Rotor Noise Research Articles

Aeroacoustic modeling of helicopter transonic rotor noise

The noise propagation of an helicopter rotor with blade tip in transonic regime is numerically simulated. To calculate the acoustic characteristics of the helicopter rotor, a novel CFD/Penetrable Surface Ffowcs Williams-Hawkings (PS-FWH) hybrid approach is established. The approach is characterized by the solution of the Reynolds-Averaged Navier-Stokes (RANS) equations on a Chimera system of overset meshes, coupled with an in-house acoustic code (ROCAAP), which employs the PS-FWH method with a Marching-Cube Emission-Surface (MCES) integral algorithm, for the noise propagation. The hybrid approach is validated through two databases. The well-known Caradonna-Tung hovering rotor test is used to assess the pressure distribution at each cross-section along the blade. The transonic acoustic survey of the UH-1H rotor is adopted to evaluate the time domain noise signal with different blade tip Mach numbers during hover. The capabilities of the hybrid method to predict helicopter transonic rotor noise are demonstrated through the achieved good correlation with experimental data.

Numerical analysis of rotor aeroacoustic characteristics during collective pitch aperiodic variation in hover

Based on the FW-H equations and the CFD method, the rotor aeroacoustic characteristics during collective pitch aperiodic variation in hover are calculated and analyzed. First, a set of analysis method for aperiodic rotor aeroacoustic characteristics is developed. The aerodynamic and aeroacoustic characteristics of the BO-105 rotor in hover, the AH-1G rotor in forward flight and the NACA rotor in a ramp increase of collective pitch are calculated, and the employed numerical analysis method is validated through comparisons with experimental data. Then, the aeroacoustic characteristics of the NACA rotor during an oscillating collective pitch are analyzed, and the sound pressure peak at different collective pitch is discussed in detail. Finally, the BO-105 rotor in hover during collective pitch aperiodic variation is calculated. In addition, parameters, such as the collective pitch change rate, are quantified, and some conclusions are obtained. The configuration of collective pitch change rate will influence the directivity of rotor noise in the collective pitch increasing process, and the loading noise will be smaller if the change rate is decreasing.

Epistemic and aleatoric uncertainties reduction with rotation variation for medical image segmentation with ConvNets

The deep convolutional neural network (ConvNet) achieves significant segmentation performance on medical images of various modalities. However, the isolated errors in a large testing set with various tumor conditions are not acceptable in clinical practice. This is usually caused in inadequate training and noise inherent during data collection, which are recognized as epistemic and aleatoric uncertainties in deep learning-based approaches. In this paper, we analyze the two types of uncertainties in medical image segmentation tasks and propose a reduction method by training models with data augmentation. The shelter zones in images are reduced with 2D imaging on surfaces of different angles from 3D organs. Rotation transformation and noise are estimated by Monte Carlo simulation with prior parameter distributions, and the aleatoric uncertainty is quantized in this process. Experiments on segmentation of computed tomography images demonstrate that overconfident incorrect predictions are reduced through uncertainty reduction and that our method outperforms prediction baselines based on epistemic and aleatoric estimation.

Multi-sensory sound source enhancement for unmanned aerial vehicle recordings

A method to effectively perform sound source enhancement from an unmanned aerial vehicle (UAV)-mounted audio recording system is proposed. The objective of this study is to utilise audio recordings and non-acoustical UAV rotor characteristics to improve rotor noise power spectral density (PSD) estimation accuracy and robustness. The improved rotor noise PSD estimate in turn improves the effectiveness of the rotor noise postfilter, leading to improvement in source enhancement performance. The performance of the proposed source enhancement algorithm is evaluated via experiments, with recordings made in an outdoor environment with an in-flight UAV. Experiment results show PSD estimation accuracy to within 1.5 dB log spectral distortion regardless of the given input conditions, such as the presence of surrounding sound sources. The method also achieves a consistent ∼20–25 dB improvement in source enhancement performance, where the effects of rotor noise from the noisy microphone recordings are significantly reduced. The proposed method also outperforms existing state-of-the-art such as the beamformer with postfilter and speech distortion weighted multichannel Wiener filter frameworks.

