Effect of streamwise vane treatments on the noise reduction performance of trailing edge serrations under aerodynamic loading conditions.

Trailing edge serrations have shown remarkable ability to reduce noise, but their effectiveness can be significantly impacted by flow misalignment, particularly under aerodynamic loading conditions. This paper presents a comprehensive study on the effect of incorporating streamwise vane treatments at the root of the trailing edge serrations on its noise reduction performance. Experiments were performed on a 100 mm chord NACA 0012 wing model with sawtooth type trailing edge serration. The aeroacoustic performance was investigated for serrations with non-zero flap angle at various angles of attack between -5° and 8.5°. The findings reveal that streamwise vanes can reduce the high-frequency noise by over 5 dB when placed at the root. Furthermore, particle image velocimetry measurements in the wall-normal plane demonstrate a significant decrease in cross-flow and turbulence generation when the treatment was placed near the root of serrations. In addition, the load measurements indicate no noticeable variation in the lift coefficient and up to 6% increase in the drag coefficients in the pre-stall region.

The impact of COVID-19 on online medical education: a knowledge graph analysis based on co-term analysis.

BACKGROUND: This study aims to identify the characteristics and future directions of online medical education in the context of the novel coronavirus outbreak new through visual analytics using CiteSpace and VOSviewer bibliometric methods. METHOD: From Web of Science, we searched for articles published between 2020 and 2022 using the terms online education, medical education and COVID-19, ended up with 2555 eligible papers, and the articles published between 2010 and 2019 using the terms online education, medical education and COVID-19, and we ended up with 4313 eligible papers. RESULTS: Before the COVID-19 outbreak, Medical students and care were the most frequent keywords and the most cited author was BRENT THOMA with 18 times. The United States is the country with the greatest involvement and research impact in the field of online medical education. The most cited journal is ACAD MED with 1326 citations. After the COVID-19 outbreak, a surge in the number of research results in related fields, and ANXIETY and four secondary keywords were identified. In addition, the concentration of authors of these publications in the USA and China is a strong indication that local epidemics and communication technologies have influenced the development of online medical education research. Regarding the centrality of research institutions, the most influential co-author network is Harvard Medical School in the United States; and regarding the centrality of references, the most representative journal to which it belongs is VACCINE. CONCLUSION: This study found that hey information such as keywords, major institutions and authors, and countries differ in the papers before and after the COVID-19 outbreak. The novel coronavirus outbreak had a significant impact on the online education aspect. For non-medical and medical students, the pandemic has led to home isolation, making it difficult to offer face-to-face classes such as laboratory operations. Students have lost urgency and control over the specifics of face-to-face instruction, which has reduced the quality of teaching. Therefore, we should improve our education model according to the actual situation to ensure the quality of teaching while taking into account the physical and psychological health of students.

Biomechanical study of the stiffness of the femoral locking compression plate of an external fixator for lower tibial fractures.

BACKGROUND: A locking compression plate (LCP) of the distal femur is used as an external fixator for lower tibial fractures. However, in clinical practice, the technique lacks a standardized approach and a strong biomechanical basis for its stability. METHODS: In this paper, internal tibial LCP fixator (Group IT-44), external tibial LCP fixator (Group ET-44), external distal femoral LCP fixator (Group EF-44, group EF-33, group EF-22), and conventional external fixator (Group CEF-22) frames were used to fix unstable fracture models of the lower tibial segment, and anatomical studies were performed to standardize the operation as well as to assess the biomechanical stability and adjustability of the distal femoral LCP external fixator by biomechanical experiments. RESULTS: It was found that the torsional and flexural stiffnesses of group EF-44 and group EF-33 were higher than those of group IT-44 and group ET-44 (p < 0.05); the flexural stiffness of group EF-22 was similar to that of group IT-44 (p > 0.05); and the compressive stiffness of all three EF groups was higher than that of group ET-44 (p < 0.05). In addition, the flexural and compressive stiffnesses of the three EF groups decreased with the decrease in the number of screws (p < 0.05), while the torsional stiffness of the three groups did not differ significantly between the two adjacent groups (p > 0.05). Group CEF-22 showed the highest stiffnesses, while group ET-44 had the lowest stiffnesses (P < 0.05). CONCLUSIONS: The study shows that the distal femoral LCP has good biomechanical stability and adjustability and is superior to the tibial LCP as an external fixator for distal tibial fractures, as long as the technique is used in a standardized manner according to the anatomical studies in this article.

Efficacy of Biological and Physical Enhancement on Targeted Muscle Reinnervation.

