Acoustic Particle Velocity Field Research Articles

Flow structure and aerodynamic forces of finned cylinders during flow-induced acoustic resonance

This experimental study investigated the impact of self-excitation of acoustic resonance on the fluctuating lift force, flow structure, and aeroacoustic energy transfer of spirally finned cylinders with crimps in cross-flow. Three different finned cylinders with varying fin pitch-to-root diameter ratios (p/Dr) were compared with their equivalent diameter (Deq) bare cylinders. The study found that self-excitation of acoustic resonance occurred at a lower reduced flow velocity for the finned cylinders. During acoustic resonance excitation, finned cylinders with lower p/Dr experienced an abrupt increase in the fluctuating lift force coefficient and acoustic pressure. The decomposition of the fluctuating lift force with respect to the acoustic pressure showed that the earlier onset of resonance and the abrupt increase in the lift force and acoustic pressure were due to the increased susceptibility to flow-induced acoustic resonance. Phase-locked particle image velocimetry (PIV) measurements of the vorticity field supported this finding by showing a well-organized vorticity field in the wake of these finned cylinders. However, the fins and crimps had a significant effect on the distribution of the acoustic particle velocity field compared to the bare cylinder. The presence of the fins and crimps caused the acoustic particle velocity magnitude to be concentrated within the fins and rapidly decline away from the cylinder base. As a result, the acoustic power generated was lower in the case of the finned cylinders. This ultimately resulted in a lower peak acoustic pressure and fluctuating lift force being generated in the case of the crimped spirally finned cylinders.

MVDR beamforming using one bi-axial velocity-sensor

A bivariate velocity-sensor measured two Cartesian components of the acoustic particle velocity field, which represents the spatial gradient of the acoustic pressure field. One such bi-axial unit provides azimuth-polar two-dimensional spatial directivity. This work presents the MVDR beam-pattern for such a sensing unit.

Theoretical Estimation of the Acoustic Energy Generation and Absorption Caused by Jet Oscillation

We investigate the energy transfer between the fluid field and acoustic field caused by a jet driven by an acoustic particle velocity field across it, which is the key to understanding the aerodynamic sound generation of flue instruments, such as the recorder, flute, and organ pipe. Howe’s energy corollary allows us to estimate the energy transfer between these two fields. For simplicity, we consider the situation such that a free jet is driven by a uniform acoustic particle velocity field across it. We improve the semi-empirical model of the oscillating jet, i.e., exponentially growing jet model, which has been studied in the field of musical acoustics, and introduce a polynomially growing jet model so as to apply Howe’s formula to it. It is found that the relative phase between the acoustic oscillation and jet oscillation, which changes with the distance from the flue exit, determines the quantity of the energy transfer between the two fields. The acoustic energy is mainly generated in the downstream ar...

Flow-Excited Acoustic Resonance of Trapped Modes of a Ducted Rectangular Cavity

Flow over ducted cavities can lead to strong resonances of the trapped acoustic modes due to the presence of the cavity within the duct. Aly and Ziada (2010, “Flow-Excited Resonance of Trapped Modes of Ducted Shallow Cavities,” J. Fluids Struct., 26(1), pp. 92–120; 2011, “Azimuthal Behaviour of Flow-Excited Diametral Modes of Internal Shallow Cavities,” J. Sound Vib., 330(15), pp. 3666–3683; and 2012, “Effect of Mean Flow on the Trapped Modes of Internal Cavities,” J. Fluids Struct., 33, pp. 70–84) investigated the excitation mechanism of acoustic trapped modes in axisymmetric cavities. These trapped modes in axisymmetric cavities tend to spin because they do not have preferred orientation. The present paper investigates rectangular cross-sectional cavities as this cavity geometry introduces an orientation preference to the excited acoustic mode. Three cavities are investigated, one of which is square while the other two are rectangular. In each case, numerical simulations are performed to characterize the acoustic mode shapes and the associated acoustic particle velocity fields. The test results show the existence of stationary modes, being excited either consecutively or simultaneously, and a particular spinning mode for the cavity with square cross section. The computed acoustic pressure and particle velocity fields of the excited modes suggest complex oscillation patterns of the cavity shear layer because it is excited, at the upstream corner, by periodic distributions of the particle velocity along the shear layer circumference.

