The scattering of multipole near-field sound by a gas bubble

1970 ◽  
Vol 314 (1518) ◽  
pp. 363-385 ◽  
Keyword(s):  
Near Field ◽  
Dipole Source ◽  
Gas Bubble ◽  
Underwater Sound ◽  
Near Field Sound ◽  
Acoustic Output ◽  
Scattering Centre ◽  
Strong Scattering ◽  
Dipole Sources ◽  
Good Agreement

A gas bubble very close to an underwater sound source can have a profound influence on the sound radiated by the source. It is well known that when source and bubble are far apart, the bubble is a strong scattering centre for the incident sound, but when they are close together the bubble responds also to the near-field flow. This near field is scattered by the bubble from a non-propagating to a propagating mode so that the acoustic output of a multipole source, which has a relatively high near-field, can be considerably increased by the scattering process. A theory is developed for the scattering by a single spherical gas bubble near ( a ) a compressible spherical monopole source and ( b ) a compressible spherical dipole source, but it can be applied to non-spherical sources and bubbles by ascribing to them an ‘equivalent spherical radius’. The theory is in good agreement with experiments using bubbles near a spherical monopole source and in fair agreement with experiments on bubbles near non-spherical monopole and dipole sources.

Hydrodynamic Noise and Radiated Mechanism of 3D Cavity

2012 ◽  
Vol 217-219 ◽  
pp. 2590-2593 ◽  
Author(s):  
Yu Wang ◽  
Bai Zhou Li
Keyword(s):  
Sound Pressure ◽  
Near Field ◽  
Low Frequency ◽  
Dipole Source ◽  
Fluctuating Pressure ◽  
Common Structure ◽  
Hydrodynamic Noise ◽  
Near Field Sound ◽  
Equivalent Sources ◽  
Field Sound

The flow past 3D rigid cavity is a common structure on the surface of the underwater vehicle. The hydrodynamic noise generated by the structure has attracted considerable attention in recent years. Based on LES-Lighthill equivalent sources method, a 3D cavity is analyzed in this paper, when the Mach number is 0.0048. The hydrodynamic noise and the radiated mechanism of 3D cavity are investigated from the correlation between fluctuating pressure and frequency, the near-field sound pressure intensity, and the propagation directivity. It is found that the hydrodynamic noise is supported by the low frequency range, and fluctuating pressure of the trailing-edge is the largest, which is the main dipole source.

An Investigation Into Acoustic Dipole Source Calibration Methods for Turbomachinery Sound Radiation in Reverberant Fields

2016 ◽  
Author(s):  
Michael L. Jonson ◽  
Steven D. Young
Keyword(s):  
Near Field ◽  
Dipole Source ◽  
Acoustic Source ◽  
In Situ Calibration ◽  
Calibration Methods ◽  
Penn State ◽  
Radiated Sound ◽  
Reverberant Field ◽  
Dipole Sources

In-situ calibration methods using a single spherical-shaped transmitting hydrophone (idealized as a monopole acoustic source) have traditionally been used for radiated sound measurements of turbomachinery performed in the Garfield Thomas 1.22-m diameter water tunnel located at The Pennsylvania State University’s Applied Research Laboratory (ARL Penn State). In this reverberant field, the monopole source containing known transmitting characteristics was used to calibrate acoustic sensors that were either near or far from the source. This method typically works well when the type of source is monopole in nature; however, many acoustics sources can be dipole or quadrupole in nature. In this study we investigated the applicability of using dipole sources in a space such as a well-characterized reverberant tank, and we found through a virtual dipole method that the radiation still appears monopole in the reverberant field. The method was extended for the vibration of a panel (a known dipole source) and once again the monopole assumption for the in-situ calibration for a near-field hydrophone and conventional reverberant hydrophones remained consistent.

A Comparative Study on the Difference between the Multi-dipole Sources and Vector Synthesis Source

2020 ◽  
Vol 25 (4) ◽  
pp. 529-543
Author(s):  
Xian-Xiang Wang ◽  
Ju-Zhi Deng
Keyword(s):  
Electromagnetic Field ◽  
Near Field ◽  
Electromagnetic Response ◽  
Dipole Source ◽  
Mountainous Area ◽  
Field Zone ◽  
Specific Direction ◽  
Single Dipole ◽  
The Difference ◽  
Dipole Sources

CSAMT exploration generally adopts a single dipole as the transmitter. The single dipole source has the apparent disadvantages–there are weak areas for all components, Ey and Hx are weak in the area where Ex and Hy are reliable. Moreover, it is hard to deploy the source with a specific direction in a rugged mountainous area. Given the shortcomings of the single dipole source, multi-dipole sources are introduced into CSAMT exploration. Although the dipole sources follow the principle of vector synthesis, the length of the source in actual exploration can last for several kilometers and the offset is generally a few kilometers. In this case, the source can no longer be regarded as a single dipole in the near-field zone. The electromagnetic field in this region becomes relatively complicated. We first compare the similarities and differences of electromagnetic field generated by vector synthesis source and multi-dipole source through the Ex radiation patterns. Then, we study the factors that affect electromagnetic response due to the substitution of the double-dipole source with the vector synthesis source. The measured EM fields is affected by the source length, frequency, the source angle, the offset, and the resistivity.Finally, we apply the double-dipole source to the 1D and 3D geological model and compare the difference between the electromagnetic field generated by the double-dipole source and that generated by the vector synthesis source. Usually, the difference is very obvious in the near-field zone, and is almost negligible in the far-field zone.

