A machine learning-guided design and manufacturing of wearable nanofibrous acoustic energy harvesters

Nanofibrous acoustic energy harvesters(NAEHs)have emerged as promising wearable platforms for efficient noise-to-electricity conversion in distributed power energy systems and wearable sound amplifiers for assistive listening devices.However,their reallife efficacy is hampered by low power output,particularly in the low-frequency range(<1 kHz).This study introduces a novel approach to enhance the performance of NAEHs by applying machine learning(ML)techniques to guide the synthesis of electrospun polyvinylidene fluoride(PVDF)/polyurethane(PU)nanofibers,optimizing their application in wearable NAEHs.We use a feed-forward neural network along with solving an optimization problem to find the optimal input values of the electrospinning(applied voltage,nozzle-collector distance,electrospinning time,and drum rotation speed)to generate maximum output performance(acoustic-to-electricity conversion efficiency).We first prepared a dataset to train the network to predict the output power given the input variables with high accuracy.Upon introducing the neural network,we fix the network and then solve an optimization problem using a genetic algorithm to search for the input values that lead to the maximum energy harvesting efficiency.Our ML-guided wearable PVDF/PU NAEH platform can deliver a maximal acoustoelectric power density output of 829μW/cm^(3) within the surrounding noise levels.In addition,our system can function stably in a broad frequency(0.1-2 kHz)with a high energy conversion efficiency of 66%.Sound recognition analysis reveals a robust correlation exceeding 0.85 among lexically akin terms with varying sound intensities,contrasting with a diminished correlation below 0.27 for words with disparate semantic connotations.Overall,this work provides a previously unexplored route to utilize ML in advancing wearable NAEHs with excellent practicability.

Modeling and volume rendering approach of underwater three-dimensional acoustic energy field

The underwater acoustics is primary and most effective method for underwater object detection and the complex underwater acoustics battlefield environment can be visually described by the three-dimensional(3D)energy field.Through solving the 3D propagation models,the traditional underwater acoustics volume data can be obtained,but it is large amount of calculation.In this paper,a novel modeling approach,which transforms two-dimensional(2D)wave equation into 2D space and optimizes energy loss propagation model,is proposed.In this way,the information for the obtained volume data will not be lost too much.At the same time,it can meet the requirements of data processing for the real-time visualization.In the process of volume rendering,3D texture mapping methods is used.The experimental results are evaluated on data size and frame rate,showing that our approach outperforms other approaches and the approach can achieve better results in real time and visual effects.

Experimental investigation on synergetic prediction of granite rockburst using rock failure time and acoustic emission energy

The frequent occurrence of rockburst and the difficulty in predicting were considered in deep engineering and underground engineering.In this work,laboratory experiments on rockburst under true triaxial conditions were carried out with granite samples.Combined with the deformation characteristics of granite,acoustic emission(AE)technology was well applied in revealing the evolution law of micro-cracks in the process of rockburst.Based on the comprehensive analysis of acoustic emission parameters such as impact,ringing and energy,the phased characteristics of crack propagation and damage evolution in granite were obtained,which were consistent with the stages of rock deformation and failure.Subsequently,based on the critical point theory,the accelerated release characteristics of acoustic emission energy during rockburst were analyzed.Based on the damage theory,the damage evolution model of rock under different loading conditions was proposed,and the prediction interval of rock failure time was ascertained concurrently.Finally,regarding damage as an intermediate variable,the synergetic prediction model of rock failure time was constructed.The feasibility and validity of model were verified.

High-Temperature and Low-Frequency Acoustic Energy Absorption by a Novel Porous Metamaterial Structure

In this paper,we propose a novel porous metamaterial structure with an improved acoustic energy absorption performance at high-temperature and in the low-frequency range.In the proposed novel porous metamaterial structure,a porous material matrix containing periodically perforated cylindrical holes arranged in a triangular lattice pattern is applied,and additional interlayers of another porous material are introduced around these perforations.The theoretical model is established by adopting the double porosity theory for the interlayer and the cylindrical hole which form an equivalent inclusion and then applying the homogenization method to the porous metamaterial structure formed by the equivalent inclusion and the porous matrix.The temperature-dependent air and material parameters are considered in the extended theoretical model,which is validated by the finite element results obtained by COMSOL Multiphysics.The acoustic or sound energy absorption performance can be improved remarkably at very low frequencies and high temperature.Furthermore,the underlying acoustic energy absorption mechanism inside the unit-cell is investigated by analyzing the distribution of the time-averaged acoustic power dissipation density and the energy dissipation ratio of each constituent porous material.The results reveal that regardless of the temperature,the acoustic energy is mostly dissipated in the porous material with a lower airflow resistivity,while the acoustic energy dissipated in the porous material with a higher airflow resistivity also becomes considerable in the high-frequency range.The novel porous metamaterial structure proposed in this paper can be efficiently utilized to improve the acoustic energy absorption performance at high temperature.

