High Frequency Acoustic Excitation Research Articles

Onset of Visible Capillary Waves from High-Frequency Acoustic Excitation.

Remarkably, the interface of a fluid droplet will produce visible capillary waves when exposed to acoustic waves. For example, a small (∼1 μL) sessile droplet will oscillate at a low ∼102 Hz frequency when weakly driven by acoustic waves at ∼106 Hz frequency and beyond. We measured such a droplet's interfacial response to 6.6 MHz ultrasound to gain insight into the energy transfer mechanism that spans these vastly different time scales, using high-speed microscopic digital transmission holography, a unique method to capture three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that low-frequency capillary waves are driven into existence via a feedback mechanism between the acoustic radiation pressure and the evolving shape of the fluid interface. The acoustic pressure is distributed in the standing wave cavity of the droplet, and as the shape of the fluid interface changes in response to the distributed pressure present on the interface, the standing wave field also changes shape, feeding back to produce changes in the acoustic radiation pressure distribution in the cavity. A physical model explicitly based upon this proposed mechanism is provided, and simulations using it were verified against direct observations of both the microscale droplet interface dynamics from holography and internal pressure distributions using microparticle image velocimetry. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.

Application of Statistical Energy Analysis (SEA) in Estimating Acoustic Response of Panels With Non-Uniform Mass Distribution

Statistical Energy Analysis (SEA) is generally used in estimating the responses of structures to high frequency acoustic excitation. Though it has been successfully applied for panels having uniform mass, its usage is limited when the mass distribution is not uniform, as seen in equipment panels of a spacecraft. Results for such panels are seldom reported. In this work, an attempt is made to address this gap. A methodology to estimate the responses of such panels in SEA framework is presented and demonstrated for an equipment panel of a spacecraft, thus widening its application. This is accomplished through SEA along with the information on the standing waves generated due to the change in the structural properties. The acceleration responses of a typical equipment panel when subjected to a diffused acoustic field in a reverberation chamber are measured. The responses of the same panel are theoretically estimated using the methodology presented and a reasonably good prediction is seen.

Effect of syngas composition on high frequency combustion instability in a non-premixed turbulent combustor

An experimental investigation on the effect of syngas composition (H2/CO) on thermodynamic instability of a bluff-body combustor is studied. Three syngas compositions, namely, 25%H2–75%CO, 50%H2–50%CO and 75%H2–25%CO, and pure hydrogen and 75%H2–25%CH4 are acoustically characterized by varying the inlet Reynolds number, Re, in the range of 2200–8100. The combustor is observed to undergo two modes of thermo-acoustic instability across the above Re range for all syngas compositions and pure hydrogen, whereas for the H2CH4 composition, the instability manifests at a single frequency ∼130 Hz. For all the syngas compositions, the pressure oscillations exhibit low frequency (130 Hz) at low Re, and bifurcate to higher frequency (∼500 Hz) at mid-range Re values, while pure hydrogen exhibits bifurcation from the low frequency to the first harmonic (∼260 Hz). The high frequency observed with the syngas compositions is at the third harmonic natural acoustic mode of the combustor and matches thrice the Strouhal frequency of the hydrodynamic oscillations of the shear layer separating from the bluff body. Further investigation into the nature of the dynamic behaviour across the listed fuels is performed using simultaneous time resolved imaging of OH* and CO2* chemiluminescence and two-component particle image velocimetry. The peak in the CO2* chemiluminescent intensity is found to be axially staggered downstream of that of OH*. This is due to sequential oxidation of H2 and CO, the former producing OH as an intermediate consumed by the latter. Spatio-temporal analysis of the time-resolved data reveals that the presence of the flame in the shear layer of the bluff-body plays a significant role in modulating the high frequency acoustic excitation. Further, a spatial Rayleigh map is derived based on both the OH* and CO2* chemiluminescence, which reveals the bluff-body shear layer dynamics to yield the key driving and damping regions in the present study.

High Energy Vibration for Gas Piping

In September 2016, a gas compressor in offshore Sarawak has its rotor changed out. Prior to this change-out, pipe vibration study was carried-out by the project team to evaluate any potential high energy pipe vibration problems at the compressor’s existing relief valve downstream pipes due to process condition changes after rotor change out.This paper covers high frequency acoustic excitation (HFAE) vibration also known as acoustic induced vibration (AIV) study and discusses detailed methodologies as a companion to the Energy Institute Guidelines for the avoidance of vibration induced fatigue failure, which is a common industry practice to assess and mitigate for AIV induced fatigue failure. Such detailed theoretical studies can help to minimize or totally avoid physical pipe modification, leading to reduce offshore plant shutdown days to plant shutdowns only being required to accommodate gas compressor upgrades, reducing cost without compromising process safety.

