Acoustic Pressure Measurements Research Articles

On the equivalence of scalar-pressure and vector-based acoustic dosage measures as derived from time-limited signal waveformsa).

The dynamic (acoustic pressure) and kinematic (acoustic acceleration and velocity) properties of time-limited signals are studied in terms of acoustic dose metrics as might be used to assess the impact of underwater noise on marine life. The work is relevant for the study of anthropogenic transient acoustic signals, such as airguns, pile driving, and underwater explosive sources, as well as more generic transient signals from sonar systems. Dose metrics are first derived from numerical simulations of sound propagation from a seismic airgun source as specified in a Joint Industry Programme benchmark problem. Similar analyses are carried out based on at-sea acoustic measurements on the continental shelf, made with a vector sensor positioned 1.45 m off the seabed. These measurements are on transient time-limited signals from multiple underwater explosive sources at differing ranges, and from a towed, sonar source. The study demonstrates, both numerically and experimentally, that under many realistic scenarios, kinematic based acoustic dosage metrics within the water column can be evaluated using acoustic pressure measurements.

Robust orbital-angular-momentum-based underwater acoustic communication with dynamic modal decomposition method.

Recently, acoustic communication employing orbital angular momentum (OAM) opens another avenue for efficient data transmission in aquatic environments. Current topological charge (TC) detection of OAM beams relies on the orthogonality among different-order OAM beams. However, such strategy requires measurements of the complete azimuthal acoustic pressure, which inevitably reduces the efficiency and increases the bit error rate (BER). To address these challenges, this study proposes a modified dynamic modal decomposition (DMD) method by partially sampling the acoustic field for precise TC detection. Numerical simulations confirm the accuracy of this approach in extracting single or multiple TCs magnitudes within a partially sampled acoustic field. We theoretically compare the performance of the modified DMD approach with conventional orthogonal decoding method. Simulation results indicate that our modified DMD scheme exhibits lower BER under the same noise interference and is more robust to the array misalignment. This research introduces an efficient demodulation solution for acoustic OAM communication, offering potential benefits for simplifying receiver array design and enhancing long-distance underwater data transmission.

Self-excited intermittent thermoacoustic fluctuations in an annular combustor exhibiting flame transient phenomena: Physical mechanisms and modeling

We experimentally examine the physical mechanisms causing intermittent thermoacoustic fluctuations in a turbulent annular combustor exhibiting flame transient phenomena (FTP). The combustor consists of 12 burners. Flames are stabilized by conical bluff bodies, resembling the afterburner and ramjet burner configurations. The combustor exhibits a dominant 1A–1L (first azimuthal–first longitudinal, ∼630 Hz) thermoacoustic mode. Instability manifests as intermittent fluctuations in acoustic pressure. From the heat release and acoustic pressure measurements, FTPs are found to create large amplitude heat release fluctuations occurring on a slow timescale compared with 1A–1L thermoacoustic mode. The heat release, in turn, leads to the observed intermittent acoustic pressure fluctuations. Four FTPs are found to occur in the combustor: (1) near blow-off, (2) flame extinction, (3) successful reignition, and (4) unsuccessful reignition. Their occurrences and the associated time spans are found to be random. The time span follows an unimodal probability distribution, peaking around 100 ms. A low-order model is developed by incorporating the distribution for the FTP time span in the heat release. An additive Gaussian white noise is added to represent background turbulent fluctuations. The model qualitatively reproduces the experimentally observed probability density function of the acoustic pressure fluctuations. This indicates that the stochastic FTP time span and turbulence are essential for reproducing intermittent thermoacoustic fluctuations.

A Thermoacoustic Instability Precursor Based on the Acoustic Flux at the Combustion Chamber Inlet

ABSTRACT The acoustic flux at the inlet of the combustion chamber is used to indicate the proximity to impending thermo-acoustic instabilities (TAI) in a swirled spray combustor. This acoustic power is integrated over all frequencies and is deduced from acoustic pressure and velocity measurements made upstream the combustion chamber. Results with this TAI precursor are compared to those obtained by considering the root mean square value of the acoustic pressure inside the combustion chamber, the peak of the power spectral density of the combustion chamber acoustic pressure and the phase lag between the acoustic pressure inside the chamber and the OH* chemiluminescence. It is demonstrated that the acoustic flux outperforms the other indicators in terms of order of magnitude change and shape evolution when approaching TAI while being relatively insensitive to signal processing parameter modifications.

