New decoupling zmethod for spiral phase array HTS coil

published_or_final_versio

Sound Radiated by a Supercritical Airfoil Operating in the Incompressible Regime

Measurements of far-field sound radiated by two and three-dimensional supercritical airfoils (ONERA OAT 15A) placed in a low Mach number flow were performed in an anechoic open-jet facility. The chord-based Reynolds numbers were between 156,000 and 468,000, while the Mach numbers ranged between 0.04 and 0.13. For the three-dimensional airfoil, two different aspect ratios (span-to-chord ratio) of 1.0 and 1.5 were considered. For comparison, the sound radiated by a symmetric NACA 0012 and cambered NACA 2412 airfoil was also measured under the same conditions. The noise from the two-dimensional airfoil was found to scale on the fifth power of Mach number. The noise generated by the cross-flow across the tip was the dominant noise source for the three-dimensional airfoils, particularly under high-lift conditions where it exceeds the noise from the mid-span portion across the considered frequency range. The tip noise spectra for the supercritical airfoils exhibit a prominent peak that scales with the free-stream velocity, but its frequency is a weak function of the lift on the airfoil and the aspect ratio. No such peak was observed for the NACA profiles even for higher lift conditions. The beamformed source maps for NACA profiles reveal an intense high-frequency noise source near the tip leading-edge which is much weaker for the supercritical airfoil due to differences in the curvature of the profiles. The tip noise spectra for the supercritical airfoil can be scaled on the fourth power of Mach number and the length-scale associated with the spectral peak. The tip noise peak magnitude and frequencies were found to be nearly independent of the airfoil aspect ratio; however, a reduction in AR was found to shift the tip noise source region further inboard. Stereo particle image velocimetry (PIV) measurements performed in a cross-plane behind the three-dimensional airfoils show that this is because a reduction in AR also shifts vortex core towards the mid-span. The PIV results were also used to quantify the meander of the tip vortex and it is shown that the amplitude of vortex meandering is independent of the angle of attack and the aspect ratio with a value between 0.5 – 0.6% of the chord-length for all cases considered

Ultrasonic phased array with dispersion compensation for monitoring multiple damages in structures

Multiple-damage-inflicted scattering signals usually overlap with each other due to Lamb wave dispersion and multi-mode characteristics. As a result, it is difficult to accurately distinguish damages that occur relatively close to each other using the conventional ultrasonic phased array method. In order to solve this problem, an improved linear mapping (ILM) dispersion compensation method is proposed and is applied to enhance the ultrasonic phased array monitoring resolution. Through a uniform linear array arrangement, the damage scattering signals are collected in a round-robin pattern of ultrasonic phased array, and then compensated based on the linear relation wavenumber curve from actual measurement. At last, the scan can be obtained by monitoring the energy scattered by the damages using delay-and-sum method. To verify the proposed method, experiments are performed on an aluminum (LY-12) plate. Two results of multiple artificial damages show that the proposed method can effectively compensate the dispersion characteristics of Lamb waves. The direction estimation error and distance estimation error are less than 4° and 2 cm, respectively

Detection of multiple defects based on structural health monitoring of pipeline using guided waves technique

Monitoring and inspecting the health condition and state of the pipelines are significant processes for an early detection of any leaking or damages for avoiding disasters. Although most Non Destructive Test (NDT) techniques are able to detect and locate damage during the maintenance intervals, interrupted services could result in high cost and lots of time consumed. In addition, most NDTs are utilized to detect and locate single damage such as axial crack, circular crack, or vertical crack only. Unfortunately, these NDTs are unable to detect or localize multi-type of damages, simultaneously. In this research, the proposed method utilizes the Structural Health Monitoring (SHM) based on guided wave techniques for monitoring steel pipeline continuously in detecting and locating multi-damages. These multi damages include the circumference, hole and slopping cracks. A physical experimental works as well as numerical simulation using ANSYS were conducted to achieve the research objectives. The experimental work was performed to validate the numerical simulation. An artificial neural network was used to classify the damages into ten classes for each type of damage including circumference, hole and sloping cracks. The obtained results showed that the numerical simulation was in agreement with the experimental work with relative error of less than 1.5%. In addition, the neural network demonstrated a feasible method for classifying the damages into classes with the accuracy ranged from 75% to 82%. These results are important to provide substantial information for active condition monitoring activities