Chip-based size-selective sorting of biological cells using high frequency acoustic excitation

This work presents the size-selective sorting of single biological cells using the assembly process known as templated assembly by selective removal (TASR). We have demonstrated experimentally, for the first time, the selective placement and sorting of single SF9 cells (clonal isolate derived from Spodoptera frugiperda (Fall Armyworm) IPLB-Sf21-AE cells) into patterned hemispherical sites on rigid assembly templates using TASR. Nearly 100% of the assembly sites on the template were filled with matching cells (with assembly density as high as 900 sites per mm(2)) within short time spans of 3 minutes. 3-D reconstruction of cell profiles and volume analysis of cells trapped inside assembly sites demonstrates that only those cells that match the assembly site precisely (within 0.5 μm) in size are assembled on the template. The assembly conditions are also compatible with the extension of TASR to mammalian cells. TASR-based size-selective structuring and sorting of biological systems represents a valuable tool with potential for implementation in biological applications such as cell sorting for medical research or diagnostics, templating for artificial tissue replication, or isolation of single cells for the study of biological or mechanical behavior.

Communication: High-frequency acoustic excitations and boson peak in glasses: A study of their temperature dependence

The results of a combined experimental study of the high-frequency acoustic dynamics and of the vibrational density of states (VDOS) as a function of temperature in a glass of sorbitol are reported here. The excess in the VDOS at approximately 4.5 meV over the Debye, elastic continuum prediction (boson peak) is found to be clearly related to anomalies observed in the acoustic dispersion curve in the mesoscopic wavenumber range of few nm(-1). The quasiharmonic temperature dependence of the acoustic dispersion curves offers a natural explanation for the observed scaling of the boson peak with the elastic medium properties.

The shoving model for the glass-former LiCl·6H2O: A molecular dynamics simulation study

Abstract Molecular dynamics (MD) simulations of LiCl·6H2O showed that the diffusion coefficient D, and also the structural relaxation time , follow a power law at high temperatures, D−1 ∝ (T − To)−μ, with the same experimental parameters for viscosity (To = 207 K, μ = 2.08). Decoupling between D and occurs at Tx ∼ 1.1To. High frequency acoustic excitations for the LiCl·6H2O model were obtained by the calculation of time correlation functions of mass current fluctuations. The temperature dependence of the instantaneous shear modulus, G∞(T), was considered in the shoving model for supercooled liquids [J.C. Dyre, T. Christensen, N.B. Olsen, J. Non-Cryst. Solids 352 (2006) 4635] resulting in a linear relationship log (D−1) vs. G∞/T.

Crystal-Like Nature of Acoustic Excitations in Glassy Ethanol

We report on inelastic x-ray scattering experiments on crystalline and glassy phases of ethanol in order to directly compare the influence of disorder on high frequency acoustic excitations. We find that both the dispersion and the line-width of the longitudinal acoustic excitations in the glass are the same as in the polycrystal in the reciprocal space portion covering the 1st and 2nd Brillouin zones. The structural disorder is found to play little role apart from an intrinsic angular averaging, and the nature of these excitations must essentially be the same in both glass and poly crystal.

High frequency acoustic excitations in ordered diblock copolymer studied by inelastic x-ray scattering.

The phonon propagation in lamellar nanostructures formed via self-assembling of short styrene-b-isoprene (SI) as well as of its more incompatible styrene-b-(ethylene-alt-propylene) (SEP) counterpart was studied by inelastic x-ray scattering. Irrespective of the physical state of the block copolymers, a single acoustic phonon was observed in SI (ordered and disordered) and SEP (ordered). At GHz frequencies, inelastic light scattering from the same samples revealed very small dispersion in the sound phase velocity but a short phonon lifetime.

Flow around a Circular Cylinder with Periodic Disturbance

The flow around a circular cylinder in the uniform flow exposed to a simple and high frequency acoustic excitation as a minute disturbance are studied. In order to clarify the effect of the disturbance on the force exerted by a fluid and to examine its mechanism, mean and fluctuating pressure distributions on the cylinder surface and the velocity field of the cylinder vicinity are carefully investigated. When the periodic disturbance close to the frequency of the transition wave in the separated shear layer is added, the diffusion of the shear layer is promoted or restrained and the fluctuating force associated with the Karman vortex shedding increases or decreases as compared with the case of no excitation.

Observation of the Crossover to Strong Scattering of Acoustic Phonons in Densified Silica

Inelastic x-ray scattering is used to observe high frequency acoustic excitations in densified silica glass. Longitudinal acoustic phonons are seen up to an energy $\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approx}9\mathrm{meV}$, corresponding to a scattering vector $Q\ensuremath{\approx}2.2{\mathrm{nm}}^{\ensuremath{-}1}$. At higher $Q$, the nature of the signal changes rapidly, indicating a crossover to strong scattering. The absence of spectral intensity near $\ensuremath{\omega}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ shows that these excitations remain long-lived compared to what might be inferred from their apparent line widths.