Underwater soundscape observations and associated demersal fish movement during a seismic airgun survey in coastal Oregon

Sound is a crucial aspect of the underwater environment for fishes—various species use sound to communicate, identify predators, navigate, and many other activities needed for survival in their habitat. In the summer of 2021, a seismic survey passed nearby Southern Oregon to map the Cascadia Subduction Zone using an array of airguns. To evaluate the effect of the seismic survey on the behavior of demersal fish, acoustic pressure and particle motion measurements were collected within a marine protected area, accompanied by tagging of two rockfish species, Black Rockfish (Sebastes melanops) and China Rockfish (S. nebulosus). Acoustic measurements were obtained using a vector sensor array, deployed in 25 m deep water. It was found that while cumulative sound exposure levels were dominated by wind noise, other metrics such as peak sound pressure, kurtosis, acoustic complexity index, crest factor and acoustic entropy showed clear signals associated with the seismic survey. Animal behavior and spatial use by the two rockfish species were also evaluated. Overall, results indicate that there are slight differences in movement and spatial use during times of seismic survey noise presence, but observed differences are reduced after only a few days.

Aerodynamic noise reduction of a blunt flat plate by trailing-edge blowing

This study investigates the influence of air blowing on the aerodynamic noise generated by a flat plate with a blunt trailing edge. The uniform blowing was applied through evenly spaced holes along the span at the base of the flat plate. Acoustic pressure measurements were conducted in an anechoic wind tunnel using a free-field microphone at Reynolds numbers ranging from 2.8×105 to 6.5×105. Particle image velocimetry measurements were performed to obtain time-resolved evolution and statistics of the flow velocity field, providing a detailed understanding of the noise reduction mechanisms. The results demonstrated that air blowing effectively reduces both the amplitude and bandwidth of tonal and broadband noise within a limited frequency range. This phenomenon was attributed to the stabilization of separated shear layers over a longer distance by air blowing and a decrease in turbulent kinetic energy in the near-wake region. It was shown that air blowing suppresses the tonal peak of vertical velocity fluctuations and narrows the wake width, which accounts for the increase of vortex shedding frequency and a potential drag reduction. Generally, the effects of air blowing on vortex shedding at the blunt trailing edge are analogous to those of a splitter plate. The unaffected convective velocity of the large-scale vortex structures and the enhanced spanwise coherence in the vortex formation region provide further evidence to this analogy.

Experimental Analysis of Rotor Blade Noise in Edgewise Turbulence

This paper presents an experimental investigation into the effects of turbulence ingestion on the aerodynamic noise characteristics of rotor blades in edgewise flight. A small-scaled, two-bladed rotor was used in the study. The test utilised two turbulence-generating grids, to generate turbulence inflows with different characteristics, and to compare them to the baseline configuration of the laminar inflow. The experiments were set at forwarding edgewise flight configuration, with freestream inflow velocity ranging from 10 m/s to 22 m/s. Simultaneous measurements of far-field acoustic pressure and load were conducted, along with a separate flow measurement using particle image velocimetry. The acoustic spectra demonstrated a larger contribution to the tonal noise radiation at blade passing frequency, and to the broadband noise radiation at the mid-frequency domain, due to turbulence ingestion. However, the broadband responses in the high-frequency domain were comparable between the tested laminar and turbulence inflow cases, with similar broadband humps featuring in the acoustic spectra. The directivity patterns of the overall sound pressure level showed that the noise radiation was lowest near the plane of rotation, and highest downstream. Turbulence ingestion effects could also be seen in the elevated noise levels throughout the observation positions for the grid inflow cases, particularly at larger advance ratios.

