Approximate calculation of low‐frequency three‐dimensional magnetotelluric responses using a multiple plate model

Geophysics ◽  
1992 ◽  
Vol 57 (1) ◽  
pp. 106-115 ◽  
Author(s):  
Carlos Flores ◽  
R. Nigel Edwards
Keyword(s):  
Three Dimensional ◽  
Low Frequency ◽  
Test Site ◽  
Observation Point ◽  
Skin Depth ◽  
Approximate Algorithm ◽  
Published Data ◽  
Low Frequencies ◽  
Severe Problem ◽  
Layered Earth

The numerical methods now used to calculate the magnetotelluric (MT) response of three‐dimensional (3-D) conductive inhomogeneities are still expensive in terms of computing cost. This is a more severe problem in many actual field conditions where multiple conductors exist. There is then a clear need for the development of faster algorithms for data interpretation. We can rapidly compute the 3-D MT response of a multiple‐conductor structure embedded in a layered earth at low frequencies using an approximate algorithm where each conductor is simulated by a set of plates. There are two limitations to the technique. The first low‐frequency limitation is a requirement that the induction number of any plate in the model be small compared with unity. The second condition specifies that the range from an observation point on the surface of the earth to a typical plate must be small compared with the skin depth in the layered background medium. These conditions allow us to ignore local self‐induction within the plates and mutual induction between the plates and the host medium. The secondary anomalous fields generated by any plate are assumed to be due to a distribution of current dipoles in the plane of the plate. The strengths of the distributed dipoles are found by solving a Fredholm integral equation of the second kind with the layered earth MT electric field at depth as the driving function. MT responses obtained with our approximate algorithm for three prismatic models compare favorably with previously published data obtained with complete algorithms for the same models. The use of the technique to model multiple conductors is illustrated with controlled source MT data collected at the Cavendish Test Site.

Active Control of Low Frequency Modes in an Aircraft Fuselage Using Spatially Weighted Arrays

2000 ◽  
Vol 122 (3) ◽  
pp. 227-234 ◽  
Author(s):  
Steven A. Lane ◽  
Robert L. Clark ◽  
Steve C. Southward
Keyword(s):  
Three Dimensional ◽  
Pressure Sensors ◽  
Low Frequency ◽  
Single Mode ◽  
Open Loop ◽  
H2 Control ◽  
Low Frequencies ◽  
Control Approach ◽  
Aircraft Fuselage ◽  
Transducer Arrays

Acoustic enclosures such as aircraft cabins often display lightly damped modal behavior at low frequencies where passive treatments are impractical due to mass and volume constraints. This work presents a feedback control approach using dynamic H2 controllers implemented with spatially weighted arrays of collocated pressure sensors and constant volume-velocity actuators. The open-loop system is shaped using spatially weighted transducer arrays to yield increased pole-zero separation, which results in better closed-loop performance. The transducer arrays are weighted to emphasize coupling to a particular acoustic mode or modes, which facilitates global control of the targeted dynamics. This work presents a method to determine the spatial weighting vectors for the transducer arrays from frequency response measurements. The development and implementation of low-order, dynamic H2 control laws is also discussed. Experimental results are presented for a single-mode and a multiple-mode controller implemented on an aircraft fuselage section, and demonstrate significant reduction of the targeted acoustic modes. To the best of the authors’ knowledge, this is the first experimental implementation of a feedback controller of this type capable of achieving such levels of global reduction in a three-dimensional acoustic system. [S0739-3717(00)02303-5]

Receptivity to free-stream vorticity of flow past a flat plate with elliptic leading edge

2010 ◽  
Vol 653 ◽  
pp. 245-271 ◽  
Author(s):  
L.-U. SCHRADER ◽  
L. BRANDT ◽  
C. MAVRIPLIS ◽  
D. S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Three Dimensional ◽  
Low Frequency ◽  
Leading Edge ◽  
Sound Waves ◽  
Streamwise Vorticity ◽  
Low Frequencies ◽  
Free Stream Turbulence

