Propagation In Ducts Research Articles

Effect of eddy on acoustic propagation from the surface duct perspective

Eddy-acoustical effects were investigated from the surface duct perspective. The presence of the eddy would influence sound propagation in surface duct by modifying the mix layer depth (MLD). Altimetry data along with the Argo profiles were used to obtain statistics of the effects of eddy on MLD. Results suggest that in the Northwestern Pacific Ocean, the mean MLD anomaly brought by warm eddy was 26.7 m and the value was 8.6 m for the cold eddy. Acoustic data from an acoustic experiment conducted in a warm eddy in South China Sea (SCS) were analyzed to study the effects of eddy on surface duct energy leakage (SDEL) phenomenon. Numerical simulation was carried out to study the sensitivity of SDEL on MLD variation. Results suggest that the SDEL phenomenon was highly sensitive to the MLD variation when the frequency was lower than the duct-thickness-related critical frequency. The signal attenuation rate of the SDEL signal received out of the duct related with MLD and frequency were also calculated. Acoustic data collected in the experiment agrees well with the simulation results. Given that the warm eddy would increase the MLD for almost 26.7 m on an average level, it is favorable for SDEL phenomenon.

C band transhorizon signal characterisations in evaporation duct propagation environment over Bohai Sea of China

Evaporation duct is an anomalous mechanism of transhorizon propagation in marine and coastal regions. In order to better understand the characteristics of signal fading, a C band signal transhorizon propagation experiment was carried out with a 107km path length over the Bohai Sea of China. Using an iron tower with meteorological and oceanographic (METOC) sensors at multiple altitudes, a direct diagnostic method of evaporation duct is presented. The dependence of signal strength on evaporation duct height, antenna height, and METOC parameters is analysed. The experimental results indicate that the signal strength tends to be linearly dependent on the evaporation duct height when the transmitter and receiver antennas are submerged within the duct layer. Normally, the signal strength is stronger when the relative humidity is lower. Stronger signal strength tends to occur under near-neutral or stable atmospheric conditions or when the air-sea temperature difference is higher. The results indicate that the signal strength distribution of fast fading is close to Rayleigh distribution, and the probability density of fade slope is symmetric at 0dB/s. These results have important implications for the design of ducting communications system.

Estimation of surface-based duct parameters from automatic identification system using the Levy flight quantum-behaved particle swarm optimization algorithm

Atmospheric duct propagation has a significant influence on the performance of wireless communication and radar systems in the maritime environment that makes it essential to master the detailed knowledge of the atmospheric refractivity profile. Automatic identification system (AIS) is a maritime navigation safety communication system that operates in the very high frequency mobile band and the propagation path of AIS signals will be influenced by different atmospheric conditions. In this paper, a new refractivity estimation method based on the AIS signal level is proposed and the Levy flight quantum-behaved particle swarm optimization (LFQPSO) algorithm is presented and applied in the new estimation method to estimate the surface based duct parameters. Numerical simulations demonstrate that the LFQPSO algorithmhas good robustness and the new refractivity estimation method based on the AIS signal level can provide near-real-time estimation ofatmospheric refractivity.

A Comprehensive Study on Maximum Wavelength of Electromagnetic Propagation in Different Evaporation Ducts

The maximum wavelength describes trapping features when considering electromagnetic wave propagation in an evaporation duct. However, the results vary between the existing methods used to calculate the maximum wavelength in an evaporation duct. Hence, this study provides a comprehensive analysis of the maximum wavelength of electromagnetic propagation in an evaporation duct. First, the existing methods are introduced and the differences between these methods are discussed. Second, the frequency responses from 1 to 20 GHz of the evaporation duct channel are analyzed. The trapping features of different evaporation duct channels are investigated. Based on the frequency responses, the existing methods are compared and evaluated. Finally, the frequency response of the evaporation duct channel was measured in the South China Sea along a 149-km-long propagation path in 2014. The data collected in the experiment are used to verify the conclusion. Both the simulation and experimental results show that the Kerr’s method and Kukushkin’s method do not represent the cutoff conditions for electromagnetic propagation, while Brekhovskikh’s method and Hall’s method can calculate the maximum wavelength in an evaporation duct. The recommended value k for using the Turton’s method is also given.