131 An Alternative Method for Measuring Sample Pump Pulsation

Abstract Sampling pumps are an essential part of the chemical exposure assessment process, as the flowrate is used for the calculation of the atmospheric concentration of pollutants. Flowrate stability is important for its contribution to uncertainty, but also because some of sample trains require a specific flow rate value to properly operate, especially for particle sampling. Pump principles are based on rotary or alternative movements which involves a very short variation in flowrate, called “pulsation”, which lasts few milliseconds and is not perceptible with a conventional flowmeter. This pulsation is a critical parameter to be measured for the characterization of the pump. The international standard ISO 13137 specifies the method for evaluating the pulsation which is based on the use of a rapidly responding hotwire anemometer. This method is particularly precise and sensitive but complex to set up and calibrate. We have developed an alternative method for measuring pump pulsation based on the determination of acoustic pressure. Assuming that the sample pumps connected to a solid tube are plane wave generators with infinite impedance, the acoustic pressure measurement is the sum of the incident and reflected waves. Using five microphones in the the acoustic pressure can be measured in the tube to calculate the acoustic velocity and therefore the dynamic flow with a very high frequency. 13 different individual pumps were tested with both hotwire anemometry and acoustic pressure methods. Pulsation result comparison with both methods exhibits a great agreement. Acoustic pressure measurement is therefore a very convenient alternative method for the personal pump pulsation determination.

An experimental modal decomposition method to compute sound power of multi-dimensional acoustic waves from turbocharger compressors

The automotive turbocharger compressor in the present experimental study features a ported shroud casing treatment, which is known to elevate tonal noise at the blade-pass frequency (BPF) while reducing broadband whoosh noise and providing higher boost pressures at low mass flow rates. The high operating rotational speeds of such modern turbocharger compressors push the BPF to ranges where acoustic wave propagation is multi-dimensional within the compressor ducting. Simultaneously propagating acoustic modes at the BPF result in strong circumferential and axial variation of in-duct sound pressure levels. This poses a challenge for acoustic characterization and comparison of different hardware since typical measurement techniques do not consider the sensitivity of acoustic pressure to the measurement location. The current work utilizes a steady-flow turbocharger gas stand with a unique rotating compressor inlet duct fitted with multiple wall-mounted dynamic pressure transducers capable of performing a modal decomposition of the acoustic field. The decomposition is done using time-resolved acoustic pressure measurements from different orientations of the rotating inlet duct during steady compressor operation. The resulting modal amplitudes are then used to determine the sound power level, a quantity that is independent of the acoustic pressure measurement locations. Therefore, in addition to revealing the modal content of noise at the compressor inlet, the rotating inlet duct experimental setup better characterizes the acoustic field with sound power levels across the operating flow range at various compressor rotational speeds.

Near-field acoustic holography for underwater moving surfaces

Near-field acoustic holography has been the standard technique to accurately image static structure vibration from nearby acoustic pressure measurements. The classical application bases the method on the stable solution of an integral surface representation of the acoustic pressure. In this work, we will be concerned with the extension of NAH to image a vibrating structure moving underwater. In an underwater medium, even at slow speeds, the action between the flow and structure produces many radiation sources that could severely limit the accuracy of the integral surface representation used for static NAH. Using the Lighthill acoustic analogy, we use an integral surface representation for the structure’s radiated sound and a quadrupole source that models the corresponding flow-induced sound. Finally, we will justify the validity of the proposed method for COMSOL numerically generated pressure data that results from the internal excitation of a cylindrical structure moving over an underwater medium. This work has been supported by the Office of Naval Research.

