Propagation In Ducts Research Articles

Observation of whistlers and associated ionospheric parameters during St. Patrick’s Day storm at low Latitude, Varanasi

The paper presents the analysis of a large number of whistlers recorded during the St. Patrick's Day geomagnetic storm (17–23 March 2015) at Indian low latitude ground station Varanasi (geomag. lat. 140 55/ N, geomag. long. 1530 54/ E, L = 1.078). The increased occurrence of the recorded whistlers has been explained in terms of duct formations for the propagation of whistlers during the geomagnetic storm. The whistler’s activity extended up to the 19th March 2015 during the daytime. The ducted propagation of the recorded whistlers is supported by the analysis of the World-Wide Lightning Location Network (WWLLN) data. In this paper, the result showed that the occurrence time of causative sferics of more than 80 % was in good agreement with that of WWLLN lightning discharges within the propagation time. The origins of these lightning strikes are lying within a 500 km radius of the conjugate point of the Varanasi. By analyzing recorded whistlers, we have computed different ionospheric parameters of Earth. The dispersion for the nighttime whistlers lies between 11 and 15 sec½ whereas dispersion for the daytime whistlers lies between 12 and 25 sec½. The columnar ionospheric electron contents (TECs) computed for the recorded whistlers during nighttime lie between 25.47 TECU and 42.84 TECU whereas TECs of the daytime whistlers lie between 30.08 TECU and 125.48 TECU. These calculated columnar ionospheric electron contents are found to be of the same order of magnitude as the IRI-2012 modeled TEC over Varanasi and that of conjugate of Varanasi whereas GPS-TEC are found to be comparatively 8–15 TECU lower over Varanasi.

Noise propagation and attenuation in L-shape duct with flow

Clarifying duct noise is gathering attention for various industries. Duct noise is one of the noise problems in the equipment such as air conditioner and ventilation system. In the duct noise, the noise generated by the sources due to the fan or motor are included, and these noise propagate in the L-shape duct with flow. However the propagation mechanism of these noise are not clarified, especially in the case of the propagation in the duct with flow. In this paper, we investigated the propagation and attenuation of the noise generated by the speaker in the duct with or without a flow both experimentally and numerically. The results show that in the case of the duct with the flow, the noise generated by the speaker at frequency more than 1200 Hz are damped more than in the case of the duct without the flow.In the numerical simulations, the results shows that the strength of the noise sources on the duct wall decreases in the case of the duct with a flow more than in the case of the duct without a flow. In the numerical simulations, we applied the coupling method using CFD and acoustic BEM solvers.

Alpha Transmitter Signals Observed by the Van Allen Probes: Ducted Versus Nonducted Propagation

AbstractThe interaction of very low frequency transmitter signals with radiation belt electrons depends ultimately on their wave normal angles. In the equatorial interaction region, these can be either low (ducted propagation) or comparatively large (nonducted propagation). Experimentally distinguishing the two modes is complicated, as multicomponent spacecraft data typically do not extend to high enough frequencies with a sufficient frequency resolution. One exception that we exploit are 11.9 kHz signals from Alpha transmitters detectable by the Van Allen Probes spacecraft. We use multicomponent burst mode measurements to distinguish between the ducted and nonducted modes of propagation and to evaluate their relative importance. While the ducted waves are detected less often, they tend to have larger Poynting fluxes. The total power propagating in the two modes is thus comparable. Magnetic local time and in‐situ density fluctuations are main parameters controlling the relative fraction of ducted waves.

Statistical Characteristics of High‐Frequency Gravity Waves Observed by an Airglow Imager at Andes Lidar Observatory

AbstractThe long‐term statistical characteristics of high‐frequency quasi‐monochromatic gravity waves are presented using multi‐year airglow images observed at Andes Lidar Observatory (ALO, 30.3°S, 70.7°W) in northern Chile. The distribution of primary gravity wave parameters including horizontal wavelength, vertical wavelength, intrinsic wave speed, and intrinsic wave period are obtained and are in the ranges of 20–30 km, 15–25 km, 50–100 m s−1, and 5–10 min, respectively. The duration of persistent gravity wave events captured by the imager approximately follows an exponential distribution with an average duration of 7–9 min. The waves tend to propagate against the local background winds and show evidence of seasonal variations. In austral winter (May–August), the observed wave occurrence frequency is higher, and preferential wave propagation is equator‐ward. In austral summer (November–February), the wave occurrence frequency is lower, and the waves mostly propagate pole‐ward. Critical‐layer filtering plays a moderate role in determining the preferential propagation direction in certain months, especially for waves with a smaller observed phase speed (less than typical background winds). The observed wave occurrence and preferential propagation direction are related to the locations of convection activities nearby and their relative distance to ALO. However, direct wave generations are less likely due to the large distance between the ALO and convective sources. Other mechanisms such as secondary wave generation and possible ducted propagation should be considered. The estimated mean momentum fluxes have typical values of a few m2 s−2.

