The flow-induced acoustic source of an axisymetric shallow cavity, coupled to longitudinal acoustic modes in a pipeline is characterized experimentally for circular pipes conveying fully developed turbulent flow. This aeroacoustic source is generated due to the interaction of the cavity shear layer oscillation with the resonant acoustic field in the pipe. The distributed aeroacoustic source is modeled by a lumped acoustic dipole source which is dependent on the Strouhal number and the acoustic particle velocity at the cavity. The amplitude and phase of this source are determined experimentally and presented in the form of a dimensionless complex source term.

This paper experimentally investigates the flow-sound interaction mechanisms in a T-junction combining the flow from its two co-axial side-branches into the central branch. The T-junction has a sudden area expansion at each side-branch entrance. Flow separation at these area expansions forms free shear layers which are shown to excite the acoustic mode(s) of the branches over several ranges of flow velocity, each of which results from the coupling of the acoustic mode with a different shear layer oscillation mode. Phase-locked particle image velocimetry is utilized to detail the unsteady flow field over the acoustic cycle for the oscillation mode which resulted in the strongest acoustic resonance. Finite element analysis is used to characterize the excited acoustic mode shape and its associated particle velocity field. In-depth analysis of the flow-sound interaction mechanism inside the T-junction is performed by means of Howe’s acoustic analogy. It is concluded that the flow-sound interaction mechanism in the entrance region of the T-junction produces a spatially alternating pattern of acoustic energy generation and absorption. This alternating pattern of energy exchange between the flow and sound fields results in a minimal amount of net acoustic power being generated in the entrance region. However, the increasing orthogonality between the acoustic particle streamlines and the flow streamlines near the exit of the T-junction at its center results in the majority of the generated sound power which sustains the acoustic resonance.

No abstract

No abstract

The aeroacoustic sources generated by flow over a cylindrical shallow cavity in a pipeline are examined in the presence of plane sound waves at various Strouhal numbers and sound intensities. The cavity is exposed to high Reynolds number fully developed turbulent flow. Extensive PIV flow measurements are performed to characterize the unsteady velocity and vorticity fields at various time instants within the sound cycle. Finite element analysis is used to obtain the particle velocity field of the sound field. The results of the PIV flow measurements and those of the finite element simulations are combined into Howe’s aeroacoustic integrand to compute the spatial and temporal distributions of the aeroacoustic sources resulting from the cavity shear layer interaction with the sound field. The results are compared with the measured aeroacoustic source strength obtained by means of the Sound Wave Method (SWM) [1]. The advantages and disadvantages of both techniques are discussed.