Theoretical and Experimental Study on the Acoustic Wave Energy After the Nonlinear Interaction of Acoustic Waves in Aqueous Media

Based on the Burgers equation and Manley-Rowe equation, the derivation about nonlinear interaction of the acoustic waves has been done in this paper. After nonlinear interaction among the low-frequency weak waves and the pump wave, the analytical solutions of acoustic waves＇ amplitude in the field are deduced. The relationship between normalized energy of high-frequency and the change of acoustic energy before and after the nonlinear interaction of the acoustic waves is analyzed. The experimental results about the changes of the acoustic energy are presented. The study shows that new frequencies are generated and the energies of the low-frequency are modulated in a long term by the pump waves, which leads the energies of the low-frequency acoustic waves to change in the pulse trend in the process of the nonlinear interaction of the acoustic waves. The increase and decrease of the energies of the low-frequency are observed under certain typical conditions, which lays a foundation for practical engineering applications.

Analysis of Dynamic Tensile Process of Fiber Reinforced Concrete by Acoustic Emission Technique

The fiber reinforced concrete has good dynamic mechanical properties. But corresponding research lacks the dynamic damage characteristics of the polypropylene fiber（fiber of low elastic modulus） and steel fiber（fiber of high elastic modulus） reinforced concrete under medium strain rate（10-6 s-1-10-4 s-1）. In order to study the effect of strain rate on the damage characteristics of fiber reinforced concrete during the full curve damage process, the real time dynamic acoustic emission（AE） technique was applied to monitor the damage process of fiber reinforced concrete at three strain rates. The AE wavelet energy spectrum in ca8 frequency band and average AE peak frequency at three strain rates were analyzed. With the accumulation of damage, the AE wavelet energy spectrum in ca8 frequency band increased first and then decreased, and the average AE peak frequency increased gradually. With the increase of strain rate, the AE wavelet energy spectrum in ca8 frequency band and average AE peak frequency decreased gradually. The polypropylene fiber content has more obvious effect on the Dynamic increase factor（DIF） of the peak stress than the steel fiber content. The theoretical basis was provided for the monitoring of dynamic damage of fiber reinforced concrete based on the AE technique.

Characterisation of the Coherent Infrasound Sources Recorded by the Infrasound International Monitoring System Station I48TN in Tunisia (Mines &Quarries)

The I48TN is one of the 60 International Monitoring System (IMS) stations of the Comprehensive nuclear Test Ban Treaty Organization (CTBTO), characterized by its location in the heart of the IMS Infrasound network. The ability of the International Monitoring System (IMS) infrasound network to detect atmospheric nuclear explosions and other signals of interest is strongly dependent on station-specific ambient noise. This ambient noise, includes both incoherent wind noise and real coherent infrasonic waves. Infrasound analysis software detects tens to hundreds of events per day which consume a lot of time for the Infrasound analysts, to define and categorize events where around 90% of the detections are coherent noise. This study analyzed the importance of the synergy between infrasound and seismic data, and provided the infrasound data analyst with the most important local coherent infrasound sources in the region as recorded by the IMS station I48TN, in order to reduce the workload of the analysts and give them a clear view on the coherent noise affecting this station for better discrimination between events of interest like nuclear explosions and coherent sources. DTK_GPMCC and DIVA software were used to perform this study. Geotool software from the International Data Centre (IDC) was used in analysing seismic data from the Tunisian IMS station KEST. The result of this study allowed the characterization of the most important coherent local infrasound sources (Mines and Quarries) which are considered as coherent noise to I48TN station and correct parameters in some reference events in the Reference Event Database source of the International Data Centre.

Ion-bunch energy acoustic tracing by modulation of the depth-dose curve

Characterizing exact energy density distributions for laser-accelerated ion bunches in a medium is challenging due to very high beam intensities and the electro-magnetic pulse emitted in the laser-plasma interaction.Ion-bunch energy acoustic tracing allows for reconstructing the spatial energy density from the ionoacoustic wave generated upon impact in water.We have extended this approach to tracing ionoacoustic modulations of broad energy distributions by introducing thin foils in the water reservoir to shape the acoustic waves at distinct points along the depth-dose curve.Here,we present first simulation studies of this new detector and reconstruction approach,which provides an online read-out of the deposited energy with depth within the centimeter range behind the ion source of state-of-the-art laser-plasma-based accelerators.

