Duct Termination Research Articles

Assessing the sound directivity of ducts based on time delay spectrometry

The sound directivity factor is an important parameter for describing the spatial distribution of acoustic energy radiated by duct terminations. Nevertheless, the available standard techniques for measuring the directivity factor rely on the free-field condition, which implies on the availability of a non-reflective environment, such as an anechoic chamber. In this paper, a technique known as time delay spectrometry is used to determine the directivity factor and the sound reflection coefficient of an open unflanged pipe in a normal reflective environment. The results show a good agreement between the experimental data and the theory. Details of the experimental procedures, as well as the discussion on the advantages and shortcomings of the method are presented in this letter.

Sound radiation from a flanged inclined duct

A simple method to calculate sound radiation from a flanged inclined duct is presented. An inclined annular duct is terminated by a rigid vertical plane. The duct termination is representative of a scarfed exit. The concept of a scarfed duct has been examined in turbofan aero-engines as a means to, potentially, shield a portion of the radiated sound from being transmitted directly to the ground. The sound field inside the annular duct is expressed in terms of spinning modes. Exterior to the duct, the radiated sound field owing to each mode can be expressed in terms of its directivity pattern, which is found by evaluating an appropriate form of Rayleigh's integral. The asymmetry is shown to affect the amplitude of the principal lobe of the directivity pattern, and to alter the proportion of the sound power radiated up or down. The methodology detailed in this article provides a simple engineering approach to investigate the sound radiation for a three-dimensional problem.

Development of Non-Reflecting Boundary Condition for Application in 3D Computational Fluid Dynamics Codes

Numerical computations are commonly used for better understanding the unsteady processes in internal combustion engine components and their acoustic behavior. The acoustic characterization of a system requires that reflections from duct terminations are avoided, which is achieved either by using highly dissipative terminations or, when an impulsive excitation is used, by placing long ducts between the system under study and the duct ends. In the latter case, the simulation of such a procedure would require a large computational domain with the associated high computational cost, unless non-reflecting boundary conditions are used. In this paper, first the different non-reflecting boundary conditions available in ANSYS-FLUENT are evaluated. Then, the development and implementation of an anechoic termination in a 3D-CFD code is presented. The performance of the new implementation is first validated in the classic Sod’s shock tube problem, and then checked against numerical and experimental results of the flow and acoustic fields in automotive exhaust mufflers. The results obtained compare favorably with those from the conventional CFD approach and experiments, while the computational cost is significantly reduced.

Bile duct duplication as a cause of distal bowel gas in neonatal duodenal obstruction

Purpose The aim of this study is to investigate the clinical characteristics of cases of duodenal atresia (DA) which present with bowel gas distal to a typical double-bubble sign through an anomalous bile duct conduit. Methods Medical records of 57 neonates with duodenal obstruction (atresia or stenosis), presenting with a double-bubble sign and treated at our institute from 1978 to 2010, were retrospectively reviewed. Results Thirteen (23%) of 57 neonates presented with bowel gas distal to the double-bubble sign. Passage of gas through the duodenal stenosis may have occurred in 3 cases, whereas in 9 cases, gas may have bypassed the atresia through an anomalous bifurcated bile duct termination and through the pancreatic duct from the accessory to the main pancreatic duct in one case. A preoperative upper gastrointestinal series was performed in 9 cases, and an anomalous bifurcated bile duct conduit was demonstrated in 5 cases. Severe and prolonged cholestasis necessitating evaluation for biliary atresia was found in 2 patients with anomalous bile duct anatomy. Conclusions Neonatal DA presenting with distal bowel gas via an anomalous bifurcated bile duct conduit is more common than initially thought and occurs more frequently than duodenal stenosis. These patients might be at risk for cholestasis, possibly owing to duodeno-biliary reflux through the ampulla.

New Numerical Procedure for Impedance Eduction in Ducts Containing Mean Flow

A new impedance eduction method is presented and validated against a benchmark method, and the effects of measurement uncertainty errors on the impedances educedwith this newmethod are assessed.Unique features of the newmethod include the following: 1) the upstream and downstream boundary conditions contain higher-order duct modes, 2) the impedance spectra of unique nonuniform test liners on opposite walls may be educed simultaneously, and 3) the measured data for the impedance eduction are acquired only at the source and duct termination planes. The validation exercise is performedwith a rigid-wall insert and a conventional liner over a range of frequencies and flowMach numbers in the NASALangleyResearchCenter’s grazing flow impedance tube. The primary conclusions of the study are that the impedance spectra of the rigid-wall insert and the conventional liner that were educed from the newmethod are in very good agreement with those that were educed by using the benchmarkmethod. However, the effects of measurement uncertainty on the educed impedance are greater at the lower frequencies and the higher Mach numbers for the newmethod. All indications are that this occurs because the newmethod 1) uses significantly less data to perform the impedance eduction than the benchmark and 2) is currently based on a rather crude approximation to the measured pressure gradient, which is more sensitive to the refractive effects of the boundary layer than the measured lower-wall pressure that is required in the benchmark method.

