Monopole Radiation Research Articles

On the computation of sound in free and wall-bounded domains

A new Dispersion-Relation-Preserving (DRP) scheme has been developed using the Lax–Wendroff methodology. Two collocated grids are placed in a staggered formation and a staggered DRP scheme is used to calculate the spatial differentiation of the propagation and convection terms. A staggered filtering scheme of a six points stencil is developed to complete the transformation from one grid to another. Existing DRP Runge–Kutta schemes are used for the time marching. Stability limits and accuracy issues are investigated using a simple 1D advection equation. The new method is then tested for monopole and quadrupole radiation, diffraction effects of an aperture in a wall, and convection effects of shear flow. All demonstrate the good accuracy and numerical stability of the new method.

Active local control of propeller-aircraft run-up noise.

Engine run-ups are part of the regular maintenance schedule at Vancouver International Airport. The noise generated by the run-ups propagates into neighboring communities, disturbing the residents. Active noise control is a potentially cost-effective alternative to passive methods, such as enclosures. Propeller aircraft generate low-frequency tonal noise that is highly compatible with active control. This paper presents a preliminary investigation of the feasibility and effectiveness of controlling run-up noise from propeller aircraft using local active control. Computer simulations for different configurations of multi-channel active-noise-control systems, aimed at reducing run-up noise in adjacent residential areas using a local-control strategy, were performed. These were based on an optimal configuration of a single-channel control system studied previously. The variations of the attenuation and amplification zones with the number of control channels, and with source/control-system geometry, were studied. Here, the aircraft was modeled using one or two sources, with monopole or multipole radiation patterns. Both free-field and half-space conditions were considered: for the configurations studied, results were similar in the two cases. In both cases, large triangular quiet zones, with local attenuations of 10 dB or more, were obtained when nine or more control channels were used. Increases of noise were predicted outside of these areas, but these were minimized as more control channels were employed. By combining predicted attenuations with measured noise spectra, noise levels after implementation of an active control system were estimated.

Mechanism of sound generation by low Mach number flow over a wall cavity

An analysis is made of the mechanism of sound production by nominally steady low Mach number flow over a rigid shallow wall cavity. At very low Mach numbers the dominant source of sound is the unsteady drag, and the aeroacoustic dipole source accompanying this force. A monopole source dependent on the compression of fluid within the cavity is smaller by a factor of the order of the flow Mach number M. The directivity of the dipole sound peaks in directions upstream and downstream of the cavity, and there is a radiation null in the direction normal to the plane of the wall. However, numerical simulations for M as small as 0.1 have predicted significant radiation in directions normal to the wall. This anomaly is investigated in this paper by means of an acoustic Green's function tailored to cavity geometry that accounts for possible aeroacoustic contributions from both the drag-dipole and from the lowest order cavity resonance. The Green's function is used to show that these sources are correlated and that their strengths are each proportional to the unsteady drag generated by vorticity interacting with the cavity trailing edge. When M∼0.01, the case in most underwater applications, the monopole strength is always negligible (for a cavity with rigid walls). At low Mach numbers exceeding about 0.05 it is shown that the cavity monopole radiation is O( M 2)≪1 relative to the dipole at low frequencies. At higher frequencies, near the resonance frequency of the cavity, the monopole and dipole have similar orders of magnitude, and the combination produces a relatively uniform radiation directivity, with substantial energy radiated in directions normal to the wall. Illustrative numerical results are given for a wall cavity subject to ‘shear layer mode’ excitation by the Rossiter ‘feedback’ mechanism.

Experimental subwavelength localization of scatterers by decomposition of the time reversal operator interpreted as a covariance matrix.

The D.O.R.T. method (French acronym for Decomposition of the Time Reversal Operator) is an active remote sensing technique using arrays of antennas for the detection and localization of scatterers [Prada et at., J. Acoust. Soc. Am. 99, 2067-2076 (1996)]. The analogy between the time reversal operator and the covariance matrix used for classical sources separation in passive remote sensing [Bienvenu et al., IEEE Trans. ASSP 31, 1235-1247 (1983)] is established. Then, an experiment of subwavelength detection and localization of point-like scatterers with a linear array of transducers is presented. Using classical estimators in reception like Maximum-Likelihood and Multiple Signal Characterization (MUSIC), two point-like scatterers separated by lambda/3 and placed at 100lambda from the array of transducers are resolved. In these experiments, the role of multiple scattering and the existence of additional eigenvectors associated with dipolar and monopolar radiation of each scatterer is discussed.