Local characteristics and acoustical energy of radiation on vibrating surface: Ⅰ. Theory

From a point of view of the auto-spectrum, the local characteristics and the acous-tical energy of radiating on a vibrating surface are theoretically studied in this paper. The point radiation impedance at any point on a vibrating surface is defined a-s the ratio of the sound pressure to the vibrating velocity, which establishes the relation between the vibration of the surface and the sound field. Applying the Cauchy iategral theorem, the chromatic disper-sion relation between the real and imaginary components of the point radiation impedance is given, and some characteristics are discussed. The discussion about two typical sound sotirces,pulsating and oscillating spheres, supports the arguments of this paper.

Local characteristics and acoustical energy of radiation on vibrating surface: Ⅱ. application

Applying the theory put forward in Ref. [3], the radiation mechanism of sound energy on a vibrating steel plate was studied. Between the point radiation resistance efficiency and the point radiation reactance efficiency exists the chromatic dispersion relation that is one to one. The chromatic dispersion relation fits not only smooth curves, but also the dispersed curves with sharp-pointed peaks. While the vibrating surface radiates the sound energy into field, it absorbs some energy from the field.

Calculation of vibrational energy transition rates in acoustic relaxation processes for excitable gas molecules

To research the correlation between vibrational energy transition rates and acoustic relaxation processes in excitable gases, the vibrational relaxation theory provided by Tanczos [J. Chem. Phy3. 25, 439 （1956）] is applied to calculate the energy transition rates of Vibrational- Vibrational （V-V） and Vibrational-Translational （V-T） energy transfer in gas mixtures. The results of calculation for the multi-relaxation processes in various gas mixtures, consisting of carbon dioxide, methane, chlorine, nitrogen, and oxygen at room temperature, demonstrate that the acoustic energy stagnated in every vibrational mode is coupled with each other through V-V energy exchanges. The vibrational excitation energy will relax through the V-T de-excitation path of the lowest mode because of its fastest V-T transition rate, resulting in that only one absorption peak can be measured for most of excitable gas mixtures. Thus, an effective model is provided to analyze how the vibrational energy transition rates affect the characteristics of acoustic relaxation processes and acoustic propagation in excitable gas mixtures.

Temperature Difference and Stack Plate Spacing Effects on Thermodynamic Performances of Standing-Wave Thermoacoustic Engines Driven by Cryogenic Liquids and Waste Heat

The standing-wave thermoacoustic engines(TAE)are applied in practice to convert thermal power into acoustic one to generate electricity or to drive cooling devices.Although there is a number of existing numerical researches that provides a design tool for predicting standing-wave TAE performances,few existing works that compare TAE driven by cryogenic liquids and waste heat,and optimize its performance by varying the stack plate spacing.This present work is primarily concerned with the numerical investigation of the performance of TAEs driven by cryogenic liquids and waste heat.For this,three-dimensional(3-D)standing-wave TAE models are developed.Mesh-and time-independence studies are conducted first.Model validations are then performed by comparing with the numerical results available in the literature.The validated model is then applied to simulate the standing-wave TAEs driven by the cryogenic liquids and the waste heat,as the temperature gradientΔT is varied.It is found that limit cycle oscillations in both systems are successfully generated and the oscillations amplitude is increased with increasedΔT.Nonlinearity is identified with acoustic streaming and the flow reversal occurring through the stack.Comparison studied are then conducted between the cryogenic liquid-driven TAE and that driven by waste heat in the presence of the same temperature gradientΔT.It is shown that the limit cycle frequency of the cryogenic liquid system is 4.72%smaller and the critical temperatureΔT_(cri)=131 K is lower than that of the waste heat system(ΔT_(cri)=187 K).Furthermore,the acoustic power is increased by 31%and the energy conversion efficiency is found to increase by 0.42%.Finally,optimization studies on the stack plate spacing are conducted in TAE system driven by cryogenic liquids.It is found that the limit cycle oscillation frequency is increased with the decreased ratio between the stack plate spacing and the heat penetration depth.When the ratio is set to between 2 and 3,the overall performance of the cryogenic liquid-driven TAE has been greatly improved.In summary,the present model can be used as a design tool to evaluate standing-wave TAE performances with detailed thermodynamics and acoustics characteristics.The present findings provide useful guidance for the design and optimization of high-efficiency standing-wave TAE for recovering low-temperature fluids or heat sources.