Wide bandwidth focal plane array receiver for radio astronomy

Reflective antennas equipped with phase array feeds (PAFs) have been proposed as part of the Square Kilometre Array reference design, since they offer a wide Field of View (FoV) and large collecting area. To achieve a contiguous FoV, and to cancel spill-over radiation, the Focal Plane Array (FPA) must sample the focal field of the reflector at least every half-wavelength at the upper operating frequency. Low-noise operation over a wide bandwidth requires appropriate impedance matching to the low-noise amplifiers, and this is a challenging research problem since the input impedance of the FPA elements can vary strongly with frequency.Advanced broadband design techniques for antenna arrays have resulted in antenna shapes getting more complex. Modelling of these antennas can only be carried out using numerical computational electromagnetic methods (CEM), and accurate modelling of complex antennas demand the full-wave analysis with fields and currents that vary in three dimensional space. The Frequency Domain Integral Equation model is adopted in this study and used via the Method of Moments (MoM) technique for simulation and modelling of the FPA. The "MoM Antenna Development Toolbox" (MoMADT), 64 bit version of the modelling software, is specifically developed in this study for designing, building and modelling of complex antenna and electromagnetic structures. MoMADT utilizes surface and volume integral equations and provides functions for generating precise meshes and accurate method of moments solutions. MoMADT enables structures to be assembled in an array formation to consist of conductive or dielectric materials, or a combination of both.Study of the wide bandwidth FPA receiver was achieved through analysis of broadband planar antenna structures. This research investigates a unique design solution for a FPA utilizing the diamond planar strip antenna elements arranged to provide three vectors of polarization (triple-polarized FPA). The most promising FPA identified is the 77 Hexagonal Diamond Tripole (HDT) array. This array yields an operating frequency range of 550 to 2100 MHz, providing bandwidth ratio of 3.8:1. It is shown that adequate impedance match can be achieved across the indicated frequency range with desired directivity and gain. In addition, the 77 HDT array offers optimized efficiency and allows the polarization to be distinguished at any angle about the axis normal to the antenna plane to within a theoretical uncertainty of ± 2.2°. This is also true for any function of the FoV allowed by the surface area of the FPA

Classification of Low Velocity Impactors Using Spiral Sensing of Acousto-Ultrasonic Waves

The non-linear elastodynamics of a flat plate subjected to low velocity foreign body impacts is studied, resembling the space debris impacts on the space structure. The work is based on a central hypothesis that in addition to identifying the impact locations, the material properties of the foreign objects can also be classified using acousto-ultrasonic signals (AUS). Simultaneous localization of impact point and classification of impact object is quite challenging using existing state-of-the-art structural health monitoring (SHM) approaches. Available techniques seek to report the exact location of impact on the structure, however, the reported information is likely to have errors from nonlinearity and variability in the AUS signals due to materials, geometry, boundary conditions, wave dispersion, environmental conditions, sensor and hardware calibration etc. It is found that the frequency and speed of the guided wave generated in the plate can be quantized based on the impactor\u27s relationship with the plate (i.e. the wave speed and the impactor\u27s mechanical properties are coupled). In this work, in order to characterize the impact location and mechanical properties of imapctors, nonlinear transient phenomenon is empirically studied to decouple the understanding using the dominant frequency band (DFB) and Lag Index (LI) of the acousto-ultrasonic signals. Next the understanding was correlated with the elastic modulus of the impactor to predict transmitted force histories. The proposed method presented in this thesis is especially applicable for SHM where sensors cannot be widely or randomly distributed. Thus a strategic organization and localization of the sensors is achieved by implementing the geometric configuration of Theodorous Spiral Sensor Cluster (TSSC). The performance of TSSC in characterizing the impactor types are compared with other conventional sensor clusters (e.g. square, circular, random etc.) and it is shown that the TSSC is advantageous over conventional localized sensor clusters. It was found that the TSSC provides unbiased sensor voting that boosts sensitivity towards classification of impact events. To prove the concept, a coupled field (multiphysics) finite element model (CFFEM) is developed and a series of experiments were performed. The dominant frequency band (DBF) along with a Lag Index (LI) feature extraction technique was found to be suitable for classifying the impactors. Results show that TSSC with DBF features increase the sensitivity of impactor\u27s elastic modulus, if the covariance of the AUS from the TSSC and other conventional sensor clusters are compared. It is observe that for the impact velocity, geometric and mechanical properties studied herein, longitudinal and flexural waves are excited, and there are quantifiable differences in the Lamb wave signatures excited for different impactor materials. It is found that such differences are distinguishable only by the proposed TSSC, but not by other state-of-the-art sensor configurations used in SHM. This study will be useful for modeling an inverse problem needed for classifying impactor materials and the subsequent reconstruction of force histories via neural network or artificial intelligence. Finally an alternative novel approach is proposed to describe the Probability Map of Impact (PMOI) over the entire structure. PMOI could serve as a read-out tool for simultaneously identifying the impact location and the type of the impactor that has impacted the structure. PMOI is intended to provide high risk areas of the space structures where the incipient damage could exist (e.g. area with PMOI \u3e 95%) after an impact

