Rotor Noise Research Articles

Homogenization of Doppler broadening in spin-noise spectroscopy

The spin noise of cesium atoms vapor with admixture of buffer gas is experimentally investigated by measuring the spin induced Faraday rotation fluctuations in the vicinity of D 2 line. The line, under these conditions, is known to be strongly inhomogeneously broadened due to the Doppler effect. Despite that, optical spectrum of the spin noise power, as we have found, has the characteristic shape of the homogeneously broadened line with the dip at the line center. This fact is in stark contrast with the results of previous studies of inhomogeneous quantum dot ensembles. In addition, the two-color experiments, where correlations of the Faraday rotation fluctuations for two probe wavelengths were measured, have shown, in a highly spectacular way, that these fluctuations are either correlated, or anticorrelated depending on whether the two wavelengths lie on the same side, or on different sides of the resonance. The experimental data are explained within the developed theoretical model which takes into account both kinetics and spin dynamics of Cs atoms. It is shown that the unexpected behavior of the optical Faraday rotation noise spectra and effective homogenization of the optical transition in the spin-noise measurements are related to smallness of the momentum relaxation time of the atoms as compared with their spin relaxation time.

Drivetrain Noise of The Gyroplane I-28

Abstract Gyroplanes, as ultralight aircraft, are popular transport vehicles recently. Ultralight aircraft flights take place at a low altitude – their noise is not without effect on people and nature. The localization of the sources of noise and a possibility to decrease the noise of an gyroplane are described in this paper. The rules of design and exploitation of gyroplanes do not define the limits of emitted noise. Gyroplanes are not noisy aircraft vehicles but for their silencing the knowledge about the sources and frequency range of noise is necessary. The goal of the conducted measurement was to determine the gyro-plane noise properties and the noise measurement methods. The evaluation of the noise sources was made by acoustic beamforming and the directional emission with single microphones at various engine speeds. The supplement of these tests should be the rotor noise measurement but that investigation should be performed on a special stand, on which the rotor propulsion noise would not disturb the measurement.

Spin and reoccupation noise in a single quantum dot beyond the fluctuation-dissipation theorem

We report on the nonequilibrium spin noise of a single InGaAs quantum dot charged by a single hole under strong driving by a linearly polarized probe light field. The spectral dependency of the spin noise power evidences a homogeneous broadening and negligible charge fluctuations in the environment of the unbiased quantum dot. Full analysis of the spin noise spectra beyond the fluctuation-dissipation theorem yields the heavy-hole spin dynamics as well as the trion spin dynamics. Moreover, the experiment reveals an additional much weaker noise contribution in the Kerr rotation noise spectra. This additional noise contribution has a maximum at the quantum dot resonance and shows a significantly longer correlation time. Magnetic-field-dependent measurements in combination with theoretical modeling prove that this additional noise contribution unveils a charge reoccupation noise which is intrinsic in naturally charged quantum dots.

Chinese Character CAPTCHA Recognition and performance estimation via deep neural network

Abstract To identify machine and human, Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) is increasingly used in many web applications. The classical English and digital characters based CAPTCHAs are recognized with high accuracy. Due to the complication of Chinese characters which greatly enhance the difficulty of automatic recognition, an increasing number of Chinese web sites use Chinese Character CAPTCHAs. To recognize Chinese Character CAPTCHAs, we propose a Convolution Neural Network (CNN) based approach to learn strokes, radicals and character features of Chinese characters, and prove that our network structure is superior to LENET-5 in this task. Furthermore, we formulate the relation among accuracy, the number of training samples and iterations, which is used to estimate the performance of our approach. Firstly, this approach greatly improves the recognition accuracy of Chinese Character CAPTCHAs with distortion, rotation and background noise. Our experiments results show that this approach achieves over 95% accuracy for single Chinese character and 84% accuracy for three types of Chinese Character CAPTCHAs with four Chinese characters. Secondly, our experiment results and theoretical analysis show that the accuracy of recognition has the exponential relationship with the product of the number of training samples and iterations in the condition of enough and representative training samples. Therefore, we can estimate the training time for a certain accuracy. Finally, we certify that our approach is superior to the most famous Chinese Optical Character Recognition (OCR) software, Hanvon, in Chinese Character CAPTCHAs recognition.

