scholarly journals Lifetime estimate for plasma turbulence

2017 ◽  
Author(s):  
Yasuhito Narita ◽  
Zoltán Vörös
Keyword(s):  
Decay Rate ◽  
Time Scale ◽  
Interplanetary Space ◽  
Spatial Scales ◽  
Turnover Time ◽  
Magnetic Field Data ◽  
Theoretical Predictions ◽  
Non Gaussian ◽  
Wigner Spectrum ◽  
Spectrum Model

Abstract. A method is proposed to experimentally determine the intrinsic time scale or a decay rate of turbulent fluctuations. The method is based on the assumption that the Breit-Wigner spectrum model with a non-Gaussian frequency broadening is valid in the data analysis. The decay rate estimate is applied to the multi-spacecraft magnetic field data in interplanetary space, yielding that the result that the decay rate on spatial scales of about 1000 km (about 10 times larger than the ion inertial length) is higher than the theoretical predictions from the random sweeping time scale of the eddy turnover time. The faster decay of fluctuation components in interplanetary space is interpreted as a realization of plasma physical (and not fluid mechanical) process.

scholarly journals Lifetime estimate for plasma turbulence

2017 ◽  
Vol 24 (4) ◽  
pp. 673-679 ◽  
Author(s):  
Yasuhito Narita ◽  
Zoltán Vörös
Keyword(s):  
Decay Rate ◽  
Interplanetary Space ◽  
Spatial Scales ◽  
Turnover Time ◽  
Rate Estimate ◽  
Magnetic Field Data ◽  
Theoretical Predictions ◽  
Non Gaussian ◽  
Wigner Spectrum ◽  
Spectrum Model

Abstract. A method is proposed to experimentally determine the intrinsic timescale or a decay rate of turbulent fluctuations. The method is based on the assumption that the Breit–Wigner spectrum model with a non-Gaussian frequency broadening is valid in the data analysis. The decay rate estimate is applied to the multispacecraft magnetic field data in interplanetary space, yielding the decay rate on spatial scales of about 1000 km (about 10 times larger than the ion inertial length), which is higher than the theoretical predictions from the random sweeping timescale of the eddy turnover time. The faster decay of fluctuation components in interplanetary space is interpreted as a realization of plasma physical (and not fluid mechanical) processes.

Reynolds-number dependence of line and surface stretching in turbulence: folding effects

2007 ◽  
Vol 586 ◽  
pp. 59-81 ◽  
Author(s):  
SUSUMU GOTO ◽  
SHIGEO KIDA
Keyword(s):  
Reynolds Number ◽  
Time Scale ◽  
Isotropic Turbulence ◽  
Three Dimensional ◽  
Turnover Time ◽  
Material Object ◽  
Material Objects ◽  
Short Time Scale ◽  
Kolmogorov Scale ◽  
Reynolds Number Dependence

The stretching rate, normalized by the reciprocal of the Kolmogorov time, of sufficiently extended material lines and surfaces in statistically stationary homogeneous isotropic turbulence depends on the Reynolds number, in contrast to the conventional picture that the statistics of material object deformation are determined solely by the Kolmogorov-scale eddies. This Reynolds-number dependence of the stretching rate of sufficiently extended material objects is numerically verified both in two- and three-dimensional turbulence, although the normalized stretching rate of infinitesimal material objects is confirmed to be independent of the Reynolds number. These numerical results can be understood from the following three facts. First, the exponentially rapid stretching brings about rapid multiple folding of finite-sized material objects, but no folding takes place for infinitesimal objects. Secondly, since the local degree of folding is positively correlated with the local stretching rate and it is non-uniformly distributed over finite-sized objects, the folding enhances the stretching rate of the finite-sized objects. Thirdly, the stretching of infinitesimal fractions of material objects is governed by the Kolmogorov-scale eddies, whereas the folding of a finite-sized material object is governed by all eddies smaller than the spatial extent of the objects. In other words, the time scale of stretching of infinitesimal fractions of material objects is proportional to the Kolmogorov time, whereas that of folding of sufficiently extended material objects can be as long as the turnover time of the largest eddies. The combination of the short time scale of stretching of infinitesimal fractions and the long time scale of folding of the whole object yields the Reynolds-number dependence. Movies are available with the online version of the paper.