Guided wave propagation in cylindrical ducts with elastic walls enclosing a fluid moving with a uniform velocity

Theoretical models for elastic wave propagation in fluid filled ducts normally neglects mean fluid flow. However, in many engineering applications the velocity of the fluid may influence the modal characteristics of the duct, for example in gas pipelines, turbomachinery applications and ventilation systems. Accordingly, the influence of a mean uniform fluid flow on acoustically driven duct wall vibration is analysed here for a cylindrical geometry. The semi-analytic finite element method is used to couple the elastodynamic wave equation for the duct wall to the convected wave equation for sound propagation in a uniform fluid flow. A one dimensional finite element approach is described and this is used to find the coupled eigenmodes for the duct. Under certain conditions, a uniform mean flow is seen to significantly affect the phase speed for different eigenmodes, and it is shown that this may cause energy to transfer from the fluid to the surrounding wall at frequencies much lower than those seen without mean flow. This behaviour has the potential to increase sound radiation from ducts at lower frequencies when mean flow is present.

A Numerical Study of Gravity Wave Propagation Characteristics in the Stratospheric Thermal Duct

AbstractWe developed a fully nonlinear, two‐dimensional, numerical model to simulate the long horizontal distance propagations of atmospheric gravity waves in a stratospheric thermal duct. The numerical results show that after the wind disturbance excited by the initial wave forcing enters the duct completely, symmetric and antisymmetric modes can be identified in the horizontal direction. The frequency‐wave number spectrum indicates that various modes with different spatial structures can exist simultaneously in the duct. The primary parameters (horizontal wavelength, vertical wavelength and wave frequency) of the ducted wave packets in a given thermal duct are mainly determined by the horizontal wavelength of the initial wave forcing. The mean ratio of the upward propagating energy to the downward propagating energy in the thermal duct was regarded as an approximation of the power standing wave ratio (PSWR). The time variation of PSWR indicates that the standing wave was formed with the propagation of the ducted waves. During the gravity waves ducted propagation, most wave energy is restricted to the ducting region and dissipates slowly with time. The higher the wave frequency (or the shorter the horizontal wavelength) of the initial wave forcing, the slower the wave energy dissipates.

Analytical coupled vibro-acoustic modeling of a cavity-backed duct-membrane system with uniform mean flow.

Sound propagation in a flow duct is a complex and technically challenging problem. The presence of flexible vibrating walls inside the duct creates additional difficulties to the problem due to the complex vibro-acoustic and aero-acoustic couplings involved in the system. An accurate prediction of the coupled system response is of great importance for a good understanding of the underlying physics as well as the optimal design of relevant noise suppression devices. In the present work, a unified energy formulation is proposed for the fully coupled structural-acoustic modelling of a duct-mounted membrane backed by an acoustic cavity with a grazing flow. Sufficiently smoothed admissible functions, taking the form of a combination of Fourier series and supplementary polynomials, are constructed to overcome the differential discontinuities for various boundary and/or coupling conditions. The formulation allows the obtention of all relevant vibro-acoustic field information in conjunction with the generalized Lighthill equation and Rayleigh-Ritz procedure. The validation and convergence studies show the accuracy and the efficiency of the proposed model. Results show the strong structural-acoustic interaction in such a duct-membrane-cavity system, and the flow affects resonant amplitude of membrane-dominant modes significantly. Some cross-zones can be observed for the membrane kinetic energy frequency response with low Mach number cases, especially when a higher tension is applied to the membrane. Analyses on the structural-acoustic coupling strength indicate that the coupling between the odd-even structural modes becomes more significant at a higher Mach number compared with odd-odd and even-even mode pairs. It is also shown that adjusting the boundary constraint of the membrane or imposing a higher tensile force allows impairing the adverse influence of the flow in the duct on sound attenuation.

A highly efficient approach to model acoustics with visco-thermal boundary losses

For devices such as hearing aids, microphones, micro loudspeakers, and compression drivers, thermal and viscous boundary layer effects are often highly noticeable. These effects can be modeled in the linear regime by the linearized, compressible Navier-Stokes equations. However, the need for resolution of the very thin boundary layers typically makes numerical solutions of these equations computationally very expensive. Based on a boundary-layer analysis, we have derived for the pressure Helmholtz equation what appears to be a new boundary condition that accurately takes visco-thermal boundary losses into account. The model is valid when the wavelength and the minimum radius of curvature of the wall is much larger than the boundary layer thicknesses. In the special case of sound propagation in a cylindrical duct, the model collapses to the classical Kirchhoff solution. We assess the model in the case of sound propagation through a compression driver, a kind of transducer that is commonly used to feed horn loudspeakers. The transmitted power spectrum through the device calculated numerically using our model agrees extremely well with computations using a hybrid model, where the full linearized, compressible Navier-Stokes equations are solved in the narrow regions of the device and the pressure Helmholtz equations elsewhere. However, our model needs two orders of magnitude less memory and computational time than the more complete model.

