Effects of low-frequency fluctuations on the growth or decay of high frequency modes

1976 ◽  
Vol 19 (2) ◽  
pp. 256 ◽  
Author(s):  
D. F. Escande
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Frequency Fluctuations ◽  
High Frequency Modes

scholarly journals On the Origin of High–Frequency Magnetic Fluctuations in the Interplanetary Medium: A Brownian–like Approach

2021 ◽  
Vol 9 ◽  
Author(s):  
Vincenzo Carbone ◽  
Fabio Lepreti ◽  
Antonio Vecchio ◽  
Tommaso Alberti ◽  
Federica Chiappetta
Keyword(s):  
High Frequency ◽  
Interplanetary Medium ◽  
Interplanetary Space ◽  
Plasma Heating ◽  
Low Frequency ◽  
Solar Wind Plasma ◽  
Magnetic Fluctuations ◽  
Fluctuation Dissipation ◽  
Frequency Fluctuations ◽  
Collisionless Dissipation

Low–frequency fluctuations in the interplanetary medium have been extensively investigated and described in the framework of turbulence, and the observed universal scaling behavior represents a clear signature of the underlying energy cascade. On the contrary, the interpretation of observations of plasma fluctuations at high frequencies, where wave–wave coupling, collisionless dissipation, and anomalous plasma heating play a key role, still represents a challenge for theoretical modeling. In this paper the high frequency fluctuations occurring in the interplanetary space are described through a Brownian–like approach, where the plasma dynamics at small scales is described through a stochastic process. It is shown that a simple model based on this framework is able to successfully reproduce the main features of the spectrum of the observed magnetic fluctuations. Moreover, the Fluctuation-Dissipation Relation, derived by our model, leads to a power law between dissipation rate and temperature, which is compatible with the occurrence of Landau damping, interpreted thus as the main mechanism of dissipation in the solar wind plasma.

scholarly journals X-ray diffraction data as a source of the vibrational free-energy contribution in polymorphic systems

IUCrJ ◽  
2019 ◽  
Vol 6 (4) ◽  
pp. 558-571 ◽  
Author(s):  
Phillip Miguel Kofoed ◽  
Anna A. Hoser ◽  
Frederik Diness ◽  
Silvia C. Capelli ◽  
Anders Østergaard Madsen
Keyword(s):  
Thermodynamic Properties ◽  
Normal Mode ◽  
Diffraction Data ◽  
High Frequency ◽  
Low Frequency ◽  
General Question ◽  
Vibrational Entropy ◽  
X Ray Diffraction ◽  
X Ray ◽  
High Frequency Modes

In this contribution we attempt to answer a general question: can X-ray diffraction data combined with theoretical computations be a source of information about the thermodynamic properties of a given system? Newly collected sets of high-quality multi-temperature single-crystal X-ray diffraction data and complementary periodic DFT calculations of vibrational frequencies and normal mode vectors at the Γ point on the yellow and white polymorphs of dimethyl 3,6-dichloro-2,5-dihydroxyterephthalate are combined using two different approaches, aiming to obtain thermodynamic properties for the two compounds. The first approach uses low-frequency normal modes extracted from multi-temperature X-ray diffraction data (normal coordinate analysis), while the other uses DFT-calculated low-frequency normal mode in the refinement of the same data (normal mode refinement). Thermodynamic data from the literature [Yang et al. (1989), Acta Cryst. B45, 312–323] and new periodic ab initio DFT supercell calculations are used as a reference point. Both approaches tested in this work capture the most essential features of the systems: the polymorphs are enantiotropically related, with the yellow form being the thermodynamically stable system at low temperature, and the white form at higher temperatures. However, the inferred phase transition temperature varies between different approaches. Thanks to the application of unconventional methods of X-ray data refinement and analysis, it was additionally found that, in the case of the yellow polymorph, anharmonicity is an important issue. By discussing contributions from low- and high-frequency modes to the vibrational entropy and enthalpy, the importance of high-frequency modes is highlighted. The analysis shows that larger anisotropic displacement parameters are not always related to the polymorph with the higher vibrational entropy contribution.

scholarly journals Accounting for Quasi-Static Coupling in Nonlinear Dynamic Reduced-Order Models

