基于深度特征融合的鸟鸣识别方法及其可解释性分析

鸟鸣识别是生态监测的重要手段,为进一步提升鸟鸣识别的准确性和鲁棒性,本文提出了1种新的基于深度特征融合的鸟鸣识别方法。该方法首先利用深度特征提取网络对鸟鸣的对数梅尔谱图和补充特征集的深度特征进行提取,再将两种深度特征进行融合,最后使用轻量级梯度提升机(light gradient boosting machine,light GBM)分类器进行分类。本文充分利用深度神经网络的特征提取能力以及light GBM的分类性能,将特征提取和特征分类过程进行分离,从而实现了高准确率的鸟鸣识别。实验结果显示,本文提出的方法在北京百鸟数据集中取得了目前已知的最佳结果,模型的平均准确率达到了98.70%,平均F1分数达到了98.84%。相比传统方法,深度融合特征在鸟鸣识别任务上准确率提升了5.62%以上。同时,引入的light GBM分类器使分类准确率提升了3.02%。此外,在CLO-43SD和Bird CLEF2022比赛的数据集中,本文方法也展现出卓越的性能,分别取得了98.32%和91.12%的平均准确率。本文还引入了类激活图对不同类型鸟鸣的识别结果进行可解释性分析,揭示了神经网络对不同类型鸟鸣的注意力区域差异,为后续的特征选择和模型优化提供了理论依据。研究结果表明,本文方法有效提高了鸟鸣识别的准确率,在3个数据集的测试中均展现出较好的性能,能够为基于鸟鸣识别的生态监测提供有力的技术支撑。

Stealth performance evaluation of helicopter against airborne early warning radar considering trimming control

Airborne pulse Doppler radar is a key threat to the military helicopter,and assessing the stealth performance of helicopter against airborne early warning radar is helpful to the helicopter’s stealth design and operational planning.In this paper,the Shooting and Bouncing Ray(SBR)and Uniform Theory of Diffraction(UTD)based high-frequency algorithms are used to calculate the Radar Cross Section(RCS)of helicopter,and the radar range equations are used to evaluate the stealth performance.In order to account for the effects of rotor flapping motions during actual flight,the aerodynamics model of whole helicopter is established and the attitudes and controls of helicopter at different flight states are trimmed and input into the RCS calculation module.The effects of helicopter flight speed,flying direction and operational environment on radar stealth performance are studied in focus.It is demonstrated by the results that the trimming control does have a great influence of more than 5 dB on the RCS of helicopter,and the introduction of the trim calculation brings the helicopter’s returns calculation closer to the reality.Variations in flight speed lead to the changes in the stealth performance of helicopter against Early Warning Aircraft(EWA),and the helicopter flight speed can be planned according to the operational requirements to minimize exposure distance or exposure time.Variations in flying direction mainly affect the detection properties of helicopter returns,and flying in the same direction with EWA usually gives the helicopter better low-observability than flying head-on.Variations in operational environment mainly affect the radar detection performance and the sensitivity of the detection performance to external factors;the same amount of change in some external factor causes a different amount of change in the helicopter’s detectability in different environments.

Aerodynamic and structural characteristics of helicopter rotor in circling flight

To investigate the distinct properties of the helicopter rotors during circling flight,the aerodynamic and dynamic models for the main rotor are established considering the trim conditions and the flight parameters of helicopters.The free wake method is introduced to compute the unsteady aerodynamic loads of the rotor characterized by distortions of rotor wakes,and the modal superposition method is used to predict the overall structural loads of the rotor.The effectiveness of the aerodynamic and the structural methods is verified by comparison with the experimental results,whereby the influences of circling direction,radius,and velocity are evaluated in both aerodynamic and dynamic aspects.The results demonstrate that the circling condition makes a great difference to the performance of rotor vortex,as well as the unsteady aerodynamic loads.With the decrease of the circling radius or the increment of the circling velocity,the thrust of the main rotor increases apparently to balance the inertial force.Meanwhile,the harmonics of aerodynamic loads in rotor disc change severely and an evident aerodynamic load shock appears at high-order components,which further causes a shift-of-peak-phase bending moment in the flap dimension.Moreover,the advancing side of blade experiences second blade/vortex interaction,whose intensity has a distinct enhancement as the circling radius decreases with the motion of vortexes.

