Propagation In Ducts Research Articles

Robust and Adaptive Feedback Noise Attenuation in Ducts

In this brief, the attenuation of sound propagation in an air-handling duct using robust and adaptive feedback active noise control (ANC) strategies is investigated. The case of multiple narrow-band disturbances located in distinct frequency regions and the interference occurring in the presence of disturbances with very close frequencies are considered. The active control uses a loudspeaker as a compensatory system. The objective is to minimize the residual noise at the end of the duct segment considered. The system does not use any additional sensors for receiving real-time information upon the disturbances. This brief illustrates the application of the techniques for active vibration control presented by Landau et al. to this problem. A hierarchical feedback control approach will be used. At the first level, a robust linear controller will be designed taking advantage of the knowledge of the domains of variation of the frequencies of the noise disturbances. To further improve the performance, a direct adaptive control algorithm will be added. Its design is based on the use of the internal model principle combined with the Youla–Kucera parameterization of the controller. Guidelines for the design of the baseline (central) controller are provided. Both robust and adaptive controls require the knowledge of the discrete-time model of the compensation path, which is obtained by identification from experimental data. Experimental results on a relevant duct ANC test bench will illustrate the performance of the proposed methodology.

Effects of particle size on flame structures through corn starch dust explosions

The effects of particle size on the structures of flame reaction zone and preheat zone of corn starch dust were investigated. Corn starch dust with three different average particle sizes were chosen to conduct explosion experiments in a half-closed vertical duct. A high-speed camera was used to record the flame propagation process and direct light emission images of flame propagation in duct were obtained. The structures of flame reaction zone and preheat zone were detected by an ion current probe and a fine thermocouple. The experimental results show that the structures of reaction zone and preheat zone vary with the particle size. There is a positive correlation between the particle size and the thickness of reaction zone and preheat zone. Meanwhile, the thickness of the two flame structures are determined under these experimental conditions. Additionally, the mass concentrations of dust clouds in duct are founded as a critical common factor for flame structure variation.

Acoustic Boundary Layer Attenuation in Ducts With Rigid and Elastic Walls Applied to Cochlear Mechanics

The cochlea is the most important part of the hearing system, due to the fact that it transforms sound guided through air, bone, and lymphatic fluid to vibrations of the cochlear partition which includes the organ of Corti with its sensory cells. These send nerve impulses to the brain leading to hearing perception. The work presents the wave propagation in rigid ducts filled with air or water including viscous-thermal boundary layer damping. In extension, a mechanical box model of the human cochlea represented by a rectangular duct limited by the tapered basilar membrane at one side is developed and evaluated numerically by the finite element method. The results match with rare experiments on human temporal bones without using the physically unfounded assumption of Rayleigh damping. A forecast on the concept of the traveling wave parametric amplification is given to potentially explain the high hearing sensitivity and otoacoustic emissions.

表面ダクト内音波伝搬における音線, モード, 干渉波の役割

表面ダクト内音波伝搬における音線, モード, 干渉波の役割

Test of single degree of freedom acoustic treatment impedance models for multimodal acoustic propagation in duct with flow.

Passive acoustic treatments, also called liners, are the main solution to noise problems. The Single Degree Of Freedom (SDOF) acoustic treatment, composed of a thin material (perforated plate) affixed to air cavities with a rigid bottom, constitutes a solution. Predicting sound level reduction by an SDOF treatment requires reliable acoustic impedance models. An experimental/numerical method has been developed for a duct with an acoustic treatment to test acoustic impedance models of SDOF treatment with a multimodal propagation in the presence of a mean flow. This method is based on the comparison of experimental results from an aeroacoustic bench composed of a circular duct with a treated area, and numerical results from an FEM-PML axisymmetric model based on Galbrun's equation. The numerical results are confronted with experimental results to test impedance models up to M0=±0.25.

