634 research outputs found
Experimental study of the impedance of a short dipole in a plasma for parallel and perpendicular orientation with respect to the dc magnetic field
Impedance measurements on small dipole immersed in anisotropic plasma with external electric fiel
On the output of acoustical sources
Contents: (1) a theoretical basis for local power calculation; (2) source radiation in the presence of a half-plane; (3) radiation from a line source near an edge at which a Kutta condition holds; (4) radiation by a point source above a plane independence boundary; and (5) power output of a point source in a uniform flow
Systematic design of antennas using the theory of characteristic modes
El principal objetivo de esta tesis es demostrar que la TeorÃa de los Modos CaracterÃsticos puede ser empleada de forma sistemática para diseñar antenas de hilo y antena planas. La gran ventaja de los modos caracterÃsticos, frente a otros métodos de diseño, es la clara visión fÃsica que proporcionan de los fenómenos que contribuyen a la radiación de la antena.
A través de numerosos ejemplos se demostrará como los modos caracterÃsticos permiten comprender mejor el funcionamiento de una antena, de forma que el diseño de la misma se puede realizar de forma justificada y coherente. También se mostrará como la información proporcionada por los modos caracterÃsticos puede ser aprovechada para seleccionar la forma más apropiada para el elemento radiante, al igual que para elegir una configuración de alimentación óptima que maximice el ancho de banda de impedancia.
La TeorÃa de los Modos CaracterÃsticos fue inicialmente formulada por Garbacz en 1968, y posteriormente refinada por Harrington y Mautz en 1971. Tradicionalmente, los modos caracterÃsticos han sido empleados para sintetizar formas de antena, y para controlar la difracción de objetos mediante carga reactiva. Sin embargo, en la actualidad, la TeorÃa de los Modos CaracterÃsticos ha caÃdo prácticamente en el olvido, a pesar de que permite obtener una solución modal para la corriente, que es de gran utilidad a la hora de analizar problemas de análisis, sÃntesis y optimización de antenas y difractores.
La TeorÃa de los Modos CaracterÃsticos parte de la definición de un problema de autovalores que involucra la matriz de impedancia generalizada de la estructura, y que tras ser resuelto proporciona un conjunto de modos de corriente reales, que son los denominados modos caracterÃsticos. Estos modos se corresponden con las resonancias naturales de la estructura y pueden ser obtenidos numéricamente para cuerpos conductores de forma arbitraria.
Por otra parte, los modos caracterÃsticos forman un conjunto de funciones cerCabedo Fabrés, M. (2007). Systematic design of antennas using the theory of characteristic modes [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1883Palanci
What can we learn about GW Physics with an elastic spherical antenna?
A general formalism is set up to analyse the response of an arbitrary solid
elastic body to an arbitrary metric Gravitational Wave perturbation, which
fully displays the details of the interaction antenna-wave. The formalism is
applied to the spherical detector, whose sensitivity parameters are thereby
scrutinised. A multimode transfer function is defined to study the amplitude
sensitivity, and absorption cross sections are calculated for a general metric
theory of GW physics. Their scaling properties are shown to be independent of
the underlying theory, with interesting consequences for future detector
design. The GW incidence direction deconvolution problem is also discussed,
always within the context of a general metric theory of the gravitational
field.Comment: 21 pages, 7 figures, REVTeX, enhanced Appendix B with numerical
values and mathematical detail. See also gr-qc/000605
Quantifying Volume Changing Perturbations in a Wave Chaotic System
A sensor was developed to quantitatively measure perturbations which change
the volume of a wave chaotic cavity while leaving its shape intact. The sensors
work in the time domain by using either scattering fidelity of the transmitted
signals or time reversal mirrors. The sensors were tested experimentally by
inducing volume changing perturbations to a one cubic meter mixed chaotic and
regular billiard system. Perturbations which caused a volume change that is as
small as 54 parts in a million were quantitatively measured. These results were
obtained by using electromagnetic waves with a wavelength of about 5cm,
therefore, the sensor is sensitive to extreme sub-wavelength changes of the
boundaries of a cavity. The experimental results were compared with Finite
Difference Time Domain (FDTD) simulation results, and good agreement was found.