2020 ◽  
Vol 15 (7) ◽  
Author(s):  
Evangelia Nicolaidou ◽  
Venkata R. Melanthuru ◽  
Thomas L. Hill ◽  
Simon A. Neild
Keyword(s):  
High Frequency ◽  
Nonlinear Dynamic ◽  
Nonlinear Dynamic Analysis ◽  
Low Frequency ◽  
Nonlinear Effects ◽  
Fe Model ◽  
Engineering Structures ◽  
Reduced Order ◽  
Modal Coupling ◽  
High Frequency Modes

Abstract Engineering structures are often designed using detailed finite element (FE) models. Although these models can capture nonlinear effects, performing nonlinear dynamic analysis using FE models is often prohibitively computationally expensive. Nonlinear reduced-order modeling provides a means of capturing the principal dynamics of an FE model in a smaller, computationally cheaper reduced-order model (ROM). One challenge in formulating nonlinear ROMs is the strong coupling between low- and high-frequency modes, a feature we term quasi-static coupling. An example of this is the coupling between bending and axial modes of beams. Some methods for formulating ROMs require that these high-frequency modes are included in the ROM, thus increasing its size and adding computational expense. Other methods can implicitly capture the effects of the high-frequency modes within the retained low-frequency modes; however, the resulting ROMs are normally sensitive to the scaling used to calibrate them, which may introduce errors. In this paper, quasi-static coupling is first investigated using a simple oscillator with nonlinearities up to the cubic order. ROMs typically include quadratic and cubic nonlinear terms, however here it is demonstrated mathematically that the ROM describing the oscillator requires higher-order nonlinear terms to capture the modal coupling. Novel ROMs, with high-order nonlinear terms, are then shown to be more accurate, and significantly more robust to scaling, than standard ROMs developed using existing approaches. The robustness of these novel ROMs is further demonstrated using a clamped–clamped beam, modeled using commercial FE software.

Understanding the influence of low-frequency vibrations on the hydrogen bonds of acetic acid and acetamide dimers

2017 ◽  
Vol 19 (36) ◽  
pp. 24866-24878 ◽  
Author(s):  
Christopher Copeland ◽  
Omkaran Menon ◽  
D. Majumdar ◽  
Szczepan Roszak ◽  
Jerzy Leszczynski
Keyword(s):  
Hydrogen Bond ◽  
Acetic Acid ◽  
Hydrogen Bonds ◽  
High Frequency ◽  
Low Frequency ◽  
Weakly Interacting ◽  
Bond Strengths ◽  
High Frequency Modes

Low-frequency vibrations coupled to high-frequency modes are known to influence the hydrogen bond strengths in a weakly interacting dimer.

Energy Transfer From High-to Low-Frequency Modes in a Flexible Structure via Modulation

1994 ◽  
Vol 116 (2) ◽  
pp. 203-207 ◽  
Author(s):  
S. A. Nayfeh ◽  
A. H. Nayfeh
Keyword(s):  
Cross Section ◽  
High Frequency ◽  
Natural Frequencies ◽  
Circular Cross Section ◽  
Low Frequency ◽  
Cantilever Beams ◽  
Axially Symmetric ◽  
Internal Resonances ◽  
Planar Motions ◽  
High Frequency Modes

An experimental study of the response of axially-symmetric (i.e., circular cross-section) cantilever beams to planar external excitations is presented. Because of the axial symmetry, one-to-one internal resonances occur at each natural frequency. These resonances cause the planar motions to lose stability and nonplanar (whirling) motions are observed. Under certain conditions, periodically-and chaotically-modulated motions may occur. In addition, when the beam is excited near one of its high natural frequencies, large first-mode responses accompanied by slow modulations of the amplitudes and phases of high-frequency modes are observed. This interaction between high-and low-frequency modes may be extremely dangerous because the amplitudes of the responses of the low-frequency modes can be very large compared with those of the directly excited high-frequency modes.