Ambient noise field and propagation in an Arctic fjord Kongsfjorden, Svalbard

Ambient noise field and propagation in a glacierized Arctic fjord, Kongsfjorden is investigated using time series measurements of noise and Conductivity Temperature Depth (CTD) measurements between July 2015–April 2016. The hydrophone recorded noise for 10 months, with a duty cycle of 1 min every 3 h, covering summer, fall, winter and spring season. Noise field is seen to be variable with highest levels recorded during summer-fall (112 dB at 0.125 kHz) after which there is a decrease in maximum recorded noise levels. The spectral distribution of noise deviates from the theoretical Gaussian assumption, at varying degrees over the frequency band (0.125–4 kHz). Low frequency band is dominated by marine traffic during summer and fall, and ice wave interactions during winter and spring months. Propagation studies reveal variable nature in propagation with different scenarios observed between seasons. During the summer, refracted paths are observed and a weak channel is formed around the sound channel axis (∼100 m), and a well defined surface duct is revealed during the fall (depth of ∼60 m), characterized by a lower half channel bounded by the sea surface and the sonic layer depth. Surface duct propagation is common in winter with an upward profile and weakens with the onset of spring, as a consequence of the hydrology at the region. The studies will be useful in the context of climate change owing to the critical location of Kongsfjorden which is more prone to experience ongoing warming.

Features of Propagation Characteristics of Ultrahigh Frequency Radio Waves in an Above-Water Duct as Functions of the Duct Parameters and Wavelength

A new numerical method for finding roots of the characteristic equation in problems of radio wave propagation in tropospheric ducts with a piecewise linear profile of the refractive index is proposed. For the selected model of the vertical profile of the refractive index of an surface duct, dynamics of propagation constants of modes and features of the behavior of the attenuation function of the electromagnetic field as functions of the troposphere parameters and the wavelength are studied. The contribution of the whispering gallery modes is also estimated.

Acoustic boundary layers as boundary conditions

The linearized, compressible Navier–Stokes equations can be used to model acoustic wave propagation in the presence of viscous and thermal boundary layers. However, acoustic boundary layers are notorious for invoking prohibitively high resolution requirements on numerical solutions of the equations. We derive and present a strategy for how viscous and thermal boundary-layer effects can be represented as a boundary condition on the standard Helmholtz equation for the acoustic pressure. This boundary condition constitutes an O(δ) perturbation, where δ is the boundary-layer thickness, of the vanishing Neumann condition for the acoustic pressure associated with a lossless sound-hard wall. The approximate model is valid when the wavelength and the minimum radius of curvature of the wall is much larger than the boundary layer thickness. In the special case of sound propagation in a cylindrical duct, the model collapses to the classical Kirchhoff solution. We assess the model in the case of sound propagation through a compression driver, a kind of transducer that is commonly used to feed horn loudspeakers. Due to the presence of shallow chambers and thin slits in the device, it is crucial to include modeling of visco-thermal losses in the acoustic analysis. The transmitted power spectrum through the device calculated numerically using our model agrees well with computations using a hybrid model, where the full linearized, compressible Navier–Stokes equations are solved in the narrow regions of the device and the inviscid Helmholtz equations elsewhere. However, our model needs about two orders of magnitude less memory and computational time than the more complete model.

A novel 2.5D spectral approach for studying thin-walled waveguides with fluid-acoustic interaction

This paper presents a novel formulation of two spectral elements to study guided waves in coupled problems involving thin-walled structures and fluid-acoustic enclosures. The aim of the proposed work is the development of a new efficient computational method to study problems where geometry and properties are invariant in one direction, commonly found in the analysis of guided waves. This assumption allows using a two-and-a-half dimensional (2.5D) spectral formulation in the wavenumber-frequency domain. The novelty of the proposed work is the formulation of spectral plate and fluid elements with an arbitrary order in 2.5D. A plate element based on a Reissner-Mindlin/Kirchhoff-Love mixed formulation is proposed to represent the thin-walled structure. This element uses C0 approximation functions to overcome the difficulties to formulate elements with an arbitrary order from C1 functions. The proposed element uses a substitute transverse shear strain field to avoid shear locking effects. Three benchmark problems are studied to check the convergence and the computational effort for different h-p strategies. Accurate results are found with an appropriate combination of element size and order of the approximation functions allowing at least six nodes per wavelength. The effectiveness of the proposed elements is demonstrated studying the wave propagation in a water duct with a flexible side and an acoustic cavity coupled to a Helmholtz resonator.