A Simulated and Experimental Analysis of Evaporation Duct Effects on Microwave Communications in the Irish Sea

This article presents a simulated and experimental analysis of the effects of the evaporation duct on microwave propagation in the Irish Sea. The evaporation duct is a phenomenon that occurs almost permanently over all the world’s oceans and allows electromagnetic waves to travel beyond the horizon. Weather data logged over several years from four buoys off the east coast of Ireland has been analyzed to find the probability and strength of the evaporation duct. Signal propagation in the evaporation duct has been simulated using the parabolic equation model and compared to results obtained from an experimental setup in the Irish Sea. The best antenna heights and frequencies to maximize signal propagation at this location are also found. Results show that the evaporation duct can be used to provide high bandwidth communications beyond the horizon with an uptime of approximately 40%, and that weather data from buoys can be used to predict the performance of the communications link. While this method of communication is not one that can be relied on all the time, it could be useful to reduce dependency on expensive satellite links or provide nontime critical communications.

Observations of Anomalous Over-the-Horizon Propagation in the Evaporation Duct Induced by Typhoon Kompasu (202118)

Evaporation ducts, which form as a result of evaporation from the ocean, have a significant impact on electromagnetic (EM) propagation in coastal and marine environments. Theoretically, typhoons would substantially increase the mass, momentum, and heat exchanges at the air-sea interface and thus inevitably influence these EM propagation environments, including the duct conditions and propagation boundary conditions. However, the effects of typhoons on EM propagation have seldom been reported. In this study, anomalous propagation in the evaporation duct induced by Typhoon Kompasu (202118) was recorded by using an EM propagation system and automatic weather station. The length of the propagation link was 53 km, which was beyond the horizon. The evaporation duct's impact on EM propagation during Typhoon Kompasu was studied based on model simulation results. Propagation far beyond the horizon indicated that ducting was present during the typhoon passage. Furthermore, the approaching of the typhoon caused a decrease in path loss of about 20 dB. This was because the abnormal marine meteorological conditions induced by typhoons result in a high evaporation duct, which leads to a small path loss for EM propagation.

Transmission loss for the Beaufort Lens and the critica frequency for mode propagation during ICEX-18.

Complexities of acoustic propagation in ducts have long been known, e.g., shallow water environments and deep waters off Gibraltar. The "Beaufort Lens" (Lens) is a duct north of Alaska with nominal depths between 60 and 200 m and is reachable by oceanographic instruments and underwater unmanned vehicles and submarines. Propagation within the ducts is governed by waveguide physics. The frequencies must be high enough to support the modes within them such that there is a "critical frequency" (CF) where modes start to "detach" from surface loss mechanisms. Therefore, transmission losses (TLs) can abruptly decrease once a mode "fits" within a duct. This paper describes an experimental part of Ice Exercise 2018 supported by the U.S. Navy's Arctic Submarine Laboratory. The signals were transmitted from Camp Sargo north of Prudhoe Bay to the submarines SSN Hartford, SSN Connecticut, and HMS Trenchant. The data indicate low TLs near 100 Hz and an abrupt 10 dB decrease in TLs 244-280 Hz, both suggesting CFs. Modeling suggests CFs for modes 1 near 100 Hz and a higher CF when modes 3-6 "cascade" into the Lens starting near 250 Hz. There are also abrupt increases in TLs at other frequencies, which are explained by nulls in the product of the mode functions.

Viability of the mixed layer duct with observed dynamics

While the upper ocean of the north Pacific is largely mixed due to air-sea interaction, a 1000 km sea-soar transect shows the existence of stratification in the mixed layer that persists over many tens of kilometers. The effects of this stratification on mixed layer acoustic propagation depends on the sound speed differences across the different density layers, and observations show the sound speed varies rapidly both between and along the isopycnals. Acoustic models are used to separately quantify the importance of the mixed layer stratification and the density compensated sound speed variability on propagation in the mixed layer duct. The observed sound speed field is modeled as the superposition of three fields: a smooth background, the stratification field with slow isopycnal sound speed variability, and the remnant unstratified sound speed variation. The sound speed variation in the stratified mixed layer is attributed to internal waves and eddy filaments and in the unstratified field to ocean spice. Both types of observed sound speed variation significantly change, or completely block, mixed layer propagation. Examples are presented of blocking features in the tilt or spice fields separately, and where they exist only in the superposition of the two fields.