Receptivity of the two-dimensional boundary layer on a flat plate with elliptic leading edge is studied by numerical simulation. Vortical perturbations in the oncoming free stream are considered, impinging on two leading edges with different aspect ratio to identify the effect of bluntness. The relevance of the three vorticity components of natural free-stream turbulence is illuminated by considering axial, vertical and spanwise vorticity separately at different angular frequencies. The boundary layer is most receptive to zero-frequency axial vorticity, triggering a streaky pattern of alternating positive and negative streamwise disturbance velocity. This is in line with earlier numerical studies on non-modal growth of elongated structures in the Blasius boundary layer. We find that the effect of leading-edge bluntness is insignificant for axial free-stream vortices alone. On the other hand, vertical free-stream vorticity is also able to excite non-modal instability in particular at zero and low frequencies. This mechanism relies on the generation of streamwise vorticity through stretching and tilting of the vertical vortex columns at the leading edge and is significantly stronger when the leading edge is blunt. It can thus be concluded that the non-modal boundary-layer response to a free-stream turbulence field with three-dimensional vorticity is enhanced in the presence of a blunt leading edge. At high frequencies of the disturbances the boundary layer becomes receptive to spanwise free-stream vorticity, triggering Tollmien–Schlichting (T-S) modes and receptivity increases with leading-edge bluntness. The receptivity coefficients to free-stream vortices are found to be about 15% of those to sound waves reported in the literature. For the boundary layers and free-stream perturbations considered, the amplitude of the T-S waves remains small compared with the low-frequency streak amplitudes.

Impedance spectroscopy and circuit modeling of Southern pine above 20% moisture content

Holzforschung ◽  
2008 ◽  
Vol 62 (6) ◽  
Author(s):  
Samuel L. Zelinka ◽  
Douglas R. Rammer ◽  
Donald S. Stone
Keyword(s):  
Moisture Content ◽  
Low Frequency ◽  
Charge Carriers ◽  
Electrode Spacing ◽  
Published Data ◽  
Southern Pine ◽  
Low Frequencies ◽  
Warburg Impedance ◽  
Mineral Ions ◽  
Pinus Spp

Abstract Impedance spectra were collected from Southern pine (Pinus spp.) equilibrated to a moisture content 20% and a saturated condition. Cylindrical graphite electrodes were embedded in the wood so that they met nearly end-to-end along a line parallel to the grain, and impedance properties were characterized as functions of electrode spacing and electrode contact pressure at frequencies between 1×10-1 and 3×105 Hz. The data exhibit a low-frequency tail that can be fit with a Warburg element, indicative of a diffusive motion of charge carriers. Values of the Warburg impedance were compared with previously published data on molar conductivities of endogenous mineral ions in wood. The data suggest that mineral ions are the predominant charge carriers in wood at low frequencies and high moisture contents based on the strong correlation between the Warburg impedance and molar conductivities.

Propagation of Low Frequency Elastic Disturbances in a Three-Dimensional Composite Material

1975 ◽  
Vol 42 (1) ◽  
pp. 159-164 ◽  
Author(s):  
W. Kohn
Keyword(s):  
Secular Equation ◽  
Displacement Vector ◽  
Three Dimensional ◽  
Low Frequency ◽  
Three Dimensions ◽  
Low Frequencies ◽  
One Dimensional ◽  
Coupled Partial Differential Equations ◽  
Constant Coefficients ◽  
The Mean

This paper is a generalization to three dimensions of an earlier study for one-dimensional composites. We show here that in the limit of low frequencies the displacement vector ui(r,t) can be written in the form ui (r,t) = (∂ij + vijl (r) ∂/∂xl + …) Uj (r,t). Here Uj (r,t) is a slowly varying vector function of r and t which describes the mean displacement of each cell of the composite. Its components satisfy a set of three coupled partial differential equations with constant coefficients. These coefficients are obtainable from the three-by-three secular equation which yields the low-lying normal mode frequencies, ω(k). Information about local strains is contained in the function vijl(r), which is characteristic of static deformations, and is discussed in detail. Among applications of this method is the structure of the head of a pulse propagating in an arbitrary direction.

scholarly journals Into the dark: patterns of middle ear adaptations in subterranean eulipotyphlan mammals

2017 ◽  
Vol 4 (9) ◽  
pp. 170608 ◽  
Author(s):  
Daisuke Koyabu ◽  
Misato Hosojima ◽  
Hideki Endo
Keyword(s):  
Middle Ear ◽  
Three Dimensional ◽  
Low Frequency ◽  
Frequency Noise ◽  
Terrestrial Environment ◽  
Low Frequencies ◽  
Geometric Morphometric ◽  
Key Innovation ◽  
Ear Ossicles ◽  
Stapes Footplate

