Journal articles: 'Far-field directivity'

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Jiang Shuang-Feng, Kong Fan-Min, Li Kang, and Gao Hui. "Study of far-field directivity of optical dipole antenna." Acta Physica Sinica 60, no. 4 (2011): 045203. http://dx.doi.org/10.7498/aps.60.045203.

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Shahbazi, Shahrokh, Iman Mansouri, Jong Wan Hu, Noura Sam Daliri, and Armin Karami. "Seismic Response of Steel SMFs Subjected to Vertical Components of Far- and Near-Field Earthquakes with Forward Directivity Effects." Advances in Civil Engineering 2019 (April 3, 2019): 1–15. http://dx.doi.org/10.1155/2019/2647387.

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In the near-field earthquake, forward directivity effects cause long-period pulse with a short effective time and a large domain in the velocity time history. This issue increases the ductility needs of structures, and in recent decades, the destructive effects of these kinds of records have been evaluated in comparison with far-field earthquakes. This brings about the necessity to compare a structure’s behavior subjected to vertical components of near-field (NF) earthquakes, including forward directivity effects vs. the effects of vertical components of far-field (FF) earthquakes. The present study investigated 3-, 5-, 8-, and 20-story steel moment frames with special ductility (SMF) through which modeling effects of panel zone have been applied, subjected to vertical component of near-field (NF) earthquakes with forward directivity and the vertical component of far-field earthquakes. By investigating the results, it can be clearly seen that the average values of the maximum displacement, shear force of the stories, and the velocity of each story under the impact of the near-field earthquake are greater than the amount of that under the effect of a far-field earthquake. However, this comparison is not valid for the amount of acceleration, axial force, and moments in the columns of the structures accurately.

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OBRIST, DOMINIK. "Directivity of acoustic emissions from wave packets to the far field." Journal of Fluid Mechanics 640 (November 10, 2009): 165–86. http://dx.doi.org/10.1017/s0022112009991297.

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We investigate the acoustic emission from wave packets to the far field. To this end, we develop a theory for one- and two-dimensional source fields in the shape of wave packets with Gaussian envelopes. This theory is based on an approximation to Lighthill's acoustic analogy for distant observers. It is formulated in the spectral domain in which a Gaussian wave packet is represented again by a Gaussian. This allows us to determine the directivity of the acoustic emission (e.g. superdirectivity and Mach waves) by simple geometric constructions in the spectral domain. It is shown that the character of the acoustic emission is mainly governed by the aspect ratio and the Mach number of the wave packet source. To illustrate the relevance of this theory we use it to study two prominent problems in subsonic jet aeroacoustics.

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Post, John T., and Elmer L. Hixson. "Measurement of throat impedance and far‐field directivity for acoustic horns." Journal of the Acoustical Society of America 96, no. 5 (November 1994): 3290. http://dx.doi.org/10.1121/1.410911.

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Guasch, Oriol, and Patricia Sánchez-Martín. "Far-field directivity of parametric loudspeaker arrays set on curved surfaces." Applied Mathematical Modelling 60 (August 2018): 721–38. http://dx.doi.org/10.1016/j.apm.2018.04.002.

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Chiselev, Ana-Maria, and Luminita Moraru. "A Study of Far Field Directivity Pattern of Bio-Inspired EMFit Emitters." IEEE Sensors Journal 12, no. 5 (May 2012): 1372–76. http://dx.doi.org/10.1109/jsen.2011.2172600.

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Soh, W. Y. "Numerical simulation of free shear flows and far-field sound pressure directivity." International Journal for Numerical Methods in Fluids 18, no. 4 (February 28, 1994): 337–59. http://dx.doi.org/10.1002/fld.1650180403.

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Bakker, Maarten C. M., and Martin D. Verweij. "An approximation to the far field and directivity of elastic wave transducers." Journal of the Acoustical Society of America 111, no. 3 (March 2002): 1177–88. http://dx.doi.org/10.1121/1.1428542.

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Zhong, Siyang, and Xin Zhang. "A sound extrapolation method for aeroacoustics far-field prediction in presence of vortical waves." Journal of Fluid Mechanics 820 (May 8, 2017): 424–50. http://dx.doi.org/10.1017/jfm.2017.219.

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Off-surface integral solutions to an inhomogeneous wave equation based on acoustic analogy could suffer from spurious wave contamination when volume integrals are ignored for computation efficiency and vortical/turbulent gusts are convected across the integration surfaces, leading to erroneous far-field directivity predictions. Vortical gusts often exist in aerodynamic flows and it is inevitable their effects are present on the integration surface. In this work, we propose a new sound extrapolation method for acoustic far-field directivity prediction in the presence of vortical gusts, which overcomes the deficiencies in the existing methods. The Euler equations are rearranged to an alternative form in terms of fluctuation variables that contains the possible acoustical and vortical waves. Then the equations are manipulated to an inhomogeneous wave equation with source terms corresponding to surface and volume integrals. With the new formulation, spurious monopole and dipole noise produced by vortical gusts can be suppressed on account of the solenoidal property of the vortical waves and a simple convection process. It is therefore valid to ignore the volume integrals and preserve the sound properties. The resulting new acoustic inhomogeneous convected wave equations could be solved by means of the Green’s function method. Validation and verification cases are investigated, and the proposed method shows a capacity of accurate sound prediction for these cases. The new method is also applied to the challenging airfoil leading edge noise problems by injecting vortical waves into the computational domain and performing aeroacoustic studies at both subsonic and transonic speeds. In the case of a transonic airfoil leading edge noise problem, shocks are present on the airfoil surface. Good agreements of the directivity patterns are obtained compared with direct computation results.

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Vatanshenas, Ali, Mohammad Sadegh Rohanimanesh, and Ehsan Mohammadiha. "Investigating the Performance of Viscoelastic Dampers (VED) Under Nearfield Earthquakes with Directivity Feature." Civil and Environmental Engineering 14, no. 1 (June 1, 2018): 21–27. http://dx.doi.org/10.2478/cee-2018-0003.