Radome‐protected low‐profile GSM antenna on small ground plane

AbstractA small radiating element, designed to work in the GSM frequency band, made up entirely of square parts on air substrate is described. The dimensions of the ground plane are only 1.3 times the dimensions of the radiating element. By stacking two square wire patches, the characteristics of compactness, large bandwidth, and monopolar wire radiation pattern can be achieved. Circular designs have been already been presented, however, compared to the circular plates solution, the new structure described in this paper allows an easier realization as well as a protective dielectric radome. Owing to its low‐profile, low‐cost, and radioelectric properties such as large bandwidth and dipolar‐type radiation pattern, this antenna is very suitable for automobile or indoor ceiling applications which require a very small ground plane. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 328–331, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11051

A Study on Tire-Structure-Borne Sound

A set of theoretical models has been prepared which describes the noise generated by tire/road interaction. The mechanisms are considered to be air pumping and carcass vibration. The models begin with a set of thin shell equations describing the motion of the belt of a radial ply tire, as derived by Böhm. The structural quantities required for these equations are derived from the material properties of the tire. The rolling shape of a tire is computed from the steady-state limit of these equations. Air pumping (monopole radiation from head voids) is calculated by assuming that tread elements move passively on the deformed tire. The vibration response of the tire is treated using full time-dependent shell equations. The force input at the tire/road interface is calculated on the basis of tread geometry and the distribution of contact patch pressure. This input is physically equivalent to the impulse distribution models widely used in the tire industry for tread pitch randomization. The subsequent radiation of sound is calculated by a Raleigh integral. These models have been embodied into a unified set of computer programs. Using the programs, the effect on noise of various tire design variations is determined and discussed. Trends that lead to low-noise design are identified. A series of experiments are planned which will test the validity of the models and provide a basis for their refinement before final documentation and dissemination.

A wide-band monopolar plate-patch antenna

A new design of a monopolar plate-patch antenna is proposed and experimentally studied. The proposed antenna is a planar rectangular monopole top-loaded with a shorted square or circular patch, which is shorted to the antenna ground by two ground plates placed symmetrically on each side of the planar monopole. Prototypes have been constructed and experimental results show, that with an antenna height less than 10% of the free-space wavelength of the center operating frequency, the proposed antenna can have a monopole radiation pattern with an impedance bandwidth larger than three times that of a corresponding monopolar wire-patch antenna.

Instability of scalar charges in (1 + 1) and (2 + 1) dimensions

We study the phenomenon of mass loss by a scalar charge—a point particle which acts as a source for a (non-interacting) scalar field—in (1 + 1)-dimensional and (2 + 1)-dimensional flat spacetimes. We find that such particles are unstable against self interaction: they lose their mass through the emission of monopole radiation. This is in sharp contrast with scalar charges in (3 + 1)-dimensional flat spacetime, where no such phenomenon occurs.

OPTIMIZED DIFFERENTIATION SCHEMES ON NON-UNIFORM GRIDS FOR COMPUTATIONAL AEROACOUSTICS

Dispersion-relation-preserving (DRP) spatial differentiation schemes are investigated for staggered non-uniform finite-volume grid type. It is shown that the staggering improves the numerical accuracy and stability whether the grid is uniform or non-uniform. The effect of the non-uniformity in the grid on the optimization procedure of the schemes is discussed. The spatial differentiation schemes are tested for the aeroacoustic problems of monopole and quadrupole radiation, showing the effectiveness of the non-uniform grid and the optimized high-order scheme.

Optimized Differentiation Schemes on Non-Uniform Grids for Computational Aeroacoustics

Dispersion-relation-preserving (DRP) spatial differentiation schemes are investigated for staggered non-uniform finite-volume grid type. It is shown that the staggering improves the numerical accuracy and stability whether the grid is uniform or non-uniform. The effect of the non-uniformity in the grid on the optimization procedure of the schemes is discussed. The spatial differentiation schemes are tested for the aeroacoustic problems of monopole and quadrupole radiation, showing the effectiveness of the non-uniform grid and the optimized high-order scheme.

Dual band hybrid vehicular telephone antenna

A dual band vehicular telephone antenna with a low profile and wide bandwidth is presented. Operating at the 900 MHz and 1800 MHz telephony bands, with a VSWR of better than 1.7, it is a hybrid construction combining a monopole with a top loading patch shorted to the ground-plane. Extra pins provide the upper frequency band resonance. The antenna has monopole radiation characteristics and can be hidden under a plastic panel or mounted on the vehicle roof.