Ocean acoustics in the rapidly changing Arctic ocean

The Arctic ocean is undergoing rapid climate change, in fact it is the most rapidly warming area of our planet. Underwater sound is a uniquely effective tool for Arctic monitoring, because it can be used under the ice where satellites are blind. The most striking change has been the reduction of summer ice cover, but recent acoustic experiments have started to gather evidence for other changes as well, such as the reduction of ice scattering from the loss of multiyear ice and the formation of subsurface ducted propagation in the Beaufort Sea. The ambient soundscape is also of great interest as human activity increases in the region and as biological activity responds. Due the strategic importance of the Arctic during the cold war, there is a history of experimental work in the area but this short talk will try to show a few recent examples.

Analytical and experimental investigation of flexural waves in horizontal pipes conveying two-phase periodic intermittent flow

Two-phase flow in pipes can generate different geometric distribution of phases depending on the flow conditions, called flow patterns. The identification of such flow patterns is fundamental for designing operational conditions of complex oil and gas industrial plants for production and transport of such fluids by pipes. Amongst them, the intermittent flow is the most relevant for industrial applications as it can lead to increased vibration levels with catastrophic failures, and also to several flow assurance problems. In this flow condition, an elongated gas bubble travels along the pipe followed by a liquid slug with or without dispersed bubbles. The two-phase flow induced vibration research have been mostly focused on the modal approaches, which can be very sensitive to boundary and operational conditions thus limiting its practical applicability in field conditions. On the other hand, wave-based approaches can overcome this limitation since they are based on local properties. In this paper, bending wave propagation in horizontal ducts conveying intermittent flow is investigated. A periodic unit cell model is assumed to describe the intermittent flow and the mass density along the duct is thus given by a Fourier series, representing space and time fluctuations. Subsequently, the wave dispersion characteristics can be analytically calculated for a duct in bending conveying intermittent flow, assuming small mixture velocity when compared to the bending waves. The periodic flow creates a periodic dispersion diagram which is distorted by the flow velocity. This effect accounts for the asymmetry in the wave propagating along and against the flow direction. The wave dispersion curves are experimentally estimated for typical intermittent flow conditions and it is shown that they can be approximated by equivalent homogeneous fluid. In addition, this curve is sensitive to the gas fraction of the two-phase fluid. The presented approach introduces a new perspective on two-phase flow induced vibrations of pipes and opens the way for better understanding the effect of the periodic two-phase flow in the dynamic behaviour of pipes.

A Novel 3D Non-Stationary Maritime Wireless Channel Model

In this paper, a novel 3-dimensional (3D) non-stationary geometry-based stochastic model (GBSM) is proposed to mimic the ship-to-ship multiple-input multiple-output (MIMO) communication channels. To reflect the realistic maritime propagation environment, the effects of rough sea surface scattering and evaporation duct propagation are investigated in the proposed channel model. The model considers the movements of transmitter (Tx), receiver (Rx), and scatterers, and is capable of capturing the channel characteristics, including non-stationary characteristics, spatial consistency, location-dependent property, etc. Through taking the array cluster evolution into account, the proposed channel model can describe massive MIMO channels and can easily switch to conventional MIMO channel model by adjusting corresponding parameters. Based on the proposed model, key statistical properties like delay/angular/Doppler power spectrum density (PSD), space-time correlation function (STCF), stationary interval, and root mean square (RMS) Doppler/delay spreads in multi-scenarios are derived. The usefulness and accuracy of the proposed model are demonstrated by comparing theoretical results, simulation results, and corresponding measurement results.

