Imaging of Acoustically Coupled Oscillations Due to Flow Past a Shallow Cavity: Effect of Cavity Length Scale

Abstract

Flow-acoustic interactions due to fully turbulent inflow past a shallow axisymmetric cavity mounted in a pipe, which give rise to flow tones, are investigated using a technique of high-image-density particle image velocimetry in conjunction with unsteady pressure measurements. This imaging leads to patterns of velocity, vorticity, streamline topology, and hydrodynamic contributions to the acoustic power integral. Global instantaneous images, as well as time-averaged images, are evaluated to provide insight into the flow physics during tone generation. Emphasis is on the manner in which the streamwise length scale of the cavity alters the major features of the flow structure.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 to maintain a flow tone. In addition, combined image analysis and pressure measurements allow categorization of the instantaneous flow patterns that are associated with t ypes of time traces and spectra of the fluctuating pressure. In contrast to consideration based solely on pressure spectra, it is demonstrated that lockedon tones may actually exhibit intermittent, non-phase-locked images, apparently due to low damping 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. Depend ing on which of these regimes occur, the time-averaged Q (quality) -factor and the dimensionless peak pressure are substantially altered.

Characteristics of Oscillation in Cavity of Helmholtz Nozzle Generating Self-excited Pulsed Waterjet

“…Additionally, with regard to amplitude, the local maximum in the pressure amplitude spectrum suggests the coupling between the acoustic standing waves and the shear layer [8]. Notably, the resonance amplitude of the lock-on frequency was approximately one order of Figure 4 Speed-dependent spectrogram measured at cavity wall magnitude higher than the pressure fluctuation in the hydrodynamic mode.…”

“…Additionally, cavitation occurs in the cavity owing to the unique structure of the axisymmetric cavity, which results in a rapid decline in the wave velocity of water. (8…”

“…Regarding experimental research pertaining to cavity flow oscillations caused by flows passing various cavities, primarily flow image display and flow measurement technology are typically prioritized. In terms of methods for measuring the dynamic pressure of walls, hot-wire experiments and particle image velocimetry are typically performed to measure the shear layer of the cavity and the flow in the cavity [8,9].…”