The Acoustical Impedance at the Junction of Non-Coaxial Cylindrical Ducts

1992 ◽  
Vol 11 (4) ◽  
pp. 114-123 ◽  
Author(s):  
Keith S. Peat
Keyword(s):  
Plane Wave ◽  
Cross Section ◽  
Plane Waves ◽  
Low Frequency ◽  
Evanescent Waves ◽  
Wave Analysis ◽  
Cross Sectional ◽  
Low Frequencies ◽  
Duct Systems

At low frequencies, only plane waves can continuously propagate along uniform ducts, but evanescent, non-planar waves arise from discontinuities in the duct cross-section. The effect of these evanescent waves can be considered as an acoustical impedance to the propagation of plane waves. It is then possible to increase the accuracy of low frequency plane-wave analysis of duct systems with cross-sectional discontinuities, by inclusion of these impedance corrections. This paper considers the derivation of the acoustical impedance at the junction of non-coaxial circular ducts, a common feature within silencer systems.

Procedure to Estimate the In-Duct Sound Power in the High Frequency Range With Non-Plane Waves

2012 ◽  
Author(s):  
Antti Hynninen ◽  
Mats Åbom
Keyword(s):  
Plane Wave ◽  
Gas Turbines ◽  
Internal Combustion Engines ◽  
Plane Waves ◽  
Low Frequency ◽  
Sound Power ◽  
Combustion Engines ◽  
Duct Systems ◽  
Acoustic Characterization ◽  
Wave Range

The acoustic characterization of fluid machines, e.g., internal combustion engines, compressors, or fans is of great importance when designing the connected duct systems and its silencers. For machines connected to large ducts where also the non-plane wave range is important, for instance large diesels and gas turbines, a suitable way to characterize the source is to determine the sound power under reflection free conditions. For the low frequency plane wave range in-duct sound power can be measured with the widely used two microphone method. The goal of this study is to investigate how, starting from the two-microphone approach, a suitable wall mounted microphone configuration can be defined and used to estimate the propagating in-duct sound power also beyond the plane wave range. For this purpose an acoustic source test-rig was built and numerical simulations were also conducted. The in-duct sound power from monopole, dipole, and quadrupole source types was determined using twelve wall mounted microphones and cross-spectra averaging methods. The in-duct results were compared against sound power measured using the reverberation room method (ISO 3741). Based on the simulations and the experimental results the best microphone positions and weighting factors were determined.

PIV Analysis of Dynamic Velocity Profiles in Non-Newtonian Drilling Fluids Exposed to Oscillatory Motion

2018 ◽  
Author(s):  
Maduranga Amaratunga ◽  
Roar Nybø ◽  
Rune W. Time
Keyword(s):  
Low Frequency ◽  
Velocity Profiles ◽  
Liquid Column ◽  
Oscillatory Motion ◽  
Velocity Shear ◽  
Drilling Fluids ◽  
Cross Sectional ◽  
Low Frequencies ◽  
Wide Range ◽  
Dynamic Velocity

Drilling fluids experience a wide range of shear rates and oscillatory motion while circulating through the well and also during the operations for solids control. Therefore, it is important to investigate the influence of oscillatory fields on the velocity profiles, shear rate and resulting rheological condition of non-Newtonian polymers, which are additives in drilling fluids. In this paper, we present the dynamic velocity profiles within both Newtonian (deionized water) and non-Newtonian liquids (Polyanionic Cellulose – PAC) exposed to oscillatory motion. A 15 cm × 15 cm square cross-sectional liquid column was oscillated horizontally with very low frequencies (0.75–1.75 Hz) using a laboratory made oscillating table. The dynamic velocity profiles at the bulk of the oscillating liquid column were visualized by the Particle Image Velocimetry (PIV) method, where the motion of fluid is optically visualized using light scattering “seeding” particles. Increased frequency of oscillations lead to different dynamic patterns and ranges of velocity-shear magnitudes. The experiments are part of a comprehensive study aimed at investigating the influence of low frequency oscillations on particle settling in non-Newtonian drilling fluids. It is discussed, how such motion imposed on polymeric liquids influences both flow dynamics as well as local settling velocities of cuttings particles.