Evolution of the middle ear ossicles was a key innovation for mammals, enhancing the transmission of airborne sound. Radiation into various habitats from a terrestrial environment resulted in diversification of the auditory mechanisms among mammals. However, due to the paucity of phylogenetically controlled investigations, how middle ear traits have diversified with functional specialization remains unclear. In order to identify the respective patterns for various lifestyles and to gain insights into fossil forms, we employed a high-resolution tomography technique and compared the middle ear morphology of eulipotyphlan species (moles, shrews and hedgehogs), a group that has radiated into various environments, such as terrestrial, aquatic and subterranean habitats. Three-dimensional geometric morphometric analysis was conducted within a phylogenetically controlled framework. Quantitative shapes were found to strongly reflect the degree of subterranean lifestyle and weakly involve phylogeny. Our analyses demonstrate that subterranean adaptation should include a relatively shorter anterior process of the malleus, an enlarged incus, an enlarged stapes footplate and a reduction of the orbicular apophysis. These traits arguably allow improving low-frequency sound transmission at low frequencies and inhibiting the low-frequency noise which disturbs the subterranean animals in hearing airborne sounds.

scholarly journals Investigation of Acoustic Properties on Wideband Sound-Absorber Composed of Hollow Perforated Spherical Structure with Extended Tubes and Porous Materials

2020 ◽  
Vol 10 (24) ◽  
pp. 8978
Author(s):  
Dengke Li ◽  
Zhongcheng Jiang ◽  
Lin Li ◽  
Xiaobo Liu ◽  
Xianfeng Wang ◽  
...  
Keyword(s):  
Sound Absorption ◽  
Three Dimensional ◽  
Low Frequency ◽  
Acoustic Properties ◽  
Low Frequencies ◽  
Frequency Sound ◽  
Sound Absorber ◽  
Spherical Structure ◽  
Absorption Performance ◽  
Melamine Foam

Traditional porous media such as melamine foam absorb sound due to their three-dimensional porous struts. However, the acoustic properties at low frequencies are greatly related to its thickness. In this paper, a novel type of thin and lightweight sound absorber composed of melamine foam and hollow perforated spherical structure with extended tubes (HPSET) is introduced to enhance the sound absorption performance at low frequencies. A theoretical model for the normal absorption coefficient of the HPSET with melamine foam is established. Good agreements are observed between the simulated and the experimental results. Compared with the virgin melamine foam, the proposed absorber can greatly improve the low-frequency sound absorption and retain the mid- to high-frequency sound absorption, while the thickness of the proposed absorber is less than 1/28 of the wavelength.

scholarly journals Stochastic BEM for the Vibroacoustic Analysis of Three-Dimensional Structures

2011 ◽  
Vol 2011 ◽  
pp. 1-12 ◽  
Author(s):  
R. D'Amico ◽  
A. Pratellesi ◽  
M. Pierini ◽  
N. Baldanzini
Keyword(s):  
Crucial Role ◽  
Three Dimensional ◽  
Low Frequency ◽  
Wide Frequency Range ◽  
Frequency Range ◽  
Small Perturbations ◽  
Low Frequencies ◽  
Response Sensitivity ◽  
Basic Assumptions ◽  
Prediction Reliability

Nowadays, extending the NVH prediction reliability to the whole frequency range is an attractive goal of vibroacoustics. Deterministic methodologies are well established for the low-frequency range, but, decreasing the wavelength, energy-based methods are necessary. In such a range, a crucial role is played by small perturbations which highly influence the response sensitivity. Moreover, taking into account these variations allows to make the product design more robust and even quicker. Introducing geometrical uncertainties within the classic BEM formulation allows to obtain the so-called stochastic BEM. As a result, the solution shows deterministic behaviour at low frequencies; decreasing the wavelength, the effect of the uncertainties smooths the response. Consequently, it is possible to obtain an averaged trend over the whole frequency range which asymptotically tends to the deterministic one. In this paper, we deal with three-dimensional acoustic SBEM. First, the formulation and its basic assumptions are presented. Secondly, they are applied to academic cases to show its potentialities in predicting vibroacoustic behaviour over a wide frequency range.