Acoustic Modeling of Charge Air Coolers

The necessity of reducing CO2 emissions has lead to an increased number of passenger cars that utilize turbocharging to maintain performance when the internal combustion (IC) engines are downsized. Charge air coolers (CACs) are used on turbocharged engines to enhance the overall gas exchange efficiency. Cooling of charged air increases the air density and thus the volumetric efficiency and also increases the knock margin (for petrol engines). The acoustic properties of charge coolers have so far not been extensively treated in the literature. Since it is a large component with narrow flow passages, it includes major resistive as well as reactive properties. Therefore, it has the potential to largely affect the sound transmission in air intake systems and should be accurately considered in the gas exchange optimization process. In this paper, a frequency domain acoustic model of a CAC for a passenger car is presented. The cooler consists of two conical volumes connected by a matrix of narrow ducts where the cooling of the air takes place. A recently developed model for sound propagation in narrow ducts that takes into account the attenuation due to thermoviscous boundary layers and interaction with turbulence is combined with a multiport representation of the tanks to obtain an acoustic two-port representation where flow is considered. The predictions are compared with experimental data taken at room temperature and show good agreement. Sound transmission loss increasing from 5 to over 10 dB in the range 50–1600 Hz is demonstrated implying good noise control potential.

Potential impact of global warming on sound propagation in monsoon induced surface mixed layers

Acoustic surface ducts in mixed layers can be an important mode of sonar propagation. The precipitation from monsoons seasonally create mixed surface layers near the equator in every major ocean. Although not as robust as mixed layers that occur at higher latitudes, they may provide the best examples of the possible global warming impact on sound propagation. This would include acoustic duct propagation properties such as attenuation, surface loss, cutoff frequency, and ambient noise. This is just a first attempt, based on preliminary estimates, to evaluate possible change. To be considered are direct effects such as ocean acidification and warming water; the seasonal impact on monsoons, like greater precipitation and duration; and finally individual transient events such as storm intensity and number (including typhoons). Given the usual monsoon variability, it may be challenging to sort out changes, but monsoon duct acoustics may be a promising new area for investigators.

Amplitude–frequency characteristics of ion–cyclotron and whistler-mode waves from Van Allen Probes data

Using two-hour (from 2300 UT January 25, 2013 to 0100 UT January 26, 2013) measurement data from Van Allen Probes on fluxes of energetic particles, cold plasma density, and magnetic field magnitude, we have calculated the local growth rate of electromagnetic ion–cyclotron and whistler-mode waves for field-aligned propagation. The results of these calculations have been compared with wave spectra observed by the same Van Allen Probe spacecraft. The time intervals when the calculated wave increments are sufficiently large, and the frequency ranges corresponding to the enhancement peak agree with the frequency–time characteristics of observed electromagnetic waves. We have analyzed the influence of variations in the density and ionic composition of cold plasma, fluxes of energetic particles, and their pitch-angle distribution on the wave generation. The ducted propagation of waves plays an important role in their generation during the given event. The chorus VLF emissions observed in this event cannot be explained by kinetic cyclotron instability, and their generation requires much sharper changes (“steps”) for velocity distributions than those measured by energetic particle detectors on Van Allen Probes satellites.

Experimental study on a comparison of typical premixed combustible gas-air flame propagation in a horizontal rectangular closed duct

Research surrounding premixed flame propagation in ducts has a history of more than one hundred years. Most previous studies focus on the tulip flame formation and flame acceleration in pure gas fuel-air flame. However, the premixed natural gas-air flame may show different behaviors and pressure dynamics due to its unique composition. Natural gas, methane and acetylene are chosen here to conduct a comparison study on different flame behaviors and pressure dynamics, and to explore the influence of different compositions on premixed flame dynamics. The characteristics of flame front and pressure dynamics are recorded using high-speed schlieren photography and a pressure transducer, respectively. The results indicate that the compositions of the gas mixture greatly influence flame behaviors and pressure. Acetylene has the fastest flame tip speed and the highest pressure, while natural gas has a faster flame tip speed and higher pressure than methane. The Bychkov theory for predicting the flame skirt motion is verified, and the results indicate that the experimental data coincide well with theory in the case of equivalence ratios close to 1.00. Moreover, the Bychkov theory is able to predict flame skirt motion for acetylene, even outside of the best suitable expansion ratio range of 6<E<8.