Furthermore, the sensor was tested using a frequency domain approach on a
numerical model of the star graph, which is a representative wave chaotic
system. These results open up interesting applications such as: monitoring the
spatial uniformity of the temperature of a homogeneous cavity during heating up
/ cooling down procedures, verifying the uniform displacement of a fluid inside
a wave chaotic cavity by another fluid, etc.Comment: 13 pages, 13 figure
Reliable Fast Frequency Sweep for Microwave Devices via the Reduced Basis Method
International audienceIn this paper, a reduced basis approximation-based model order reduction for fast and reliable sweep in time-harmonic Maxwell's equations is detailed. Contrary to what one may expect by observing the frequency response of different microwave circuits, the electromagnetic field within these devices does not drastically vary as frequency changes in a band of interest. Thus, instead of using computationally inefficient, large dimension, numerical approximations such as finite element or boundary element methods for each frequency in the band, the point in here is to approximate the dynamics of the electromagnetic field itself as frequency changes. A much lower dimension, reduced basis approximation sorts this problem out. Not only rapid frequency evaluation of the reduced order model is carried out within this approach, but also special emphasis is placed on a fast determination of the error mesure for each frequency in the band of interest. This certifies the accurate response of the reduced order model. The same scheme allows us, in a offline stage, to adaptively select the basis functions in the reduced basis approximation and automatically select the model order reduction process whenever a preestablished accuracy is required throughout the band of interest. Finally, real-life applications will illustrate the capabilities of this approach
Energy loss mechanism for suspended micro- and nanoresonators due to the Casimir force
A so far not considered energy loss mechanism in suspended micro- and
nanoresonators due to noncontact acoustical energy loss is investigated
theoretically. The mechanism consists on the conversion of the mechanical
energy from the vibratory motion of the resonator into acoustic waves on large
nearby structures, such as the substrate, due to the coupling between the
resonator and those structures resulting from the Casimir force acting over the
separation gaps. Analytical expressions for the resulting quality factor Q for
cantilever and bridge micro- and nanoresonators in close proximity to an
underlying substrate are derived and the relevance of the mechanism is
investigated, demonstrating its importance when nanometric gaps are involved
Propagation Of Sound In The Vicinity Of Rigid Porous Interfaces
Propagation of sound in the vicinity of rigid porous interfaces is investigated systematically to facilitate the acoustical characterization of sound absorption materials for noise reduction applications. Various rigid porous interfaces are considered: (1) a semi-infinite porous layer; (2) a porous hard-backed surface; and (3) a porous impedance-backed layer. A closed-form solution and numerical methods are derived with respect to each rigid porous interface condition.
A modified saddle-point method is exploited to investigate the sound field emanating from a monopole source above and below a rigid porous interface. The solutions can be expressed in a form that resembles the classical Weyl-Van der Pol formula. A heuristic method is then proposed to remove the singularity within the asymptotic solution via application of the double saddle-point method. Its relative simplicity and accuracy demonstrates the advantage of the double saddle-point method whenever the approximation is valid. Following this, the sound field within a hard-backed rigid porous medium due to an airborne source is examined. The accuracy of the proposed asymptotic solutions has been confirmed by comparison with benchmark numerical solutions and through indoor sound propagation experiments. Measurement data and theoretical predictions suggest that when the receiver is positioned near the top surface of the hard-backed layer, the ground reflection of the refracted wave contributes greatly to the total sound field.
Taking into account source characteristics, an asymptotic formula is derived for predicting the sound field from a dipole source above and below an extended reaction ground. The directional effect of the dipole source on each term within the asymptotic solutions is interpreted. Further analysis shows that an accurate asymptotic solution can provide a good starter field for the Parabolic Equation--Finite Element Method (PE/FEM). The PE/FEM marching schemes are derived based on linear and cubic finite element discretization along both the vertical and horizontal directions. The Perfectly Matched Layer (PML) technique is applied to the PE/FEM, resulting in a substantial reduction in computational time. Comparison with experimental data for snow covered grounds is made and good agreement was demonstrated, which validates the accuracy of the proposed PE/FEM approach
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