Hidden Antennas for Vehicle Telematics Systems

This paper describes three different antenna systems for mobile vehicle applications. The antennas are either patches or derivatives in all cases. The first is a dual-band telephone antenna with a low profile and wide bandwidth. Operating at 900 MHz and 1800 MHz, with a VSWR of better than 1·7, it is a hybrid construction combining a monopole with a top-loading patch shorted to the ground-plane. Extra-shorted pins provide the upper frequency band coverage. It provides monopole radiation characteristics and can be hidden under a plastic panel or mounted on the vehicle roof. The second is a microstrip patch antenna integrated into a laminated glass windscreen for a vehicle. It is fed using a coplanar waveguide feed printed on the innermost layer of the glass, avoiding the need for a contacting feed within the laminate. The patch and ground plane are meshed for manufacturing in the glass to avoid distorting the heat profile when the glass is shaped and laminated. The patch is easily fed from inside the vehicle and is potentially a very low cost design. The final antenna discussed is a dual-band patch antenna specifically designed for the Globalstar satellite telephone system at 1·6 GHz and 2·45 GHz. It also covers the Iridium band at 1·6 GHz. A single circularly polarised patch is used. Dual-band operation results from truncating the corners of the square patch and judiciously placed slots to achieve a band spacing of 1·5.

A broadband low‐profile cylindrical monopole antenna

AbstractA broadband low‐profile cylindrical monopole antenna top‐loaded with a shorted circular patch is presented. The shorted circular patch is connected to the antenna ground by two shorting cylinders placed symmetrically on each side of the cylindrical monopole. Experiments have been carried out, and obtained results show that, with an antenna height only about 0.07 times the free‐space wavelength of the center operating frequency, the proposed antenna can have a monopole radiation pattern with an impedance bandwidth of 14.8%, which is much greater than that (3%) of a corresponding monopolar wire‐patch antenna. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 77–79, 2001.

Gravitational Waves from the Collapse and Bounce of a Stellar Core in Tensor‐Scalar Gravity

Tensor-scalar theory of gravity allows the generation of gravitational waves from astrophysical sources, like supernovae, even in the spherical case. That motivated us to study the collapse of a degenerate stellar core, within tensor-scalar gravity, leading to the formation of a neutron star through a bounce and the formation of a shock. This paper discusses the effects of the scalar field on the evolution of the system, as well as the appearance of strong nonperturbative effects of this scalar field (the so-called spontaneous scalarization). As a main result, we describe the resulting gravitational monopolar radiation (form and amplitude) and discuss the possibility of its detection by the gravitational detectors currently under construction, taking into account the existing constraints on the scalar field. From the numerical point of view, it is worthy to point out that we have developed a combined code that uses pseudo-spectral methods for the evolution of the scalar field and High-Resolution Shock-Capturing schemes, as well as for the evolution of the hydrodynamical system. Although this code has been used to integrate the field equations of that theory of gravity, in the spherically symmetric case, a by-product of the present work is to gain experience for an ulterior extension to multidimensional problems in Numerical Relativity of such numerical strategy.

Photonic crystal-based resonant antenna with a very high directivity

We investigate the radiation properties of an antenna that was formed by a hybrid combination of a monopole radiation source and a cavity built around a dielectric layer-by-layer three-dimensional photonic crystal. We measured a maximum directivity of 310, and a power enhancement of 180 at the resonant frequency of the cavity. We observed that the antenna has a narrow bandwidth determined by the cavity, where the resonant frequency can be tuned within the band gap of the photonic crystal. The measured radiation patterns agree well with our theoretical results.

Neutron star transition to a strong-scalar-field state in tensor-scalar gravity

Spherical neutron star models are studied within tensor-scalar theories of gravity. Particularly, it is shown that, under some conditions on the second derivative of the coupling function and on star's mass, for a given star there exist two strong-scalar-field solutions as well as the usual weak-field one. This last solution happens to be unstable and a star, becoming massive enough to allow for all three solutions, evolves to reach one of the strong field configurations. This transition is dynamically computed and it appears that the star radiates away the difference in energy between both states (a few $10^{-3} M_\odot c^2$) as gravitational radiation. Since part of the energy ($\sim 10^{-5} M_\odot c^2$) is injected into the star as kinetic energy, the velocity of star's surface can reach up to $10^{-2} c$. The waveform of this monopolar radiation is shown as well as the oscillations undergone by the star. These oscillations are also studied within the slowly-rotating approximation, in order to estimate an order of magnitude of the resulting quadrupolar radiation.

Research on circularly polarized conical-beam antennas

This antenna is intended for communication between a stationary satellite and mobile stations, and is designed to possess circularly polarized conical beams. It generates a beam in the desired direction, by means of changing the inclination angle of the linear-antenna elements, and the spacing between the antenna elements, without phase shifters. The antenna inclination angle (/spl alpha/), the antenna length (e), the distance between antenna elements (d), and the radius of the circular reflector (R), are adopted as parameters, and optimum values for the directivity of the research objective are obtained by calculation. Four elements of the antenna are fed by in-phase signals, the distances between elements are set to appropriate lengths, and by providing spatial phase differences of 90/spl deg/, circularly polarized waves are obtained. The antenna elements are fundamentally monopole radiators. Ultimately, the antenna parameters were determined as follows: /spl Lscr/=0.64/spl lambda/ (120 mm), R=0.43/spl lambda/ (80 mm), /spl alpha/=45/spl deg/, and d=0.48/spl lambda/ (90 mm). In the calculation, the frequency was set to 1.6 GHz. It was seen that the beam radiation intensity took the maximum value in the angular range of /spl theta/=30/spl deg/ to 60/spl deg/, and that the /spl theta/ and /spl phi/ components of the electric field were approximately equal. In other words, the radiated waves were circularly polarized. An experiment for measuring the directivity of the circularly polarized conical-beam antenna was carried out by using four monopole antennas, and the effectiveness of the theoretical analysis was confirmed. Experiments were carried out for the impedance and directivity.