Targeted muscle reinnervation (TMR) is a microsurgical repair technique to reconstruct the anatomical structure between the distal nerve and the muscle stump to provide more myoelectric information to the artificially intelligent prosthesis. Postoperative functional electrical stimulation treatment of the patient's denervated muscle or proximal nerve stump as well as nerve growth factor injection is effective in promoting nerve regeneration and muscle function recovery. In this experiment, we successfully established a TMR rat model and divided Sprague-Dawley (SD) adult male rats into TMR group, TMR + FES group, and TMR + NGF group according to TMR and whether they received FES treatment or NGF injection after surgery, and the recovery effect of rat neuromuscular function was assessed by analyzing EMG signals. Through the experiments, we confirmed that growth factor supplementation and low-frequency electrical stimulation can effectively promote the regeneration of the transplanted nerve as well as significantly enhance the motor function of the target muscle and have a positive effect on the regeneration of the transplanted nerve.

An experimental investigation of noise characteristics of overlapping propellers.

Multiple propellers arranged in overlapping configurations can reduce the sizes of unmanned aerial vehicles for compact architectures. However, the resulting aerodynamic interaction between the propellers could lead to extra noise and calls for extra studies on acoustic physics. In this work, we conducted aeroacoustic tests of two overlapping propellers in an anechoic chamber, focussing on the influence of the axial separation distance and lateral tip spacing at different tip Mach numbers, ranging from 0.18-0.23 and Reynolds numbers ranging from 5.2 × 104 to 6.7 × 104. The results indicate that, compared with the coplanar configurations, the propeller thrusts of the overlapping propellers are deteriorated. The rear propeller under the impingement of the upstream wake flows is significantly affected, leading to the mean value of the thrust being reduced by up to 8.2% and the thrust fluctuation being increased. Consequently, the tonal noise is intensified by up to 10 dB at the low observation angles (referenced to the rotation axis). However, the broadband noise is slightly changed within 3 dB. Also, the lateral tip spacing has a primary impact on the noise characteristics, while the effect of the axial separation distance is secondary.

Efficient computation of broadband noise propagation using Gaussian beam tracing method.

The Gaussian beam tracing method has advantages in computing long-distance noise propagation in outdoor environments due to its high efficiency. Usually, repeated computations are required if the target sound wave is broadband or contains multiple frequencies because the method was developed in the frequency domain. This paper presents an efficient computation of broadband noise propagation using Gaussian beam tracing. The approach is based on the fact that the ray behaviors and source properties can be decoupled at high frequencies, where the wave equation is asymptotically solved. Consequently, only a single computation is needed to determine the frequency-independent ray properties, including ray-centered coordinates and beam dynamics. Then sound waves at different frequencies can be simultaneously introduced in a single computation. By separating the processes of determining the ray properties and incorporating the broadband noise source in the numerical implementation, the computational cost can be largely saved. In this work, several benchmark problems are studied, showing that the efficiency could be increased in comparison to the multiple individual computations. A computation acceleration up to 35-fold could be achieved when 200 frequency components are applied. The method is also applied to study the impact of broadband noise due to multiple drone propellers in urban environments. Applications to stochastic broadband problems are also discussed.

On the efficient modeling of generic source directivity in Gaussian beam tracing.

For large-distance sound propagation with complex obstacles, the Gaussian beam tracing (GBT) method is often applied. An omnidirectional source model is commonly implemented in GBT, which, however, neglects the influence of generic directivity patterns of practical acoustic problems. There have been efforts to synthesize or reproduce the target directivity pattern over an observation surface by using multiple distributed point sources. However, the efficiency and applicability of general applications still call for improvement. More specifically, rays from each of the point sources and their traces in the space should be computed and superposed to estimate the sound field, making the computational cost largely dependent on the complexity of the source directivity pattern. In this work, a complex-valued radiation function model is developed to realize the generic source directivity for GBT computation. One advantage of the method is that only one source is required such that computation cost can be greatly reduced. Rays are emitted from the source with direction-dependent amplitude and phase to realize the target directivity pattern. The development of the radiation function is associated with the GBT method. The verification cases show that this method can give good agreement with analytical or wave-based numerical solutions. Capabilities of modeling a complex source model of the spinning sound field to mimic the propeller noise are studied, and the result matches well with analytical solutions. Finally, a demonstration case of a four-propeller-powered drone in an urban region is conducted.

An efficient rectilinear Gaussian beam tracing method for sound propagation modelling in a non-turbulent medium.

