New model for acoustic waves propagating through a vortical flow

2017 ◽  
Vol 823 ◽  
pp. 658-674 ◽  
Author(s):  
Jim Thomas
Keyword(s):  
Asymptotic Analysis ◽  
Wave Field ◽  
Time Scale ◽  
Acoustic Waves ◽  
High Frequency ◽  
Spatial Scales ◽  
Amplitude Equation ◽  
Vortical Flow ◽  
Reduced Model ◽  
Scale Separation

A new amplitude equation is derived for high-frequency acoustic waves propagating through an incompressible vortical flow using multi-time-scale asymptotic analysis. The reduced model is derived without an explicit spatial-scale separation ansatz between the wave and vortical fields. As a consequence, the model is seen to capture very well the features of the wave field in the regime where the spatial scales of the wave and vortical fields are comparable, a regime for which an optimal reduced model does not seem to be available.

scholarly journals Asymptotics of near-cloaking

2020 ◽  
pp. 1-21
Author(s):  
J. R. OCKENDON ◽  
H. OCKENDON ◽  
B. D. SLEEMAN ◽  
R. H. TEW
Keyword(s):  
Asymptotic Analysis ◽  
Acoustic Waves ◽  
High Frequency ◽  
Ray Theory ◽  
Elastic Materials ◽  
Eigenfunction Expansions ◽  
Linear Elastic

This paper describes how asymptotic analysis can be used to gain new insights into the theory of cloaking of spherical and cylindrical targets within the context of acoustic waves in a class of linear elastic materials. In certain cases, these configurations allow solutions to be written down in terms of eigenfunction expansions from which high-frequency asymptotics can be extracted systematically. These asymptotics are compared with the predictions of ray theory and are used to describe the scattering that occurs when perfect cloaking models are regularised.

2MHz ultrasound enhances t-PA-mediated thrombolysis: comparison of continuous versus pulsed ultrasound and standing versus travelling acoustic waves

2003 ◽  
Vol 89 (03) ◽  
pp. 583-589 ◽  
Author(s):  
Stefan Pfaffenberger ◽  
Branka Devcic-Kuhar ◽  
Karem El-Rabadi ◽  
Martin Gröschl ◽  
Walter Speidl ◽  
...  
Keyword(s):  
Wave Field ◽  
Acoustic Waves ◽  
High Frequency ◽  
Travelling Wave ◽  
High Frequency Ultrasound ◽  
Pulsed Ultrasound ◽  
Application Mode ◽  
Frequency Ultrasound

SummaryIn addition to fibrinolytic enzymes, ultrasound has the potential to enhance thrombolysis. High frequency ultrasound has the advantage that a combination of diagnostic and therapeutic ultrasound with only one device is possible. Therefore, we investigated the optimal high frequency (2 MHz) ultrasound field characteristics and application mode in vitro. Continuous ultrasound significantly enhanced rt-PA mediated thrombolysis: in a travelling wave field thrombolysis was augmented by 49.0 ± 14.7% and in a standing wave field by 34.8 ± 7.3%. In an intermittent application mode (1Hz, 10Hz, 100Hz, 1kHz) most efficient results were obtained for both wave fields using 1 Hz (46.4 ± 10.7% and 39.1 ± 6.6%, respectively). Referring to a possible in vivo application our in vitro data suggests that an intermittent application of a 2 MHz high frequency ultrasound using a travelling wave field would be the most potent application for lysing blood clots.

scholarly journals High frequency generation in the corona: Resonant cavities

2018 ◽  
Vol 611 ◽  
pp. A10 ◽  
Author(s):  
I. C. Santamaria ◽  
T. Van Doorsselaere
Keyword(s):  
Magnetic Field ◽  
Shock Wave ◽  
Acoustic Waves ◽  
High Frequency ◽  
Spatial Scales ◽  
Wave Train ◽  
Null Point ◽  
Solar Chromosphere ◽  
High Frequency Region ◽  
Frequency Wave