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AbstractOne of the most important factors that make structures vulnerable to earthquakes is the short distance between structures and epicenter. Near-field earthquakes have special properties, such as increasing acceleration applied to the structure, which distinguishes them from far-field earthquakes. Therefore, the absorption of input energy for structures located near the faults is very important. Hence, by rotating the earthquake acceleration time history and comparing the resulting spectral acceleration response, the angle which applies the greatest force to the structure on the earthquake directivity side is obtained, and then the performance of a steel structure with viscoelastic dampers (VED) under near-field earthquakes with directivity feature is investigated. After analyzing the structure using nonlinear time history analysis, it was observed that the directivity phenomenon leads to significant increase in the force applied to the structure, but the viscoelastic dampers showed an acceptable performance in both states of with and without directivity.

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Andreyev, Yuri. "Structure of the emission field of ensemble of ultra-wideband chaotic sources." ITM Web of Conferences 30 (2019): 05029. http://dx.doi.org/10.1051/itmconf/20193005029.

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The structure of the emission field of ensemble of independent ultra-wideband chaotic sources in collective emission mode is investigated analytically and numerically, including power density, directivity, and far zone border. The waves emitted by independent individual sources are incoherent; hence in the reception point the created incoherent fields are summed by power, and this summation gives no additional directivity to the ensemble emission pattern. If all the individual antennas are equal, emission pattern of the entire ensemble is the same as the emission pattern of any of the individual emitters.

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Assaad, Jamal, and Jean Michel Rouvaen. "Numerical evaluation of the far-field directivity pattern using the fast Fourier transform." Journal of the Acoustical Society of America 104, no. 1 (July 1998): 72–80. http://dx.doi.org/10.1121/1.423305.

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Ghosh, A., J. Bridges, and F. Hussain. "Instantaneous Directivity in Jet Noise by Multipole Decomposition." Journal of Vibration and Acoustics 117, no. 2 (April 1, 1995): 172–79. http://dx.doi.org/10.1115/1.2873876.

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Noise generated by nearly axisymmetric vortex pairing in an excited low Mach number jet was studied experimentally as a prototypical jet noise source using a three-dimensional microphone array. The measured time-dependent sound field was decomposed into its multipole components using general linear regression techniques. Although the vortex interactions were very nearly axisymmetric and thus a nearly axisymmetric lateral quadrupole sound field was expected, the sound field was found to have large departures from axisymmetry. This acoustic sensitivity to symmetry breaking in the vorticity field was also shown in calculations using numerical vortex models and vortex sound theory. The directivity of the far-field sound gets smeared out in the time averaged data due to the tilting of the quadrupoles; this emphasizes the need for studying the jet sound pressure field in four dimensions (x, t) to better understand the mechanisms of noise generation.

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Wang, Shiquan. "Far-Field Performances Prediction of High Frequency Projectors Using Secondary Source Array Method." Journal of Computational Acoustics 25, no. 02 (November 2, 2016): 1750002. http://dx.doi.org/10.1142/s0218396x17500023.

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This paper investigates the prediction of the far-field performances of high frequency projectors using the second source array method (SSAM). The far-field parameters can be calculated accurately using the complex acoustic pressure data of two very close parallel planes which lie in the near-field region of the projector. The paper simulates the feasibility of predicting the far-field parameters such as transmitting voltage response and the far-field directivity pattern. The predicting results are compared with that calculated using boundary element method (BEM). It shows very good agreement between the two methods. A planar high frequency projector is measured using the near-field method. In order to verify the predicting results, the far-field measurement is performed for the same projector. The comparison of the results shows that the near-field method is capable to precisely predict the far-field parameters of the projector.

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Masumoto, Takayuki, Yosuke Yasuda, Naohisa Inoue, and Tetsuya Sakuma. "Fast Calculation of Far-Field Sound Directivity Based on Fast Multipole Boundary Element Method." Journal of Theoretical and Computational Acoustics 28, no. 04 (November 30, 2020): 1950024. http://dx.doi.org/10.1142/s2591728519500245.

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A fast method for calculating sound radiation/reflection directivities at high resolution in the infinite far field is proposed with the use of the fast multipole boundary element method (FMBEM). This method calculates directivities using direction-dependent coefficients called outgoing coefficients, which are obtained in the calculation process of the matrix-vector products in the FMBEM. Since the outgoing coefficients are generally calculated for a large number of directions high-resolution directivities can be easily obtained with extremely small computational cost and minor modifications in the FMBEM program codes. It is confirmed via comparison with the numerical results using the conventional method that the proposed method can calculate directivities at infinity. Numerical results also show that the computational time for the proposed method is significantly shorter than that for the conventional method with no addition of the required memory.

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Assaad, Jamal, and Christian Bruneel. "A physical interpretation of the far‐field directivity pattern of lithium niobate bar transducers." Journal of Applied Physics 80, no. 9 (November 1996): 5489–93. http://dx.doi.org/10.1063/1.362739.

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Shi, Chuang, and Yoshinobu Kajikawa. "A convolution model for computing the far-field directivity of a parametric loudspeaker array." Journal of the Acoustical Society of America 137, no. 2 (February 2015): 777–84. http://dx.doi.org/10.1121/1.4906163.

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Hong, Seokhyeon, Young Jin Lee, Kihwan Moon, Youngsoo Kim, Eunso Shin, and Soon-Hong Kwon. "Far-Field Analysis on Reflecting Colors of Dielectric Nanosphere Metasurface." Journal of Nanomaterials 2019 (June 16, 2019): 1–8. http://dx.doi.org/10.1155/2019/6532967.