Assessment of autonomic function in humans by heart rate spectral analysis

1985 ◽  
Vol 248 (1) ◽  
pp. H151-H153 ◽  
Author(s):  
B. Pomeranz ◽  
R. J. Macaulay ◽  
M. A. Caudill ◽  
I. Kutz ◽  
D. Adam ◽  
...  
Keyword(s):  
Heart Rate ◽  
Nervous System ◽  
Spectral Analysis ◽  
High Frequency ◽  
Autonomic Function ◽  
Parasympathetic Nervous System ◽  
Low Frequency ◽  
Parasympathetic System ◽  
Heart Rate Fluctuations ◽  
Frequency Fluctuations

Spectral analysis of spontaneous heart rate fluctuations were assessed by use of autonomic blocking agents and changes in posture. Low-frequency fluctuations (below 0.12 Hz) in the supine position are mediated entirely by the parasympathetic nervous system. On standing, the low-frequency fluctuations increase and are jointly mediated by the sympathetic and parasympathetic nervous systems. High-frequency fluctuations, at the respiratory frequency, are decreased by standing and are mediated solely by the parasympathetic system. Heart rate spectral analysis is a powerful noninvasive tool for quantifying autonomic nervous system activity.

scholarly journals Amplitude Modulation of Relative Humidity by Wind in Northeast China: the Formation of Variance Annual Cycle in Relative Humidity

2021 ◽  
Author(s):  
Da Nian ◽  
Yu Huang ◽  
Zuntao Fu
Keyword(s):  
Wind Speed ◽  
Relative Humidity ◽  
Annual Cycle ◽  
Amplitude Modulation ◽  
High Frequency ◽  
Northeast China ◽  
Low Frequency ◽  
Phase Correlation ◽  
Low Frequency Oscillations ◽  
Frequency Fluctuations

Abstract Relative humidity has an important impact not only on climate change and ecosystems but also on human life. The intensity of high-frequency fluctuations in relative humidity over Northeast China shows a predominant seasonally dependent structure, which may be closely related to regional monsoon activities. However, the factors responsible for this phenomenon remain unknown. This study defines the Variance Annual Cycle (VAC) to describe this seasonally dependent intensity structure of high-frequency relative humidity fluctuations. Relative humidity VAC shows a high correlation with low-frequency oscillations of wind speed. We examine the instantaneous amplitude-phase correlation map and amplitude modulation (AM) index between relative humidity and wind speed. We find that the wind speed with a period around 140-420 days has a significant amplitude modulation effect on the relative humidity with a period around 2-90 days over most regions in Northeast China, which reveals that the low-frequency oscillations of wind speed amplitude- modulate on the high-frequency fluctuations of relative humidity. To explore the physical mechanism behind this modulation, we examine the monthly mean patterns of the atmospheric fields. The patterns indicate that this amplitude modulation is induced by the evolution and transition of East Asian winter monsoon and summer monsoon.

Anomalous transport and anomalous heating due to lower-hybrid wave fields

1988 ◽  
Vol 39 (3) ◽  
pp. 447-474 ◽  
Author(s):  
M. Krämer ◽  
N. Sollich ◽  
J. Dietrich
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Field Energy ◽  
Lower Hybrid ◽  
Hybrid Wave ◽  
Heating Rates ◽  
Lower Hybrid Wave ◽  
Drift Wave Turbulence ◽  
Frequency Fluctuations ◽  
High Frequency Waves

The microscopic and macroscopic behaviours of a linear reflex discharge in the presence of low-frequency turbulence are investigated under the action of moderate lower-hybrid wave power. The frequency and wavenumber spectra of both the low-frequency fluctuations and the high-frequency waves are measured using a correlation-analysis technique with two probes. The low-frequency fluctuations may be attributed to drift-wave turbulence. The fluctuation level is raised when RF power is coupled to the plasma, thus leading to considerably enhanced radial transport. The coupling between low-frequency fluctuations and high-frequency waves can be seen clearly from the spectra. The high-frequency wavenumber spectra measured inside the antenna are in reasonable agreement with the lower-hybrid wave dispersion. However, the wavenumbers observed in the lower-hybrid resonance region outside the antenna are – in contrast with expectation – not larger than in the plasma edge region. From the electric-field energy-density spectra and from measurements of the density and the temperatures, a detailed energy balance can be performed. The calculated heating rates are anomalously large for both the electrons and the ions. The absorption processes, relevant for the present experiment, are discussed.

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