Flocking from a quantum analogy: spin–orbit coupling in an active fluid

Systems composed of strongly interacting self-propelled particles can form a spontaneously flowing polar active fluid. The study of the connection between the microscopic dynamics of a single such particle and the macroscopic dynamics of the fluid can yield insights into experimentally realizable active flows, but this connection is well understood in only a few select cases. We introduce a model of self-propelled particles based on an analogy with the motion of electrons that have strong spin–orbit coupling. We find that, within our model, self-propelled particles are subject to an analog of the Heisenberg uncertainty principle that relates translational and rotational noise. Furthermore, by coarse-graining this microscopic model, we establish expressions for the coefficients of the Toner–Tu equations—the hydrodynamic equations that describe an active fluid composed of these ‘active spins.’ The connection between stochastic self-propelled particles and quantum particles with spin may help realize exotic phases of matter using active fluids via analogies with systems composed of strongly correlated electrons.

Development of Motor Rotation Noise Analysis App. Using a Smartphone built-in Microphone

Development of Motor Rotation Noise Analysis App. Using a Smartphone built-in Microphone

Wind turbine noise measurements in controlled conditions

To validate and reduce the uncertainty associated with noise prediction models for wind turbines, there is a need for detailed noise measurements on wind turbines in controlled conditions. However, high-quality wind tunnel campaigns on horizontal axis wind turbine models are scarce due to the large wind tunnel size needed and consequently high associated costs. To serve this purpose, an experiment using the 4.5 -m diameter Mexico turbine was set up in the large low-speed facility of the DNW wind tunnel. An overview of the experiments is given including a selection of results. Both far field microphone as well as microphone array measurements have been performed together with unsteady force measurements on five instrumented blade sections. This allows a unique insight into the relation between acoustics and the underlying aerodynamics. Overall noise characteristics of the turbine have been determined for a variety of operational conditions by varying tip speed ratio and blade pitch angles. Scaling of the noise is studied by comparing similar combinations of tip speed ratio and pitch angle for different tip speeds. The effect of blade soiling on the noise is evaluated using roughness strips, as well as the influence of yawed or misaligned inflow on the rotor noise. A comparison of calculations using a BPM model is given indicating for which operational conditions this model suffices and for which parts the noise prediction can be improved. In summary, after years of preparation, ECN and partners have performed very successful aeroacoustic experiments in the largest wind tunnel in Europe. The comprehensive high-quality database that has been obtained will be used in the international Mexnext consortium to further improve wind turbine acoustic modeling.

Prediction of the acoustic shielding by aircraft empennage for contra-rotating open rotors

This paper presents a numerical study of the acoustic shielding of aeroengine noise by the aircraft structure. More specifically, the engines considered are contra-rotating open rotors in pusher mode which are installed at the rear of the aircraft. Several empennage designs and engine positions are evaluated. An original method for the calculation of engine noise installation effects is presented and its application to contra-rotating open rotor propulsion is demonstrated. It is based on the weak coupling between computational fluid dynamics, an integral method based on Lighthill’s analogy for the calculation of acoustic radiation, and on a boundary element method for the calculation of the acoustic diffraction by the aircraft fuselage and empennage. The prediction of a pylon-installed (isolated) contra-rotating open rotor noise directivity is presented and compared to equivalent wind-tunnel measurements. In both cases, the simulation and experiment are performed at 1:7 scale. In addition, the installation effect of the wind-tunnel, being non-anechoic, is also assessed with the numerical approach presented. Finally, for each aircraft configuration of interest, equipped with two rear contra-rotating open rotors, the predicted noise impact on the ground is evaluated. Of these, the contra-rotating open rotor mounted above an L-tail empennage is found to be the most beneficial configuration in terms of acoustic shielding.