Análise de modelos de previsão do escoamento e do ruído acústico de jatos subsônicos gerados por bocais serrilhados

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Mecânica.O presente trabalho analisa métodos de previsão do escoamento e do ruído acústico de jatos subsônicos (Re=1,38x106 e M=0,9). Além disto, investigam-se também os efeitos de bocais serrilhados no desenvolvimento do jato e como mecanismo passivo de controle de ruído. A formulação híbrida de simulação adotada consiste em se resolver primeiramente o campo do escoamento turbulento e, posteriormente, avaliar o ruído acústico no campo distante. Dois modelos de turbulência baseados no conceito de média de Reynolds foram testados para a solução do escoamento: o modelo k-e cúbico e o modelo de transporte do tensor de Reynolds. Além disto, previsões do ruído foram realizadas com o emprego da analogia acústica de Lighthill e da analogia acústica de Ffowcs-Williams e Hawkings. Comparações entre resultados numéricos e experimentais de perfis de velocidade e tensões de Reynolds mostraram que os modelos de turbulência supracitados são capazes de prever o efeito do bocal serrilhado sobre o escoamento de forma satisfatória. Por outro lado, os métodos de previsão de ruído acústico implementados no código comercial CFD++/CAA++ se mostraram inadequados em todas as situações de escoamento investigadas.The present study considers an assessment of numerical methods to predict the fluid flow and the acoustic noise of subsonic jets (M = 0.9 and Re=1.38x106). Additionally, the roles of chevron nozzles on the flow development and as a passive noise control device are also investigated. The adopted hybrid simulation approach initially solves the turbulent field flow, which is then used as input data for estimates of far-field noise. Two turbulence models within the frame of the Reynolds averaging concept were tested for the field flow solution: the cubic k-e model and the Reynolds stress transport model. Moreover, predictions of acoustic noise were carried out through the Lighthill analogy and the Ffowcs-Williams and Hawkings analogy. Comparisons between numerical and experimental results for velocity and Reynolds stresses showed that the aforementioned turbulence models are capable of satisfactorily predict the effect of chevron nozzles on the field flow. On the other hand, the methods based on the two acoustic analogies implemented in the commercial software CFD++/CAA++ were seen to be inadequate for estimates of noise in all flow conditions

Compressive Sensing and Imaging of Guided Ultrasonic Wavefields

Structural health monitoring (SHM) and Nondestructive Evaluation (NDE) technologies can be used to predict the structural remaining useful life through appropriate diagnosis and prognosis methodologies. The main goal is the detection and characterization of defects that may compromise the integrity and the operability of a structure. The use of Lamb waves, which are ultrasonic guided waves (GW), have shown potential for detecting damage in specimens as a part of SHM or NDT systems. These methods can play a significant role in monitoring and tracking the integrity of structures by estimating the presence, location, severity, and type of damage. One of the advantages of GW is their capacity to propagate over large areas with excellent sensitivity to a variety of damage types while guaranteeing a short wavelength, such that the detectability of large structural damages is guaranteed. The Guided ultrasonic wavefield imaging (GWI) is an advanced technique for Damage localization and identification on a structure. GWI is generally referred to as the analysis of a series of images representing the time evolution of propagating waves and, possibly, their interaction with defects. This technique can provide useful insights into the structural conditions. Nowadays, high-resolution wavefield imaging has been widely studied and applied in damage identification. However, full wavefield imaging techniques have some limitations, including slow data acquisition and lack of accuracy. The objectives of this dissertation are to develop novel and high resolution Guided Wavefield Imaging techniques able to detect defects in metals and composite materials while reducing the acquisition time without losing in detection accuracy

