Low‐frequency propagation across the East Greenland Frontal Zone: Directional dependence of acoustic modes

1985 ◽  
Vol 78 (S1) ◽  
pp. S71-S71
Author(s):  
Leonard E. Mellberg ◽  
Donald N. Connors ◽  
Ola M. Johannessen ◽  
George Botseas ◽  
David Browning
Keyword(s):  
Frontal Zone ◽  
Low Frequency ◽  
East Greenland ◽  
Directional Dependence ◽  
Acoustic Modes

scholarly journals Low‐frequency propagation across the East Greenland and Frontal Zone: Depth dependence of transmission loss

1987 ◽  
Vol 82 (S1) ◽  
pp. S31-S31
Author(s):  
Leonard E. Mellberg ◽  
D. N. Connors ◽  
D. G. Browning ◽  
G. Botseas ◽  
O. M. Johannessen
Keyword(s):  
Frontal Zone ◽  
Transmission Loss ◽  
Low Frequency ◽  
East Greenland ◽  
Depth Dependence ◽  
Zone Depth

scholarly journals An analysis of acoustic propagation across the East Greenland Frontal Zone

1985 ◽  
Vol 77 (S1) ◽  
pp. S56-S56
Author(s):  
Leonard E. Mellberg ◽  
Donald N. Connors ◽  
Ola M. Johannessen ◽  
David G. Browning ◽  
George Botseas
Keyword(s):  
Frontal Zone ◽  
Acoustic Propagation ◽  
East Greenland

Low‐frequency propagation across an East Greenland ice‐edge eddy: Winter conditions

1986 ◽  
Vol 79 (S1) ◽  
pp. S69-S69
Author(s):  
Leonard E. Mellberg ◽  
Ola M. Johannessen ◽  
Donald N. Connors ◽  
George Botseas ◽  
David G. Browning
Keyword(s):  
Low Frequency ◽  
East Greenland ◽  
Winter Conditions ◽  
Ice Edge

Nonlinear generation of non-acoustic modes by low-frequency sound in a vibrationally relaxing gas

2010 ◽  
Vol 88 (4) ◽  
pp. 293-300 ◽  
Author(s):  
Anna Perelomova
Keyword(s):  
Degrees Of Freedom ◽  
Vibrational Energy ◽  
Low Frequency ◽  
Acoustic Energy ◽  
Dynamic Equations ◽  
Thermal Mode ◽  
Acoustic Source ◽  
Frequency Sound ◽  
Acoustic Modes ◽  
Relaxing Gas

Two dynamic equations referring to a weakly nonlinear and weakly dispersive flow of a gas in which molecular vibrational relaxation takes place, are derived. The first one governs an excess temperature associated with the thermal mode, and the second one describes variations in vibrational energy. Both quantities refer to non-wave types of gas motion. These variations are caused by the nonlinear transfer of acoustic energy into thermal mode and internal vibrational degrees of freedom of a relaxing gas. The final dynamic equations are instantaneous; they include a quadratic nonlinear acoustic source, reflecting the nonlinear character of interaction of low-frequency acoustic and non-acoustic motions of the fluid. All types of sound, periodic or aperiodic, may serve as an acoustic source of both phenomena. The low-frequency sound is considered in this study. Some conclusions about temporal behavior of non-acoustic modes caused by periodic and aperiodic sound are made. Under certain conditions, acoustic cooling takes place instead of heating .

Beta-induced Alfvén eigenmodes and geodesic acoustic modes in presence of strong tearing activity during the current ramp-down on JET

2022 ◽  
Author(s):  
Gianluca Pucella ◽  
Edoardo Alessi ◽  
Fulvio Auriemma ◽  
Paolo Buratti ◽  
Matteo Valerio Falessi ◽  
...  
Keyword(s):  
Low Frequency ◽  
Simultaneous Presence ◽  
Magnetic Islands ◽  
Linear Interaction ◽  
Acoustic Modes ◽  
Current Ramp ◽  
Alfven Eigenmodes ◽  
Non Linear ◽  
Mode Tm ◽  
Magnetic Oscillations

Abstract The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 kHz to 20 kHz, being well below the toroidal gap in the Alfven continuum and of the same order of the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered characterized by very low additional heating. The calculation of the coupled Alfven-acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfven eigenmodes (BAE) rather than beta-induced Alfven acoustic eigenmodes (BAAE). As a main novelty compared to previous works, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfven continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating tearing modes, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAM), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction among TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

A Dimensionless Quotient for Determining Coupling Strength in Modal Energy Analysis

2016 ◽  
Vol 138 (6) ◽  
Author(s):  
Peng Zhang ◽  
Qingguo Fei ◽  
Shaoqing Wu ◽  
Yanbin Li
Keyword(s):  
Frequency Band ◽  
Coupling Strength ◽  
Energy Analysis ◽  
Low Frequency ◽  
Input Power ◽  
Acoustic Modes ◽  
Acoustic Cavity ◽  
Weakly Coupled ◽  
Gyroscopic Coupling ◽  
Almost All

Modal energy analysis (MODENA) is an energy-based method recently proposed to estimate the dynamic response of a coupled structure/acoustic cavity system. The accuracy of MODENA is affected by the coupling strength between structural and acoustic modes. A dimensionless coupling quotient which is equal to the ratio of the gyroscopic coupling coefficient and the critical coefficient at modal frequencies is defined to determine the coupling strength in MODENA. The coupling strength of the system is classified as weak, moderate, or strong, according to the coupling quotient with a proposed criterion. When computing the modal input power in MODENA, the mobility of the uncoupled mode can be used if the modes are weakly coupled, but the mobility of the coupled mode should be adopted to obtain accurate results if many modes are moderately coupled. The effectiveness of the proposed criterion is validated via a numerical example where a plate is coupled with an acoustic cavity. Results show that many low-order structural and acoustic modes are moderately coupled while almost all high-order modes are weakly coupled. Errors of the energy responses appear in a low-frequency band, but accurate results are acquired in a mid- to high-frequency band when the mobility of uncoupled mode is used.

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