Impedance Eduction with an Extended Search Procedure

process in which impedance of the sample lining, duct termination impedance, and mean flow Mach number are set and the acoustic field is predicted. A search algorithm varying 5 degrees of freedom (real and imaginary parts of the twoimpedancesandMachnumber)isusedtobest fitthepredictedacoustic fieldtothemeasuredacoustic field.With Mach number as a search parameter, substantially improved data fitting is achieved as compared with the use of an a priori assumption of effective Mach number presumed to account for real flow effects. Published NASA Langley Research Center grazing incidence tube data are used in benchmark examples. Data from the Spirit AeroSystems flow duct facility are used to emphasize the quality of results achieved.

An indirect method for the characterization of locally reacting liners

An indirect technique for educing the homogenized acoustic impedance of a liner mounted on the wall of a barrel is presented. It is based on measurements and computational simulations of the multimodal scattering matrix of this lined duct. Measurements are performed with a multisource method and the use of an anechoic duct termination. The numerical computation of the scattering matrix relies on a finite element model, and assume that the duct is axisymmetric and uniformly covered by a locally reacting material. The impedance is educed by minimizing the difference between the theoretical and experimental acoustic power dissipations, which are deduced from the scattering matrix. The source is an incoming pressure field generated at one end of the duct only and composed of all cut-on modes. This technique was tested on a cylindrical barrel whose wall was partially lined with a realistic, locally reacting material made of honeycomb cells and a perforated facing sheet. Results for the acoustic impedance are compared with those deduced from semiempirical models and the experimental two microphone method. The best agreement with the indirect method is found with the semiempirical impedance results when the difference between the acoustic power dissipated by the actual lined barrel and the reference barrel is chosen as the cost function of the minimizing procedure.

Acoustic noise synthesis for duct systems

Studies on acoustical noise from duct systems are important in their design and performance. A methodology is presented for simulation of acoustical noise from a duct system which can be modeled in terms of source, duct element and termination. In this methodology first the transfer function of the given duct system is evaluated using the four pole parameters of the duct system, source strength, source acoustic and termination acoustic impedances. Then the impulse response of the predicted transfer function is convolved with source signal to obtain the acoustic noise output of the duct system in frequency domain. In order to verify this methodology, the simulated noise is compared with experimentally measured noise for two types of duct systems namely a straight pipe and a simple expansion chamber. The results show good agreement between the simulated and measured noise spectra. The methodology presented in this work provides the capability of simulating the noise of a duct system from its design before it is actually built. Also sound quality studies can be based on design that would assist in reaching the final design to be fabricated

Anatomical Variations of the Extrahepatic Ducts in Dogs: Knowledge for Surgical Procedures

Variations of the hepatic duct terminal were examined in 50 cadavers of mixed-breed dogs. The hepatic duct was formed by 4 major tributaries in 70% of the dogs and by 3 tributaries in 30% of the dogs. The order of hepatic duct termination, proximal to the gallbladder, was as follows: the duct from the right medial lobe, then the quadrate, then the common duct from the right lateral and caudate lobes and finally the duct from the left lobes in most of the dogs (90%). In only 10% of the dogs, the duct from the quadrate lobe terminated proximal to the duct from the right medial lobe. The length of the cystic duct was more than 5 mm in most dogs (88%). This study showed anatomical variations of the extrahepatic biliary tree and their preoperational imaging. These results may be useful for shortening the operative period and provide basic information for the application of laparoscopic cholecystectomies.

Comparison of Open and Choked Premixed CombustorExits During Thermoacoustic Limit Cycle

This paper compares the thermoacoustic limit cycles of a premixed laboratory combustor with acoustically open and choked exits. It is shown that the form of the downstream boundary condition can have a significant effect on the combustion chamber acoustics, with both the dominant limit cycle frequencies and the acoustic mode shapes being very different for the different combustor exits. The fundamental limit cycle frequency with the choked exit in place agrees closely with that determined by the convective time scales of entropy disturbances, as argued by other authors. A novel experimental method is then developed to examine the acoustic response of an arbitrary duct termination to incident pressure and entropy perturbations, and used to measure the response of the combustor downstream boundary condition during thermoacoustic limit cycle. The reflection coefficient for the acoustically open exit matches closely the classical result for zero mean flow. It is also shown that a choked nozzle downstream of the flame generates significant sound due to the interaction of the convected entropy perturbation with the nozzle. This final result is qualitatively in keeping with an existing analytic boundary condition for a choked nozzle, even though quantitative agreement is not observed. Reasons for this discrepancy are then suggested.