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Photonic resonances in nanostructures have been exploited in reflective or transmission color filters, which can provide vivid colors. Metallic nanostructures have been widely studied to demonstrate a variety of color filters based on strong light interaction due to plasmonic resonances. However, because of the severe absorption loss of metal in visible light, dielectric nanoparticles having Mie resonances are a popular study focus in recent years to achieve vivid colors. In contrast to the behaviors of point-like electric dipole in metallic nanoparticle, the interplay of the electric and magnetic Mie resonances in dielectric nanoparticle enables a large degree of freedom in manipulating the directivity of light scattering, reflecting/transmitting color, and spontaneous emission rates. Here, we propose a color reflector based on an array of silicon nanoparticles that shows reflectance greater than 70% and vivid colors over the entire visible spectrum range, which covers sRGB color area. Viewing angle dependencies of the color and brightness are also investigated by calculating color-resolved far-field patterns, while exhibiting maintenance of the color and high reflectance over a broad viewing angle.

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Sannikova, Natalia K., Harvey Segur, and Diego Arcas. "Influence of Tsunami Aspect Ratio on Near and Far-Field Tsunami Amplitude." Geosciences 11, no. 4 (April 16, 2021): 178. http://dx.doi.org/10.3390/geosciences11040178.

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This study presents a numerical investigation of the source aspect ratio (AR) influence on tsunami decay characteristics with an emphasis in near and far-field differences for two initial wave shapes Pure Positive Wave and N-wave. It is shown that, when initial total energy for both tsunami types is kept the same, short-rupture tsunami with more concentrated energy are likely to be more destructive in the near-field, whereas long rupture tsunami are more dangerous in the far-field. The more elongated the source is, the stronger the directivity and the slower the amplitude decays in the intermediate- and far-fields. We present evidence of this behavior by comparing amplitude decay rates from idealized sources and showing their correlation with that observed in recent historical events of similar AR.

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Bravo-Haro, M. A., J. R. Virreira, and A. Y. Elghazouli. "Inelastic displacement ratios for non-structural components in steel framed structures under forward-directivity near-fault strong-ground motion." Bulletin of Earthquake Engineering 19, no. 5 (March 2021): 2185–211. http://dx.doi.org/10.1007/s10518-021-01059-3.

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AbstractThis paper describes a detailed numerical investigation into the inelastic displacement ratios of non-structural components mounted within multi-storey steel framed buildings and subjected to ground motions with forward-directivity features which are typical of near-fault events. The study is carried out using detailed multi-degree-of-freedom models of 54 primary steel buildings with different structural characteristics. In conjunction with this, 80 secondary non-structural elements are modelled as single-degree-of-freedom systems and placed at every floor within the primary framed structures, then subsequently analysed through extensive dynamic analysis. The influence of ground motions with forward-directivity effects on the mean response of the inelastic displacement ratios of non-structural components are compared to the results obtained from a reference set of strong-ground motion records representing far-field events. It is shown that the mean demand under near-fault records can be over twice as large as that due to far-fault counterparts, particularly for non-structural components with periods of vibration lower than the fundamental period of the primary building. Based on the results, a prediction model for estimating the inelastic displacement ratios of non-structural components is calibrated for far-field records and near-fault records with directivity features. The model is valid for a wide range of secondary non-structural periods and primary building fundamental periods, as well as for various levels of inelasticity induced within the secondary non-structural elements.

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Souliez, F. J., L. N. Long, P. J. Morris, and A. Sharma. "Landing Gear Aerodynamic Noise Prediction Using Unstructured Grids." International Journal of Aeroacoustics 1, no. 2 (August 2002): 115–35. http://dx.doi.org/10.1260/147547202760236932.

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Aerodynamic noise from a landing gear in a uniform flow is computed using the Ffowcs Williams-Hawkings (FW-H) equation. The time accurate flow data on the integration surface is obtained using a finite volume low-order flow solver on an unstructured grid. The Ffowcs Williams-Hawkings equation is solved using surface integrals over the landing gear surface and over a permeable surface away from the landing gear. Two geometric configurations are tested in order to assess the impact of two lateral struts on the sound level and directivity in the far-field. Predictions from the Ffowcs Williams-Hawkings code are compared with direct calculations by the flow solver at several observer locations inside the computational domain. The permeable Ffowcs Williams-Hawkings surface predictions match those of the flow solver in the near-field. Far-field noise calculations coincide for both integration surfaces. The increase in drag observed between the two landing gear configurations is reflected in the sound pressure level and directivity mainly in the streamwise direction.

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Duren, Richard E. "A theory for marine source arrays." GEOPHYSICS 53, no. 5 (May 1988): 650–58. http://dx.doi.org/10.1190/1.1442499.

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General mathematical expressions for a marine source array’s (1) far‐field pulse spectrum, (2) radiated energy density, and (3) directivity are developed for both a source in an infinite homogeneous medium and a source operating near the ocean surface. These results, intended to assist the analysis and design of marine source arrays, apply to any marine source array when (1) individual elements radiate isotropically, (2) their individual waveforms are specified, and (3) the array geometry is specified. Arbitrary geometry and arbitrary isotropic waveforms are allowed. The theory assumes linear superposition of the individually specified waveforms, and is consistent with the “square law effect” for identical elements. For an array of small elements, expended energy agrees with the array’s radiated energy found using far‐field methods. Also, the energy radiated from an array with large element spacing is equal to the sum of the independently radiated energies. Two closely spaced identical elements radiate four times the energy contained in a single outgoing waveform over all space. The appropriate directivity definition for marine seismic sources is the ratio of the radiated energy density per unit solid angle in a particular direction to the average radiated energy density per unit solid angle. This definition allows directivity to be expressed explicitly in terms of the individual frequency spectra and geometry.

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Craster, R. V., and A. V. Shanin. "Embedding formulae for diffraction by rational wedge and angular geometries." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 461, no. 2059 (June 15, 2005): 2227–42. http://dx.doi.org/10.1098/rspa.2004.1443.

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Embedding is the process of taking the far-field directivity from a diffraction problem (or problems) involving line sources or multipoles placed at a sharp edge, and then constructing the far field, for the same geometry, for more general incidence using only this canonical problem(s). Thus far, embedding has been limited to planar, parallel scattering surfaces, for instance, collections of parallel cracks or slits; it appeared that there was a fundamental limitation to embedding, disallowing its use for angular structures. In this article, we overcome this limitation and demonstrate the use of embedding upon wedge diffraction problems and upon a simple polygonal shape; a limitation is that the final formulae are for rational wedge angles.