Duct acoustics for air-coupled ultrasonic phased arrays

Air-coupled ultrasound is used in many applications such as range inding, tactile feedback, flow metering or non-destructive testing. The transducers directivity is a crucial acoustic property for all these applications. For instance, a narrow beam width allows for higher angular resolutions, whereas a wider beam width allows for emitting the sound wave in a bigger area for obstacle detection. However, the transducers dimensions influence its directivity and its resonance frequency. In order to decouple the acoustic aperture from transducer, acoustic waveguides are investigated in this work. This way, grating lobe free phased arrays can be built for unambiguous beamforming. In this thesis, the wave propagation inside these waveguides, including coupling mechanisms from the transducer till the free-field, are investigated. First, the state of the art of duct acoustics applications in the audible range and the ultrasound range are presented. Afterwards, different duct acoustics models are derived and compared. Each model is validated for 40 kHz, a duct length of 80 mm and an aperture between 10 mm and 3.4 mm. The challenge of the simulations is to take higher modes into account while reducing the calculation times. Therefore, analytical and numerical models were investigated. As a result, the boundary element method is the most efficient approach for the given geometry wavelength ratio using the commercial software COMSOL Multiphysics. With this method, free-field calculations on a single Xeon E5-2660 v3 CPU and 256 GB RAM without the need of a cluster are possible. The model is validated with calibrated measurements in an anechoic chamber. Therefore, an automated measurement system is established where a calibrated measurement microphone moves relative to the transducer, thus characterizing the sound field in front of the transducer. This setup can measure a hemisphere with a radius of up to 6 m and has a dynamic range of 111 dB. After the validation of the numerical model, waveguide geometry optimizations were conducted. The analyzed properties were: the influence of a perpendicular output and input surface on the wave propagation inside the waveguide; the size of the output aperture; length variations of the waveguide including temperature dependence; the position of tapering and types of losses due to the waveguide. As a result, the perpendicular input is crucial for fundamental mode propagation, otherwise higher modes occur, because the input diameter is bigger compared to the wavelength. The size of the output surface can be increased for line arrays with an SPL gain of +10 dB. However, the limit of the aperture size is 3.7 × λ, otherwise higher modes occur at the output which lead to defocusing of the main lobe. The length of the waveguide can increase the SPL. However, the industrial temperature demands of −25◦C to 75◦C have the same influence on the SPL as the length optimization (±4.8 dB), and, thus, are not investigated in more detail. The positioning of the tapering has just a minor influence of ±0.4 dB. The losses of the waveguide are −10 dB with diffraction loss as the dominant part. The losses inside the waveguide (reflection and thermoviscous losses) could not be validated with measurements due to the narrow bandwidth of the transducers, since the incident and reflected wave superposed. The derived results of the geometry optimization were used to build four line arrays. First, a waveguide with equal length ducts was built as a reference. Second, a Bézier waveguide with plane input surfaces for the transducers was designed. Third, the output aperture was changed from round outputs to rectangular shapes to increase the SPL and sensitivity. Last, a shortened version of the Bézier waveguide was built which has a reduced length of 65%. All four waveguides were simulated using the boundary element method and validated with the measurement etup. As a result, in both simulation and measurement the shorten waveguide has an increased SPL of +5 dB compared to the reference waveguide with equal length ducts. Thus, it is possible to build compact waveguides for air-coupled phased arrays. Next, the influence of different duct lengths in an acoustic waveguide is analyzed in more detail. Using ducts of different length offers more design freedom for the entire waveguide for compact design, easier assembly and reduced assembly time. However, different lengths must be compensated with additional time delays. Therefore, two waveguides were compared. First, an equal length waveguide was used. Second, a waveguide with Bézier-shaped ducts was used. The time delays, due to varying duct lengths, were measured and simulated with analytic and numerical methods. Afterwards, the directivity patterns of both waveguides were compared. As a result, the time compensation has no significant impact on the beam profile regarding side lobe level and half power beam width. In addition, SPL deviation of the waveguides are within the manufacturing tolerances of the transducers. The last aspect investigated in this thesis is the water resistance of the waveguide. Since it is designed for air-coupled ultrasound, it can be clogged due to dirt, dust or liquid. Two commonly known solutions for this issue is the use of hydrophobic fabrics or thin films. Therefore, both solutions were compared. First, these two approaches showed no significant impact on the beamforming capabilities of the phased array. In addition, the IP class of the fabric reached IPX7 and the thin film achieved even IPX8. Furthermore, the fabric has a minor insertion loss of just −1.8 dB. In contrast, the film reduces the SPL by −7.5 dB. This loss can be further reduced with special effort to +0.4 dB by changing the waveguide geometry and tuning the system to the correct resonance frequency. However, this shows that the film has a high temperature dependence compared to the fabric. In conclusion, acoustic waveguides enhance the acoustic properties of ultrasonic sensors. The directivity can be decoupled from the transducer and customized for a certain application