A semianalytical solution for the propagation of electromagnetic waves in 3‐D lossy orthotropic media

Geophysics ◽  
2001 ◽  
Vol 66 (4) ◽  
pp. 1141-1148 ◽  
Author(s):  
José M. Carcione ◽  
Fabio Cavallini
Keyword(s):  
Magnetic Field ◽  
Plane Wave ◽  
Electromagnetic Waves ◽  
Plane Waves ◽  
Computer Experiment ◽  
Transverse Magnetic ◽  
Wave Analysis ◽  
Helmholtz Equations ◽  
Magnetic Surfaces ◽  
Time Histories

We derive an analytical solution for electromagnetic waves propagating in a 3‐D lossy orthotropic medium for which the electric permittivity tensor is proportional to the magnetic permeability tensor. The solution is obtained through a change of coordinates that transforms the spatial differential operator into a pure Laplace operator and the differential equations for the electric and magnetic field components into pure Helmholtz equations. A plane‐wave analysis gives the expression of the slowness and attenuation surfaces as a function of frequency and propagation direction. The transverse electric and transverse magnetic surfaces degenerate to one repeated sheet so that, in any direction, the two differently polarized plane waves have the same slowness. A computer experiment with realistic geophysical parameters has shown that the anisotropic propagation and dissipation properties emerging from plane‐wave analysis agree with the different time histories of the magnetic field computed at a number of representative receiver locations.

The low frequency scalar diffraction by an elliptic disc

1967 ◽  
Vol 63 (4) ◽  
pp. 1273-1280 ◽  
Author(s):  
B. D. Sleeman
Keyword(s):  
Plane Wave ◽  
Cross Section ◽  
Scattered Field ◽  
Low Frequency ◽  
Field Amplitude ◽  
Wave Number ◽  
Series Expansions ◽  
Far Field ◽  
Scalar Diffraction ◽  
Scattering Cross

SummaryThe problem of scalar Dirichlet diffraction of a plane wave by an elliptic disc is discussed. A scheme is given whereby the low frequency expansion of the scattered field may be readily obtained. Series expansions are obtained for the far-field amplitude up to and including the second order in the wave number. The first two terms of the scattering cross-section are also derived.

Numerical analysis of the transmission loss of dissipative mufflers with polygonal cross-section

2021 ◽  
Vol 263 (3) ◽  
pp. 3108-3117
Author(s):  
Thomas Geyer ◽  
Christopher Mai ◽  
Anna-Sophia Henke
Keyword(s):  
Cross Section ◽  
Transmission Loss ◽  
Broadband Noise ◽  
The Finite Element Method ◽  
Practical Application ◽  
Cross Sectional ◽  
Low Frequencies ◽  
Maximum Transmission ◽  
Calculation Procedures ◽  
Good Agreement

Dissipative mufflers are often used for the reduction of broadband noise transmitted in ducts. Many common calculation procedures for the transmission loss of such mufflers require conventional shapes like rectangular or circular cross-sectional areas. In an effort to analyze the effect of the cross-sectional area of dissipative mufflers on the resulting noise reduction, the transmission loss of axially uniform mufflers with polygonal cross-sectional areas was investigated using the finite element method. The mufflers are designed to have the same open area, and hence in a practical application would lead to a similar pressure drop. The results were compared to those obtained with the well known approximative method of Piening. Good agreement between simulation and estimation was found regarding basic trends at low frequencies, while notable differences were revealed regarding the maximum transmission loss.

SCATTERING OF DIRAC PARTICLES BY GRAVITATIONAL PLANE WAVES

1995 ◽  
Vol 04 (03) ◽  
pp. 291-304 ◽  
Author(s):  
DONATO BINI ◽  
VALERIA FERRARI
Keyword(s):  
Dirac Equation ◽  
Plane Wave ◽  
Cross Section ◽  
Plane Waves ◽  
Scattering Cross Section ◽  
Bogoliubov Transformation ◽  
Dirac Particles ◽  
Scattering Cross ◽  
Complete Set

We find a complete set of solutions of the Dirac equation in the background of a gravitational plane wave. The Bogoliubov transformation relating the in and the out modes and the scattering cross-section is also derived.

scholarly journals Low-Frequency Sea Surface Radar Doppler Echo

2018 ◽  
Author(s):  
Yury Yu. Yurovsky ◽  
Vladimir N. Kudryavtsev ◽  
Semyon A. Grodsky ◽  
Bertrand Chapron
Keyword(s):  
Cross Section ◽  
Modulation Transfer Function ◽  
Low Frequency ◽  
Sea Surface ◽  
Doppler Velocity ◽  
Modulation Transfer ◽  
Low Frequencies ◽  
Wave Range ◽  
Wave Slope ◽  
Backscatter Cross Section

Observed sea surface Ka-band normalized radar backscatter cross section (NRCS) and Doppler velocity (DV) exhibit energy at low frequencies (LF) below the surface wave range. It is shown that non-linearity in NRCS-wave slope Modulation Transfer Function (MTF) and inherent NRCS averaging within the footprint account for the NRCS and DV LF variance with the exception of VV NRCS for which almost half of the LF variance is attributable to wind fluctuations. Although the distribution of radar DV is quasi-Gaussian suggesting virtually little impact of non-linearity, the LF DV variations arise due to footprint averaging of correlated local DV and non-linear NRCS. Numerical simulations demonstrate that MTF non-linearity weakly affects traditional linear MTF estimate (less than 10% for |MTF|< 20). Thus the linear MTF is a good approximation to evaluate the DV averaged over large footprints typical of satellite observations.