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Sugar, Mitchell, and Paul Slaboch. "Directivity of sound propagation from an commercial supersonic engine inlet." INTER-NOISE and NOISE-CON Congress and Conference Proceedings 263, no. 2 (August 1, 2021): 4211–18. http://dx.doi.org/10.3397/in-2021-2633.

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The effects of mean flow variations on sound propagation from an axisymmetric commercial supersonic engine inlet were studied using numerical methods. A finite element model of the inlet was constructed in Ansys Fluent and used to solve for flow fields given by different initial conditions. Results from this model were fed into the aeroacoustic solver, Actran, and used to calculate far field radiated noise as well as the directivity of that noise. The acoustic source of this noise was a plane wave of a known strength placed at the fan face. In addition to assessing the effects of mean flow on the radiated noise transfer functions, the duct modes of the model were compared across different flow regimes. Relationships between mean flow parameters and the directivity of duct modes are developed. The results of this study will be used in further studies to gain a deeper understanding of how the underlying physics which govern the system create favorable or unfavorable directivity patterns.

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Zhang, Xiao Yun, Qing Song, and Xiao An Zhang. "Influences of Ground Reflection on the Elevated Box Bridge Structure Noise." Applied Mechanics and Materials 721 (December 2014): 813–17. http://dx.doi.org/10.4028/www.scientific.net/amm.721.813.

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A 32 m simply supported concrete box bridge is taken as the research object. The box bridge structure noise induced by the static loads and how the ground reflection affects it are studied by the modal acoustic transfer vector (MATV) method. The results show that it is commendable to analyze the box bridge structure noise using the MATV method; the regularities of the sound field distribution are changed obviously by the ground reflection, and the directivity is very obvious in the far-field region and there has the stratification of the vertical sound field under the box bridge; the ground reflection enhances the overall sound level of the sound field except the region above the box bridge, and the vertical sound field in the far field is affected very obviously. The buildings, especially the high-rise buildings, are mostly in the far field area, so the influences of the ground reflection on the box bridge structure noise must be paid attention to.

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Ram, Gopi, Durbadal Mandal, Rajib Kar, and Sakti Prasad Ghoshal. "Directivity maximization and optimal far-field pattern of time modulated linear antenna arrays using evolutionary algorithms." AEU - International Journal of Electronics and Communications 69, no. 12 (December 2015): 1800–1809. http://dx.doi.org/10.1016/j.aeue.2015.09.009.

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Lyu, B., and M. Azarpeyvand. "On the noise prediction for serrated leading edges." Journal of Fluid Mechanics 826 (August 3, 2017): 205–34. http://dx.doi.org/10.1017/jfm.2017.429.

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An analytical model is developed for the prediction of noise radiated by an aerofoil with leading-edge serration in a subsonic turbulent stream. The model makes use of Fourier expansion and Schwarzschild techniques in order to solve a set of coupled differential equations iteratively and express the far-field sound power spectral density in terms of the statistics of incoming turbulent upwash velocity. The model has shown that the primary noise-reduction mechanism is due to the destructive interference of the scattered pressure induced by the leading-edge serrations. It has also shown that in order to achieve significant sound reduction, the serration must satisfy two geometrical criteria related to the serration sharpness and hydrodynamic properties of the turbulence. A parametric study has been carried out and it is shown that serrations can reduce the overall sound pressure level at most radiation angles, particularly at small aft angles. The sound directivity results have also shown that the use of leading-edge serration does not significantly change the dipolar pattern of the far-field noise at low frequencies, but it changes the cardioid directivity pattern associated with radiation from straight-edge scattering at high frequencies to a tilted dipolar pattern.

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Shahmohammadi Mehrjerdi, Mohammad, Ahmad Ali Fallah, and S. T. Tabatabaei Aghda. "Numerical Modeling of Soil-Pile-Interaction with Near and Far Field Earthquake's Effects." Civil Engineering Journal 3, no. 5 (May 30, 2017): 351–64. http://dx.doi.org/10.28991/cej-2017-00000096.

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This paper studies Near and Far Field effects of the response of a column-pile to earthquakes considering Dynamic-Soil-Structure-Interaction (DSSI) effects in soft clay (Vs<180 m/s ) and stiff clay (180<Vs<375 m/s). Opensees software that can simulate the dynamic time history analysis is used. Both kinematic and inertial interactions are considered and Finite Element Method (FEM) is used to solve DSSI. The direct method applies to 3D modeling of the layered soil and column-pile. A Pressure Independ Multi Yield Surface Plasticity Model is used to simulate different kinds of clay behavior. Time history seismic analyses provide for the mass and stiffness matrices to evaluate dynamic structural response with and without directivity effects for Near and Far Field earthquakes. Results show that the Multi-Yield-Surface-Kinematic-Plasticity-Model can be used instead of bilinear springs between piles and clay soil, for both Near Field and Far Field earthquakes. In addition, comparing Near and Far Field analyses, acceleration response spectrum at the top of the structure in the Far Field increases with the softness of the soil more than that in the Near field.

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Pavlov, V. I., J. Tromp, and E. P. Tito. "Tsunami generation by ocean floor rupture front propagation: Hamiltonian description." Natural Hazards and Earth System Sciences 9, no. 1 (February 23, 2009): 217–27. http://dx.doi.org/10.5194/nhess-9-217-2009.

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Abstract. The Hamiltonian method is applied to the problem of tsunami generation caused by a propagating rupture front and deformation of the ocean floor. The method establishes an alternative framework for analyzing the tsunami generation process and produces analytical expressions for the power and directivity of tsunami radiation (in the far-field) for two illustrative cases, with constant and gradually varying speeds of rupture front propagation.

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Sodha, A. H., D. P. Soni, M. K. Desai, and S. Kumar. "Behavior of Quintuple Friction Pendulum System Under Near-Fault Earthquakes." Journal of Earthquake and Tsunami 11, no. 05 (December 2017): 1750017. http://dx.doi.org/10.1142/s1793431117500178.

