction with a special test section, which allows effective laser illumination and digital acquisition of patterns of particles in the flow. These patterns lead to wholefield images of velocity, vorticity and streamline topology, as well as hydrodynamic contributions to the acoustic power integral. Global instantaneous images, as well as phase- and time-averaged images, are evaluated to provide insight into the flow physics during tone generation.

These image-based approaches allow identification of regions of the unsteady shear layer that contribute to the instantaneous hydrodynamic component of the acoustic power, which is necessary for maintaining a flow tone. In addition, image analysis, in conjunction with pressure measurements, allows categorization of the instantaneous flow patterns that are associated with categories of time traces and spectra of the fluctuating pressure. In contrast to consideration based solely on pressure spectra, it is demonstrated that locked-on tones may actually exhibit intermittent, non-phase-locked images, apparently due to low dam ping of the acoustic resonator. Locked-on flow tones (without modulation or intermittency), locked-on flow tones with modulation, and non-locked-on oscillations with short-term, highly coherent fluctuations are defined and represented by selected cases. Depending on which of these regimes occur, the time average Q (quality) -factor and the dimensionless peak pressure are substantially altered.