scholarly journals Development and Integration of Sub-Daily Flood Modelling Capability within the SWAT Model and a Comparison with XAJ Model

Water ◽  
2018 ◽  
Vol 10 (9) ◽  
pp. 1263 ◽  
Author(s):  
Dachen Li ◽  
Simin Qu ◽  
Peng Shi ◽  
Xueqiu Chen ◽  
Feng Xue ◽  
...  
Keyword(s):  
Time Scale ◽  
Flood Control ◽  
Assessment Tool ◽  
Spatial Scales ◽  
Swat Model ◽  
Flood Forecasting ◽  
Base Flow ◽  
Flood Simulation ◽  
Promising Tool ◽  
Daily Time

To date, floods have become one of the most severe natural disasters on Earth. Flood forecasting with hydrological models is an important non-engineering measure for flood control and disaster reduction. The Xin’anjiang (XAJ) model is the most widely used hydrological model in China for flood forecasting, while the Soil and Water Assessment Tool (SWAT) model is widely applied for daily and monthly simulation and has shown its potential for flood simulation. The objective of this paper is to evaluate the performance of the SWAT model in simulating floods at a sub-daily time-scale in a slightly larger basin and compare that with the XAJ model. Taking Qilijie Basin (southeast of China) as a study area, this paper developed the XAJ model and SWAT model at a sub-daily time-scale. The results showed that the XAJ model had a better performance than the sub-daily SWAT model regarding relative runoff error (RRE) but the SWAT model performed well according to relative peak discharge error (RPE) and error of occurrence time of peak flow (PTE). The SWAT model performed unsatisfactorily in simulating low flows due to the daily calculation of base flow but behaved quite well in simulating high flows. We also evaluated the effect of spatial scale on the SWAT model. The results showed that the SWAT model had a good applicability at different spatial scales. In conclusion, the sub-daily SWAT model is a promising tool for flood simulation though more improvements remain to be studied further.

scholarly journals Asteroseismology of 36 Kepler subgiants – I. Oscillation frequencies, linewidths, and amplitudes

2020 ◽  
Vol 495 (2) ◽  
pp. 2363-2386 ◽  
Author(s):  
Yaguang Li ◽  
Timothy R Bedding ◽  
Tanda Li ◽  
Shaolan Bi ◽  
Dennis Stello ◽  
...  
Keyword(s):  
Monte Carlo ◽  
Oscillation Mode ◽  
Monte Carlo Algorithm ◽  
Scaling Relations ◽  
Mass Dependence ◽  
Seismic Parameters ◽  
Theoretical Predictions ◽  
Mixed Modes ◽  
Oscillation Frequencies ◽  
Spectrum Model

ABSTRACT The presence of mixed modes makes subgiants excellent targets for asteroseismology, providing a probe for the internal structure of stars. Here we study 36 Kepler subgiants with solar-like oscillations and report their oscillation mode parameters. We performed a so-called peakbagging exercise, i.e. estimating oscillation mode frequencies, linewidths, and amplitudes with a power spectrum model, fitted in the Bayesian framework and sampled with a Markov chain Monte Carlo algorithm. The uncertainties of the mode frequencies have a median value of 0.180 μHz. We obtained seismic parameters from the peakbagging, analysed their correlation with stellar parameters, and examined against scaling relations. The behaviour of seismic parameters (e.g. Δν, νmax, ϵp) is in general consistent with theoretical predictions. We presented the observational p–g diagrams, namely γ1–Δν for early subgiants and ΔΠ1–Δν for late subgiants, and demonstrate their capability to estimate stellar mass. We also found a log g dependence on the linewidths and a mass dependence on the oscillation amplitudes and the widths of oscillation excess. This sample will be valuable constraints for modelling stars and studying mode physics such as excitation and damping.

scholarly journals Sensitivity of the lower edge of the pair-instability black hole mass gap to the treatment of time-dependent convection

2020 ◽  
Vol 493 (3) ◽  
pp. 4333-4341 ◽  
Author(s):  
M Renzo ◽  
R J Farmer ◽  
S Justham ◽  
S E de Mink ◽  
Y Götberg ◽  
...  
Keyword(s):  
Black Hole ◽  
Gravitational Wave ◽  
Time Scale ◽  
Turnover Time ◽  
Black Hole Mass ◽  
The Third ◽  
Mass Gap ◽  
Mixing Length Theory ◽  
State Assumption ◽  
Additional Equation