Collision-Sensitive Spin Noise

Collision phenomena are ubiquitous and of importance in determining the microscopic structures and intermolecular interactions of atoms and molecules. The existing approaches are mostly based on atomic or molecular scatterings, which are hindered by the inconvenience of using ultra-high vacuum and low temperature systems. Here we demonstrate a new spin-noise spectroscopic approach by measuring optical polarization rotation noise of the probe light, which operates with simple apparatus and ambient conditions. Our approach features tens of gigahertz bandwidth and one part-per-million resolution, outperforming existing spin-noise techniques. Enabled by the new technique, we observe the collision-induced spin noise of alkali atoms, and precisely determine key collision parameters, such as collision diameter, well depth, and dominant interaction type. Our work provides a new tool to study a broad range of collision phenomena under ambient conditions.

Random Weighting Adaptive Estimation of Model Errors on Attitude Measurement for Rotary Steerable System

During drilling, a rotary steerable system (RSS) is affected by vibration, rotation, and other random noises. This paper presents a random weighting adaptive estimation of model errors on attitude measurement for RSS drilling tools. The algorithm is used to estimate the covariance matrix of the dynamic model errors and the state prediction vectors for RSS tools, to reduce the effect of abnormal noises of the dynamic model on the estimation of state parameters. The random weighting estimations of observation’s systematic error and covariance matrices of observation error vectors to control the effect of observation model noises anomaly on state parameter estimation, enhance the use efficiency of the latest observation information, indirectly weaken the impact of model error on state parameter vector, and improve the accuracy of attitude calculation of RSS. Experimental results and comparison analysis demonstrate that the proposed algorithm achieves better outcomes than the Kalman filtering and Extended Kalman filtering. The maximum solution error is controlled at 0.15°, and the tool face angle error is less than 3°. It cannot only control the covariance matrix of the observation error vector and predicted residual vector, but it can also effectively resist the disturbances of kinematic model error. It also can improve the dynamic measurement accuracy of attitude parameters, and solves the problem of uncertainty in dynamic attitude measurement of rotary steerable drilling tools.

Region-Based Hybrid Medical Image Watermarking Scheme for Robust and Secured Transmission in IoMT

With the growth in Internet and digital technology, Internet of Medical Things (IoMT) and Telemedicine have become buzzwords in healthcare. A large number of medical images and information is shared through a public network in these applications. This paper proposes a region-based hybrid Medical Image Watermarking (MIW) scheme to ensure the authenticity, authorization, integrity, and confidentiality of the medical images transmitted through a public network in IoMT. In the proposed scheme, medical images are partitioned into Region of Interest (RoI) and Region of Non-Interest (RoNI). To ascertain integrity of RoI, tamper detection and recovery bits are embedded in RoI in the medical image. RoI is watermarked using adaptive Least Significant Bit (LSB) substitution with respect to the hiding capacity map for higher RoI imperceptibility and accuracy in tamper detection and recovery. Electronic Patient Record (EPR) is compressed using Huffman coding and encrypted using a pseudo-random key (secret key) to provide higher confidentiality and payload. QR code of hospital logo, Encrypted EPR, and RoI recovery bits are interleaved in RoNI using Discrete Wavelet Transform-Singular Value Decomposition (DWT-SVD) hybrid transforms to achieve a robust watermark. The proposed scheme is tested under various geometric and non-geometric attacks such as filtering, compression, rotation, salt and pepper noise and shearing. The evaluation results demonstrate that the proposed scheme has high imperceptibility, robustness, security, payload, tamper detection, and recovery accuracy under image processing attacks. Therefore, the proposed scheme can be used in the transmission of medical images and EPR in IoMT. Relevance of the proposed scheme is established by its superior performance in comparison to some of the popular existing schemes.