The resonator problem in a spherical GW detector

This paper is a brief summary of the most relevant features of the solution to the general problem of the coupled motion of a set of resonant transducers and a solid sphere when acted upon by a GW excitation, as recently investigated by us. A remarkably elegant theory emerges out of the analysis, which fully displays the system dynamics for arbitrary resonator configurations. The power of the method can be used to consider alternative layouts to the TIGA proposal, the virtues and/or drawbacks of which can then be assessed. A specific new resonator distribution will be presented which takes advantage of the significant cross section of a spherical GW detector at its second quadrupole resonance and which is also useful for eventual monopole radiation sensing.

Microwave Applicator Design for Cardiac Tissue Ablations

A variety of microwave applicators were designed, fabricated and tested for catheter applications: I-radiators, U-radiators, O-radiators, forward helical coil radiator, reverse helical coil, double coil radiator, loaded monopole radiator, leaky coaxial radiator and tee radiators. The comparative and relative radiation characteristics of these applicators were tested in a saline bath and tissues. Most radiators designed produced larger lesions than have been described previously.

Comparison of higher resolution Euler schemes for aeroacoustic computations

Modified finite volume methods of Osher and Chakravarthy (MOC) and Sanders and Li (MSL) and a finite element method of Lin and Chin are investigated on several aeroacoustic problems. Higher order accuracy is obtained with the monotonic upwind-centered scheme for conservation laws approach. The tested problems include oblique shock reflection, linear wave convection, monopole radiation, vortex preservation, and blade-vortex interaction. Based on the order of accuracy, stability, grid nonuniformity, and dissipation property of each scheme, it is concluded that the MOC scheme is the most suitable scheme among the schemes tested for aeroacoustic computations. We also conclude that the MSL scheme needs to be improved on problems of convergence and small wiggles before it is used in computational aeroacoustics. In the blade-vortex interaction problem, two sound waves, transonic and compressibility waves, found in recent experiments are simulated. N recent years, considerable progress has been made in the numerical analysis of fluid dynamics. Usually, numerical methods which solve the Euler/Navier-Stokes equations for aerodynamic flows fall into three major classes: finite difference, finite volume, and finite element methods. Recently, successful methods have employed higher order upwind interpolations and limiter functions to obtain algorithms possessing higher resolution and higher stability bounds. The inherently dissipative nature of upwind schemes and limiter functions is beneficial at or near shock waves. Specifically, the implementation of limiter functions make schemes more stable for computing solutions with strong shock waves. In general, an upwind scheme with a limiter function is formally second- or thirdorder accurate but is locally first-order accurate in regions of high localized gradients (such as shocks) and in many cases at local extrema also. The loss of formal accuracy near shocks is not usually serious, but for aeroacoustic calculations the dissipation at extrema will be harmful. Upwind schemes may be classified into two classes: monotonic upwind-centered scheme for conservation laws (MUSCL) and nonMUSCL.1'4 The present work is focused on the MUSCL-based schemes. We have studied the performance of the following three basic schemes: 1) the finite element method of Lin and Chin (LC)5 and Lin et al.,6 2) the finite volume method of Osher and Chakravarthy (OC)7, and 3) the finite volume method of Sanders and Li (SL). 8'9 The popularity of the OC scheme for aeroacoustic computations is likely to increase since it is formally of third order and is stable in the computation of strong shock waves. The SL scheme is formally of fourth order and is a suitable method for the aeroacoustic computation of low-speed flows. An overall assessment of this scheme is available in Refs. 8 and 9. The LC scheme is formally of second order for both uniform and nonuniform meshes. This scheme has been extensively tested on both inviscid and viscous flows.5'6 It has been shown that the scheme is capable of computation of steady and unsteady flows. In this paper, we have modified the limiter functions in the OC and SL schemes to obtain two new schemes, the modified OC (MOC) and the modified SL (MSL) schemes. In this effort a detailed investigation is performed to evaluate the capability of those schemes. Several test problems are studied, including oblique shock reflection, linear wave convection, monopole radiation, vortex preservation, and blade-vortex interaction. Details