Method for the Extraction of Helicopter Main and Tail Rotor Noise

In this paper a method for the extraction of main and tail rotor noise from an acoustic pressure time history of a helicopter is described. The method is based on the classic Fourier series; however, modifications are made to account for nonperiodic and nonstationary data. Convergence of the method is demonstrated by the application of the method to artificial data. Thereafter the method is applied to wind tunnel data and flight test data for two different helicopters, one with a traditional tail rotor (Bo105) and one with a modern Fenestron (EC135). The results illustrate that a traditional tail rotor displays a rather tonal noise signature very similar to that of the main rotor, only smaller in amplitude and on a shorter time scale. The Fenestron displays a more chaotic noise signature with both a tonal and a broadband noise characteristic.

Novel phase-based descriptor using bispectrum for texture classification

In this paper, we propose a novel rotation invariant and noise tolerant descriptor for texture classification. This descriptor is based on circular statistics of Fourier phase. This one is recovered from the third order spectrum namely the bispectrum, known to provide invariance properties and to preserve phase information. The computational complexity of two dimensional image is reduced using Radon transform. At first, the input image is decomposed into a set of 1D radon projections. Then, for each projection the bispectrum is computed and the phase vector is recovered. Features vectors contain circular statistics of each phase vector recovered from bispectrum of each 1D projection. The proposed descriptor is evaluated on three test suites from the database “Outex” and compared with three descriptors also based on the phase. According to the classification experiments, our descriptor achieves highest rates under noise, illumination and rotation changes.

Identifying natural images and computer generated graphics based on binary similarity measures of PRNU

Aiming at the identification of natural images and computer generated graphics, an image source pipeline forensics method based on binary similarity measures of PRNU (photo response non-uniformity) is proposed. As PRNU is a unique attribute of natural images, binary similarity measures of PRNU are used to represent the differences between natural images and computer generated graphics. Binary Kullback-Leibler distance, binary minimum histogram distance, binary absolute histogram distance and binary mutual entropy are calculated from PRNU in RGB three channels. With a total of 36 dimensions of features, LIBSVM is used for classification. Experimental results and analysis indicate that it can achieve an average identification accuracy of 99.83%, and the capability of identifying natural images and computer generated graphics is balanced. Meanwhile, it is robust against JPEG compression, rotation and additive noise.

Noise from a rotor ingesting a thick boundary layer and relation to measurements of ingested turbulence

Haystacked peaks in rotor noise spectra are produced by the correlated unsteady loading on neighboring blades. This noise can be predicted with knowledge of the turbulent inflow and blade response function, but this is not trivial, especially for complex turbulent inflows that are both inhomogeneous and anisotropic. This paper details the radiated noise and direct measurement of the unsteady upwash correlation in the rotating frame of a 10-bladed, 457mm diameter rotor immersed in planar wall boundary layers of different thickness at non-thrusting and thrusting advance ratios. At low thrust conditions, the measured upwash correlation can be predicted using the fixed frame space-time correlation function of the undisturbed inflow. However, as the advance ratio is lowered, predictions progressively deviate from measurements. This is shown to be due to both the distortion of the approaching turbulence and the formation of a separation region on the wall beneath the rotor. At these low advance ratios, haystacks at the blade passage frequency and harmonics are observed in the blade-to-blade upwash coherence spectra at spanwise locations near the blade tips. Also, a lateral contraction of the turbulence is not observed in measurement of the spanwise coherence with increasing thrust. Finally, increasing the boundary layer thickness increases spectral levels of the radiated noise at higher advance ratios near zero thrust by a factor similar to the increase in boundary layer thickness. At low advance ratios, peaks in the measured noise have the same approximate magnitude for similar rotational and freestream velocities regardless of the boundary layer thickness as these are likely produced by interaction with vortex structures in the separated region.

Efficient Method to Predict Rotor Noise Scattered by an Axisymmetric Body

This paper presents an efficient method for predicting noise radiated from rotating sources and scattered by an axisymmetric body, in which a frequency-domain numerical method and a null-field method are employed to compute the acoustic radiation and scattering phenomena, respectively. The novelty of the present research is that an advanced method is proposed to locate fictitious interior observers inside the scattering surface, contributing to that the incident acoustic pressure received by the fictitious interior observers can be efficiently computed. Numerical cases show that the developed method ensures high computational accuracy and significantly improves the computational efficiency compared with existing methods, showing an appealing application potential in efficiently predicting rotor noise scattered by the shaft and casing. Parametric studies are carried out to investigate the effects of the hub–tip ratio and the rotating frequency on a rotor noise scattered by a hub.