Airfoil–Vortex Interaction Noise Control Mechanism Based on Active Flap Control

AbstractBlade-vortex interaction (BVI) noise is a significant source of rotor noise. In recent years, active flap control (AFC) has been used successfully to control BVI by adding a trailing edge f...

Shielding of rotor noise by plates and wings

A method of noise reduction proposed for the next generation of aircraft is to shield noise from the propulsion system, by positioning the noise source over a wing or another surface. In this paper, an approximate analysis is developed for the acoustic field far from a circular source placed near the edge of a semi-infinite plate, a model problem for shielding of noise by a wing and for scattering by a trailing edge. The approximation is developed for a source of small radius and is found to be accurate when compared to full numerical evaluation of the field.

Identification of Turbomachinery Noise Sources via Processing Beamforming Data Using Principal Component Analysis

Complex turbomachinery systems produce a wide range of noise components. The goal is to identify noise source categories, determine their characteristic noise patterns and locations. Researchers can then use this information to quantify the impact of these noise sources, based on which new design guidelines can be proposed. Phased array microphone measurements processed with acoustic beamforming technology provide noise source maps for pre-determined frequency bands (i.e., bins) of the investigated spectrum. However, multiple noise generation mechanisms can be active in any given frequency bin. Therefore, the identification of individual noise sources is difficult and time consuming when using conventional methods, such as manual sorting. This study presents a method for combining beamforming with Principal Component Analysis (PCA) methods in order to identify and separate apart turbomachinery noise sources with strong harmonics. The method is presented through the investigation of Counter-Rotating Open Rotor (CROR) noise sources. It has been found that the proposed semi-automatic method was able to extract even weak noise source patterns that repeat throughout the data set of the beamforming maps. The analysis yields results that are easy to comprehend without special prior knowledge and is an effective tool for identifying and localizing noise sources for the acoustic investigation of various turbomachinery applications.

Fundamental investigation using active plasma control to reduce blade–vortex interaction noise

Blade vortex interaction noise is a problematic and dominant component of rotor noise. Plasma actuators strategically placed at the tip of the rotor blades can reduce the strength of the tip vortices. This reduction has the potential to significantly reduce blade vortex interaction noise. A combined experimental, numerical, and theoretical program shows supporting evidence that low power plasma actuators can effectively lower coherence of the blade tip vortex and reduce blade vortex interaction noise over-pressure by up to 80%. For a nominal small five-bladed unmanned aerial vehicle, we predict an approximate 8.88 maximum ΔdB reduction for a 150 m/s tip speed. Experimental, computational, and acoustic modeling support these predictions. This study represents a fundamental investigation in the fixed-frame, which provides evidence for higher level research and testing in a rotating framework.

Noise of small-scale contra rotating rotors

This article presents the results of an experimental study of the noise characteristics of small-scale coaxial counter-rotating rotors. The tests were carried out in an anechoic soundproof chamber for F7/A7 counter-rotating rotors with 11 and 9 blades on the front and rear rotors, respectively. The intensity of noise emission by rotors was compared with the constancy of the thrust of the power units. Acoustic studies have shown the presence of tonal and broadband noise components at low rotation speeds. In addition to the usual noise components, tonal components were also observed for the rotor blade running combinations. The observed noise levels at these frequencies are equal, and in some directions are significantly higher than the noise levels at the frequencies of the blades of the first and second rows. In experiments, it was found that, while maintaining the equality of the rods, with an increase in the diameter, the noise levels at the rotors decrease due to a decrease in peripheral speeds. A decrease in the noise levels of rotors with an increase in the number of blades was established due to a decrease in aerodynamic loads on the blades and peripheral speeds. An important parameter here is the filling factor of the space swept by the blades. With an increase in this coefficient, the rotation noise decreases, but the vortex noise increases significantly.