Experimental investigation on aerodynamic and noise characteristics of Fenestron

It is difficult to simulate the strong interference and serious flow separation of Fenestron by the CFD method based on the widely used RANS equation,and the detailed experimental data,which could be used to validate the aerodynamic and noise numerical methods,is unavailable.The experimental investigation on the aerodynamic and noise characteristics of Fenestron is carried out.In view of the complex internal flow field in duct and the relative motion between the stationary duct and the rotating rotor,a comprehensive aerodynamics and pressure measurement scheme is designed based on the bottom support rig.In this measurement scheme,the thrust generated by the rotating rotor can be measured by the rotating shaft balance,the thrusts from Fenestron are measured by the external balance and the pressures on duct inner wall are monitored by a pressure measuring system.To fully capture the noise directionality of Fenestron,a series of noise observers located at an arc array are arranged.In terms of the Fenestron test models,the baseline model,the performance improvement model based on the high-performance tail rotor and the noise reduction model based on the non-uniform blade distribution are designed respectively.By the designed measurement scheme,aerodynamic forces and pressure distributions and noise were measured for the three different Fenestron models.The results show that the aerodynamic thrusts of the tail rotor and duct increase greatly and the noise increases slightly for the performance improvement model because of the larger aerodynamics.The rotor aerodynamic performance of the noise reduction model is reduced,but the modulation effect of the tail rotor improves the forces of the duct.The noise radiated by the noise reduction model is reduced and a good noise reduction result is obtained in frequency domain.

High-fidelity simulation of blade vortex interaction of helicopter rotor based upon TENO scheme

Numerical simulation methods for unsteady vortex field of helicopter rotor with high resolution and low dissipation TENO8-AA primitive variables reconstruction schemes are established based on moving-embedded grid and Navier-Stokes equations.Firstly,the Targeted Essentially Non-Oscillatory(TENO)scheme are developed by employing ENO-like candidate stencil selection strategy,and the candidate stencil is adopted with optimal weight in smooth region while it is discarded completely in discontinuous region,which reduces the dissipation and dispersion errors and approaches better spectral properties.Then,the aerodynamic characteristics of Helishape-7A model rotor in Blade Vortex Interaction(BVI)state and the flowfield of Lynx rotor in hover are simulated,which validates that the blade tip vortex trajectory with larger wake age and more details of vortex can be captured by TENO8-AA scheme with only a quarter of grid points and half time comparing to WENO-JS scheme.Moreover,the simulation accuracy of thrust coefficient is improved by up to 36%.Finally,the analyses for BVI and aeroacoustic characteristics of Operational Loads Survey(OLS)rotor are conducted,and the different forms of interaction mechanism are explored,such as oblique and parallel interactions.The results indicate that TENO scheme not only ensures the resolution of simulation in discontinuous region,but also minimizes the numerical dissipation in smooth region dominated by blade tip vortex.Therefore,the acoustic pressure peak prediction error of rotor in BVI state is significantly reduced to 5.6%and 0.8%at two microphone locations,respectively.

Robust Navier-Stokes method for predicting unsteady flowfield and aerodynamic characteristics of helicopter rotor

A robust unsteady rotor flowfield solver CLORNS code is established to predict the complex unsteady aerodynamic characteristics of rotor flowfield. In order to handle the difficult problem about grid generation around rotor with complex aerodynamic shape in this CFD code,a parameterized grid generated method is established, and the moving-embedded grids are constructed by several proposed universal methods. In this work, the unsteady Reynolds-Averaged Navier-Stokes（RANS） equations with Spalart-Allmaras are selected as the governing equations to predict the unsteady flowfield of helicopter rotor. The discretization of convective fluxes is accomplished by employing the second-order central difference scheme, third-order MUSCL-Roe scheme, and fifth-order WENO-Roe scheme. Aimed at simulating the unsteady aerodynamic characteristics of helicopter rotor, the dual-time scheme with implicit LU-SGS scheme is employed to accomplish the temporal discretization. In order to improve the computational efficiency of holecells and donor elements searching of the moving-embedded grid technology, the ‘‘disturbance diffraction method＂ and ‘‘minimum distance scheme of donor elements method＂ are established in this work. To improve the computational efficiency, Message Passing Interface（MPI） parallel method based on subdivision of grid, local preconditioning method and Full Approximation Storage（FAS） multi-grid method are combined in this code. By comparison of the numerical results simulated by CLORNS code with test data, it is illustrated that the present code could simulate the aerodynamic loads and aerodynamic noise characteristics of helicopter rotor accurately.