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The Quintuple Friction Pendulum (QTFP) system is a new generation sliding isolation having six spherical sliding surfaces with five effective pendula. Due to multiple sliding surfaces, QTFP system shows highly adaptive behavior under different hazard level of earthquakes, despite being a passive system. The paper describes mathematical model and seismic response of QTFP system under 60 earthquake records consisting of service level, design basis and maximum considered earthquakes. To study the effect of directivity focusing and fling step, additional 15 records consist of far-field, near-fault with forward directivity and fling step effect are also considered. Three types of effective period and effective damping in combination with two different displacement capacities of QTFP bearing resulting in six isolator designs are considered. The seismic demand parameters like base shear, top floor absolute acceleration and isolator displacement have been studied. It is found that the QTFP bearing stiffens at low input, softens with increasing input, and then stiffens again at higher levels of input. Thus, it shows highly adaptive behavior under different hazard levels of earthquake. Further, due to forward and backward momentum conveyed by the directivity pulse, near-fault directivity effect imposes higher demand compared to fling step containing only forward momentum.

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Kurzon, Ittai, Vladimir Lyakhovsky, and Yehuda Ben-Zion. "Earthquake source properties from analysis of dynamic ruptures and far-field seismic waves in a damage-breakage model." Geophysical Journal International 224, no. 3 (October 23, 2020): 1793–810. http://dx.doi.org/10.1093/gji/ggaa509.

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SUMMARY We present results on earthquake source properties using simulations of dynamic rupture and radiated seismic waves in a continuum damage-breakage rheological model. The source properties are derived by (1) calculation of source parameters directly from the simulated ruptures and (2) observational processing of the far-field radiated waves. The seismic potency, moment, damage-related source term, rupture velocity and effective rigidity are estimated directly from the simulated sources, while the radiation pattern, dominant frequency, directivity, rupture velocity and seismic potency are calculated through analysis of the radiated waves. The potencies calculated directly from the sources are used to validate those estimated by wave analysis. The effective rigidity at the rupture zone during failure is about four times smaller than that of the intact surrounding rocks. Rupture velocity can be estimated by far-field measurements for sources with unidirectional ruptures with prominent rupture directivity. The dominant frequencies for P and S waves $f_d^S/f_d^P$ reflect clearly the rupture duration and have a ratio in the range 0.87–1.12. Seismic potencies obtained through processing the P or S waves have an overall ±15 per cent difference from the source reference values. The calculated values of the coefficient ${\rm{\kappa }}$, relating rupture length to corner or dominant frequency, have strong dependency on the source geometry. Using a strain-rate dependent ${\rm{\kappa }}$, we obtain much weaker dependencies of strain-drop on the dominant frequencies, $\Delta {\rm{\varepsilon }} \propto {( {{f_d}} )^{3/4}}$, than the classical cube-dependency between stress drop and corner frequency, and corresponding weak dependency of average slip on dominant frequency, ${\rm{\bar{D}}} \propto {( {{f_d}} )^{1/2}}$. The obtained analysis procedure and relations can be used to reduce the uncertainty of source properties derived from far-field seismic waves.

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Férand, Mélissa, Thomas Livebardon, Stéphane Moreau, and Marlène Sanjosé. "Numerical Prediction of Far-Field Combustion Noise from Aeronautical Engines." Acoustics 1, no. 1 (February 19, 2019): 174–98. http://dx.doi.org/10.3390/acoustics1010012.

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A hybrid methodology combining a detailed Large Eddy Simulation of a combustion chamber sector, an analytical propagation model of the extracted acoustic and entropy waves at the combustor exit through the turbine stages, and a far-field acoustic propagation through a variable exhaust temperature field was shown to predict far-field combustion noise from helicopter and aircraft propulsion systems accurately for the first time. For the single-stream turboshaft engine, the validation was achieved from engine core to the turbine exit. Propagation to the far field was then performed through a modeled axisymmetric jet. Its temperature modified the acoustic propagation of combustion noise significantly and a simple analytical model based on the Snell–Descarte law was shown to predict the directivity for axisymmetric single jet exhaust accurately. Good agreement with measured far-field spectra for all turboshaft-engine regimes below 2 kHz stresses that combustion noise is most likely the dominant noise source at low frequencies in such engines. For the more complex dual-stream turbofan engine, two regime computations showed that direct noise is mostly generated by the unsteady flame dynamics and the indirect combustion noise by the temperature stratification induced by the dilution holes in the combustion chamber, as found previously in the turboshaft case. However, in the turboengine, direct noise was found dominant at the combustor exit for the low power case and equivalent contributions of both combustion noise sources for the high power case. The propagation to the far-field was achieved through the temperature field provided by a Reynolds-Averaged Navier–Stokes simulation. Good agreement with measured spectra was also found at low frequencies for the low power turboengine case. At high power, however, turboengine jet noise overcomes combustion noise at low frequencies.

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Guasch, Oriol, Patrícia Sánchez-Martín, and Marc Arnela. "Far field directivity of an omnidirectional parametric loudspeaker consisting of piezoelectric ultrasonic transducers set on a sphere." Journal of the Acoustical Society of America 141, no. 5 (May 2017): 3650. http://dx.doi.org/10.1121/1.4987892.

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Zhang, X., X. X. Chen, and C. L. Morfey. "Acoustic Radiation from a Semi-Infinite Duct With a Subsonic Jet." International Journal of Aeroacoustics 4, no. 1-2 (January 2005): 169–84. http://dx.doi.org/10.1260/1475472053730075.