Experimental assessment of the noise characteristics of propellers for commercial drones

Multi-copters or drones are engaged in a wide range of industrial applications for their flexibility, safety and low-cost. The noise emission is becoming an issue with the expanding applications, among which the propellers that drive the drones are the major sources of noise. In this work, the noise characteristics of small-scale propellers is experimentally investigated using the advanced rotor aerodynamics and aeroacoustics test platform in an anechoic chamber at the Hong Kong University of Science and Technology (HKUST). The study will focus on the representative off-the-shelf propellers. The rotor noise will be measured by a linear array with 20 microphones, and the aerodynamic forces will be acquired by using the high-accuracy load cells. The dependence of both the tonal and broadband noise radiation with the thrust and rotation speed at various conditions will be tested. The study will enhance our understanding of the noise features of the multi-rotor powered drones, and will provide us with a better understanding of the status of the drone noise impact on the environment.

Control and controllability of microswimmers by a shearing flow.

With the continuing rapid development of artificial microrobots and active particles, questions of microswimmer guidance and control are becoming ever more relevant and prevalent. In both the applications and theoretical study of such microscale swimmers, control is often mediated by an engineered property of the swimmer, such as in the case of magnetically propelled microrobots. In this work, we will consider a modality of control that is applicable in more generality, effecting guidance via modulation of a background fluid flow. Here, considering a model swimmer in a commonplace flow and simple geometry, we analyse and subsequently establish the efficacy of flow-mediated microswimmer positional control, later touching upon a question of optimal control. Moving beyond idealized notions of controllability and towards considerations of practical utility, we then evaluate the robustness of this control modality to sources of variation that may be present in applications, examining in particular the effects of measurement inaccuracy and rotational noise. This exploration gives rise to a number of cautionary observations, which, overall, demonstrate the need for the careful assessment of both policy and behavioural robustness when designing control schemes for use in practice.

The multi-functional rotor aerodynamic and aeroacoustic test platform at HKUST

This paper describes the multi-functional rotor noise and aerodynamics test platform at the Hong Kong University of Science and Technology (HKUST). To investigate the noise characteristics of propellers with aerodynamic flows, the test rig is installed in the 2.5×2 (m) low-speed and low-noise wind tunnel in the Aerodynamic and Acoustic Facility (AAF) at HKUST. The wind tunnel can facilitate flow from 0 to 40 m/s. The test rig is assembled in a turntable on the ceiling of the tunnel wall, which enables the testing range of pitch angle can vary from 0° (axial flow) to 90° (parallel flow), with an accuracy of 0.1°. The noise produced by the rotor is measured by a set of wall-mounted surface microphones. Semi-empirical calibration is conducted to quantify the noise reflection by the tunnel walls. A low-noise struct has been designed and manufactured to locate a set of far-field microphones equipped with nosecone, to improve the quality of acoustic measurement inside the flow. In addition, a synchronized system is developed to conduct the phase-locking Particle Image Velocimetry (PIV) measurement on the rotor, to study the flow pattern to better understand the noise generation mechanism.

Mismatched Models to Lower Bound the Capacity of Dual-Polarization Optical Fiber Channels

Regular perturbation is applied to the Manakov equation and motivates a generalized correlated phase-and-additive noise model for wavelength-division multiplexing over dual-polarization optical fiber channels. The model includes three hidden Gauss-Markov processes: phase noise, polarization rotation, and additive noise. Particle filtering is used to compute lower bounds on the capacity of multi-carrier communication with frequency-dependent powers and delays. A gain of 0.17 bits/s/Hz/pol in spectral efficiency or 0.8 dB in power efficiency is achieved with respect to existing models at their peak data rate. Frequency-dependent delays also increase the spectral efficiency of single-polarization channels.