High-fidelity fan-noise simulations based on improved delayed detached eddy simulation

We have developed high-fidelity CFD/CAA capability for the prediction of fan tone and broadband noise from an aero-engine. With a zonal URANS—wall-modeled LES approach, referred to as the Improved Delayed Detached Eddy Simulation (IDDES), the entire geometry of NASA's Source Diagnostic Test (SDT) fan rig including the nacelle is solved at approach, cut-back and take-off conditions. Multi-block grids with high-order structured finite-volume schemes are designed to resolve turbulence in the wakes and vortices as well as sound propagation in the bypass duct through the blade rows. By applying the Ffowcs-Williams and Hawkings technique with permeable surfaces, far-field sound-power spectra are computed, and by incorporating with the duct-mode-extraction technique, the radiated sound from the inlet and exhaust is decomposed into duct modes. These predicted fan-noise characteristics as well as aerodynamic performances are validated against NASA’s wind-tunnel test data. The fan-noise associated with the rotor-stator interaction is very well characterized: The tone power levels up to the second blade passing frequencies from both inlet and exhaust are predicted well across the engine speed, and the broadband power spectra are also predicted except for those from the inlet only at supersonic tip Mach numbers.

Sensitivity of mixed layer duct propagation to deterministic ocean features: The interaction parameter

Surface duct propagation can be affected by deterministic ocean features such as fronts, eddies, filaments, and/or density compensated temperature and salinity anomalies (spice). Using the concept of duct modal structure and the Dyson series solution to the wave equation, we present a non-dimensional parameter Γmn, termed the interaction parameter, that quantifies the strength of mode coupling between a surface duct mode m and any other mode n as a function of ocean feature scales and acoustic frequency. Weak, first order coupling is given by Γmn < 1 where as strong, higher order coupling has Γmn > 1. Importantly, there is a frequency dependent resonance condition associated with the range width of the perturbations, Δ, such that Γmn goes to zero as Δ approaches 0 and infinite. The utility of the interaction parameter in estimating the stability of mixed layer propagation is demonstrated using numerical examples of a cold eddy and of a hot and salty spice feature.

Direct Evidence Reveals Transmitter Signal Propagation in the Magnetosphere

AbstractSignals from very‐low‐frequency transmitters on the ground are known to induce energetic electron precipitation from the Earth's radiation belts. The effectiveness of this mechanism depends on the propagation characteristics of those signals in the magnetosphere, and in particular whether the signals are ducted or nonducted along channels of enhanced plasma density, analogous to optical fibers. Here we perform a statistical analysis of in‐situ waveform data collected by the Van Allen Probes satellites that shows that nonducted propagation dominates over ducted propagation in both the occurrence and intensity of the waves. Ray tracing confirms that the latitudinal distribution of wavevectors corresponds to nonducted as opposed to ducted propagation. Our results show the dominant mode of propagation needed to quantify transmitter‐induced precipitation and improve the forecast of electron radiation belt dynamics for the safe operation of satellites.

Liner development at low technology readiness level utilizing the advanced noise control fan

The Advanced Noise Control Fan (ANCF) was utilized in the design, test, and evaluation for technical risk mitigation for many of the innovative ducted acoustic liner technologies developed by NASA over the past 20 years. The ANCF is a low-speed ducted fan test bed for measuring and understanding fan-generated aeroacoustics, duct propagation, and radiation to the farfield. It is considered a low Technology Readiness Level testbed. The international aeroacoustics research community employed the ANCF to facilitate advancement of liner concepts, liner measurement technologies, and for liner code validation. From 1994 to 2016, it was located in the NASA Glenn Research Center’s Applied Aero Propulsion Laboratory. In 2016 the ANCF was transferred to the University of Notre Dame where it is expected to continue to positively impact research in the liner area. This paper summarizes the capabilities and contributions of the ANCF to the field of liner development by documenting its history.

Time-domain simulations of sound propagation in a flow duct with extended-reacting liners

A general methodology to simulate acoustic propagation in ducts with extended-reacting liners in the time domain is presented, including a generic perforated sheet on the air-material interface. The Linearized Euler Equations (LEE) with a mean flow profile are solved in the duct and the linearized equations on an equivalent fluid are solved in the liner material. The auxiliary differential equation method (ADE) is used to prevent the computation of convolution integrals, and leads to a formulation compatible with high-order numerical schemes. The methodology is illustrated for the case of liners consisting of rigid-frame porous materials and a prototype of locally-resonant acoustic metamaterials. A one-dimensional (1D) test case is first used to validate the algorithm and assess the numerical error. The numerical order of the algorithm is the expected one, independently of the interface, as long as the number of poles retained in the partial fraction expansions involved in the formulation is high enough. The algorithm is then applied to a realistic two-dimensional (2D) configuration in a duct with flow, and is used to illustrate the restrained validity of the locally-reacting approximation. Finally, the impact of the flow Mach number on the acoustic performance of porous and metamaterial extended-reacting liners is briefly assessed.

