Dissertations / Theses on the topic 'Farfield'

The evaluation of how Computational Fluid Dynamics (CFD) package may be incorporated into a conceptual design method is performed. The repeatability of the CFD solution as well as the accuracy of the calculated aerodynamic coefficients and pressure distributions was also evaluated on two different wing-body models. The overall run times of three different mesh densities was also evaluated to investigate if the mesh density could be reduced enough so that the computational stage of the CFD cycle may become affordable to use in the conceptual design stage. A farfield method was derived and used in this analysis to calculate the lift and drag coefficients. The CFD solutions were also compared with two methods currently used in conceptual design - the vortex lattice based program Vorview and ACSYNT. The unstructured Euler based CFD package FELISA was used in this study.
Master of Science

Acoustical-based imaging systems have found merit in determining the behavior of vibrating structures. This thesis focuses on the continued development of the nearfield acoustic holography (NAH) approach. Conventional NAH consists of first measuring the pressure field on a two-dimensional conformal surface and then propagating this data back to the vibrating structure to obtain information about the source, such as the normal velocity distribution. Recent work has been done which incorporates particle velocity information into the traditional NAH measurements to reduce the number of measurements required. This advancement has made NAH a more affordable tool for acoustical imaging and sound field characterization. It is proposed that the inclusion of velocity information into traditional NAH can further increase its usefulness. By propagating the velocity and pressure values independently and recombining them on the reconstruction surface, the pressure field and energy density fields can be predicted at any point in the sound field. It is also proposed that the same NAH measurement can be used to predict farfield directivity. The inclusion of velocity values into the NAH technique also provides a means for predicting energy density (ED) anywhere within the acoustic field. These two developments would allow a single NAH measurement to provide much more information about an acoustic source and its radiated sound field. Experimental testing shows that NAH is successful at predicting the shape of the resulting ED field and directivity pattern with some error in amplitude. The best performance of the technique is with a planer source resulting in an average amplitude error of 18.5% over the entire field.

Measuring the farfield noise levels of full-scale rotor systems is not trivial and can be costly. Researchers prefer to perform small-scale experiments in the laboratory so that they can extrapolate the model scaled results to the larger scale. Typically Inverse Square Law (ISL) is used to extrapolate the sound pressure levels (SPL), obtained from model-scale experiments at relatively small distances to predict noise at much larger distances for larger scale systems. The assumption underlying this extrapolation is that the source itself can be treated as a point sound source. At what distance from a rotor system it can be treated as a point source has never been established. Likewise, many theoretical models of shielding by hard surfaces assume the source to be a point monopole source. If one is interested in shielding the noise of a rotor system by interposing a hard surface between the rotor and the observer, can the rotor system really be considered to be a monopole? If rotating noise sources are under consideration what is the effect of configuration and design parameters? Exploring the validity of point source assumption alluded to above for a rotor for farfield acoustic measurements with and without shielding form the backbone of the present work.

In many engineering applications, including radar, sonar, communications and seismology, the direction of impinging signal wavefronts can be used to discriminate between competing sources. Often these source signals cover a wide bandwidth and conventional narrowband beamforming techniques are ineffective, since spatial resolution varies significantly across the band. In this thesis we consider the problem of beamforming for broadband signals, primarily when the spatial response remains constant as a function of frequency. This is called a frequency invariant beamformer (FIB).¶ Rather than applying the numerical technique of multi-parameter optimisation to solve for the beamformer parameters, we attempt to address the fundamental nature of the FIB problem. The general philosophy is to use a theoretical continuous sensor to derive relationships between a desired FI beampattern and the required signal processing structure. Beamforming using an array of discrete sensors can then be formulated as an approximation problem. This approach reveals a natural structure to the FIB which is otherwise buried in a numerical optimisation procedure.¶ Measured results from a microphone array are presented to verify that the simple FIB structure can be successfully implemented. We then consider imposing broadband pattern nulls in the FI beampattern, and show that (i) it is possible to impose an exact null which is present over all frequencies, and (ii) it is possible to calculate a priori how many constraints are required to achieve a null of a given depth in a FIB. We also show that the FIB can be applied to the problem of broadband direction of arrival (DOA) estimation and provides computational advantages over other broadband DOA estimators.¶ Through the theoretical continuous sensor approach, we show that the FIB theory can be generalised to the problem of designing a general broadband beamformer (GBB) which realizes a broadband angle-versus-frequency beampattern specification. Coupled with a technique for radial beampattern transformation, the GBB can be applied to a wide class of problems covering both nearfield beamforming (in which the shape of the impinging wavefront must be considered and farfield beamforming (which is simplified by the assumption of planar wavefronts) for a broadband beampattern specified over both angle and frequency.

