Session T31: Acoustics: General & Hydroacoustics

Visualization of a shock wave travelling inside a rectangular duct using the background oriented schlieren method

Shock waves are easily generated in narrow ducts by means of the rupture of a membrane or thin sheet of certain materials. The geometry of the shock waves generated in this manner has been previously described in a qualitative manner using the shadowgraph and schlieren methods. In the present study, the background-oriented schlieren method is used to observe the propagation of a fully-formed shock inside a narrow rectangular duct. In conjunction with measurements of the acoustic signal inside the duct, it is possible, employing the Gladstone-Dale relation, to obtain a two-dimensional portrait of the density field inside the duct. The propagation speed of the shock is also calculated directly from visualization of the shock at different times.   

Background Noise Characterization and Event Detection at an Infrasound Array in Oklahoma

It has been shown that there are many natural and manmade sources of infrasound, or sound below the threshold of human hearing (20 Hz). Some of these sources include severe storms, oceans, volcanos, earthquakes, and explosions. These infrasound signals can propagate hundreds to thousands of kilometers depending on the frequency and atmospheric conditions. However, detecting these signals can be difficult depending on the local wind conditions, background noise, and frequency band of interest.  The current work investigates these background conditions and the detection of known events at a 3 microphone infrasound array located in Stillwater, OK between 2016 and 2021. The background noise was investigated by attempting to identify local or consistent infrasound sources as well as quantify the array's sensitivity to wind. Characterizing the background noise allows for better detection of other infrasound sources of interest. Therefore, this work will report the findings on the local "noise floor" as well as possible sources for infrasound contamination. Additionally, several events that were detected utilizing f/k analysis and an adaptive f detector will be shown.    

Not Participating

Tornado-producing storms have been observed to emit infrasound, sound at frequencies below 20 Hz, up to two hours before tornadogenesis. Due to low atmospheric attenuation of low frequency, infrasound may be detected over large distances. If the received signals can be correlated with storm and/or tornado properties, passive infrasound monitoring could improve and augment the monitoring and/or prediction of severe weather. Previously, our team has focused on observations from a stationary array and a mobile array deployed near weather radar. To improve our database of observations, we have mined the available infrasound data from the IRIS Transportable Array (TA), which consisted of a grid of several hundred broadband seismographs that traversed the continental United States from 2003 to 2016. The widespread use of infrasound sensors on stations began in 2011. Given the high number of tornadoes from 2011 to 2016, the TA provides an untapped vein of infrasound recordings of severe storms. Within the present work, infrasound recordings from TA stations during several events will be presented alongside radar and local atmospheric conditions. These recordings will be considered within the light of potential mechanisms for tornado infrasound proposed in previous literature.   

Not Participating

The parasitoid fly Ormia ochracea famously locates both its prey and predators by sound to an accuracy 2° in the horizontal plane despite having an hearing organ spacing of only 500 μm, via mechanical coupling. The existing one dimensional mechano-mathematical model for O. ochracea’s hearing response accurately predicts the interaural amplitude difference (IAD) between the tympana at all azimuthal incident sound angles but cannot correctly predict the interaural time difference (ITD) at high azimuthal angles. Recent synchrotron radiation microtomography (SR-μCT) revealed more information about the 3D morphology of O. ochracea tympana. In contrast to previous assumptions of near two-dimensionality, these results displayed a complex, highly three-dimensional structure. Inspired by our observations of the 3D morphology of O. ochracea tympana, we created a finite-element analysis model to develop a novel microphone design that integrates acoustic sensing in multiple planes. Through this updated design, we increased the amplitude of response at high incident sound angles compared to existing planar designs, leading to a new ormia-inspired device with improved directional sensing capabilities.   

Acoustophoresis of non-spherical microparticles using strong resonant acoustic fields of asymmetric acoustofluidic devices

Acoustofluidic devices use ultrasound to create a force field for manipulation of particles in microchannels. We present a combined numerical and experimental method for design and characterization of asymmetric microfluidic chips capable of exciting acoustic resonances that are stronger by two orders of magnitude compared to their more common symmetric counterparts. Properly damped, multiphysics simulations of silicon-glass devices capture the complex interplay of physical phenomena leading to development of robust acoustophoresis that is crucial for rapid assembly of microparticle structures. Numerical models of isolated non-spherical microparticles show a variance in acoustic force of up to 50 percent based on particle orientation and aspect ratio alone. Calculated acoustic torque emphasizes that suspended non-spherical particles have a single preferred orientation depending on their geometry. These findings offer insight and an added layer of control for generating more complex micro-assemblies using acoustofluidics. This information is critical for technologies that benefit from gentle and robust handling of microparticles for applications like additive manufacturing and bio 3D printing.   