Parametric analyses on dynamic stall control of rotor airfoil via synthetic jet

The effects of synthetic jet control on unsteady dynamic stall over rotor airfoil are investigated numerically. A moving-embedded grid method and an Unsteady Reynolds Averaged Navier-Stokes（URANS） solver coupled with k-x Shear Stress Transport（SST） turbulence model are established for predicting the complex flowfields of oscillatory airfoil under jet control. Additionally, a velocity boundary condition modeled by sinusoidal function has been developed to fulfill the perturbation effect of periodic jet. The validity of present CFD method is evaluated by comparisons of the calculated results of baseline dynamic stall case for rotor airfoil and jet control case for VR-7 B airfoil with experimental data. Then, parametric analyses are conducted emphatically for an OA212 rotor airfoil to investigate the effects of jet control parameters（jet location, dimensionless frequency, momentum coefficient, jet angle, jet type and dual-jet） on dynamic stall characteristics of rotor airfoil. It is demonstrated by the calculated results that efficiency of jet control could be improved with specific momentum coefficient and jet angle when the jet is located near separation point of rotor airfoil. Furthermore, the dual-jet could improve control efficiency more obviously on dynamic stall of rotor airfoil with respect to the unique jet, and the influence laws of dual-jet＇s angles and momentum coefficients on control effects are similar to those of the unique jet. Finally,unsteady aerodynamic characteristics of rotor via synthetic jet which is located on the upper surface of rotor blade in forward flight are calculated, and as a result, the aerodynamic characteristics of rotor are improved compared with the baseline. The results indicate that synthetic jet has the capability in improving aerodynamic characteristics of rotor.

An adaptive integration surface for predicting transonic rotor noise in hovering and forward flights

In this paper, a new permeable adaptive integration surface is developed in order to evaluate transonic rotor noise in accordance with FW-H_pds equations(Ffowcs Williams-Hawkings equations with penetrable data surface). Firstly, a nonlinear near-field solution is computed on the basis of Navier-Stokes equations, which is developed on moving-embedded grid methodology.The solution calculated through the present CFD method is used as the input for acoustic calculations by FW-H_pds equations. Then, two criteria for constructing integration surfaces are established based on the analysis of the quadrupole source strength and the nonlinear characteristic.A new surface is determined adaptively by the pressure gradient or density in a given flowfield,eschewing the uncertainties associated with determining cylinder-shaped integration surfaces. For varying hover cases, transonic noises are simulated with new integration surfaces for a UH-1 model rotor. Furthermore, numerical results of the new integration surface derived from the density perturbation value conform better to experimental data than results derived from the pressure gradient.Finally, the integration surface given by jrqj being 0.1, which is an applicable criterion obtained from hover cases, is used to predict transonic rotor noise in forward flight. The computational accuracy of the new integration surface method has been validated in predicting transonic rotor noise of an AH-1 model rotor at different advance ratios.

Aeroelastic analysis and structural parametric design of composite rotor blade

Based on FEM theory,a method of dynamic analysis for hingeless rotors considering anisotropic composite materials is established.A parametric modeling method of composite blade with typical profile and high simulation degree for design is proposed.Through the finite element method,the profile characteristics of rotor blade can be obtained efficiently and accurately,and the synchronization of parametric design and finite element analysis of structural characteristics can be realized.Then a 23-degrees of freedom non-linear beam element is used to simulate the extended one-dimensional beam,thereby a nonlinear differential equation describing the elastic motion of the rotor is established.To obtain the crosssectional target characteristics of the blades,an inverse design method is proposed for cross-section components based on combinatorial optimization algorithm.The calculation and validation work show that the proposed model can effectively analyze the aeroelastic characteristics of general composite rotors.Further,the influence of cross-sectional parameters on the aeroelastic stability and hub loads of hingeless rotor is analyzed and some remarkable conclusions are obtained.

Light field salient object detection:A review and benchmark

Salient object detection(SOD)is a long-standing research topic in computer vision with increasing interest in the past decade.Since light fields record comprehensive information of natural scenes that benefit SOD in a number of ways,using light field inputs to improve saliency detection over conventional RGB inputs is an emerging trend.This paper provides the first comprehensive review and a benchmark for light field SOD,which has long been lacking in the saliency community.Firstly,we introduce light fields,including theory and data forms,and then review existing studies on light field SOD,covering ten traditional models,seven deep learning-based models,a comparative study,and a brief review.Existing datasets for light field SOD are also summarized.Secondly,we benchmark nine representative light field SOD models together with several cutting-edge RGB-D SOD models on four widely used light field datasets,providing insightful discussions and analyses,including a comparison between light field SOD and RGB-D SOD models.Due to the inconsistency of current datasets,we further generate complete data and supplement focal stacks,depth maps,and multi-view images for them,making them consistent and uniform.Our supplemental data make a universal benchmark possible.Lastly,light field SOD is a specialised problem,because of its diverse data representations and high dependency on acquisition hardware,so it differs greatly from other saliency detection tasks.We provide nine observations on challenges and future directions,and outline several open issues.All the materials including models,datasets,benchmarking results,and supplemented light field datasets are publicly available at https://github.com/kerenfu/LFSOD-Survey.