An architecture for multi-robot localization and mapping in the Gazebo/Robot Operating System simulation environment

Robots are an important part of urban search and rescue tasks. World wide attention has been given to developing capable physical platforms that would be beneficial for rescue teams. It is evident that use of multi-robots increases the effectiveness of these systems. The Robot Operating System (ROS) is becoming a standard platform for the robotics research community for both physical robots and simulation environments. Gazebo, with connectivity to the ROS, is a three-dimensional simulation environment that is also becoming a standard. Several simultaneous localization and mapping algorithms are implemented in the ROS; however, there is no multi-robot mapping implementation. In this work, two multi-robot mapping algorithm implementations are presented, namely multi-robot gMapping and multi-robot Hector Mapping. The multi-robot implementations are tested in the Gazebo simulation environment. Also, in order to achieve a more realistic simulation, every incremental robot movement is modeled with rotational and translational noise.

On a two-dimensional mode-matching technique for sound generation and transmission in axial-flow outlet guide vanes

The present work deals with the analytical modeling of two aspects of outlet guide vane aeroacoustics in axial-flow fan and compressor rotor-stator stages. The first addressed mechanism is the downstream transmission of rotor noise through the outlet guide vanes, the second one is the sound generation by the impingement of the rotor wakes on the vanes. The elementary prescribed excitation of the stator is an acoustic wave in the first case and a hydrodynamic gust in the second case. The solution for the response of the stator is derived using the same unified approach in both cases, within the scope of a linearized and compressible inviscid theory. It is provided by a mode-matching technique: modal expressions are written in the various sub-domains upstream and downstream of the stator as well as inside the inter-vane channels, and matched according to the conservation laws of fluid dynamics. This quite simple approach is uniformly valid in the whole range of subsonic Mach numbers and frequencies. It is presented for a two-dimensional rectilinear-cascade of zero-staggered flat-plate vanes and completed by the implementation of a Kutta condition. It is then validated in sound generation and transmission test cases by comparing with a previously reported model based on the Wiener-Hopf technique and with reference numerical simulations. Finally it is used to analyze the tonal rotor-stator interaction noise in a typical low-speed fan architecture. The interest of the mode-matching technique is that it could be easily transposed to a three-dimensional annular cascade in cylindrical coordinates in a future work. This makes it an attractive alternative to the classical strip-theory approach.

Memory effects in funnel ratchet of self-propelled particles

The transport of self-propelled particles with memory effects is investigated in a two-dimensional periodic channel. Funnel-shaped barriers are regularly arrayed in the channel. Due to the asymmetry of the barriers, the self-propelled particles can be rectified. It is found that the memory effects of the rotational diffusion can strongly affect the rectified transport. The memory effects do not always break the rectified transport, and there exists an optimal finite value of correlation time at which the rectified efficiency takes its maximal value. We also find that the optimal values of parameters (the self-propulsion speed, the translocation diffusion coefficient, the rotational noise intensity, and the self-rotational diffusion coefficient) can facilitate the rectified transport. When introducing a finite load, particles with different self-propulsion speeds move to different directions and can be separated.

Blade tip vortex measurements on actively twisted rotor blades

Active rotor control concepts, such as active twist actuation, have the potential to effectively reduce the noise and vibrations of helicopter rotors. The present study focuses on the experimental investigation of active twist for the reduction of blade–vortex interaction (BVI) effects on a model rotor. Results of a large-scale smart-twisting active rotor test under hover conditions are described. This test investigated the effects of individual blade twist control on the blade tip vortices. The rotor blades were actuated with peak torsion amplitudes of up to $$2^\circ$$ and harmonic frequencies of 1–5/rev with different phase angles. Time-resolved stereoscopic particle image velocimetry was carried out to study the effects of active twist on the strength and trajectories of the tip vortices between $$\psi _\text {v}= 3.6^\circ$$ and $$45.7^\circ$$ of vortex age. The analysis of the vortex trajectories revealed that the 1/rev active twist actuation mainly caused a vertical deflection of the blade tip and the corresponding vortex trajectories of up to $$1.3\%$$ of the rotor radius R above and $$-1\%R$$ below the unactuated condition. An actuation with frequencies of 2 and 3/rev significantly affected the shapes of the vortex trajectories and caused negative vertical displacements of the vortices relative to the unactuated case of up to $$2\%R$$ within the first $$35^\circ$$ of wake age. The 2 and 3/rev actuation also had the most significant effects on the vortex strength and altered the initial peak swirl velocity by up to $$-34$$ and $$+31\%$$ relative to the unactuated value. The present aerodynamic investigation reveals a high control authority of the active twist actuation on the strength and trajectories of the trailing blade tip vortices. The magnitude of the evoked changes indicates that the active twist actuation constitutes an effective measure for the mitigation of BVI-induced noise on helicopters.