Acoustic radiation forces and torques on compressible micro rings in standing waves

Acoustic radiation forces and torques on spherical particles and spheroids have been studied extensively. Here, we present a numerical study to calculate acoustic radiation forces and torques on micro rings in an ultrasonic standing wave in an inviscid fluid based on the perturbation method. The acoustic radiation force is due to the background pressure field and scattered waves from ring segments. Effects of various parameters: geometry, orientation, and position as well as density and compressibility ratios of the ring material and the fluid on acoustic radiation forces and torques are investigated. The results show that the acoustic radiation forces and torques depend on the position of the rings with respect to the pressure node of the standing waves but also depend on the particle's material and its surrounding medium. Rings that are not co-planar with the standing waves are subject to non-zero torque. Lastly, numerical results are compared with the chain of spheres (CoS) model, which is based on a segmentation of the ring and treating each segment as a spherical particle. Our study sheds light on acoustic radiation forces and torques that will improve the acoustic control of ring-shaped microparticles.   

Experimental Response of Condenser Microphones Under Hyperbaric Conditions

Acoustic phenomena of practical interest typically take place either in air at sea level, high altitude, or underwater. Accordingly, devices such as microphones and hydrophones have been developed to accurately measure sound in these conditions. In air at elevated pressures, typical condenser microphones should continue to operate, though the details of their performance are not readily available. Characterizing the response of microphones at high pressure will allow for a new regime of experimental aeroacoustics to be explored, including upcoming work which utilizes small model rotor blades in hyperbaric conditions to achieve aerodynamic scale similarity with full-size rotorcraft. To evaluate the response of condenser microphones in high pressure air, a pistonphone calibrator was developed to provide known reference signals over a wide range of ambient pressures and frequencies. The design of the pistonphone calibrator will be outlined and methods presented for evaluating the response of typical condenser microphones over a range of frequencies and elevated pressures up to 100 bar.   

Application of Acoustic Techniques to Fluid-Particle Flows in Fluidised bed

The characterisation of dispersed flows is of great importance to many industrial sectors, including energy, and pharmaceutical industries. Recently, acoustic techniques have attracted considerable attention for the characterizarion of dispersed particle flows.This is attributed  to their ability to obtain measurements in real-time and at high particle concentrations.

In this paper, we have applied acoustic techniques to the study of solid-liquid fluidized beds.The signal processing for obtaining velocity profiles, particle size distribution, and volume fraction are discussed.The experiment was conducted in a vertical tube with a 5cm inner-diameter, made of borosilicate glass, and it contains 0.55kg glass particles.The techniques are based on the measurement of ultrasound attenuation coefficient spectroscopy, sound speed, and frequency shift of the propagated sound wave.The ultrasound propagation speed in the fluidized bed was measured to be between 1504m/s to 1565m/s for the glass particle volume fractions  from 27.1% to  60.9%  in tap water.The solid velocity profiles were measured for fluid flow rates varying from 1L/min to 5L/min.The particle size distributions were measured for four different sizes of glass particles ranging from 500μm to 1250μm at a volume fraction of 35%.   

Multiscale articulating differentials analysis of one-dimensional fast acoustic streaming

Classical approaches to modeling acoustic streaming flows involve perturbative expansions that depend on the slowness of the streaming velocity relative to the driving acoustics. This "slow streaming" approach was first described by Rayleigh as a tractable means for extracting the streaming field from the governing nonlinear equations. In modern acoustofluidics applications, order of magnitude separation between the acoustics and the resulting streaming flow cannot be generally relied upon. In these extremal systems, classical methods fail to extract the essential dynamics. We describe a theoretical framework with greater flexibility via direct, explicit consideration and exploitation of drastic spatiotemporal scale disparities typifying microacoustofluidic systems. This is achieved with multiscale partitioning of differential operations. The framework is generally applicable to nonlinear continuous media and is particularly well suited to acoustofluidics modeling. We briefly demonstrate application of the framework to various media before delving more deeply into its application to a one-dimensional acoustofluidics problem of semiinfinite extent characterized by order of magnitude equivalence between the constituent flow fields—the "fast streaming" condition. The resulting Burgers-Riccati model is used to explain the fundamental characteristics of fast bulk acoustic streaming. We derive from the model a number of important flow properties including a remarkably simple non-constitutive upper bound on the energetic conversion efficiency of the driving acoustics to the resultant maximum streaming flow magnitude. The theory is confirmed with a broad survey of experimental data from the recent literature.   