Effect of Plate Distance on Steady and Unsteady Characteristics of Impinging Rectangular Jet

Acoustic and unsteady characteristics of a rectangular jet impinging on a flat plate were investigated. Experiments were performed at the nozzle pressure ratio of 2 for the impingement distances , 4, 8, and 12. The mean and unsteady flow properties of the impinging jet were studied using the steady and unsteady pressures on the impinging surface, time-resolved schlieren images, and acoustic measurements at different directivity angles. Mean schlieren images elucidate the flow features including redirected flow, and the plate shock for the impingement distance . The mean surface pressures show unidentical pressure profiles in the and planes. The pressure profile in the plane is found to be identical to the free-jet case. Proper orthogonal decomposition performed on the time-resolved schlieren images shows that the impinging jet is dominated by the flapping instability mode for a given range of impingement distances. Flow features such as plate shock and the redirected flow were assessed using proper orthogonal decomposition and time-resolved schlieren images. Synchronized acoustic and unsteady pressure measurements reveal the coupling between the upstream acoustic waves and the unsteadiness arising from the impinging surface.

Coupled droplet dynamics: Investigation of acoustic-spray interactions in a reacting flow field

Droplet dynamics in a self-excited thermoacoustic instability are not well understood due to the tight coupling of competing effects and difficulty in taking high-quality data. In this investigation, droplet data from a combustion instability is used to illustrate how a spray couples with a thermoacoustic instability. Commonly accepted ensemble averaging techniques such as the Sauter mean diameter (D32) is shown to be inadequate in describing the coupling phenomena, and instead, a new approach based on droplet surface area is used to characterize the spray. The present study extends literature concerning nonreacting acoustic-spray interactions and considers these in a fully self-excited instability. A lean direct injection (LDI) geometry is used at atmospheric pressure and phased averaged Phase Doppler Particle Analyzer (PDPA) measurements, acoustic pressure measurements, and OH* chemiluminescence images are taken to characterize the flow field. It is shown that while some of the trends found in nonreacting experiments apply to reacting systems, the mechanisms are notably different due to the presence of a flame. Flame response is shown to be important due to its influence on the evaporation of droplets.

Ultrasonic melt processing upon direct-chill casting of aluminium alloys

Ultrasonic melt processing (USP) has been known for decades for beneficial effects in as-cast aluminium alloys, including degassing, grain and structure refinement. In the last 10 years the authors performed a series of research projects dedicated to understanding the nature of these effects, by dedicated advanced experiments and by developing numerical models that adequately reflect the complicated physics involved, aiming at further optimising the technology so that it becomes feasible for scale-up applications and attractive for industrial use. Based on the main USP mechanisms previously studied by in-situ observations coupled with acoustic pressure measurements, i.e. fragmentation, deagglomeration and dispersion of the solid phases and inclusions, technological approaches are suggested and tested for grain refinement upon direct-chill casting of Al alloys. Results showed that USP in the melt flow in the launder significantly improves the as-cast structure of a billet, opening the way for upscaling.

Dual frequency ultrasonic cavitation in various liquids: High-speed imaging and acoustic pressure measurements

Ultrasonic cavitation is used in various processes and applications, utilizing powerful shock waves and high-speed liquid jets generated by the collapsing bubbles. Typically, a single frequency source is used to produce the desired effects. However, optimization of the efficiency of ultrasound reactors is necessary to improve cavitation activity in specific applications such as for the exfoliation of two dimensional materials. This research takes the next step to investigate the effect of a dual frequency transducer system on the bubble dynamics, cavitation zone, pressure fields, acoustic spectra, and induced shock waves for four liquids with a range of physical properties. Using ultra-high-speed imaging and synchronized acoustic pressure measurements, the effect of ultrasonic dual frequencies on bubble dynamics was investigated. The addition of a high frequency transducer (1174 kHz) showed that the bubble fragments and satellite bubbles induced from a low frequency transducer (24 kHz) were able to extend their lifecycle and increase spatial distribution, thus, extending the boundaries of the cavitation zone. Furthermore, this combination of ultrasonic frequencies generated higher acoustic pressures (up to 180%) and enhanced the characteristic shock wave peak, indicating more bubble collapses and the generation of additional shock waves. The dual frequency system also enlarged the cavitation cloud size under the sonotrode. These observations specifically delineated the enhancement of cavitation activity using a dual frequency system pivotal for optimization of existing cavitation-based processing technologies.