Doppler Shifted Alpha Transmitter Signals in the Conjugate Hemisphere: DEMETER Spacecraft Observations and Raytracing Modeling

AbstractAlpha navigation transmitters are very low frequency (VLF) transmitters operating at mid‐latitudes, which use a specific discrete radiation pattern at three distinct frequencies (11.9, 12.6, and 14.9 kHz). The transmitters are located in the northern hemisphere, but the radiated signals propagate through the magnetosphere to the conjugate hemisphere, where they are detectable by low‐altitude spacecraft. We present an analysis of such signals detected by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft at an altitude of about 660 km. It is found that, due to a Doppler shift, the observed signal frequencies can be at times rather different than the radiated frequencies. This indicates wave propagation at large wave normal angles (close to the resonance cone). Simultaneous observations of the same signal with different Doppler shifts reveal three distinct ways of signal propagation: (i) ducted propagation, (ii) unducted propagation, and (iii) propagation interpreted as only partially ducted. A raytracing analysis is employed to obtain typical wave trajectories corresponding to the individual ways of signal propagation and respective Doppler shifts. A reasonable agreement between the observed and calculated Doppler shifts is obtained. Our results demonstrate the peculiarities of VLF signal propagation throughout the magnetosphere and the possibility of using Doppler shifts to estimate wave normal angles.

Sensitivity of mixed layer duct propagation to deterministic ocean features.

Here, the problem of mode coupling in a mixed layer (ML) surface duct is considered where the coupling is induced by deterministic upper ocean features such as eddies, filaments, and/or density compensated temperature and salinity anomalies (spice). The single scatter Dyson series solution for mode energy is used to define a non-dimensional mode interaction parameter Γmn that quantifies the strength of coupling between modes m and n as a function of environmental factors and frequency. Direct coupled mode simulations at 400 and 1000 Hz show weak, first order coupling and small ML transmission loss (TL) variability when Γmn<1, while for Γmn>1, there is strong, higher order coupling with large changes in ML TL. Importantly, there is a frequency dependent resonance condition associated with the range width of the perturbations, Δ, such that Γmn→0 as Δ→0 and ∞.

Estimation of co-channel interference between cities caused by ducting and turbulence

With the rapid development of the fifth-generation (5G) mobile communication technology, the application of each frequency band has reached the extreme, causing mutual interference between different modules. Hence, there is a requirement for detecting filtering and preventing interference. In the troposphere, over-the-horizon propagation occurs in atmospheric ducts and turbulent media. The effects of both ducting and turbulence can increase the probability of occurrence of long-distance co-channel interference (CCI), in turn, severely affecting the key performance indicators such as system access, handover and drop. In the 5G era, to ensure communication channels and information security, CCI must be reduced. This paper introduces a scattering parabolic equation algorithm for calculating signal propagation in atmospheric ducts on irregular terrain boundaries. It combines Hitney’s radio physical optical model and Wagner’s nonuniform turbulent scattering model for calculating the tropospheric scattering in an evaporation duct or a surface-based duct. The new model proposes a tropospheric scattering parabolic equation algorithm for various tropospheric duct environments. Finally, as a specific case, the topographical boundaries between several cities in the East China Plain were considered, and the over-the-horizon propagation loss was simulated for various ducting and turbulent environments. The simulation results were used to evaluate whether CCI would occur between cities in a specific environment.

Gasdynamic behaviours of a radial turbine with pulsating incoming flow

This paper investigates turbine performance and gasdynamic behaviours of a radial turbine confronted by pulsating inflow via detailed flow measurement. Firstly, pressure propagation in the volute over circumferential direction is captured by six pressure transducers. The propagation speed is confirmed to be the sum of bulk flow velocity and sonic velocity by the experiment. Local flow parameters also propagate at the same speed with pressure pulse, which indicates that pulsations of the flow parameters are driven by the pulse of pressure. An evident phase-shift is observed between input power and output work, which is mainly resulted from the pulse propagation in the inlet duct. Furthermore, the pressure pulse at rotor inlet is evidently out-of-phase with that at rotor exit, producing a larger hysteresis loop of swallowing capacity. Specifically, the cycle-averaged input work is about 22.2% higher than the case with constant exit pressure. It is found to be resulted from the phase-flip of the pulse at rotor exit due to the gasdynamic behaviour at the interface between the rotor and exit duct, where sudden expansion happens. This paper unveils the gasdynamic behaviours of turbine confronted by pulsating inflow and can have a guide effect for performance enhancement design methodology.