Keywords: Cavity acoustics, Acoustic-hydrodynamic interactions, Pipe-cavity jets, Pipe-cavity
jet noise, Turbulence mixing noise, Screech, Broadband shock associated noise,
Cavity tones, Schlieren flow visualization, pipe-cavity modes, Non-linear
interactions

Axisymmetric cavities play a vital role in aerospace, pipeline, power plant industries.
Compressible flows through cavities exhibit high amplitude self-sustained oscillations leading to
dynamic acoustic loading. Geometric and flow parameters decide the resonance frequency of the
axisymmetric cavity. An incorrectly expanded pipe-cavity jet emits strong cavity tones and
shock associated noise that may damage adjacent nozzle structures and are highly annoying. In
order to effectively attenuate the cavity associated tones and noise, the cavity noise generation

mechanism needs to be understood. Hence the present work is motivated to understand the pipe-
cavity noise generation mechanism and the effects of upstream cavity hydrodynamic fluctuations

on the downstream jet flow and the far-field acoustics. This work also attempts to reduce the
cavity noise using a passive control method.
Firstly, an experimental investigation is conducted with different depths of the axisymmetric
cavity to show the effect of the cavity aspect ratio on the jet noise over a wide range of Nozzle
Pressure Ratios (NPRs)/Mach numbers. Hydrodynamic fluctuations are measured in the cavity
while simultaneously measuring the far-field noise. Further, flow visualization is carried to study
the jet structure and the role of cavity hydrodynamics on the far-field noise and pipe-cavity jet
dynamics. Theoretical predictions and finite element analyses are carried out to study the cavity
resonance frequency and corresponding mode shapes. The fluid resonance oscillation mechanism
plays a major role in governing the pipe-cavity resonance at higher NPRs/Mach numbers. The
experimentally measured resonance frequency is close to the first tangential mode rather than
higher modes. Scalogram and higher-order spectral analysis is implemented to unravel the mode
switching and nonlinear interactions. It is also noted that the mode switching from pure
tangential mode to combined mode is observed for higher depth cavities. Schlieren imaging
method, POD, and FFT analyses are implemented to unravel the effect of the upstream cavity on
jet flow structure and to provide a link to the far-field acoustics. The experimental investigation is further extended to study the effect of the shallowest and
deepest upstream cavity on low and moderately underexpanded jets. Linear and higher-order
spectral analyses are implemented on the unsteady cavity pressure to comprehend the nature of
the cavity acoustics and nonlinear interactions of different acoustic modes of the pipe-cavity
system. At higher NPRs/Mach numbers, the higher depth cavities exhibit nonlinear interaction of
pipe and cavity acoustic modes. In the case of moderately underexpanded jet condition, the
power spectra of pipe-cavity jets show the combined effect of cavity and shock associated tones.
The amplitudes of the Broadband Shock Associated Noise (BSAN) and high-frequency mixing
noise are considerably increased due to the upstream cavity. Proper orthogonal and dynamic
mode decomposition methods are used to extract the spatial and dynamic modes of the jet
structure. These methods are used to segregate the cavity associated jet dynamics and screech
dynamics. DMD is used to extract the spatial structure at a particular frequency of oscillations.
Hence, the spatial structures of nonlinear modes and screech cases are obtained.
Further, an experimental study is carried out with different inlet pipe lengths to show the
effects on pipe-cavity jet noise. An extension of inlet pipe length acts as a tool for an increase in
the thickness of the initial shear layer at the leading edge of the cavity. The thicker shear layer
impinges upon the trailing edge of the cavity, leading to suppression of instabilities compared to
a thinner one. Also, an increase in initial shear layer thickness is capable of reducing the tonal
noise due to an increase in the stability of the shear layer. In the present study, an increase in
shear layer thickness tends to attenuate the cavity tones, with a small increase in screech tonal
noise. An increase in upstream pipe length leads to a decrease in overall sound pressure levels
and acoustic power of the jet noise.
Lastly, the passive control method of cavity noise using protrusion at various locations in the
cavity is presented. The experiments are carried out for protrusion with different locations for
fixed dimensions. Also, the pipe-cavity without protrusion is compared. Once the location is
optimized, further investigations are carried out with varying lengths of the protrusion. The
device aims to alleviate the feedback mechanism and the recirculation flow in the cavity system.
Significant noise reduction is observed with the protrusions cases when compared to the baseline
(without protrusion). The protrusion at the trailing edge effectively attenuates the cavity noise at
both cavity pressure fluctuations and far-field acoustics. Besides helping gain a deeper understanding of the flow and acoustic physics, the results of the present work may help in
designing pipe-cavity systems for various flow conditions and noise considerations.