An eco-friendly solution for liquid phase exfoliation of graphite under optimised ultrasonication conditions

Ultrasonic assisted liquid phase exfoliation (ULPE) is a promising method for the large scale production of 2D materials. Currently, toxic solvents such as N-Methyl-2-pyrrolidone (NMP) are commonly used for the production of graphene. In this paper four solvents; three green solvents (water, ethanol and water/ethanol) plus NMP for comparison, were sonicated and examined in terms of their bubble dynamics and acoustic emissions. Advanced fundamental analysis was conducted using high-speed imaging synchronised with acoustic pressure measurements complemented by shadowgraphic photography of the emitted shockwaves, in order to determine a suitable eco-friendly solvent medium from a cavitation bubbles dynamics perspective. Thereafter, ULPE of graphite in the optimum solvent took place for 2 h under controlled ultrasonication parameters. The produced graphene samples were characterised by employing a series of techniques consisting of Ultraviolet–visible (UV–Vis) and Raman spectroscopy as well as transmission electron microscopy (TEM). A mixture of deionised water and ethanol was shown to produce a yield twice that of pure water, comprising of high quality few layer graphene (3–5 Ls) with an average area of ∼1.15 (μm)2 and stability of ∼78% for the duration of six months. This combination is a promising eco-friendly substitute for future commercial manufacturing of graphene.

Temperature as a key parameter for graphene sono-exfoliation in water

Graphene dispersions in water are highly desirable for a range of applications such as biomedicines, separation membranes, coatings, inkjet printing and more. Recent novel research has been focussed on developing a green approach for scalable production of graphene. However, one important parameter, which is often neglected is the bulk temperature of the processing liquid. This paper follows our earlier work where optimal sono-exfoliation parameters of graphite in aqueous solutions were determined based on the measured acoustic pressure fields at various temperatures and input powers. Here, we take the next step forward and demonstrate using systematic characterisation techniques and acoustic pressure measurements that sonication-assisted liquid phase exfoliation (LPE) of graphite powder can indeed produce high quality few layer graphene flakes in pure water at a specific temperature, i.e. 40 °C, and at an optimised input generator power of 50%, within 2-h of processing. UV–vis analysis also revealed that the exfoliation, stability and uniformity of dispersions were improved with increasing temperature. We further confirmed the successful exfoliation of graphene sheets with minimal level of defects in the optimized sample with the help of Raman microscopy and transmission electron microscopy. This study demonstrated that understanding and controlling processing temperature is one of the key parameters for graphene exfoliation in water which offers a potential pathway for its large-scale production.

Real-time thermoacoustic data assimilation

Low-order thermoacoustic models are qualitatively correct, but typically, they are quantitatively inaccurate. We propose a time-domain bias-aware method to make qualitatively low-order models quantitatively (more) accurate. First, we develop a Bayesian ensemble data assimilation method for a low-order model to self-adapt and self-correct any time that reference data become available. Second, we apply the methodology to infer the thermoacoustic states and heat-release parameters on the fly without storing data (real time). We perform twin experiments using synthetic acoustic pressure measurements to analyse the performance of data assimilation in all nonlinear thermoacoustic regimes, from limit cycles to chaos, and interpret the results physically. Third, we propose practical rules for thermoacoustic data assimilation. An increase, reject, inflate strategy is proposed to deal with the rich nonlinear behaviour; and physical time scales for assimilation are proposed in non-chaotic regimes (with the Nyquist–Shannon criterion) and in chaotic regimes (with the Lyapunov time). Fourth, we perform data assimilation using data from a higher-fidelity model. We introduce an echo state network to estimate in real time the forecast bias, which is the model error of the low-fidelity model. We show that: (i) the correct acoustic pressure, parameters, and model bias can be inferred accurately; (ii) the learning is robust as it can tackle large uncertainties in the observations (up to 50 % of the mean values); (iii) the uncertainty of the prediction and parameters is naturally part of the output; and (iv) both the time-accurate solution and statistics can be inferred successfully. Data assimilation opens up new possibility for real-time prediction of thermoacoustics by combining physical knowledge and experimental data synergistically.