Aims. Null points are prominent magnetic field singularities in which the magnetic field strength strongly decreases in very small spatial scales. Around null points, predicted to be ubiquitous in the solar chromosphere and corona, the wave behavior changes considerably. Null points are also responsible for driving very energetic phenomena, and for contributing to chromospheric and coronal heating. In previous works we demonstrated that slow magneto-acoustic shock waves were generated in the chromosphere propagate through the null point, thereby producing a train of secondary shocks escaping along the field lines. A particular combination of the shock wave speeds generates waves at a frequency of 80 MHz. The present work aims to investigate this high frequency region around a coronal null point to give a plausible explanation to its generation at that particular frequency. Methods. We carried out a set of two-dimensional numerical simulations of wave propagation in the neighborhood of a null point located in the corona. We varied both the amplitude of the driver and the atmospheric properties to investigate the sensitivity of the high frequency waves to these parameters. Results. We demonstrate that the wave frequency is sensitive to the atmospheric parameters in the corona, but it is independent of the strength of the driver. Thus, the null point behaves as a resonant cavity generating waves at specific frequencies that depend on the background equilibrium model. Moreover, we conclude that the high frequency wave train generated at the null point is not necessarily a result of the interaction between the null point and a shock wave. This wave train can be also developed by the interaction between the null point and fast acoustic-like magneto-acoustic waves, that is, this interaction within the linear regime.

scholarly journals On Singular Perturbations of Flexible and Variable-Speed Wind Turbines

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
R. Oulad Ben Zarouala ◽  
C. Vivas ◽  
J. Á. Acosta ◽  
L. El Bakkali
Keyword(s):  
Singular Perturbations ◽  
Time Scale ◽  
Reduced Model ◽  
Physical Parameters ◽  
Scale Separation ◽  
Reduced Order ◽  
Physical Mechanisms ◽  
Time Scale Separation ◽  
Successful Approach ◽  
Mechanical Dynamics

A model for the mechanical dynamics of a wind turbine is developed, which is the composition of three physical mechanisms: flexion, torsion, and rotational dynamics. A first contribution is the identification of the essential physical parameters that provide a time-scale separation of these three mechanisms. Under the assumption of singular perturbations the time-scale separation allows to work with a reduced model of order one. This reduction has been essential for the control of this system allowing to control designers to take into account only the reduced-order model. A second contribution consists in employing a measurement of the fore-aft nacelle acceleration with the reduced model, together with a Kalman filter to estimate the flexible DOFs of the system (tower and average blade deflection). The successful approach is tested on high-order nonlinear aeroelastic simulator (FAST).

scholarly journals Crack-Length Estimation for Structural Health Monitoring Using the High-Frequency Resonances Excited by the Energy Release during Fatigue-Crack Growth

Sensors ◽  
2021 ◽  
Vol 21 (12) ◽  
pp. 4221
Author(s):  
Roshan Joseph ◽  
Hanfei Mei ◽  
Asaad Migot ◽  
Victor Giurgiutiu
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Length ◽  
Crack Growth ◽  
Health Monitoring ◽  
Acoustic Waves ◽  
High Frequency ◽  
Fem Analysis ◽  
Signal Frequency ◽  
Propagation Pattern

Acoustic waves are widely used in structural health monitoring (SHM) for detecting fatigue cracking. The strain energy released when a fatigue crack advances has the effect of exciting acoustic waves, which travel through the structures and are picked up by the sensors. Piezoelectric wafer active sensors (PWAS) can effectively sense acoustic waves due to fatigue-crack growth. Conventional acoustic-wave passive SHM, which relies on counting the number of acoustic events, cannot precisely estimate the crack length. In the present research, a novel method for estimating the crack length was proposed based on the high-frequency resonances excited in the crack by the energy released when a crack advances. In this method, a PWAS sensor was used to sense the acoustic wave signal and predict the length of the crack that generated the acoustic event. First, FEM analysis was undertaken of acoustic waves generated due to a fatigue-crack growth event on an aluminum-2024 plate. The FEM analysis was used to predict the wave propagation pattern and the acoustic signal received by the PWAS mounted at a distance of 25 mm from the crack. The analysis was carried out for crack lengths of 4 and 8 mm. The presence of the crack produced scattering of the waves generated at the crack tip; this phenomenon was observable in the wave propagation pattern and in the acoustic signals recorded at the PWAS. A study of the signal frequency spectrum revealed peaks and valleys in the spectrum that changed in frequency and amplitude as the crack length was changed from 4 to 8 mm. The number of peaks and valleys was observed to increase as the crack length increased. We suggest this peak–valley pattern in the signal frequency spectrum can be used to determine the crack length from the acoustic signal alone. An experimental investigation was performed to record the acoustic signals in crack lengths of 4 and 8 mm, and the results were found to match well with the FEM predictions.

scholarly journals Research of the process of vegetable oil extracting under the influence of a high frequency wave field

2020 ◽  
Vol 941 ◽  
pp. 012052
Author(s):  
V Yu Ovsyannikov ◽  
A A Berestovoy ◽  
N N Lobacheva ◽  
V V Toroptsev ◽  
S A Trunov
Keyword(s):  
Wave Field ◽  
Vegetable Oil ◽  
High Frequency ◽  
Frequency Wave ◽  
High Frequency Wave
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