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The radiation of high-order spinning modes from a semi-infinite exhaust duct is studied numerically. The issues involved have applications to noise radiation from the exhaust duct of an aircraft engine. The numerical method is based on solutions of linearised Euler equations (LEE) for propagation in the duct and near field, and the acoustic analogy for far field radiation. A 2.5D formulation of a linearised Euler equation model is employed to accommodate a single spinning mode propagating over an axisymmetric mean flow field. In the solution process, acoustic waves are admitted into the propagation area surrounding the exit of an axisymmetric duct and its immediate downstream area. The wave admission is realised through an absorbing non-reflecting boundary treatment, which admits incoming waves and damps spurious waves generated by the numerical solutions. The wave propagation is calculated through solutions of linearised Euler equations, using an optimised prefactored compact scheme for spatial discretisation. Far field directivity is estimated by solving the Ffowcs Williams-Hawkings equations. The far field prediction is compared with analytic solutions with good agreement.

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Samanta, Arnab, and Jonathan B. Freund. "A model supersonic buried-nozzle jet: instability and acoustic wave scattering and the far-field sound." Journal of Fluid Mechanics 778 (July 30, 2015): 189–215. http://dx.doi.org/10.1017/jfm.2015.354.

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We consider sound source mechanisms involving the acoustic and instability modes of dual-stream isothermal supersonic jets with the inner nozzle buried within an outer shroud-like nozzle. A particular focus is scattering into radiating sound waves at the shroud lip. For such jets, several families of acoustically coupled instability waves exist, beyond the regular vortical Kelvin–Helmholtz mode, with different shapes and propagation characteristics, which can therefore affect the character of the radiated sound. In our model, the coaxial shear layers are vortex sheets while the incident acoustic disturbances are the propagating shroud modes. The Wiener–Hopf method is used to compute their scattering at the sharp shroud edge to obtain the far-field radiation. The resulting far-field directivity quantifies the acoustic efficiency of different mechanisms, which is particularly important in the upstream direction, where the results show that the scattered sound is more intense than that radiated directly by the shear-layer modes.

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Bianchi, S., A. Corsini, and A. G. Sheard. "Experimental Characterisation of the Far-Field Noise in Axial Fans Fitted with Shaped Tip End-Plates." ISRN Mechanical Engineering 2012 (May 15, 2012): 1–9. http://dx.doi.org/10.5402/2012/212358.

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The authors investigate the far-field noise emissions of a datum fan blade fitted with tip end-plate geometries, originally designed to control the leakage vortex swirl level. The end-plate geometries influence the tip-leakage flow, vortex formation, and swirl level. In doing so, the end-plate geometries influence the sound-power levels. After an evaluation of fan rotors' aerodynamic performance, the study compares the rotors' far-field noise signature characterised in terms of sound-power and pressure-level spectra to enable and assess the end-plate acoustic pay-off. The investigation attempts to establish a cause-and-effect relationship between the tip-flow dynamics and the radiated sound fields, exploring the diverse directivity patterns. The authors found a tonal reduction, due to the enhanced blade-tip end-plates and clarified the relevance of the tip features influencing the radial distribution of the noise sources using coherence analysis. The modified multiple-vortex breakdown end-plate design was effective in reducing the broadband noise, giving an improvement in the frequency range of the turbulent noise.

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37

Lowis, C. R., P. F. Joseph, and A. J. Kempton. "Estimation of the far-field directivity of broadband aeroengine fan noise using an in-duct axial microphone array." Journal of Sound and Vibration 329, no. 19 (September 2010): 3940–57. http://dx.doi.org/10.1016/j.jsv.2010.03.021.

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Ram, Gopi, Durbadal Mandal, Rajib Kar, and Sakti Prasad Ghoshal. "Directivity improvement and optimal far field pattern of time modulated concentric circular antenna array using hybrid evolutionary algorithms." International Journal of Microwave and Wireless Technologies 9, no. 1 (June 25, 2015): 177–90. http://dx.doi.org/10.1017/s1759078715001075.

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In this paper time modulated nine-ring concentric circular antenna array (TMCCAA) using fitness based novel hybrid adaptive differential evolution with particle swarm optimization (ADEPSO) has been studied. ADEPSO is hybrid of fitness based adaptive differential evolution and particle swarm optimization (PSO). Differential evolution is a simple and robust evolutionary algorithm but sometimes causes instability problem; PSO is also a simple, population based robust evolutionary algorithm but has the problem of sub-optimality. ADEPSO has overcome the above individual disadvantages faced by both the algorithms and is used for the design of TMCCAA. The comparative case studies as Case-1 and Case-2 are made with three control parameters like uniform inter-element spacing in rings, inter-ring radii and the switching “ON” times of rings. The same array radiates at various harmonic frequencies. The first two harmonic frequencies have been considered in this work. The numerical results show Case-2, outperforms Case-1 with respect to better side lobe level (SLL), and more improved directivity. Apart from this, the authors have computed powers radiated at the center/fundamental frequency and the first two sideband frequencies, and dynamic efficiency. It is found that power radiated by any sideband frequency is very less as compared with the power radiated at the center frequency. It has been observed that as the sideband frequency increases, side band level decreases to the greater extent as compared with SLL. The aperture size is a very important constraint for the array, since there is an upper limit for the aperture size of a given array in real-life environment. Hence, in our optimization design, the maximum radius of the concentric ring array is constrained.

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Yedeg, Esubalewe Lakie, Eddie Wadbro, and Martin Berggren. "Layout optimization of thin sound-hard material to improve the far-field directivity properties of an acoustic horn." Structural and Multidisciplinary Optimization 55, no. 3 (July 12, 2016): 795–808. http://dx.doi.org/10.1007/s00158-016-1536-9.

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Groeneweg, J. F., and E. J. Rice. "Aircraft Turbofan Noise." Journal of Turbomachinery 109, no. 1 (January 1, 1987): 130–41. http://dx.doi.org/10.1115/1.3262058.

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Recent advances in the understanding of turbofan noise generation and suppression in aircraft engines are reviewed with particular emphasis on NASA research. The review addresses each link in the chain of physical processes which connect unsteady flow interactions with fan blades to far field noise. Mechanism identification and description, duct propagation, radiation, and acoustic suppression are discussed. Recent advances in the experimental technique of fan inflow control assure that inflight generation mechanisms are not masked by extraneous sources in static tests. Rotor blade surface pressure and wake velocity measurements aid the determination of the types and strengths of the generation mechanisms. Approaches to predicting or measuring acoustic mode content, optimizing treatment impedance to maximize attenuation, translating impedance into porous wall structure, and interpreting far field directivity patterns are illustrated by comparisons of analytical and experimental results. A persistent theme of the review is the interdependence of source and acoustic treatment design to minimize far field noise. Areas requiring further research are discussed and the relevance of aircraft turbofan results to quieting other turbomachinery installations is addressed.