Kalman filter temperature estimation with a photoacoustic observation model during the hyperthermia treatment of cancer

Hyperthermia with laser heating can be a noninvasive treatment for tumors near the body surface. Thermal damage of tissues depends on temperature levels and time of exposure to high temperatures, thus requiring accurate techniques to measure the temperature in the target region of interest. In this computational work, we couple the laser hyperthermia treatment of cancer to the photoacoustic temperature measurement of internal tissues, by solving a state estimation problem with the Kalman filter. The region of interest is considered to be continuously heated by a laser for the hyperthermia treatment, while another laser is sequentially pulsed over the same heated surface to generate acoustic pressure waves. The observation model is derived from the photoacoustic problem, which is linear with respect to the local internal temperatures of the tissues. The evolution model given by the bioheat transfer problem is also supposed linear and uncertainties are modeled as additive and Gaussian. Therefore, the Kalman filter provides the optimal solution of the state estimation problem considered in this work. Simulated nonintrusive acoustic pressure measurements are used in inverse analyses for two test cases, with different dimensions and locations of a tumor in a two-dimensional region. The simulated tumors are supposed loaded with nanoparticles, in order to enhance the local absorption of the laser energy and to reduce thermal damage to healthy cells. The Kalman filter estimated accurate and stable temperature distributions, even in the tumor region that involved large gradients. Moreover, uncertainties in the Kalman filter estimates were smaller than those resulting from the direct simulation of the photoacoustic thermometry problem.

Quantitative Analysis of Acoustic Pressure for Sonophoresis and Its Effect on Transdermal Penetration.

Ultrasound facilitates the penetration of macromolecular compounds through the skin and offers a promising non-invasive technique for transdermal delivery. However, technical difficulties in quantifying ultrasound-related parameters have restricted further analysis of the sonophoresis mechanism. In this study, we devise a bolt-clamped Langevin transducer-based sonophoresis device that enables us to measure with a thin lead zirconate titanate (PZT) sensor. One-dimensional acoustic theory accounting for wave interaction at the skin interface indicates that the acoustic pressure and cavitation onset on the skin during sonophoresis are sensitive to the subcutaneous support, meaning that there is a strong need to perform the pressure measurement in an experimental environment replacing the human body. From a series of the experiments with our new device, the transdermal penetration of polystyrene, silica and gold nanoparticles is found to depend on the size and material of the particles, as well as the hardness of the subcutaneous support material. We speculate from the acoustic pressure measurement that the particles' penetration results from the mechanical action of cavitation.

On the impact of H[formula omitted]-enrichment on flame structure and combustion dynamics of a lean partially-premixed turbulent swirling flame

Large Eddy Simulation (LES) with Conjugate Heat Transfer (CHT) is used to analyze the impact of H2-enrichment on the flame structure and combustion dynamics of a lean partially-premixed turbulent CH4/Air swirling flame. Experimentally, the combustor is operated at atmospheric pressure with H2 fuel fractions of up to 50%, by volume. LES-CHT results are compared and validated against time-resolved stereo PIV, OH* chemiluminescence, OH-PLIF imaging and acoustic pressure measurements. In terms of dynamics, for the pure CH4 and 20% of H2 enrichment cases, no thermoacoustic oscillation is observed in either the experimental or numerical data. As the fuel fraction of hydrogen is increased, the flame length reduces due to the increase in laminar flame speed and the heat release rate distribution becomes more compact. CHT simulations reveal that H2-enrichment leads to higher temperatures at the centerbody tip. At 50% H2, in agreement with experiments, LES predicts a bi-modal thermoacoustic oscillation, with two main frequencies corresponding to the quarter and chamber modes of the system. Dynamic Mode Decomposition is performed on the measured OH-PLIF images and LES 3D fields to extract each mode contribution to the overall flame dynamics. It is observed that both modes are characterized by local variations of equivalence ratio, while only the higher frequency (chamber) mode is characterized by vortices periodically detaching from the backplane and the centerbody walls causing a strong periodic wrinkling of the flame front during the thermoacoustic oscillation.