Acoustic Design of a Local Scatterer in a Phase-Space

1995 ◽  
Author(s):  
John J. McCoy ◽  
Ben Zion Steinberg
Keyword(s):  
Mechanical Property ◽  
Phase Space ◽  
Mass Density ◽  
Sound Field ◽  
Design Strategy ◽  
Local Region ◽  
Small Scale ◽  
Far Field ◽  
Acoustic Design ◽  
Surrounding Fluid

Abstract A spatially local region of mechanical property heterogeneity is a source of scattering, by which a structure-borne mechanical wavefield is released as sound, to a surrounding fluid. We consider the case of a scatterer which is of the order of the size of the wavelength of a plate-wave field for a frequency which is below coincidence. A design strategy for reducing the strength of the scattered sound field in the fluid, at far-field distances from the scatterer, by adding a small-scale structure to the heterogenity, is presented. The design is accomplished in a wavelet-based phase-space. Emphasized is a significant distinction required of the added structure, depending on the heterogeneity applying to a measure of the local mass density or the local bending stiffness.

scholarly journals Radiation in the neighbourhood of a double layer

2014 ◽  
Vol 32 (6) ◽  
pp. 677-687 ◽  
Author(s):  
R. Pottelette ◽  
M. Berthomier ◽  
J. Pickett
Keyword(s):  
Phase Space ◽  
Electron Distribution ◽  
Electron Distribution Function ◽  
Source Region ◽  
Double Layers ◽  
Small Scale ◽  
High Time Resolution ◽  
Local Electron ◽  
Nonlinear Phase ◽  
Electron Holes

Abstract. In the auroral kilometric radiation (AKR) source region, acceleration layers narrow in altitude and associated with parallel field-aligned potential drops of several kV can be identified by using both particles and wave-field high time-resolution measurements from the Fast Auroral SnapshoT explorer spacecraft (FAST). These so-called double layers (DLs) are recorded around density enhancements in the auroral cavity, where the enhancement can be at the edge of the cavity or even within the cavity at a small scale. Once immersed in the plasma, DLs necessarily accelerate particles along the magnetic field lines, thereby generating locally strong turbulent processes leading to the formation of nonlinear phase space holes. The FAST data reveal the asymmetric character of the turbulence: the regions located on the high-potential side of the DLs are characterized by the presence of electron holes, while on the low-potential side, ion holes are recorded. The existence of these nonlinear phase space holes may affect the AKR radiation pattern in the neighbourhood of a DL where the electron distribution function is drastically different from a horseshoe shape. We present some observations which illustrate the systematic generation of elementary radiation events occurring significantly above the local electron gyrofrequency in the presence of electron holes. These fine-scale AKR radiators are associated with a local electron distribution which presents a pronounced beam-like shape.

scholarly journals Observations of interstellar Lyman-α absorption

1970 ◽  
Vol 36 ◽  
pp. 281-301 ◽  
Author(s):  
Edward B. Jenkins
Keyword(s):  
Neutral Hydrogen ◽  
Local Region ◽  
Line Of Sight ◽  
Small Scale ◽  
Line Broadening ◽  
Interstellar Gas ◽  
B Stars ◽  
Hydrogen Density ◽  
The Galaxy ◽  
To Come

Absorption at the Lyman-α transition from interstellar neutral hydrogen has been observed in the ultraviolet spectra of 18 nearby O and B stars. Radiation damping is the dominant cause of line broadening, which makes the derived line-of-sight column densities proportional to the square of the observed equivalent widths. An average hydrogen density on the order of 0.1 atom cm−3 has been found for most of the stars observed so far. This is in contrast to the findings from surveys of 21-cm radio emission, which suggest 0.7 atom cm−3 exists in the local region of the Galaxy. Several effects which might introduce uncertainties into the Lyman-α measurements are considered, but none seems to be able to produce enough error to explain the disagreement with the 21-cm data. The possibility that small-scale irregularities in the interstellar gas could give significantly lower values at Lyman-α is explored. However, a quantitative treatment of the factor of ten discrepancy in Orion indicates the only reasonable explanation requires the 21-cm flux to come primarily from small, dense, hot clouds which are well separated from each other. The existence of such clouds, however, poses serious theoretical difficulties.

scholarly journals Fluid Dynamics of Ballistic Strategies in Nematocyst Firing

Fluids ◽  
2020 ◽  
Vol 5 (1) ◽  
pp. 20 ◽  
Author(s):  
Christina Hamlet ◽  
Wanda Strychalski ◽  
Laura Miller
Keyword(s):  
Immersed Boundary Method ◽  
Short Distance ◽  
Reynolds Numbers ◽  
Two Dimensions ◽  
Immersed Boundary ◽  
Small Scale ◽  
Viscous Damping ◽  
Final Velocity ◽  
Inertial Regime ◽  
Surrounding Fluid