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Zhang, Chang Hao, Wei Wang, Hu Wang, and Xun Tao Wang. "The Influence of near-Fault Ground Motions on the Seismic Response of Reinforced Concrete Frame." Applied Mechanics and Materials 90-93 (September 2011): 2633–39. http://dx.doi.org/10.4028/www.scientific.net/amm.90-93.2633.

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This paper examined the engineering characteristics of the near-fault ground motion. The four-story reinforced concrete frame was designed under Code for seismic design of building (GB50011-2010).The SAP2000 software was applied to model it, and the nonlinear time history analyses of structure were implemented. Near-fault ground motions with forward directivity and fling-step and far field ground motions were selected as seismic inputs．The results show that in terms of some structural dynamic response parameters, such as the vertex displacement, between the corner of the layer displacement, and the base shear et al., the structural responses to the ground motion with near-fault are increased by considerable magnitudes when the seismic responses of structures step into the elastic-plastic stage, compared with far-field ground motion, and the influence of damaging the mid-lower structure is significantly greater.

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ZHOU, Q., J. XIE, B. FANG, and Y. PAN. "PARAMETRIC STUDY OF PROPELLER TONE NOISE DUE TO NONUNIFORM FLOWS." Journal of Computational Acoustics 21, no. 03 (July 3, 2013): 1350005. http://dx.doi.org/10.1142/s0218396x13500057.

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This paper investigates the tone noise generation by submarine propellers operating in nonuniform flows. A frequency domain numerical approach is proposed. The flow nonuniformities due to submarine wakes are obtained from Computational Fluid Dynamics method. The results show that the velocity field and its harmonics depend greatly on the hull geometry and its appendages. The prediction of the radiated far-field tone noise due to a seven-bladed, highly-skewed propeller has been performed. A prediction of the acoustic pressure level for various blade skew angles shows that the tone noise reduction amount increases as the inflow velocity increases. The predicted dependence of the radiated noise on the inflow velocity, propeller diameter and blade rotational speed shows that the mean square sound pressure at the first-order blade passing frequency varies as the 3.3 power of the characteristic inflow velocity, the third power of the propeller diameter and with the second power of the blade rotational speed. Effects of blade number on the propeller tone noise directivity are also discussed, which shows that the characteristic coupling order between the main inflow harmonic and the rotating blades is the key to determine the directivity pattern.

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Long, Shuang Li, Hong Nie, and Xin Xu. "Aeroacoustic Study on a Simplified Nose Landing Gear." Applied Mechanics and Materials 184-185 (June 2012): 18–23. http://dx.doi.org/10.4028/www.scientific.net/amm.184-185.18.

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Simulation analysis and experiment research are performed on the aeroacoustic noise of a landing gear component in this paper. Detached Eddy Simulation (DES) is used to produce the flow field of the model. The Ffowcs-Williams/Hawkings (FW-H) equation is used to calculate the acoustic field. The sound field radiated from the model is measured in the acoustic wind tunnel. A comparison shows that the simulation results agree well with the experiment results under the acoustic far field condition. The results show that the noise radiated from the model is broadband noise. The directivity of the noise source is like a type of dipole. The wheel is the largest contributor and the strut is the least contributor to the landing gear noise. The results can provide some reference for low noise landing gear design.

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Zhu, Wei Jun, Jens Nørær Sørensen, and Wen Zhong Shen. "An Aerodynamic Noise Propagation Model for Wind Turbines." Wind Engineering 29, no. 2 (March 2005): 129–41. http://dx.doi.org/10.1260/0309524054797168.

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A model based on 2-D sound ray theory for aerodynamic noise propagation from wind turbine rotating blades is introduced. The model includes attenuation factors from geometric spreading, sound directivity of source, air absorption, ground deflection and reflection, as well as effects from temperature and airflow. At a given receiver point, the sound pressure is corrected by taking into account these propagation effects. As an overall assumption, the noise field generated by the wind turbine is simplified as a point source placed at the hub height of the wind turbine. This assumtion is reasonable, for the receiver is located in the far field, at distances from the wind turbine that are much longer than the diameter of the rotor.

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Song, Lianning, and Zaiping Nie. "A Truncated Singular Value Decomposition Enhanced Nested Complex Source Beam Method." International Journal of Antennas and Propagation 2017 (2017): 1–7. http://dx.doi.org/10.1155/2017/9072421.

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This work presents a novel matrix compression algorithm to improve the computational efficiency of the nested complex source beam (NCSB) method. The algorithm is based on the application of the truncated singular value decomposition (TSVD) to the multilevel aggregation, translation, and disaggregation operations in NCSB. In our implementation, the aggregation/disaggregation matrices are solved by the truncated far-field matching, which is based on the directional far-field radiation property of the complex source beams (CSBs). Furthermore, the translation matrices are obtained according to the beam width of CSBs. Due to the high directivity of the radiation patterns of CSBs, all the far-field related interaction matrices are low-ranked. Therefore, TSVD can be employed and a new set of equivalent sources can be constructed by a linear combination of the original CSBs. It is proved that the radiation power of the new sources is proportional to the square of the corresponding singular values. This provides a theoretical guideline to drop the insignificant singular vectors in the calculation. In doing so, the efficiency of the original NCSB method can be much improved while a reasonably good accuracy is maintained. Several numerical tests are conducted to validate the proposed method.

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TINNEY, C. E., L. S. UKEILEY, and M. N. GLAUSER. "Low-dimensional characteristics of a transonic jet. Part 2. Estimate and far-field prediction." Journal of Fluid Mechanics 615 (November 25, 2008): 53–92. http://dx.doi.org/10.1017/s0022112008003601.