Wind machine rotor acoustic prediction method

Wind machines are commonly used in vineyards in New Zealand during a radiation frost to draw warm air from above a crop and blow it over the crop to prevent frost damage. The primary source of noise produced by the wind machine has been identified as being aero-acoustic noise from the rotor. This paper describes a theoretical method for predicting both the broadband and tonal rotor noise. An empirical model that accounts for the effects of different blade tip geometries is derived from an experimental investigation and is combined with previously developed empirical methods to predict the broadband noise of the rotors. Several purely theoretical models are developed and used to predict the tone noise which is produced by rotating steady loading and thickness sources on the rotor blades as well as the interaction of the blades with the flow disturbance caused by the tower. The model is validated against measurements. As the method can calculate the noise level produced by a wind machine rotor very quickly, it is well suited for use in an optimization method where many different rotor designs are evaluated.

Improving Encryption Digital Watermark by Using Blue Monkey Algorithm

Watermarking enables the users to share the digital contents in public domain without any issue. In the present day, the tremendous development in the digital technologies and networks caused an increase in the threats of unauthorized copying, tampering of digital media and image theft. To face these threats, digital watermark technology can be applied. However, the current paper uses new technique with two main algorithms that are the following ones: improved honey algorithm that is used to encrypt the digital watermark and blue monkey meta-heuristic algorithm which is used to find the best location in the host image to hide the digital watermark. Furthermore, in order to check the security and the robustness of the proposed method against various common image processing attacks such as Gaussian noise, Rotation, Salt and pepper noise, Sharpen, Median filter, Average filter, compression and Cropping is computed, certain performance metrics such as Peak to Signal Noise Ratio (PSNR)and Mean Square Error (MSE) also are computed. Likewise, Normalized Correlation (NC) is used to check similarity between the original and extracted digital watermark. The results demonstrate that the proposed method is efficient, provides high security and robustness against most attacks compared to the pervious methods.

A watermarking scheme for source authentication, ownership identification, tamper detection and restoration for color medical images

This paper proposes a blind watermarking scheme that has ability of confidentiality, source authentication, ownership identification, integrity, tampering detection and restoration of the medical images. It embeds the hospital logo, electronic patient record and perceptual hash value of region of interest in the mid-frequency coefficients of discrete cosine transform of the region of non-interest of medical image. The region of interest of the image is restored against attacks including the desynchronization attacks and impulse noise attack, and the severity of tampering due to any attack is measured by comparing the difference between the original perceptual hash value of the region of interest and the extracted perceptual hash value of region of interest of watermarked image. Restoration is done that tampers the region of interest of the images. Number of medical image watermarking schemes that show resilient to many different types of singular and hybrid attacks is very few. The proposed method has inbuilt restoration schemes against different attacks such as rotation, scaling, translation, shearing, horizontal reflection, vertical reflection and impulse noise attacks. Comparison with the latest and state-of-art medical image watermarking schemes shows that the performance of the proposed method is superior to other methods.

Aerodynamic and aeroacoustic investigations of multi-copter rotors with leading edge serrations during forward flight

Inspired by the silent flight of owl wings, this paper adopts leading edge (LE) serrations for noise reduction of isolated multi-copter rotors. By choosing different serration parameters of height and wavelength, aerodynamic loading and noise emissions of these multi-copter rotors are measured and analyzed at the forward flight condition through a force balance and microphones in an anechoic wind tunnel. Rotors with add-on LE serrations generate slightly lower thrust than the Baseline (BL) rotor at the same rotation speed. In order to assess the noise reduction of different serration designs from the application perspective, a comparison of rotor noise emission is conducted at the same thrust. All serrated rotors considered in this study show lower tonal, broadband, and overall noise levels than the BL rotor. The noise reductions of all serrated rotors are observed to generally follow a scaling law with the Strouhal number in the mid-to-high frequency range. The height-to-wavelength ratio is a potential critical parameter that determines the coupled aerodynamic and aeroacoustic performance of the serrated rotors. The directivity of the rotor noise reduction is also shown through the data of a circular microphone array.