Experimental determination of end reflection loss (ERL) over a range of common types and configurations of duct terminations

The two-microphone impedance tube method ASTM E1050 was applied to directly measure the termination impedance of rectangular duct configurations of different aspect ratios and end conditions over a 25- to 500-Hz frequency band. Measured values of the acoustic impedance, and hence the reflection coefficient, of the duct termination permits computation of end reflection loss. ERL is essentially the insertion loss of duct termination. The measurement program is challenging, as the precision required for accurate ERL computation is much higher than that for accurate determination of the reflection coefficient. The measurement program is part of a project to assess the available published data on duct ERL. The ASHRAE Handbook contains a tabulation of ERL data derived from predictions of the radiation impedance of a circular plane piston, and there have been some validation measurements made on a limited set of circular duct configurations and orifice plate terminations. Results to date closely conform to published ERL values. [Work supported by ASHRAE.]

Active control of buzz-saw tones: Experimental results from a laboratory-scale, no-flow rig

This paper details the results of an experiment on a small scale no-flow rig to control a spinning mode with similar characteristics to the modes generated by buzz-saw noise. The spinning mode (m=−3, n=1) is generated by a mode synthesizer ring which has similar characteristics to that of a low frequency buzz-saw mode. A control system using seven microphones and seven control actuators is used to minimize the sum of the squared pressures at the duct wall using a filtered reference least mean squares (FXLMS) algorithm. It is shown that at certain frequencies the sound power transmitted to the duct termination is reduced by 14.5dB. The control mechanisms at different frequency ranges are identified and discussed with reference to computer simulations. It is shown that cut-off modes excited by the secondary actuator array have a significant effect on control performance at frequencies close to the cut-on frequency of the (m=−4, n=1) mode.

Wave reflections from duct terminations.

The reflection coefficients and inertial end corrections of several duct terminations, including finite length duct extensions perpendicular to an infinite wall, as well as at a number of angles, curved interface surfaces, and annular cavities, are determined and analyzed in the absence of flow by employing the boundary element method. Predictions for the classical unflanged and flanged circular ducts show good agreement with analytical and computational results available in the literature. The predictions for curved interface surfaces (bellmouth or horn) are also consistent with the available experimental data. In view of its high reflection coefficient, the duct termination with an annular cavity may be suggested for the suppression of noise radiation in a specific frequency band or for an effective wave reflection from the termination.

On HVAC duct acoustical end reflection

Duct end reflection (ER) is the apparent loss of sound power resulting from an abrupt change in a cross-sectional area of the duct. In most references, the magnitude of ER is only a function of frequency and duct size, because of assumed plane-wave propagation. If, between the source and duct termination, there are short duct runs, 90-deg duct bends, offset ducts, T-sections, various end diffusers, high aspect ratio cross sections, the presence of airflow, etc., the relative effects on ER are unknown. The American Society of Heating, Refrigeration, and Air-Conditioning Engineer’s Applications Handbook presents tables that provide the power loss due to ER. However, the assumptions used and the limitations of the table are lacking. This paper describes results of laboratory-measured changes in reverberation room sound-pressure level (SPL) created by common system variables that are associated with ER: duct length between noise source and reverb room termination, and several duct termination conditions were investigated using small diameter duct. In a controlled laboratory environment, these system conditions were found to create an apparent random variability (up to 10 dB) in the resulting SPL, for both a high impedance fan reference sound source and a low impedance loudspeaker source.

Propagation of Spinning Acoustic Modes in Turbofan Exhaust Ducts

Atheoretical and experimental study of sound propagation in a turbofan exhaust duct is presented. The effect of duct-area change on the propagation of rotor-stator interaction tones is studied. The pressureree ection coefe cient and far-e eld directivity shape variations as a result of duct termination, duct-e ow Mach number, and exhaust hub-to-tip radius ratio are examined. A e nite element method and a modal analysis technique that is employed in induct mode measurements are used. The computational results show that even with large changes in duct area there is no transfer of modal power from one mode to another, as it propagates through varying cross-sectional area. However, as the exhaust hub-to-tip radius ratio increases, higher-order radial modes get cut off. Exponential horn termination keeps the principal lobe peak angle e xed for the e rst-order radial modes at the design speed. Increases in e ow Mach number and exhaust hub-to-tip radius ratio move the principal lobe peak angle away from the exhaust duct axis. This reduces the axial beaming of acoustic energy of low-order spinning modes and enhancesabsorption by acousticliners. The computed axial variation of acousticpowerin theductand phaseangle of acoustic pressure show good agreement with the experimental data.