Large-distance sound propagation with high-frequency noise sources, multiple obstacles/geometry with varying acoustic impedance is common in real-life applications. To resolve the acoustic governing equations directly is often computationally costly, especially in three-dimensional space. Methods based on geometric acoustics can be more rapid. However, efforts are still being made to improve the efficiency, robustness, and the capability for complex configurations of such methods. In this paper, an efficient implementation of the rectilinear Gaussian beam tracing method is conducted, which combines rectilinear ray tracing with a proposed efficiency-matched dynamic ray tracing algorithm. A continuous medium stratification method is employed to improve the robustness. Also, a ray compression algorithm is proposed to save computation time. Numerical tests show that computation acceleration up to tenfold is achieved, benefiting rapid estimation of large-distance sound propagation. A standard octree data structure is employed in the code, which accelerates ray tracing in the testing cases with complex geometries. The efficiency and capability of the solver are demonstrated by studying several benchmark problems with varying complexity.

An experimental study of trailing edge noise from a heaving airfoil.

In this study, the far-field noise and near-field flow properties from a heaving NACA 0012 airfoil at the Reynolds number of 6.6×104 were investigated experimentally in a 0.4 m2 anechoic wind tunnel. The airfoil had an incident angle of 0° and followed a sinusoidal heaving motion. The Strouhal number, controlled by changing the heaving frequency and amplitude, varied from 0.0024 to 0.008. The acoustic properties were measured by a free-field microphone placed at a distance of 1.2 m away from the tunnel central line, and the flow structures near the trailing edge were acquired using the particle image velocimetry. It was found that the heaving motion could reduce the sound pressure level (SPL) of the primary peak in the time-averaged spectra. The spectrograms obtained by the short-time Fourier transform revealed that the discrete tones were produced when the airfoil passed through the maximum heaving position. During the corresponding period, a sequence of large-scaled vortices convected on the airfoil surface was observed, and then was shed from the trailing edge to the wake region at the same frequency as the primary tone of the induced sound. With the increase of Strouhal number, the sound signals tended to be broadband, and the overall SPL was increased in the far field.

A revisit of the tonal noise of small rotors.

In this study, asymptotic analysis of the frequency-domain formulation to compute the tonal noise of the small rotors in the now ubiquitously multi-rotor powered drones is conducted. Simple scaling laws are proposed to evaluate the impacts of the influential parameters such as blade number, flow speed, rotation speed, unsteady motion, thrust and observer angle on the tonal noise. The rate of noise increment with thrust (or rotational speed) is determined by orders of blade passing frequency harmonics and the unsteady motion. The axial mean flow influence can be approximated by quadratic functions. At given thrust, the sound decreases rapidly with the radius and blade number as the surface pressure becomes less intensive. The higher tonal harmonics are significantly increased if unsteady motions, although of small-amplitude, are existed, as indicated by the defined sensitivity function, emphasizing that the unsteady motions should be avoided for quiet rotor designs. The scaling laws are examined by comparing with the full computations of the rotor noise using the frequency-domain method, the implementation of which has been validated by comparing with experiments. Good data collapse is obtained when the proposed scaling laws, which highlights the dominant influence of the design parameters, are incorporated.

Extensions to the acoustic scattering analysis for cloaks in non-uniform mean flows.

Most of the acoustic cloak designs are based on the stationary medium, which, however, may be inaccurate in many practical applications with non-uniform flows. The optimization of the acoustic cloaking performance was described as an active noise control problem, and the theoretical model by Huang, Zhong, and Stalnov [J. Acoust. Soc. Am. 135(5), 2571-2580 (2014b)] was employed for the quick evaluation of the sound scattering. In this work, extensions are made to address the unsolved but essential issues in the model. First, the impact of the discontinuities at the interface between the cloak and surrounding fluids is investigated. Second, the high-order Born's approximation is employed to solve the sound governing equation, which can quickly improve the prediction accuracy. Finally, the optimized cloaking strategy is applied to airfoils in turbulent flows to demonstrate the capability of the proposed modelling for cases of practical importance. Also, it is found that the performance of the optimized cloak is insensitive to the frequency of the incident wave. The promising results suggest that an optimized cloaking design can effectively suppress the sound scattering, providing the confidence with the mathematical framework for the potential aeronautics and marine applications.

On the effect of streamwise disturbance on the airfoil-turbulence interaction noise.

Usually the prediction of airfoil-gust interaction noise is mainly based on the flat plate solution, and only the transverse velocity disturbance that is perpendicular to the flat plate surface is considered. However, the solution might underestimate the sound generation in non-uniform mean flows for finite thickness airfoils, in which the velocity disturbance in the streamwise direction could probably produce sound as well. The effect is studied in this work by using both numerical simulations and theoretical analysis. To highlight the effect of streamwise disturbance, the anisotropic turbulences with constant transverse velocity components while varying streamwise components are employed, which can be realised by adjusting the integral length scales in the Gaussian energy spectra. The simulation results explicitly show that more sound is produced by the streamwise components for airfoils at higher Mach numbers when the flow non-uniformities are more significant. An analytical model based on acoustic analogy to account for the sound generation by the coupling between the gust and non-uniform mean flows is proposed, which explains the observations in the numerical simulation well. Also, a correction to Amiet's flat plate solution using the proposed model yields better agreement with the numerical results, especially for anisotropic turbulences with larger integral length scale in the streamwise direction.