This thesis considers the design of a beamformer which can enhance desired signals in an environment consisting of broadband nearfield and/or farfield sources. The thesis contains: a formulation of a set of analysis tools which can provide insight into the intrinsic structure of array processing problems; a methodology for nearfield beamforming; theory and design of a general broadband beamformer; and a consideration of a coherent nearfield broadband adaptive beamforming problem. To a lesser extent, the source localization problem and background noise modeling are also treated. ¶: A set of analysis tools called modal analysis techniques which can be used to a solve wider class of array signal processing problems, is first formulated. The solution to the classical wave equation is studied in detail and exploited in order to develop these techniques. ¶: Three novel methods of designing a beamformer having a desired nearfield broadband beampattern are presented. The first method uses the modal analysis techniques to transform the desired nearfield beampattern to an equivalent farfield beampattern. A farfield beamformer is then designed for a transformed farfield beampattern which, if achieved, gives the desired nearfield pattern exactly. The second method establishes an asymptotic equivalence, up to complex conjugation, of two problems: (i) determining the nearfield performance of a farfield beampattern specification, and (ii) determining the equivalent farfield beampattern corresponding to a nearfield beampattern specification. Using this reciprocity relationship a computationally simple nearfield beamforming procedure is developed. The third method uses the modal analysis techniques to find a linear transformation between the array weights required to have the desired beampattern for farfield and nearfield, respectively. ¶: An efficient parameterization for the general broadband beamforming problem is introduced with a single parameter to focus the beamformer to a desired operating radius and another set of parameters to control the actual broadband beampattern shape. This parameterization is derived using the modal analysis techniques and the concept of the theoretical continuous aperture. ¶: A design of an adaptive beamformer to operate in a signal environment consisting of broadband nearfield sources, where some of interfering signals may be correlated with desired signal is also considered. Application of modal analysis techniques to noise modeling and broadband coherent source localization conclude the thesis.

A dispersão dos efluentes do emissário submarino de Santos foi estudada por meio de modelagem numérica, acoplando-se modelos hidrodinâmico, de dispersão no campo próximo e no campo distante. As simulações foram realizadas para o projeto original do emissário de 1979, para a nova configuração de 2009 e para terceiro arranjo sugerido por este estudo, que dobra o comprimento do duto em relação ao projeto original. As diluições foram modeladas em cenários ambientais com propriedades marinhas e estuarinas climatológicas de verão, juntamente com forçantes atmosféricas climatológicas da mesma estação, simulando ausência de vento, presença de vento médio e presença de vento médio seguido por passagem de frente fria climatológica (obtida por este estudo). Com as devidas validações do modelo hidrodinâmico, a diluição, tanto no campo próximo quanto no campo distante, foi fortemente regida pela estratificação da coluna dágua e pela velocidade das correntes. O cenário sem vento apresentou as maiores estratificações, menores velocidades e originou as menores diluições em oposição ao cenário que simulou a passagem de frente fria. Dentre os projetos de emissário analisados, aquele proposto por este estudo apresentou plumas com menores comprimentos horizontais e menores profundidades, atingindo a costa uma só vez. O projeto original e o novo que será implantado em 2009 apresentaram, comparativamente, plumas mais extensas e profundas, interceptando a costa em diversas situações.
Sewage plume dispersion for the Santos outfall was studied using numerical models, coupling hydrodynamic, near field and far field models. Experiments were conducted for the 1979 original outfall design, for the new 2009 outfall configuration and also for a third arrangement suggested here, which duplicates the emissary length comparatively to the original 1979 project. Dilutions were modeled in environmental sceneries with summer climatological estuarine and marine properties, together with climatological atmospheric forcing for the same season, simulating no wind conditions, mean wind forcing and mean wind followed by a climatological cold front (developed at this study) passage. After the appropriate hydrodynamic numerical model validation, the dilution, at the near field as well as at the far field, was strongly determined by the vertical water stratification and by the current velocities. The no-wind scenery presented the highest stratifications, smallest velocities and originated the smallest dilutions, in opposition to the cold-front-scenery. Among the sewage outfall projects considered, the one presented at this study showed the shallowest plumes, with the smallest horizontal lengths, reaching the coast only once. The original 1979 project and the 2009 configuration showed, comparatively, the deepest and widest plumes reaching the coast in several situations.