scholarly journals Hypersensitivity of Vestibular System to Sound and Pseudoconductive Hearing Loss in Deaf Patients

2014 ◽  
Vol 2014 ◽  
pp. 1-5 ◽  
Author(s):  
Seyede Faranak Emami
Keyword(s):  
Vestibular System ◽  
Low Frequency ◽  
Vestibular Function ◽  
Multiple Comparisons ◽  
Vestibular Evoked Myogenic Potentials ◽  
Cross Sectional ◽  
Low Frequencies ◽  
Anova Test ◽  
Hearing Thresholds ◽  
One Way Anova

The objective of this cross-sectional study is to compare bone-conducted low-frequency hearing thresholds (BClf) to cervical vestibular evoked myogenic potentials (cVEMPs) findings in prelingual adult deaf patients. The fifty participants (100 ears) included twenty healthy controls and thirty other subjects selected from patients who presented with bilateral prelingual deafness to Department of Audiology of Hamadan University of Medical Sciences and Health Services (Hamadan, Iran). Assessments comprised of audiological evaluations, cVEMPs, and computerized tomography scans. Twenty deaf patients (forty affected ears) with bilateral decreased vestibular excitability as detected by abnormal cVEMPs revealed that BClf hearing thresholds were completely absent. Ten deaf patients (twenty unaffected ears) with normal cVEMPs reported a sensation of the sound at BClf hearing thresholds (the mean for 250 Hz=41 dBHL and for 500 Hz=57.75 dBHL). Multiple comparisons of mean p 13 latencies, mean n23 latencies and peak-to-peak amplitudes between three groups were significant (P = 0.01 for all, one-way ANOVA test). Multiple Comparisons of mean BClf between three groups were significant (P = 0.00, One-way ANOVA test). Conclusion. Hypersensitivity of vestibular system to sound augments BClf hearing thresholds in deaf patients. The sensation of the sound at low frequencies may be present in patients with total deafness and normal vestibular function (predominantly saccule). This improvement disappears when saccular function is lost.

The Ultra Weak Variational Formulation Using Bessel Basis Functions

2012 ◽  
Vol 11 (2) ◽  
pp. 400-414 ◽  
Author(s):  
Teemu Luostari ◽  
Tomi Huttunen ◽  
Peter Monk
Keyword(s):  
Plane Wave ◽  
Variational Formulation ◽  
Bessel Functions ◽  
Plane Waves ◽  
Rectangular Domain ◽  
Evanescent Waves ◽  
Basis Functions ◽  
Test Cases ◽  
Helmholtz Problem ◽  
Shaped Domain

AbstractWe investigate the ultra weak variational formulation (UWVF) of the 2-D Helmholtz equation using a new choice of basis functions. Traditionally the UWVF basis functions are chosen to be plane waves. Here, we instead use first kind Bessel functions. We compare the performance of the two bases. Moreover, we show that it is possible to use coupled plane wave and Bessel bases in the same mesh. As test cases we shall consider propagating plane and evanescent waves in a rectangular domain and a singular 2-D Helmholtz problem in an L-shaped domain.

Scaling Behavior Of Dynamic Permeability In Porous Media

1988 ◽  
Vol 137 ◽  
Author(s):  
Ping Sheng ◽  
Min-Yao Zhou ◽  
E. Charlaix ◽  
A. Kushnick ◽  
J. P. Stokes
Keyword(s):  
Cross Section ◽  
High Frequency ◽  
Low Frequency ◽  
Universal Function ◽  
The Other ◽  
Formation Factor ◽  
Geometric Information ◽  
Cross Sectional ◽  
Dynamic Permeability ◽  
Dimensionless Function

AbstractWe show that the scaling of the dynamic permeability κ(ω) by its static value κ0 and the frequency ω by a characteristic frequency ω0 particular to the medium results in a dimensionless function , with , which is dominated by the geometry of the throat regions in a porous medium. If the pore cross sectional area S varies slowly near the throat, i.e. dS/dz ≃ 0 where z is the distance normal to the cross section, then is an approximate universal function independent of microstructures. When scaling holds, the dynamic permeability κ(ω) is shown to contain only two pieces of geometric information, and the knowledge of either the low-frequency or the high-frequency asymptotic constants of κ(ω) would enable one to deduce the other missing parameters. In particular, since the high-frequency asymptotic parameters of κ(ω) can be related to the electrical formation factor and the volume-to-surface ratio, the static permeability value κ0 may be directly deduced from such non-permeability measurements.

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