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Complementary low-dimensional techniques are modified to estimate the most energetic turbulent features of a Mach 0.85 axisymmetric jet in the flow's near-field regions via spectral linear stochastic estimation. This model estimate is three-dimensional, comprises all three components of the velocity field and is time resolved. The technique employs the pressure field as the unconditional input, measured within the hydrodynamic periphery of the jet flow where signatures (pressure) are known to comprise a reasonable footprint of the turbulent large-scale structure. Spectral estimation coefficients are derived from the joint second-order statistics between coefficients that are representative of the low-order pressure field (Fourier-azimuthal decomposition) and of the low-order velocity field (proper orthogonal decomposition). A bursting-like event is observed in the low-dimensional estimate and is similar to what was found in the low-speed jet studies of others. A number of low-dimensional estimates are created using different velocity–pressure mode combinations from which predictions of the far-field acoustics are invoked using Lighthill's analogy. The overall sound pressure level (OASPL) directivity is determined from the far-field prediction, which comprises qualitatively similar trends when compared to direct measurements at r/D=75. Retarded time topologies of the predicted field at 90° and 30° are also shown to manifest, respectively, high- and low-frequency wave-like motions when using a combination of only the low-order velocity modes (m=0, 1, 2). This work thus constitutes a first step in developing low-dimensional and dynamical system models from hydrodynamic pressure signatures for estimating and predicting the behaviour of the energy-containing events that govern many of the physical constituents of turbulent flows.

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Qu, Ke, Binbin Zou, Jingjing Chen, Yingge Guo, and Runtian Wang. "Experimental Study of a Broadband Parametric Acoustic Array for Sub-Bottom Profiling in Shallow Water." Shock and Vibration 2018 (December 25, 2018): 1–8. http://dx.doi.org/10.1155/2018/3619257.

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Broadband parametric acoustic arrays appear to offer advantages for shallow water sub-bottom profiling. In this paper, the performance of a broadband parametric acoustic array system was experimentally evaluated. In tank experiments using the nonlinear parabolic wave (KZK) equation, the directivity, source level, parametric acoustic array length, and penetration depth were evaluated. Based on Berktay’s far-field solution, the system’s emission signal was designed. According to sea trials of the broadband parametric acoustic array system as designed, a clear sub-bottom profile was obtained. Moreover, buried pipelines in the seabed were effectively detected, verifying the system’s effectiveness.

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Baruah, Juhi K., Kandarpa Kumar Sarma, and Sivaranjan Goswami. "Designing of a 2x2 E-shaped Microstrip Patch Grid Antenna." WSEAS TRANSACTIONS ON COMMUNICATIONS 20 (August 10, 2021): 123–27. http://dx.doi.org/10.37394/23204.2021.20.16.

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In this work, a 2×2 grid of E-shaped patch antennas is proposed. The design of the grid is achieved through the design of a single element, the design of a 1×2 array and finally the design of the 2×2 grid on an FR4 epoxy substrate of thickness 1.5 mm. A corporate feed network of microstrip lines is used to excite the array. The performance of each stage is studied in terms of the return loss parameter, the far field gain, and the beam-widths are observed in each case from simulation results. The resonant frequency in each case is 3.8 GHz. It is observed that as the number of elements is increased, the beam-width reduces. In other words, the directivity is increased. Further, it is also observed that the gain and bandwidth is the minimum for the single patch, followed by that of the 1×2 array and the maximum for the 2×2 grid. Thus, the construction of the grid leads to increase in gain, bandwidth and directivity of the antenna.

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Margnat, F., V. Morinière, and Y. Gervais. "Noise Radiated by Flow Impingement on a Flat Plate Using DNS with a Virtual Boundary Method." International Journal of Aeroacoustics 4, no. 1-2 (January 2005): 117–34. http://dx.doi.org/10.1260/1475472053730101.

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Uniform flow impinging on the rectangular leading edge of a flat plate is simulated using an incompressible, two-dimensional direct numerical simulation with uniform grid, the solid surface being modelled by the virtual boundary method previously developed by Goldstein et al. Access is given to the hydrodynamic field at a Reynolds number of 500, which shows the separation at the edge, the development of a double recirculating bubble, a periodic vortex-shedding, and the generation of a boundary layer profile after the re-attachment point. Curle's analogy is then applied to evaluate the sound emission in the far field. Results are given in terms of far field pressure levels directivity and pressure spectra associated with surfacic or volumic sources. The emission from the surfacic sources reaches its maximum value in the direction perpendicular to the plate and acoustic frequencies tend to follow the main wall pressure frequencies of the flow, while the emission from the volumic sources exhibits a shear noise behaviour and acoustic frequencies seem to be the double of the main velocity frequencies.

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Ayton, Lorna J., and N. Peake. "On high-frequency sound generated by gust–aerofoil interaction in shear flow." Journal of Fluid Mechanics 766 (February 4, 2015): 297–325. http://dx.doi.org/10.1017/jfm.2015.21.

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AbstractA theoretical model is constructed to predict the far-field sound generated by high-frequency gust–aerofoil interaction in steady parallel shear flow, including the effects of aerofoil thickness. Our approach is to use asymptotic analysis of the Euler equations linearised about steady parallel shear flow, in the limits of high frequency and small, but non-zero, aerofoil thickness and Mach number. The analysis splits the flow into various regions around the aerofoil; local inner regions around the leading and the trailing edges where sound is generated and scattered; a surface transition region accounting for the curvature of the aerofoil; a wake transition region downstream of the aerofoil; and an outer region through which the sound propagates to the observer. Solutions are constructed in all regions, and matched using the principle of matched asymptotic expansions to yield the first two terms in the expansion of both the amplitude and the phase of the far-field pressure. Result are computed for the particular case of scattering of a gust by a symmetric Joukowski aerofoil placed in symmetric Gaussian parallel shear flow. The introduction of mean shear is shown to have a significant effect on the far-field directivity and on the total radiated power.

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Journal articles on the topic 'Far-field directivity'

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