碩士
國立交通大學
光電(科學)研究所
83
In this thesis, we discuss the resonance cavity modes of microdisk lasers and the corresponding farfield emisssion patterns. Besides conventional circular microdisk lasers, we introduce the point matching method to solve the cavity modes of noncircular microdisks. And we further discuss how the changes in disk shape influences the farfield emission pattern of microdisk lasers.

碩士
國立清華大學
動力機械工程學系
105
A farfield random array is implemented for sound source identification. Microphone positions are optimized, with the aid of the simulated annealing (SA) method as a supervised Monte Carlo approach, random samples of sensor position are drawn from Gaussian distribution to minimize the sidelobe maximum of the farfield beam-pattern. A two-stage localization and separation algorithm is devised on the basis of the equivalent source model (ESM). In the localization stage, the active source regions are located by using the delay-and-sum (DAS) method, followed by a parametric array localization procedure that is capable of locating sources with improved resolution. In the separation stage, source amplitude extraction is achieved by formulating an inverse problem based on the steering matrix relating the sound pressures received by the microphones and the source amplitudes. The number of sources is selected to be less than the number of microphones to render an overdetermined problem which can be solved by using the Tikhonov regularization (TIKR). Alternatively, the separation problem can be augmented into an underdetermined problem which can be solved using the compressive sensing (CS) technique. Furthermore, the acoustic variables including sound pressure, particle velocity, sound intensity, and sound power can be estimated based on ESM. Numerical and experimental results are presented to validate the proposed technique.

博士
國立清華大學
動力機械工程學系
102
Farfield and nearfield microphone arrays are proposed for noise source identification (NSI) and sound field visualization (SFV). Farfield acoustic imaging algorithms including the delay and sum (DAS) algorithm, the minimum variance distortionless response (MVDR) algorithm and the multiple signal classification (MUSIC) algorithm are employed to estimate direction of arrival (DOA). Results show that the MUSIC algorithm can attain the highest resolution of localizing sound sources positions. In the nearefield array signal processing, one formulation derived from discretizing the simple layer potential is termed the indirect equivalent source model (ESM)-based nearfield acoustical holography (NAH), while another formulation derived from discretizing the Kirchhoff-Helmholtz integral equation is termed the direct ESM-based NAH. In the use of ESM NAH, the choice of parameters including retract distance and average area of element is of vital importance. These parameters are optimized, with the aid of the golden section search and parabolic interpolation (GSS-PI) algorithm and the simulated annealing (SA) algorithm, for the direct and indirect ESM formulations. Instead of directly solving the inverse problem, the forward problem is solved in a recursive manner akin to the approach adopted by using recursive Wiener filtering. The approaches proposed are based on a state-space formulation employing the Kalman filter-based state observer and particle filter-based state estimator. The state observer and estimator are adaptive in nature and capable of tracking dynamic variation of sound field, even in the presence of noises and perturbations. Optimum weighting coefficients and inverse filters for microphone arrays can be accomplished, with the aid of a systematic methodology of mathematical programming. Both farfield and nearfield array problems are formulated in terms of compressive sampling (CS) and convex optimization (CVX) formalisms. CVX is applied to beamformer design, pressure field reconstruction, source separation and modal analysis with satisfactory performance in both nearfield and farfield microphone arrays. Design of optimal beamformers that withstand system errors such as channel mismatch, sensor position error, and pointing error has been a key issue in real-world applications of arrays. This thesis also examines the effects of system errors on beamformer performance from a statistical perspective. In practical applications where only patch array with scarce sensors are available, the ESM-based interpolation (ESM-IP), under-determined ESM (UD-ESM), direct basis function model (D-BFM) and BFM-based interpolation (BFM-IP) are proposed to reconstruct source velocity with sound filed interpolation. These methods were compared with the direct ESM (D-ESM) method. In the BFM-based NAH, basis functions including planar and spherical wave functions are used. CS is exploited in the BFM-IP and the D-BFM in light of CVX method. It is desirable to enhance the image resolution based on a sparse array configuration. As indicated by the simulation and experiment results, the proposed technique proved effective in identifying sources of many practical examples, including wooden box experiment, noncontact modal analysis of plate and machine tools.