CANAPE-2015 (Canada Basin acoustic propagation experiment): A pilot tomography experiment in the Beaufort Sea

During the summer of 2015, test transmissions were made from a ship-suspended source to a moored vertical line array in the Beaufort Sea. These were carried out at a variety of ranges in order to prepare for a year-long (2016-2017) tomography experiment. A major concern was the expected transmission loss (TL) under the ice for current climatic conditions in the Arctic. A greater proportion of smooth first year ice (rather than rough multi-year ice) was expected due to climate change. This should lead to less scattering and a lower TL. Acoustic propagation in the Artic includes propagation in a weak shallow duct as well as arrivals of deeper turning rays. The TL of these different regimes is compared to that of an experiment in a tropical ocean, the Philippine Sea. The initial analysis shows that the Arctic TL scales with range to the fourth power. This large TL could be because the actual ice conditions in the experimental area had a high proportion of multi-year ice in 2015.

Path-Loss and Correlation Analysis for Space and Polarization Diversity in Surface Ducts

Atmospheric ducts that are caused by the rapid decrease in the refractive index of the lower atmosphere can trap the propagating signals. The trapping effect of atmospheric ducts provides significant received power enhancements and enables signals to reach beyond-line-of-sight distances. Although wave propagation in atmospheric ducts are well studied, there is no work that provides fading and correlation analysis for atmospheric ducts. Therefore, the main motivation of this communication is to develop a large-scale path-loss model, estimate the distribution of fading, and analyze fading correlation for space and polarization diversity systems in the presence of surface ducts. Lastly, the achievable data rate results are presented to show the performance improvement with space and polarization diversity techniques.

Three-dimensional detonation cellular structures in rectangular ducts using an improved CESE scheme**Project supported by the National Natural Science Foundation of China (Grant Nos. 10732010 and 10972010).

The three-dimensional premixed H2-O2 detonation propagation in rectangular ducts is simulated using an in-house parallel detonation code based on the second-order space–time conservation element and solution element (CE/SE) scheme. The simulation reproduces three typical cellular structures by setting appropriate cross-sectional size and initial perturbation in square tubes. As the cross-sectional size decreases, critical cellular structures transforming the rectangular or diagonal mode into the spinning mode are obtained and discussed in the perspective of phase variation as well as decreasing of triple point lines. Furthermore, multiple cellular structures are observed through examples with typical aspect ratios. Utilizing the visualization of detailed three-dimensional structures, their formation mechanism is further analyzed.

A Special Boundary Integral Method for the Numerical Simulation of Sound Propagation in Flow Ducts Lined with Multi-Cavity Resonators

In this work, acoustic performances of a liner concept based on perforated screens backed by air cavities are investigated numerically for circular ducts with mean flow. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. In this case, the liner becomes nonlocally reacting and this gives rise to additional resonance effects which renders the attenuation more effective over a broader frequency range. In order to predict the mufflers’ acoustic performances, a special boundary integral method is presented. Using a tailored Green’s function for hard wall circular ducts containing uniform mean flow, the numerical technique only requires the discretization of the acoustic velocity potential on both sides of the perforated screen separating the central channel from the air cavities. Comparisons with finite element results show that the proposed method allows accurate results for a relatively modest computational cost. Influence of the mean flow in the central airway, the dimensions of the cavity as well as the nature of the incident field on acoustic performances are also shown and discussed.