scholarly journals Imaging dipole flow sources using an artificial lateral-line system made of biomimetic hair flow sensors

2013 ◽  
Vol 10 (83) ◽  
pp. 20130162 ◽  
Author(s):  
Ahmad Dagamseh ◽  
Remco Wiegerink ◽  
Theo Lammerink ◽  
Gijs Krijnen
Keyword(s):  
Lateral Line ◽  
Estimation Error ◽  
Near Field ◽  
Dipole Source ◽  
Lateral Line System ◽  
Flow Sensors ◽  
Line System ◽  
Field Environment ◽  
Measurement Results ◽  
Dipole Sources

In Nature, fish have the ability to localize prey, school, navigate, etc., using the lateral-line organ. Artificial hair flow sensors arranged in a linear array shape (inspired by the lateral-line system (LSS) in fish) have been applied to measure airflow patterns at the sensor positions. Here, we take advantage of both biomimetic artificial hair-based flow sensors arranged as LSS and beamforming techniques to demonstrate dipole-source localization in air. Modelling and measurement results show the artificial lateral-line ability to image the position of dipole sources accurately with estimation error of less than 0.14 times the array length. This opens up possibilities for flow-based, near-field environment mapping that can be beneficial to, for example, biologists and robot guidance applications.

Euler’s differential equation and the identification of the magnetic point‐pole and point‐dipole sources

Geophysics ◽  
1984 ◽  
Vol 49 (9) ◽  
pp. 1549-1553 ◽  
Author(s):  
J. O. Barongo
Keyword(s):  
Magnetic Field ◽  
Differential Equation ◽  
Magnetic Anomalies ◽  
Field Vector ◽  
Magnetic Data ◽  
Dipole Source ◽  
Point Dipole ◽  
Main Subject ◽  
Depth Extent ◽  
Dipole Sources

The concept of point‐pole and point‐dipole in interpretation of magnetic data is often employed in the analysis of magnetic anomalies (or their derivatives) caused by geologic bodies whose geometric shapes approach those of (1) narrow prisms of infinite depth extent aligned, more or less, in the direction of the inducing earth’s magnetic field, and (2) spheres, respectively. The two geologic bodies are assumed to be magnetically polarized in the direction of the Earth’s total magnetic field vector (Figure 1). One problem that perhaps is not realized when interpretations are carried out on such anomalies, especially in regions of high magnetic latitudes (45–90 degrees), is that of being unable to differentiate an anomaly due to a point‐pole from that due to a point‐dipole source. The two anomalies look more or less alike at those latitudes (Figure 2). Hood (1971) presented a graphical procedure of determining depth to the top/center of the point pole/dipole in which he assumed prior knowledge of the anomaly type. While it is essential and mandatory to make an assumption such as this, it is very important to go a step further and carry out a test on the anomaly to check whether the assumption made is correct. The procedure to do this is the main subject of this note. I start off by first using some method that does not involve Euler’s differential equation to determine depth to the top/center of the suspected causative body. Then I employ the determined depth to identify the causative body from the graphical diagram of Hood (1971, Figure 26).

The breakdown of the linearized theory and the role of quadrupole sources in transonic rotor acoustics

1991 ◽  
Vol 226 ◽  
pp. 591-624 ◽  
Author(s):  
H. Ardavan
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Impulsive Noise ◽  
Near Field ◽  
Point Of View ◽  
Rotating Frame ◽  
Homogeneous Wave ◽  
Linearized Theory ◽  
Near Field Sound ◽  
Rotor Acoustics

The retarded Green's function for the linearized version of the equation of the mixed type governing the potential flow around a rotating helicopter blade or a propeller (with no forward motion) is derived and is shown to constitute the unifying feature of the various existing approaches to rotor acoustics. This Green's function is then used to pinpoint the singularity predicted by the linearized theory of rotor acoustics which signals its experimentally confirmed breakdown in the transonic regime: the gradient of the near-field sound amplitude, associated with a linear flow which is steady in the blade-fixed rotating frame, diverges on the sonic cylinder at the dividing boundary between the subsonic and supersonic regions of the flow. Prom the point of view of the equivalent Cauchy problem for the homogeneous wave equation, this singularity is caused by the imposition of entirely non-characteristic initial data on a space—time hypersurface which, at its points of intersection with the sonic cylinder, is locally characteristic. It also emerges from the analysis presented that the acoustic discontinuities detected in the far zone are generated by the quadrupole source term in the Ffowcs Williams-Hawkings equation and that the impulsive noise resulting from these discontinuities would be removed if the flow in the transonic region were to be rendered unsteady (as viewed from the blade-fixed rotating frame).