A rapid technique to determine the internal area function of finite-length ducts using maximum length sequence analysis

This paper describes a rapid technique for reconstruction of the internal area function of a duct using blockage-induced eigenvalue shifts determined from eigenfrequencies measured under two sets of duct termination boundary conditions. A single broad band maximum length sequence (MLS) measurement of short duration is utilized to obtain the transfer function of the duct, which in turn can be utilized to determine its eigenvalue shifts and subsequently its internal area function using an inverse perturbation technique. The reconstruction results display the same order of accuracy as those obtained previously using swept sine measurements of extended duration. An expression for the determination of the area function is presented utilizing resonant frequency information alone, thus rendering duct length determination unnecessary. A computational routine further simplifies the process such that the accuracy of the technique could be ascertained for a range of configurations including longer ducts and ducts that initially have nonuniform internal cross section over their length. Development of a relationship between obstacle length and wavelength of the lowest eigenfrequency required for successful reconstruction is also described. This is an important result for longer ducts where measurement of lower eigenfrequencies may present problems using standard measurement equipment.

ACTIVE INTENSITY CONTROL FOR THE REDUCTION OF RADIATED DUCT NOISE

Mean intensity based active control for the cancellation of radiated noise out of the duct exit is studied. The active intensity control strategy is derived based on the relation of the exterior sound field radiated out of the duct termination and the interior sound field of the duct. One of the characteristics of this control strategy is that the maximum possible control performance can be maintained regardless of the sensor location, compared with the conventional local pressure control methods at either interior downstream or exterior field positions. This is a simple consequence of the active intensity at the interior downstream being not space-dependent as long as it is plane wave. A time-domain adaptive filtering method for the active intensity control is also suggested and experimental results for an open ended duct based on the adaptive filtering method are presented. For the purpose of practical comparison, experimental results for conventional sound pressure control based on the well known filtered-xLMS algorithm are also presented. The experimental results show the potential of the active intensity control strategy for reducing the emitted noise out of the duct exit.

Morphological and histological analysis of the thoracic duct at the jugulo-subclavian junction in Japanese cadavers.

The macroscopic arrangement of the termination of the thoracic duct (TD) was examined in detail in Japanese cadavers, and the distribution of various types of collagen, tenascin, laminin, and fibronectin in the framework of the wall of the thoracic duct termination was demonstrated. We identified several branching patterns and pathways of the TD (i.e., those terminating 1) at the venous angle (Type A); 2) at the end of the internal jugular vein (Type B); 3) at the the external jugular vein (Type C); or 4) in a complex with various branches (Type D). The TD often divided into two or three trunks before or after passing beneath the left brachiocephalic vein. Compared with the main trunk of the TD, fewer muscle fibers, elastic fibers, specific compounds of the extracellular matrices, and valvular connective tissues were found near the termination of the TD and the valves of the TD and veins. Smooth muscle cells were arranged irregularly in each region of the TD.

A comparison of time domain boundary conditions for acoustic waves in wave guides

We consider several types of boundary conditions in the context of time domain models for acoustic waves. Experiments with four different duct terminations (hardwall, free radiation, foam, wedge) were carried out in a wave duct from which reflection coefficients over a wide frequency range were measured. These reflection coefficients are used to estimate parameters in the time domain boundary conditions, and a comparison of the relative merits of the models in describing the data is presented. Boundary conditions that yield a good fit of the model to the experimental data were found for all duct terminations except the wedge.

A comparison of error sensor strategies for the active control of duct noise

A theoretical comparison of a number of error sensor strategies is undertaken for active control of periodic noise in a hard-walled rectangular duct. Error sensor strategies investigated include the minimization of pressure at a point, the total real acoustic power output, and an estimate of the acoustic potential energy. A new error sensor strategy is also proposed and investigated. The strategy is based upon minimization of the power flow determined by a modal decomposition of the duct sound field. The error sensor strategies are analyzed for both plane-wave and multimode sound fields, and for a range of duct termination conditions. The criterion used to assess the error sensor strategies is the minimization of the sound field downstream of the control sources.