Experimental study of premixed syngas/air flame propagation in a half-open duct

The flame propagation characteristics of premixed syngas/air mixtures are investigated in a half-open duct over a wide range of hydrogen fractions at various equivalence ratios. A high-speed camera and a pressure transducer are used to acquire the flame images and overpressure dynamics. The results indicate that the flame shape changes as the hydrogen fraction and equivalence ratio vary. There are two different styles of tulip flame inversion. When the equivalence ratios, Ф, are 2.5 and 3.0 and the hydrogen fraction, φ, is higher than 0.3, the flame inversion is more complicated than the classical tulip flame inversion. Moreover, the distorted tulip flame, which is always obtained in closed duct, is observed in a half-open duct in three cases, i.e., Ф = 1.0 and φ = 0.1, Ф = 1.0 and φ = 0.3, and Ф = 3.0 and φ = 0.1. Both the equivalence ratio and the hydrogen fraction can affect the flame propagation velocity and the pressure dynamics. Dramatic flame reacceleration and pressure oscillation occur after the flame inversion. The hydrogen fraction has a significant influence on the characteristics of premixed syngas/air flame. The influence is significant when hydrogen fraction was small (φ < 0.5), and the influence is moderate when hydrogen fraction was large (φ ≥ 0.5).

An integrated acoustic–mechanical development method for off-road motorcycle silencers: from design to sound quality test

Vehicle noise is one of the main sources of complaints because it is widespread and active in almost all environments, from urban areas to rural context. The present work aims to describe a practical procedure to design silencers for off-road motorcycles so that, once produced, they are lightweight, do not affect the engine performances and cut noise emissions to a sustainable limit. The required noise reduction, usually around 35–40 dB, can be achieved only by carefully designing the exhaust system. The first part of the paper provides the theoretical background of noise propagation in ducts, describes the main attenuation devices as tools the designer can use in the development process and outlines the workflow. In the second part, a case study of silencer optimization is presented: the noise emitted by a real off-road motorcycle is characterized and, once the sound pressure level and the mass flow generated by the engine are known, a silencer fulfilling noise attenuation, size and pressure drop requirements is designed, 3D-modeled and rendered. The outcome of the acoustic design process can be used to perform virtual sound quality tests, which can be of interest for manufacturers to catch the attention of passionate users.

Three-dimensional shallow water sound propagation and applications toward acoustical oceanography

Underwater sound propagation in areas of the continental shelf, shelf break, and continental slope can encounter strong horizontal reflection, refraction, diffraction, focusing, and/or defocusing due to a variety of environmental factors, including bathymetric variability, water column fluctuations (internal waves and shelf-break fronts), and boundary roughness. Theoretical, numerical, and experimental approaches have been taken to investigate the underlying physics of these three-dimensional (3-D) sound propagation effects and their temporal and spatial variability induced jointly by marine geological features and dynamic oceanographic processes. Some recent work on theoretical analysis, numerical modeling and field work experiments to study 3-D sound propagation in nonlinear internal wave ducts, over the continental slope and in submarine canyons will be reviewed in this talk. The ultimate goal of investing these 3-D sound propagation effects is to improve long-distance acoustical oceanographic technolo...

A quasi-one-dimensional theory of sound propagation in lined ducts with mean flow

Sound propagation in ducts with locally-reacting liners has received the attention of many authors proposing two- and three-dimensional solutions of the convected wave equation and of the Pridmore-Brown equation. One-dimensional lined duct models appear to have received less attention. The present paper proposes a quasi-one-dimensional theory for lined uniform ducts with parallel sheared mean flow. The basic assumption of the theory is that the effects of refraction and wall compliance on the fundamental mode remain within ranges in which the acoustic fluctuations are essentially uniform over a duct section. This restricts the model to subsonic low Mach numbers and Helmholtz numbers of less than about unity. The axial propagation constants and the wave transfer matrix of the duct are given by simple explicit expressions and can be applied with no-slip, full-slip or partial slip boundary conditions. The limitations of the theory are discussed and its predictions are compared with the fundamental mode solutions of the convected wave equation, the Pridmore-Brown equation and measurements where available.