Systematic Error of Acoustic Particle Image Velocimetry and Its Correction

Abstract Particle Image Velocimetry is getting more and more often the method of choice not only for visualization of turbulent mass flows in fluid mechanics, but also in linear and non-linear acoustics for non-intrusive visualization of acoustic particle velocity. Particle Image Velocimetry with low sampling rate (about 15Hz) can be applied to visualize the acoustic field using the acquisition synchronized to the excitation signal. Such phase-locked PIV technique is described and used in experiments presented in the paper. The main goal of research was to propose a model of PIV systematic error due to non-zero time interval between acquisitions of two images of the examined sound field seeded with tracer particles, what affects the measurement of complex acoustic signals. Usefulness of the presented model is confirmed experimentally. The correction procedure, based on the proposed model, applied to measurement data increases the accuracy of acoustic particle velocity field visualization and creates new possibilities in observation of sound fields excited with multi-tonal or band-limited noise signals.

Transient, planar, nonlinear acoustical holography for reconstructing acoustic pressure and particle velocity fields

A steady-state, nonlinear nearfield acoustical holography procedure, based on the Westervelt Wave Equation (WWE), was developed by the authors of this article to accurately reconstruct nonlinear acoustic “pressure” fields. Here, a “transient,” nonlinear acoustic holography algorithm is introduced that can be used to reconstruct three-dimensional, nonlinear acoustic pressure as well as particle velocity fields from two-dimensional acoustic pressure data measured on a measurement plane. This procedure is based on the Kuznetsov Wave Equation (KWE) that is directly solved by applying temporal and spatial Fourier Transforms to the KWE. When compared to the WWE-based procedure, the proposed procedure can be used to reconstruct acoustic particle velocity fields in addition to acoustic pressure fields. It can also be applied to multi-frequency source cases where each frequency component can contain both linear and nonlinear components. The KWE-based procedure is validated by conducting four numerical simulations with: (1) an infinite-size panel vibrating at a single frequency, (2) a pulsating sphere with a bifrequency excitation, (3) a finite-size, vibrating panel generating bended wave rays, and (4) an ultrasound transducer with a transient excitation. The numerical results show that holographically projected acoustic fields match well with directly calculated ones.

Experimental Results of Underwater Cooperative Source Localization Using a Single Acoustic Vector Sensor

This paper aims at estimating the azimuth, range and depth of a cooperative broadband acoustic source with a single vector sensor in a multipath underwater environment, where the received signal is assumed to be a linear combination of echoes of the source emitted waveform. A vector sensor is a device that measures the scalar acoustic pressure field and the vectorial acoustic particle velocity field at a single location in space. The amplitudes of the echoes in the vector sensor components allow one to determine their azimuth and elevation. Assuming that the environmental conditions of the channel are known, source range and depth are obtained from the estimates of elevation and relative time delays of the different echoes using a ray-based backpropagation algorithm. The proposed method is tested using simulated data and is further applied to experimental data from the Makai'05 experiment, where 8–14 kHz chirp signals were acquired by a vector sensor array. It is shown that for short ranges, the position of the source is estimated in agreement with the geometry of the experiment. The method is low computational demanding, thus well-suited to be used in mobile and light platforms, where space and power requirements are limited.

Acoustic PIV: Measurement of the acoustic particle velocity using synchronized PIV-technique

This paper outlines a technique for measuring the acoustic particle velocity and the flow field simultaneously by applying synchronized particle image velocimetry (PIV). As test set-up a squared acrylic glass chamber was chosen. One side of the test section is connected to a loudspeaker, which allows a sinusoidal excitation of the chamber. To point out constrains of this method the investigation includes an analysis of excitation amplitude and frequency as well as the effect of the mean flow magnitude. Therefore a small PC fan can be mounted inside the test section to produce an adjustable mean flow. It can be shown that for a low number of averaged images (80) reasonable results can be achieved up to a certain level of fan rotation speed. Beyond this level the turbulence sensitivity increases and more images are necessary for the calculations. However, the acoustic particle velocity can be computed in the presence of turbulent flow. The presented method called acoustic PIV is a non intrusive technique, applied successfully in measuring acoustic particle velocity fields over a wide range of conditions.