Nematocysts are stinging organelles used by members of the phylum Cnidaria (e.g., jellyfish, anemones, hydrozoans) for a variety of important functions including capturing prey and defense. Nematocysts are the fastest-known accelerating structures in the animal world. The small scale (microns) coupled with rapid acceleration (in excess of 5 million g) present significant challenges in imaging that prevent detailed descriptions of their kinematics. The immersed boundary method was used to numerically simulate the dynamics of a barb-like structure accelerating a short distance across Reynolds numbers ranging from 0.9–900 towards a passive elastic target in two dimensions. Results indicate that acceleration followed by coasting at lower Reynolds numbers is not sufficient for a nematocyst to reach its target. The nematocyst’s barb-like projectile requires high accelerations in order to transition to the inertial regime and overcome the viscous damping effects normally encountered at small cellular scales. The longer the barb is in the inertial regime, the higher the final velocity of the projectile when it touches its target. We find the size of the target prey does not dramatically affect the barb’s approach for large enough values of the Reynolds number, however longer barbs are able to accelerate a larger amount of surrounding fluid, which in turn allows the barb to remain in the inertial regime for a longer period of time. Since the final velocity is proportional to the force available for piercing the membrane of the prey, high accelerations that allow the system to persist in the inertial regime have implications for the nematocyst’s ability to puncture surfaces such as cellular membranes or even crustacean cuticle.

WAVELET-BASED POSTPROCESSING OF JET LES DATA FOR ACOUSTIC FAR-FIELD EXTRAPOLATIONS

2013 ◽  
Vol 21 (04) ◽  
pp. 1350017
Author(s):  
RAMIN KAVIANI ◽  
VAHID ESFAHANIAN ◽  
MOHAMMAD EBRAHIMI
Keyword(s):  
Large Scale ◽  
Stokes Equations ◽  
Near Field ◽  
Flow Simulation ◽  
Computational Time ◽  
Jet Flows ◽  
Small Scale ◽  
Frequency Noise ◽  
Far Field ◽  
Noise Sources

The affordable grid resolutions in conventional large-eddy simulations (LESs) of high Reynolds jet flows are unable to capture the sound generated by fluid motions near and beyond the grid cut-off scale. As a result, the frequency spectrum of the extrapolated sound field is artificially truncated at high frequencies. In this paper, a new method is proposed to account for the high frequency noise sources beyond the resolution of a compressible flow simulation. The large-scale turbulent structures as dominant radiators of sound are captured in LES, satisfying filtered Navier–Stokes equations, while for small-scale turbulence, a Kolmogorov's turbulence spectrum is imposed. The latter is performed via a wavelet-based extrapolation to add randomly generated small-scale noise sources to the LES near-field data. Further, the vorticity and instability waves are filtered out via a passive wavelet-based masking and the whole spectrum of filtered data are captured on a Ffowcs-Williams/Hawkings (FW-H) surface surrounding the near-field region and are projected to acoustic far-field. The algorithm can be implemented as a separate postprocessing stage and it is observed that the computational time is considerably reduced utilizing a hybrid of many-core and multi-core framework, i.e. MPI-CUDA programming. The comparison of the results obtained from this procedure and those from experiments for high subsonic and transonic jets, shows that the far-field noise spectrum agree well up to 2 times of the grid cut-off frequency.

scholarly journals Extended lens reconstructions with grale: exploiting time-domain, substructural, and weak lensing information

2020 ◽  
Vol 494 (3) ◽  
pp. 3253-3274
Author(s):  
Jori Liesenborgs ◽  
Liliya L R Williams ◽  
Jenny Wagner ◽  
Sven De Rijcke
Keyword(s):  
Mass Density ◽  
Gravitational Lens ◽  
Weak Lensing ◽  
Small Scale ◽  
Source Position ◽  
Cluster Galaxy ◽  
Weak Inversion ◽  
Sufficient Detail ◽  
Inversion Techniques