On the homogenization of the acoustic wave propagation in perforated ducts of finite length for an inviscid and a viscous model.

The direct numerical simulation of the acoustic wave propagation in multiperforated absorbers with hundreds or thousands of tiny openings would result in a huge number of basis functions to resolve the microstructure. One is, however, primarily interested in effective and so homogenized transmission and absorption properties and how they are influenced by microstructure and its endpoints. For this, we introduce the surface homogenization that asymptotically decomposes the solution in a macroscopic part, a boundary layer corrector close to the interface and a near-field part close to its ends. The effective transmission and absorption properties are expressed by transmission conditions for the macroscopic solution on an infinitely thin interface and corner conditions at its endpoints to ensure the correct singular behaviour, which are intrinsic to the microstructure. We study and give details on the computation of the effective parameters for an inviscid and a viscous model and show their dependence on geometrical properties of the microstructure for the example of Helmholtz equation. Numerical experiments indicate that with the obtained macroscopic solution representation one can achieve an high accuracy for low and high porosities as well as for viscous boundary conditions while using only a small number of basis functions.

Fading Characteristics in Evaporation Duct: Fade Margin for a Wireless Link in the South China Sea

Evaporation ducts (ED), which are caused by a rapid decrease in the refractive index of the lower atmosphere, are known to trap radio waves between the evaporation duct layer and the sea surface. Although signal propagation and refractivity estimation in evaporation ducts have been well studied, the fading characteristics of ED must be accurately established to help determine fade margin, required for a reliable wireless communication link. In this paper, based on the percentage of occurrences of evaporation ducts, that have been measured and reported in the literature, the statistical distribution of path loss in the South China Sea is calculated for varying distances. These path loss distributions are weather-dependent and time-varying and define several slow time-varying fading characteristics. These slow fading characteristics are, what can be termed as log-lognormal in nature. Propagation in ED, being non-line-of-sight, also has a fast-fading component that is Rayleigh in nature. Once, the complete fading characteristics are known, the overall fade margin due in part; to both slow fading and fast fading can be obtained. For a distance of 100 km, approximately 99% availability can be obtained with an 88-dB fade margin while for a shorter distance of 50 km, the required total margin for 99% availability is only 50-dB.

Noise prediction of axial fan duct using a lattice Boltzmann approach

Large axial fans in fire power plants are noise source of the plants, they are required to reduce the noise level. Fan noise propagates in a fan duct, and since it is emitted to the exterior, silencers are used as countermeasure in general. It is important for noise reduction to predict noise generation and noise propagation in fan ducts. The Lattice-Boltzmann Method (LBM) based approach was applied for the unsteady simulation of axial fan and its duct to predict fan noise generation and sound propagation simultaneously. The LBM has minute numerical viscosity, and is suitable for the analysis of the sound propagation and the flow field. In this research, aeroacoustic analysis by using applicable analysis model in the design phase has been conducted and validated compared to experimental results. The results shows good agreement with experiment within 3dB at overall value and predicted spectra are also compared with experimental results.

Infrasound propagation in tropospheric ducts and acoustic shadow zones.

Numerical computations of the Navier-Stokes equations governing acoustic propagation are performed to investigate infrasound propagation in the troposphere and into acoustic shadow zones. An existing nonlinear finite-difference, time-domain (FDTD) solver that constrains input sound speed models to be axisymmetric is expanded to allow for advection and rigid, stair-step topography. The FDTD solver permits realistic computations along a given azimuth. It is applied to several environmental models to examine the effects of nonlinearity, topography, advection, and two-dimensional (2D) variations in wind and sound speeds on the penetration of infrasound into shadow zones. Synthesized waveforms are compared to a recording of a rocket motor fuel elimination event at the Utah Test and Training Range. Results show good agreement in the amplitude, duration, and spectra of synthesized and recorded waveforms for propagation through 2D atmospheric models whether or not topography, advection, or nonlinearity is explicitly included. However, infrasound propagation through a one-dimensional, range-averaged, atmospheric model yields waveforms with lower amplitudes and frequencies, suggesting that small-scale atmospheric variability causes significant scatter within the troposphere, leading to enhanced infrasound penetration into shadow zones. Thus, unresolved fine-scale atmospheric dynamics are not required to explain infrasound propagation into shadow zones.