Low-frequency rotational isolator for airborne exploration

We have determined the performance of a passive rotational vibration isolator for a time-domain airborne electromagnetic (TDEM) receiver. The isolator uses neutrally buoyant flotation to provide very soft suspension and a very low resonant frequency of 0.065 Hz ± 0.005 Hz. One of the limitations of mapping deeper targets in areas of conductive overburden with TDEM systems is that low-frequency coil-vibration noise provides a lower bound to the transmitter base frequency (typically limited to 25 Hz). The purpose of this new isolator is to improve coil vibration related noise between 5 and 20 Hz to allow the transmitter base frequency to be reduced. A fixed-wing flight test determined that a receiver inside the new isolator had five times less rotational noise at 10 Hz than a current commercial system.

Evaluation of Microphone Array for Multirotor Helicopters

[abstFig src='/00290001/16.jpg' width='300' text='UAV with a microphone array whose performance is evaluated' ] High expectations are placed on the use of unmanned aerial vehicles (UAVs) in such tasks as rescue operations, which require a system that makes use of visual or auditory information to recognize the surrounding environment. As an example of such a system, this study examines the recognition of the environment using a helicopter mounted with a microphone array. Because the rotors of a helicopter generate high noise during operation, it is necessary to reduce the effects of this noise and those from other sources to record the audio signals coming from the ground with onboard microphones. In particular, because of helicopter body control, the rotor speed changes continuously and causes an unsteady rotor noise, which implies that it would be effective to arrange the microphones at a sufficient distance from the rotors. When a large microphone array is employed, however, the array weight may alter the helicopter’s flight characteristics and increase the noise, presenting a dilemma. This paper presents a model of rotor noise that takes into account the effect of the microphone array on the helicopter’s dynamic characteristics and proposes a method of evaluating the optimality of the array configuration, which is necessary for design. The validity of the proposed method is investigated using a multirotor helicopter mounted with a microphone array previously developed by the authors. In addition, an application example for locating sound sources on the ground using this helicopter is presented.

On-Manifold Preintegration for Real-Time Visual--Inertial Odometry

Current approaches for visual-inertial odometry (VIO) are able to attain highly accurate state estimation via nonlinear optimization. However, real-time optimization quickly becomes infeasible as the trajectory grows over time, this problem is further emphasized by the fact that inertial measurements come at high rate, hence leading to fast growth of the number of variables in the optimization. In this paper, we address this issue by preintegrating inertial measurements between selected keyframes into single relative motion constraints. Our first contribution is a \emph{preintegration theory} that properly addresses the manifold structure of the rotation group. We formally discuss the generative measurement model as well as the nature of the rotation noise and derive the expression for the \emph{maximum a posteriori} state estimator. Our theoretical development enables the computation of all necessary Jacobians for the optimization and a-posteriori bias correction in analytic form. The second contribution is to show that the preintegrated IMU model can be seamlessly integrated into a visual-inertial pipeline under the unifying framework of factor graphs. This enables the application of incremental-smoothing algorithms and the use of a \emph{structureless} model for visual measurements, which avoids optimizing over the 3D points, further accelerating the computation. We perform an extensive evaluation of our monocular \VIO pipeline on real and simulated datasets. The results confirm that our modelling effort leads to accurate state estimation in real-time, outperforming state-of-the-art approaches.