Improved Analytical Models for Single- and Multi-layer Silver Superlenses

2009 ◽  
Vol 1182 ◽  
Author(s):  
Ciaran P Moore ◽  
Richard John Blaikie ◽  
Matthew D Arnold
Keyword(s):  
Transfer Functions ◽  
Near Field ◽  
Single Layer ◽  
Analytical Models ◽  
Full Field ◽  
Accurate Performance ◽  
Imaging Performance ◽  
Performance Estimates ◽  
Continuous Metal ◽  
Good Agreement

AbstractSpatial-frequency transfer functions are regularly used to model the imaging performance of near-field �superlens� systems. However, these do not account for interactions between the object that is being imaged and the superlens itself. As the imaging in these systems is in the near field, such interactions are important to consider if accurate performance estimates are to be obtained. We present here a simple analytical modification that can be made to the transfer function to account for near-field interactions for objects consisting of small apertures in otherwise-continuous metal screens. The modified transfer functions are evaluated by comparison with full-field finite-element simulations for representative single-layer and multi-layer silver superlenses, and good agreement is found.

scholarly journals The overlapping roles of the inner ear and lateral line: the active space of dipole source detection

2000 ◽  
Vol 355 (1401) ◽  
pp. 1115-1119 ◽  
Author(s):  
Christopher B. Braun ◽  
Sheryl Coombs
Keyword(s):  
Inner Ear ◽  
Lateral Line ◽  
Near Field ◽  
Dipole Source ◽  
Source Detection ◽  
Sound Sources ◽  
Distance Range ◽  
Spatial Gradients ◽  
Theoretical Predictions ◽  
Benthic Fishes

The problems associated with the detection of sounds and other mechanical disturbances in the aquatic environment differ greatly from those associated with airborne sounds. The differences are primarily due to the incompressibility of water and the corresponding increase in importance of the acoustic near field. The near field, or hydrodynamic field, is characterized by steep spatial gradients in pressure, and detection of the accelerations associated with these gradients is performed by both the inner ear and the lateral line systems of fishes. Acceleration–sensitive otolithic organs are present in all fishes and provide these animals with a form of inertial audition. The detection of pressure gradients, by both the lateral line and inner ear, is the taxonomically most widespread mechanism of sound–source detection amongst vertebrates, and is thus the most likely primitive mode of detecting sound sources. Surprisingly, little is known about the capabilities of either the lateral line or the otolithic endorgan in the detection of vibratory dipole sources. Theoretical considerations for the overlapping roles of the inner ear and lateral line systems in midwater predict that the lateral line will operate over a shorter distance range than the inner ear, although with a much greater spatial resolution. Our empirical results of dipole detection by mottled sculpin, a benthic fish, do not agree with theoretical predictions based on midwater fishes, in that the distance ranges of the two systems appear to be approximately equal. This is almost certainly as a result of physical coupling between the fishes and the substrate. Thus, rather than having a greater active range, the inner ear appears to have a reduced distance range in benthic fishes, and the lateral line distance range may be concomitantly extended.

Noise Prediction for Multi-Element Airfoil Based on FW-H Equation

2011 ◽  
Vol 52-54 ◽  
pp. 1388-1393
Author(s):  
Jun Tao ◽  
Gang Sun ◽  
Ying Hu ◽  
Miao Zhang
Keyword(s):  
Flow Field ◽  
Sound Pressure ◽  
Near Field ◽  
Pressure Level ◽  
Aerodynamic Noise ◽  
Noise Prediction ◽  
Far Field ◽  
Acoustic Analogy ◽  
Acoustic Information ◽  
Good Agreement

In this article, four observation points are selected in the flow field when predicting aerodynamic noise of a multi-element airfoil for both a coarser grid and a finer grid. Numerical simulation of N-S equations is employed to obtain near-field acoustic information, then far-field acoustic information is obtained through acoustic analogy theory combined with FW-H equation. Computation indicates: the codes calculate the flow field in good agreement with the experimental data; The finer the grid is, the more stable the calculated sound pressure level (SPL) is and the more regularly d(SPL)/d(St) varies.

Far-Field Performances Prediction of High Frequency Projectors Using Secondary Source Array Method

2016 ◽  
Vol 25 (02) ◽  
pp. 1750002 ◽  
Author(s):  
Shiquan Wang
Keyword(s):  
High Frequency ◽  
Near Field ◽  
Directivity Pattern ◽  
Field Method ◽  
Voltage Response ◽  
Secondary Source ◽  
Far Field ◽  
Comparison Of The Results ◽  
Source Array ◽  
Good Agreement

This paper investigates the prediction of the far-field performances of high frequency projectors using the second source array method (SSAM). The far-field parameters can be calculated accurately using the complex acoustic pressure data of two very close parallel planes which lie in the near-field region of the projector. The paper simulates the feasibility of predicting the far-field parameters such as transmitting voltage response and the far-field directivity pattern. The predicting results are compared with that calculated using boundary element method (BEM). It shows very good agreement between the two methods. A planar high frequency projector is measured using the near-field method. In order to verify the predicting results, the far-field measurement is performed for the same projector. The comparison of the results shows that the near-field method is capable to precisely predict the far-field parameters of the projector.

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