ABSTRACT The information about the mass density of galaxy clusters provided by the gravitational lens effect has inspired many inversion techniques. In this article, updates to the previously introduced method in grale are described, and explored in a number of examples. The first looks into a different way of incorporating time delay information, not requiring the unknown source position. It is found that this avoids a possible bias that leads to ‘overfocusing’ the images, i.e. providing source position estimates that lie in a considerably smaller region than the true positions. The second is inspired by previous reconstructions of the cluster of galaxies MACS J1149.6+2223, where a multiply imaged background galaxy contained a supernova, SN Refsdal, of which four additional images were produced by the presence of a smaller cluster galaxy. The inversion for the cluster as a whole was not able to recover sufficient detail interior to this quad. We show how constraints on such different scales, from the entire cluster to a single member galaxy, can now be used, allowing such small-scale substructures to be resolved. Finally, the addition of weak lensing information to this method is investigated. While this clearly helps recover the environment around the strong lensing region, the mass sheet degeneracy may make a full strong and weak inversion difficult, depending on the quality of the ellipticity information at hand. We encounter ring-like structure at the boundary of the two regimes, argued to be the result of combining strong and weak lensing constraints, possibly affected by degeneracies.

A FINITE ELEMENT CODE FOR THE NUMERICAL SOLUTION OF THE HELMHOLTZ EQUATION IN AXIALLY SYMMETRIC WAVEGUIDES WITH INTERFACES

1999 ◽  
Vol 07 (02) ◽  
pp. 83-110 ◽  
Author(s):  
NIKOLAOS A. KAMPANIS ◽  
VASSILIOS A. DOUGALIS
Keyword(s):  
Finite Element ◽  
Helmholtz Equation ◽  
Nonlocal Boundary Condition ◽  
Bottom Topography ◽  
Nonlocal Boundary ◽  
Small Scale ◽  
Far Field ◽  
Axially Symmetric ◽  
Fortran Code ◽  
Iterative Linear Solvers

We consider the Helmholtz equation in an axisymmetric cylindrical waveguide consisting of fluid layers overlying a rigid bottom. The medium may have range-dependent speed of sound and interface and bottom topography in the interior nonhomogeneous part of the waveguide, while in the far-field the interfaces and bottom are assumed to be horizontal and the problem separable. A nonlocal boundary condition based on the DtN map of the exterior problem is posed at the far-field artificial boundary. The problem is discretized by a standard Galerkin/finite element method and the resulting numerical scheme is implemented in a Fortran code that is interfaced with general mesh generation programs from the MODULEF finite element library and iterative linear solvers from QMRPACK. The code is tested on several small scale examples of acoustic propagation and scattering in the sea and its results are found to compare well with those of COUPLE.

Acoustic Radiation of a Cylindrical Piezoelectric Power Transformer

2013 ◽  
Vol 80 (6) ◽  
Author(s):  
He Zhang ◽  
Guiru Ye ◽  
Zhicheng Zhang
Keyword(s):  
Acoustic Radiation ◽  
Power Transformer ◽  
Power Level ◽  
Sound Radiation ◽  
Sound Field ◽  
Piezoelectric Transducers ◽  
Radial Polarization ◽  
Metal Core ◽  
Sound Power Level ◽  
Surrounding Fluid

Theoretical analysis is performed for the sound radiation of a cylindrical power transformer composed of piezoelectric transducers with radial polarization. The transformer is driven in thickness-stretch mode, and an exact solution is obtained for the sound pressure and sound power level in the surrounding fluid. Representative examples are used to illustrate the sound field induced by the operation of the transformer. Numerical results indicate that the electrical impedance and the thickness ratio of actuator/sensor to metal core have considerable effects on sound radiation of the cylindrical power transformer.

Aerodynamic sound generation in a pipe

1968 ◽  
Vol 32 (4) ◽  
pp. 765-778 ◽  
Author(s):  
H. G. Davies ◽  
J. E. Ffowcs Williams
Keyword(s):  
Acoustic Waves ◽  
Large Scale ◽  
Sound Field ◽  
Convective Motion ◽  
Energy Output ◽  
Small Scale ◽  
Mean Flow ◽  
Limited Region ◽  
Aerodynamic Sound ◽  
Scale Turbulence

The paper deals with the problem of estimating the sound field generated by a limited region of turbulence in an infinitely long, straight, hard-walled pipe. The field is analysed in a co-ordinate system moving with the assumed uniform mean flow, and the possibility of eddy convection relative to that reference system is considered. Large-scale turbulence is shown to induce plane acoustic waves of intensity proportional to the sixth power of flow velocity. The same is true of small-scale turbulence of low characteristic frequency. In both cases convective effects increase the acoustic output and distribute the bulk of the energy in a mode propagating upstream against the mean flow. Small-scale turbulence of higher frequency excites more modes, the sound increasing with very nearly the eighth power of velocity (U7.7) as soon as the second mode is excited. In the limit, when more than about 20 modes are excited, the energy output is unaffected by the constraint of the pipe walls, increasing with the eighth power of velocity, and being substantially amplified by convective motion.

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