碩士
國立臺灣科技大學
電機工程系
100
In this thesis, we proposed two topics related to the antenna measurement systems. The first part focuses on performance improvement of an antenna matching testing system developed in previous work. The system can evaluate impedance matching conditions of one port devices. In the improved system, a high-precision data acquisition and control card is used. LabView programs are also developed to realizer system automation. Measurement speed, stability and precision are enhanced. The directional coupler, which is more compact and economical, is used to replace the circulator, while maintain similar system performance. In the second part, we proposed a planar scanning farfield antenna pattern measurement range. As wireless communication techniques progress, we can expect that new wireless standards are more likely to use the millimeter wave band. Antenna pattern measurement ranges that fit the millimeter antenna evaluation needs are required. In order to cope with the large and heavy mixer used above 60 GHz, planar 2-dimentional linear scanner can be used to perform the transmission measurement. Linearly scanned results can be transformed to fields on a spherical surface. By calibrating the effects of the sampling probe, the desired 3-dimentional pattern of the antenna-under-test can be derived. This system is suitable for directional antennas in the millimeter wave band and is easy to control.

Collision of the Yakutat Terrane with North America in southern Alaska has driven growth of the Chugach-St. Elias orogen. Glaciation of the St. Elias Range has periodically increased since the Miocene, but began dominating erosion and spurred enhanced exhumation since the mid-Pleistocene transition at ~1 Ma. Ice associated with this glacial intensification carved cross-shelf sea valleys that connect the St. Elias Range to the deep-sea Surveyor Fan. A newly increased terrigenous sediment flux into the fan triggered the formation and growth of the Surveyor Channel. The change in geomorphology observed throughout Fan sequences allows us to characterize the influence that a glaciated orogen can have in shaping margin processes and the sediment pathways from source to sink. Seismic data also reveal an isolated, large, short runout, mass-transport deposit (MTD) buried in the Surveyor Fan. The MTD geometry, size and location on a convergent margin lend support to recent studies suggesting seismic strengthening and infrequent sediment failure on active margins. This study provides insight into the magnitude and scope of events required to cause submarine mega-slides and overcome higher than normal sediment shear strength, including the influence of climate and sea level change. Beneath the Surveyor Fan, integrated geophysical data reveals massive intraplate shearing, and a lack of oceanic crust magnetic lineaments in regions of Pacific Plate crust. We argue that stress from the Yakutat-North America collision transferred outboard to the Pacific Plate is the major driver for the deformation causing these features. This stress would have resulted in significant strain in the NE corner of the Pacific Plate, creating pathways for sill formation in the crust and Surveyor Fan. The collision further intensified as the thickest Yakutat portion began to subduct during the Pleistocene, possibly providing the impetus for the creation of the Gulf of Alaska Shear Zone, a >200 km zone of shear extending out into the Pacific Plate. This study highlights the importance of farfield stress from complex tectonic regimes in consideration of large-scale oceanic intraplate deformation.
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In many engineering applications, including radar, sonar, communications and seismology, the direction of impinging signal wavefronts can be used to discriminate between competing sources. Often these source signals cover a wide bandwidth and conventional narrowband beamforming techniques are ineffective, since spatial resolution varies significantly across the band. In this thesis we consider the problem of beamforming for broadband signals, primarily when the spatial response remains constant as a function of frequency. This is called a frequency invariant beamformer (FIB). ¶ ...

This thesis considers the design of a beamformer which can enhance desired signals in an environment consisting of broadband nearfield and/or farfield sources. The thesis contains: a formulation of a set of analysis tools which can provide insight into the intrinsic structure of array processing problems; a methodology for nearfield beamforming; theory and design of a general broadband beamformer; and a consideration of a coherent nearfield broadband adaptive beamforming problem. To a lesser extent, the source localization problem and background noise modeling are also treated. A set of analysis tools called modal analysis techniques which can be used to a solve wider class of array signal processing problems, is first formulated. The solution to the classical wave equation is studied in detail and exploited in order to develop these techniques. Three novel methods of designing a beamformer having a desired nearfield broadband beampattern are presented.