Mechanism of capillary waves generation driven by high-frequency ultrasound

High-frequency thickness mode ultrasound is an energy-efficient way to atomize high-viscosity fluid at a high flow rate into fine aerosol mists of micron-sized droplet distributions. However, the complex physics of the atomization process is not well understood. It is found that with low power the droplet vibrates at low frequency (10^2 Hz) when driven by high-frequency ultrasound (10^6 Hz and above). To study the mechanism of the energy transfer that spans these vastly different timescales, we measure the droplet's interfacial response to 6.6 MHz ultrasound excitation using high-speed digital holography---a revolutionary method for capturing three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that the onset of low-frequency capillary waves is driven by feedback interplay between the acoustic radiation pressure distribution and the droplet surface. These dynamics are mediated by the Young-Laplace boundary between the droplet interior and the ambient environment. Numerical simulations are performed via global optimization against the rigorously defined interfacial physics. The proposed model is explicitly based on the pressure distribution hypothesis. For low-power acoustic excitation, the simulations reveal stable oscillatory feedback that induces capillary wave formation. The simulation results are confirmed with direct observations of the microscale droplet interface dynamics as provided by the high-resolution holographic measurements. The acoustic pressure interfacial feedback model accurately predicts the acoustic power required to initiate capillary waves, and interfacial oscillation amplitude and frequency. The radiation pressure distribution is likewise confirmed with particle migration observations. Viscous effects on wave attenuation are also studied by comparing experimental and simulated results for a pure water droplet and 90wt%-10wt% glycerol-water solution droplet.   

Not Participating

Anthropogenic noise from marine shipping and other sources poses a serious threat to marine mammals and the ocean environment. The formation and collapse of bubbles during propeller-induced cavitation is the dominant source of underwater sound produced by ships.  This work explores the physics of noise emission from deforming geometries in turbulent and multiphase flows by developing a coupled framework for flow and acoustic simulation. We are particularly interested to understand how cavitation impacts noise radiation with varying parameters.  Acoustic propagation is simulated by solving the linearized Euler equations via the discontinuous Galerkin finite element method. The acoustic solver is one way coupled to the multiphase fluid-structure interaction solver by extracting the time history of hydrodynamic pressure and density as acoustic sources. Interpolation between non-matching flow and acoustic grids is implemented to take advantage of different length scales between flow and acoustic phenomena. The acoustic solver is first validated against analytical solutions for acoustic monopole and dipole sources. The combined flow-acoustic hybrid framework is validated through the sound generation of a vortex shedding 2D cylinder at a laminar flow with Re=200. Finally, the framework is demonstrated through sound generation by a cavitating airfoil.   

Scaling analysis of linear and non-linear terms of the Ffowcs-Williams and Hawkings equation

The Ffowcs- Williams and Hawkings (FWH) equation, which rules propagation of noise generated by a rigid body in motion is composed of linear and non-linear terms.

The linear terms are associated to the noise produced by the velocity and pressure over the body in motion, the non-linear ones are associated to the wake. Since the Lighthill seminal work, scaling properties of monopole, dipole and quadrupole sources have been exploited, finding dependence with the Mach number, no matter what the fluid dynamic regime is. Such scaling laws apply to the FWH terms decaying as 1/??. We  complete the classical theory, through analysis of scaling of the additional terms (linear and non-linear) decaying as 1/??² and as 1/??³ , tipically not considered in literature. 

The analysis is carried out  in view of perfect/imperfect scaling when data obtained at laboratory scale are used for prediction at the full scale.

We found that in order to recover perfect similarity (i.e. the Mach number has to be the same at the laboratory scale and at the full scale) the speed of sound at a laboratory scale needs to be properly scaled, otherwise imperfect similarity introduces errors in the acoustic response related both to the linear terms and to the non-linear terms, the latter of great

importance when the wake is characterized by organized vorticity and turbulence.

The error associated to the imperfect scaling is quantified considering the case of a marine propeller in uniform single-phase flow condition. The flow field is obtained using Large Eddy Simulation with a wall-layer model.

We find that the near field is nearly unaffected by imperfect scaling because there the thickness term, which scales perfectly being independent on the Mach number, dominates. On the other hand, the intermediate-to far field noise, dominated by the non-linear terms which scale imperfectly may be substantially affected by the scaling procedure.