Particle velocity field measurement using an ultra-light membrane

The aim of this paper, is to present an innovative experimental approach to assess images of the acoustic particle velocity field using a laser vibrometer scanning an ultra-light membrane. Firstly, a theoretical part is devoted to the infinite membrane governing equations, and to its response to an acoustic incident plane wave. An impedance of the membrane is defined, and a mass correction factor is obtained for a plane wave in normal incidence, allowing to assess the particle velocity of the acoustic field without membrane from the measured velocity of the membrane. The practical realization of a finite membrane is reported in a second section, showing difficulties linked to membrane modes generated by an uncontrolled residual tension, also showing how those difficulties are overcome by weighing down the membrane and by applying the mass correction. Third and fourth parts of the article are dedicated to two-dimension applications, illustrating the implementation of the membrane approach in the case of a plate excited by a shaker, and in the case of a loudspeaker driven with a white noise. Results are encouraging, showing that a “heavy” membrane should be used in low frequency to avoid difficulties due to membrane modes, and a light one in high frequency to minimize the mass effect (that can be corrected only for plane wave in normal incidence) and to avoid potential interactions between the membrane and the air gap between the membrane and the source, when the membrane is placed in the near field.

On the use of acoustic particle velocity fields in adjoint-based inversion

Following the recent interest in the use of combined pressure and particle motion sensors in underwater acoustics and signal processing, some general aspects regarding the modeling and multipath phenomenology of acoustic particle velocity fields in shallow water environments have been studied. In this paper we will address a number of issues associated with the incorporation of vector sensor data (pressure and particle velocity) into adjoint-based inversion schemes. Specifically, we will discuss the ability of a semi-automatic adjoint approach to compute the necessary gradient information without the need for an analytic model of the adjoint particle velocity field. Solutions to the forward propagation of acoustic pressure are computed using an implicit finite-difference parabolic equation solver while the particle velocity is calculated locally at each grid point. Some numerical examples of vector sensor inversion results are provided. [Work supported by Royal Netherlands Navy.]

Vector intensity field scattered by a rigid prolate spheroid

The short-wavelength, steady-state vector intensity field scattered by a rigid prolate spheroid (10:1 fineness ratio) is investigated analytically. The intensity field is the product of the scalar acoustic pressure and the acoustic particle velocity fields which are computed from the Helmholtz equation in prolate spheroidal coordinates. Particular emphasis is placed on results in the forward-scattered direction. It is found that the equivalent plane wave intensity varies by less than ±0.5dB (relative to the incident acoustic intensity) in the far-forward-scattered direction with the selected parameters. This illustrates that the forward-scattered pressure is masked by and interferes with the incident wave. The reactive intensity in the forward-scattered direction is found to be −25dB relative to the incident active intensity, and the computed phase difference between pressure and particle velocity is 4° at ranges approaching 10 spheroid lengths at a reduced frequency of h=41.7; in the absence of the spheroid, the phase difference due to the incident plane wave alone is exactly 0°. The study demonstrates that unique information, extending beyond direction-of-arrival estimation, can be derived from the total acoustic intensity field and may allow inferences to be made regarding the presence or absence of the spheroid.

Self-initiating MUSIC-based direction finding in underwater acoustic particle velocity-field beamspace

This paper introduces a novel blind MUSIC-based (MUltiple SIgnal Classification) source localization algorithm applicable to an arbitrarily spaced three-dimensional array of vector hydrophones, each of which comprises two or more co-located and orthogonally oriented velocity hydrophones plus an optional pressure hydrophone. This proposed algorithm: (1) exploits the incident sources' angular diversity in the underwater acoustic particle velocity field; (2) adaptively forms velocity-field beams at each vector-hydrophone; (3) uses ESPRIT to self-generate coarse estimates of the sources' arrival angles to start off its MUSIC-based iterative search with no a priori source information; and (4) automatically pairs the x-axis direction-cosine estimates with the y-axis direction-cosine estimates. Simulation results verify the efficacy of this proposed scheme.

Modeling acoustic particle velocity in range‐dependent environments with a parabolic equation code

Measurements have been made of the acoustic particle velocity field generated by a towed source transmitting 7‐, 10‐, and 16‐Hz tones. The periodic change in the polarization of the motion as a function of source–receiver range, from prograde circular motion to retrograde circular motion and back, appears to have been disrupted by changes in slope of the bottom topographic features in the experiment. In order to test this hypothesis, the University of Miami parabolic equation code (UMPE) was modified in order to efficiently calculate acoustic particle velocity. This numerical code makes effective use of the equivalent fluid approximation for low shear speed ocean bottoms [C. T. Tindle and Z. Y. Zhang, J. Acoust. Soc. Am. 91, 3248–3256 (1992)]. The predictions of the UMPE model are compared with the actual measurements, and with the predictions made by the finite element parabolic equation (FEPE) by M. D. Collins, which explicitly incorporates the effects of bottom shear. [Work supported by ONR.]