On the frequency domain formulation of the generalized sound extrapolation method.

A frequency domain formulation of the generalized sound extrapolation method is proposed. The Fourier transform is performed to the flow variables to prepare the source terms as the input of the sound extrapolation solver, and the inhomogeneous convected Helmholtz equation is solved using the Green's functions in the frequency domain. The proposed formulations are applied to typical aeroacoustics problems including two dimensional (2D) and three dimensional monopoles in uniform mean flows, flat plate-gust interaction noise and the sound produced by a co-flowing jet. The formulations can yield close agreements with the predictions by the time domain solution. Substantial time saving can be achieved compared with a time domain solver, as the estimation of emission/retarded time, which needs to conduct interpolation of the time sequence of the flow variables, is not required. Also, the computation in the 2D space is much easier since the Green's function is used directly. The results suggest that the frequency formulation can potentially be used for sound projection computation in aeroacoustics.

Numerical investigation of nonlinear sound propagation of photoacoustic tomography imaging.

Almost all reconstruction methods in photoacoustic tomography (PAT) have been developed by assuming that sound propagation is linear, which is valid for ordinary PAT applications but would become inappropriate when the sound amplitude is higher than a certain threshold level. In the current study, we investigate the effect of nonlinear sound propagation on PAT by using a numerical method which utilizes the time-reversal (TR) technique. In the forward stage, the Euler equations are solved to simulate nonlinear sound propagation, and the flow variables (pressure, velocity and density) are recorded by an array of virtual sensors. The recorded data are used to reconstruct the initial fields within the computational domain by using both linear and nonlinear TR techniques. Furthermore, TR results constructed with and without the recorded flow velocity field, which is difficult to measure for practical applications, have also been compared. The current results show that nonlinear reconstructions produce images with superior clarity, resolution and contrast compared to those reconstructed by the linear method, particularly when the recorded velocity field is used in the reconstruction.

A Triangle Mesh Standardization Method Based on Particle Swarm Optimization.

To enhance the triangle quality of a reconstructed triangle mesh, a novel triangle mesh standardization method based on particle swarm optimization (PSO) is proposed. First, each vertex of the mesh and its first order vertices are fitted to a cubic curve surface by using least square method. Additionally, based on the condition that the local fitted surface is the searching region of PSO and the best average quality of the local triangles is the goal, the vertex position of the mesh is regulated. Finally, the threshold of the normal angle between the original vertex and regulated vertex is used to determine whether the vertex needs to be adjusted to preserve the detailed features of the mesh. Compared with existing methods, experimental results show that the proposed method can effectively improve the triangle quality of the mesh while preserving the geometric features and details of the original mesh.

Analysis of scattering from an acoustic cloak in a moving fluid.

This work develops a theoretical framework for acoustic cloak scattering analysis in a low speed non-stationary fluid that is simply described as a potential flow. The equivalent sound source induced by the moving fluid local to the cloak is analytically constructed and is then estimated using Born approximation. The far-field scattering can thereafter be obtained using the associated Green's function of the convected wave equation. The results demonstrate that the proposed analytical approach, which might be helpful in the design and evaluation of cloaking systems, effectively elucidates key characteristics of the relevant physics. In addition, it can be seen that, in a moving fluid, the so-called convected cloaking design achieves better cloaking performance than the classical cloaking design.

Compressive sensing beamforming based on covariance for acoustic imaging with noisy measurements.

Compressive sensing, a newly emerging method from information technology, is applied to array beamforming and associated acoustic applications. A compressive sensing beamforming method (CSB-II) is developed based on sampling covariance matrix, assuming spatially sparse and incoherent signals, and then examined using both simulations and aeroacoustic measurements. The simulation results clearly show that the proposed CSB-II method is robust to sensing noise. In addition, aeroacoustic tests of a landing gear model demonstrate the good performance in terms of resolution and sidelobe rejection.

Experimental evaluation of flow-induced noise in level flight of the pigeon (Columba livia).

The experimental method employed in an anechoic wind tunnel to characterize flow-induced noise of the pigeon (Columba livia) during level flight is described in this letter. A live pigeon was managed to maintain a steady level flight at the wind tunnel test flow of 15 m/s. A microphone array was fabricated, and the conventional beamforming method was then adopted to yield the corresponding narrowband acoustic images and broadband sound pressure spectral results. The results justified the experimental method developed in this work. It can be seen that the flight noise of the pigeon is mostly from the wing tips. In addition, the spectral waveform of the pigeon flight suggests a slope of -20 dB/dec between 500 Hz and 5 kHz.