Possible generation mechanisms for Pc1 pearl structures in the ionosphere based on 6 years of ground observations in Canada, Russia, and Japan

AbstractWe investigate pearl structures (amplitude modulations) of Pc1 pulsations simultaneously observed at Athabasca (ATH, 54.7°N, 246.7°E, L = 4.3) in Canada, Magadan (MGD, 60.1°N, 150.7°E, L = 2.6) in Russia, and Moshiri (MOS, 44.4°N, 142.3°E, L = 1.5) in Japan. From 6 years of ground observations, from 2008 to 2013, we selected 84 Pc1 events observed simultaneously at the longitudinally separated stations (ATH and MGD) and 370 events observed at the latitudinally separated stations (MGD and MOS), all with high coherence (&gt;0.7) of Pc1 waveforms. We calculated the cross‐correlation coefficient (similarity: r) for the Pc1 pearl structures and found that more than half of the events in both pairs had low similarity (r &lt; 0.7), indicating that most Pc1 waves exhibit different pearl structures at different stations. We found that high‐similarity Pc1 pearl structures (r &gt; 0.7) at the longitudinally separated stations are concentrated from 6 to 15 UT when both stations are in the nighttime. The similarity of Pc1 pearl structures tends to show a negative correlation with the standard deviation of the polarization angle in both pairs. The observed repetition period of Pc1 pearl structures has a clear positive correlation with the repetition period estimated from Pc1 bandwidth by assuming beating of different frequencies. From these results, we suggest that ionospheric beating effect could be a dominant process for the generation of Pc1 pearl structures. Beating processes in the ionosphere with a spatially distributed ionospheric source can cause the different shapes of Pc1 pearl structures at different observation points during ionospheric duct propagation.

Broadband noise suppression for turbofan inlet applications

Broadband noise in turbofan inlets is represented in a frequency domain analysis by a statistical distribution of propagating duct modes. The required number of modes is estimated by Rice’s approximation method. A brief overview of essential results from the theory of acoustic propagation in ducts is given, providing a starting point for Rice’s estimation of the distribution of propagating modes in a uniform unlined duct required to represent a broadband noise source. The relationship between mode cut-off ratio and attenuation in lined ducts is examined to connect cut-off ratio to modal attenuation, and it is shown by example that for arbitrary impedance at a specified frequency, modes with similar cut-off ratios have similar attenuation. It is then shown by example that the attenuation of a low-order circumferential mode, specifically the axially symmetric circumferential mode, with a large range of propagating radial mode cut-off ratios, is insensitive to impedance variations near the optimum impedance. Two models for estimating achievable attenuation for broadband noise attenuation, based on equipartition of acoustic power among propagating modes, are considered. One uses only the axially symmetric circumferential mode, and the second uses all propagating circumferential modes and their associated propagating radial modes. A new parallel architecture version of an established propagation code, designed to include all propagating modes on a statistical basis, is described. Examples for prediction of acoustic power attenuation of broadband noise at low frequency and at moderate frequency are used to compare the performance of the axially symmetric circumferential mode and the all circumferential mode models. It is shown by example that a broadband noise model based on the axially symmetric circumferential mode predicts lower attenuation than a model with all propagating circumferential modes. Rice’s correlation of mode cut-off ratio with attenuation justifies this observation.

An acoustic model of NIST’s long-wavelength acoustic flowmeter for gas flow in large-diameter pipes

NIST is investigating long-wavelength acoustic flowmeter (LWAF) technology to accurately measure gas flow in large-diameter pipes, such as smokestacks used by coal-burning power plants. To aid in the data analysis and development of the method, we constructed a lumped-element acoustic model of the LWAF based on existing theory for sound propagation in circular ducts, modified to include flow. The model calculates the ratio of the acoustic pressure amplitudes and phase differences between two locations in a partial standing wave downstream of a continuous sound source up to the duct’s cut-on frequency. We used the numerical calculations of the reflection coefficient by Munt [J. Sound Vibration 142, 413–436 (1990)] to model the radiation impedance as a function of flow speed. In the absence of flow, the model was used to calibrate the positions of several microphones in the LWAF. In the presence of flow, the model predicts qualitatively the measured amplitude ratios and phase differences as a function of flow rate. Quantitative comparison is limited by the uncertainty of the radiation impedance and its flow dependence. This limitation prompts us to investigate ways to either measure the radiation impedance or eliminate it by using multiple coherent sound sources.