Numerical modeling of the electromagnetic response of three‐dimensional conductors in a layered earth

Geophysics ◽  
1985 ◽  
Vol 50 (4) ◽  
pp. 665-676 ◽  
Author(s):  
M. E. Best ◽  
P. Duncan ◽  
F. J. Jacobs ◽  
W. L. Scheen
Keyword(s):  
Finite Element Method ◽  
Finite Element ◽  
Numerical Modeling ◽  
Complete Solution ◽  
Three Dimensional ◽  
Skin Depth ◽  
The Body ◽  
Electromagnetic Response ◽  
Layered Earth ◽  
Element Method

This paper describes a numerical modeling program which was developed for computing the electromagnetic response of a three‐dimensional conductor in a layered earth. The algorithm is a hybrid one, in the sense that a finite‐element method is combined with an integral equation approach for the complete solution of the problem. In this approach, two relatively small matrices are inverted to obtain the secondary magnetic fields at the interior nodes of the subdivided volume. The finite‐element method requires special attention at low frequencies, i.e., when the body dimensions are small compared with the skin depth. The method was tested with various models involving spheres and rectangular dikes, and comparisons have been made with analytical, numerical, and scale‐modeling data. These comparisons generally show good agreement between our numerical model and the other models tested. Examples are given which show the flexibility and usefulness of this modeling algorithm when applied to a ground or airborne prospecting system with a coil separation of 10 m.

Handheld Laser Scanner Research

2019 ◽  
Vol 952 (10) ◽  
pp. 47-54
Author(s):  
A.V. Komissarov ◽  
A.V. Remizov ◽  
M.M. Shlyakhova ◽  
K.K. Yambaev
Keyword(s):  
Point Cloud ◽  
Three Dimensional ◽  
Laser Scanner ◽  
Test Site ◽  
Optical Systems ◽  
Advantages And Disadvantages ◽  
Site Survey ◽  
Laser Scanners ◽  
Three Dimensional Models ◽  
The Stability

The authors consider hand-held laser scanners, as a new photogrammetric tool for obtaining three-dimensional models of objects. The principle of their work and the newest optical systems based on various sensors measuring the depth of space are described in detail. The method of simultaneous navigation and mapping (SLAM) used for combining single scans into point cloud is outlined. The formulated tasks and methods for performing studies of the DotProduct (USA) hand-held laser scanner DPI?8X based on a test site survey are presented. The accuracy requirements for determining the coordinates of polygon points are given. The essence of the performed experimental research of the DPI?8X scanner is described, including scanning of a test object at various scanner distances, shooting a test polygon from various scanner positions and building point cloud, repeatedly shooting the same area of the polygon to check the stability of the scanner. The data on the assessment of accuracy and analysis of research results are given. Fields of applying hand-held laser scanners, their advantages and disadvantages are identified.

scholarly journals Earless toads sense low frequencies but miss the high notes

2017 ◽  
Vol 284 (1864) ◽  
pp. 20171670 ◽  
Author(s):  
Molly C. Womack ◽  
Jakob Christensen-Dalsgaard ◽  
Luis A. Coloma ◽  
Juan C. Chaparro ◽  
Kim L. Hoke
Keyword(s):  
High Frequency ◽  
Species Differences ◽  
Low Frequency ◽  
Auditory Brainstem ◽  
Low Frequencies ◽  
Hearing Sensitivity ◽  
Sensory Pathways ◽  
Airborne Sound ◽  
Vibrational Sensitivity ◽  
Species Specific

Sensory losses or reductions are frequently attributed to relaxed selection. However, anuran species have lost tympanic middle ears many times, despite anurans' use of acoustic communication and the benefit of middle ears for hearing airborne sound. Here we determine whether pre-existing alternative sensory pathways enable anurans lacking tympanic middle ears (termed earless anurans) to hear airborne sound as well as eared species or to better sense vibrations in the environment. We used auditory brainstem recordings to compare hearing and vibrational sensitivity among 10 species (six eared, four earless) within the Neotropical true toad family (Bufonidae). We found that species lacking middle ears are less sensitive to high-frequency sounds, however, low-frequency hearing and vibrational sensitivity are equivalent between eared and earless species. Furthermore, extratympanic hearing sensitivity varies among earless species, highlighting potential species differences in extratympanic hearing mechanisms. We argue that ancestral bufonids may have sufficient extratympanic hearing and vibrational sensitivity such that earless lineages tolerated the loss of high frequency hearing sensitivity by adopting species-specific behavioural strategies to detect conspecifics, predators and prey.

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