Artificial noise systems for parametric studies of turbo-machinery aero-acoustics

The study of turbo-machinery aero-acoustics encompasses source generation, duct propagation, and radiation to the far field for the purposes of physical understanding, evaluation, and noise reduction. Further, the acoustics subset can be divided into overall, broadband, or tone emphasis. Ultimately, assessments on full-scale turbofans are required. However, for isolating specific effects, or for costs reasons, it is useful to test models. These models may be scaled versions of turbofan components depending on the physical process of interest. The advantage of using models is the lower cost allows for a wider range of conditions to be studied. Even so, the cost of manufacturing and testing scale model fans in mid-technology readiness level can be limiting. A potentially useful supplement to turbo-machinery aero-acoustics studies is the use of artificial sources to generate acoustic signatures. The advantage is that a wide range of signatures can be quickly and efficiently studied, particularly useful for noise reduction concepts, or validating prediction methodologies that are sensitive to variations in geometry or acoustic signature. A disadvantage is the lack of the ability to study source generation. This trade-off must be considered carefully when deciding on the usefulness of utilizing fan artificial noise sources for the study of turbo-machinery aero-acoustics. This paper presents two test articles that have contributed to turbo-machinery aero-acoustics studies. One is a 48 in. diameter duct (nominally full-scale) generating acoustic signatures in the audible range; the second is a 6 in. diameter duct (nominally scaled) generating acoustic signatures in the ultrasonic range.

Radial mode analysis of broadband noise in flow ducts using azimuthal sensor array

For the evaluation and improvement of noise reduction notions and the verification of broadband sound power measurement in flow ducts, it is interesting to obtain the detailed information of the in-duct acoustic mode spectrum and subsequent broadband noise sources separation. A new broadband noise mode analysis method, which uses full wall-mounted sensor array, was experimentally applied on broadband sound fields at three operation conditions, which were generated by means of high loaded single-stage axial flow fan test rig. Two axial sensor arrays were mounted wall-flush upstream of the fan. Measurements were made at operating conditions from 60% to 100% rotor design speed. On the whole, the new method behaves robustly. It delivers physically meaningful broadband mode amplitudes. Mode coherence functions are calculated between all pairs of propagating modes. This feature enables the detailed comparison of different sound fields with characteristically coupled mode pairs. For tonal noise, mode coherence results show that modes are correlated with the source and with each other, especially at blade passing frequencies. The experimental outcome proves the usefulness of the analysis technique for interpreting andunderstanding broadband sound propagation in turbomachinery flow ducts.

Influence of obstacle on electromagnetic wave propagation in evaporation duct with experiment verification**Project supported by the National Natural Science Foundation of China (Grant No. 11174235) and the Fundamental Research Funds for the Central Universities of China (Grant No. 3102014JC02010301).

In this paper, the influence of obstacle on electromagnetic wave propagation in an evaporation duct is investigated, both from numerical simulation and experimental observation. A comparison of electromagnetic wave propagation in evaporation duct with and without obstacle for a typical case is presented. The presence of obstacle causes a significant increase in path loss. The obstacle has significant impact on electromagnetic wave propagation when the frequency is higher than 5 GHz and when the evaporation duct height is higher than 10 m. The influence of an island on electromagnetic wave propagation was observed in the experiment held in the South China Sea, October 2012. The experiment result shows that the island causes about 30–40 dB increase in path loss. The discrepancy between model and measurement is analyzed and the errors of transmitting antenna height and relative humidity are the possible causes of the discrepancy.

Sound Propagation in a Duct with Wall Corrugations Having Square-Wave Profiles

Acoustic wave propagation in ducts with rigid walls having square-wave wall corrugations is considered in the context of a perturbation formulation. Using the ratio of wall corrugation amplitude to the mean duct half width, a small parameter is defined and a two levels of approximations are obtained. The first-order solution produces an analytical description of the pressure field inside the duct. The second-order solution yields an analytical estimate of the phase speed of waves transmitting through the duct. The effect of wall corrugation density on acoustic impedance and wave speeds is highlighted. The analysis reveals that waves propagating in a duct with square-wave wall corrugation are slower than waves propagating in a duct with sinusoidal wave corrugation having the same corrugation wavelength.