ABSTRACT Gravitational-wave detections are now probing the black hole (BH) mass distribution, including the predicted pair-instability mass gap. These data require robust quantitative predictions, which are challenging to obtain. The most massive BH progenitors experience episodic mass ejections on time-scales shorter than the convective turnover time-scale. This invalidates the steady-state assumption on which the classic mixing length theory relies. We compare the final BH masses computed with two different versions of the stellar evolutionary code $\tt{MESA}$: (i) using the default implementation of Paxton et al. (2018) and (ii) solving an additional equation accounting for the time-scale for convective deceleration. In the second grid, where stronger convection develops during the pulses and carries part of the energy, we find weaker pulses. This leads to lower amounts of mass being ejected and thus higher final BH masses of up to ∼$5\, \mathrm{M}_\odot$. The differences are much smaller for the progenitors that determine the maximum mass of BHs below the gap. This prediction is robust at $M_{\rm BH, max}\simeq 48\, \mathrm{M}_\odot$, at least within the idealized context of this study. This is an encouraging indication that current models are robust enough for comparison with the present-day gravitational-wave detections. However, the large differences between individual models emphasize the importance of improving the treatment of convection in stellar models, especially in the light of the data anticipated from the third generation of gravitational-wave detectors.

scholarly journals Lagrangian predictability of high-resolution regional models: the special case of the Gulf of Mexico

2004 ◽  
Vol 11 (1) ◽  
pp. 47-66 ◽  
Author(s):  
P. C. Chu ◽  
L. M. Ivanov ◽  
L. H. Kantha ◽  
T. M. Margolina ◽  
O. V. Melnichenko ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Prediction Error ◽  
Spatial Scales ◽  
Singular Values ◽  
Forecast System ◽  
Gaussian Shape ◽  
University Of Colorado ◽  
Non Gaussian ◽  
The University ◽  
Lagrangian Drifter

Abstract. The Lagrangian prediction skill (model ability to reproduce Lagrangian drifter trajectories) of the nowcast/forecast system developed for the Gulf of Mexico at the University of Colorado at Boulder is examined through comparison with real drifter observations. Model prediction error (MPE), singular values (SVs) and irreversible-skill time (IT) are used as quantitative measures of the examination. Divergent (poloidal) and nondivergent (toroidal) components of the circulation attractor at 50m depth are analyzed and compared with the Lagrangian drifter buoy data using the empirical orthogonal function (EOF) decomposition and the measures, respectively. Irregular (probably, chaotic) dynamics of the circulation attractor reproduced by the nowcast/forecast system is analyzed through Lyapunov dimension, global entropies, toroidal and poloidal kinetic energies. The results allow assuming exponential growth of prediction error on the attractor. On the other hand, the q-th moment of MPE grows by the power law with exponent of 3q/4. The probability density function (PDF) of MPE has a symmetrical but non-Gaussian shape for both the short and long prediction times and for spatial scales ranging from 20km to 300km. The phenomenological model of MPE based on a diffusion-like equation is developed. The PDF of IT is non-symmetric with a long tail stretched towards large ITs. The power decay of the tail was faster than 2 for long prediction times.

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 Modulation equations for strongly nonlinear oscillations of an incompressible viscous drop

2010 ◽  
Vol 654 ◽  
pp. 141-159 ◽  
Author(s):  
WARREN R. SMITH
Keyword(s):  
Decay Rate ◽  
Time Scale ◽  
Stokes Equations ◽  
Nonlinear Oscillations ◽  
Prolate Spheroid ◽  
Perturbation Scheme ◽  
Modulation Equation ◽  
Conservation Of Energy ◽  
Modulation Equations ◽  
Drop Radius

Large-amplitude oscillations of incompressible viscous drops are studied at small capillary number. On the long viscous time scale, a formal perturbation scheme is developed to determine original modulation equations. These two ordinary differential equations comprise the averaged condition for conservation of energy and the averaged projection of the Navier–Stokes equations onto the vorticity vector. The modulation equations are applied to the free decay of axisymmetric oblate–prolate spheroid oscillations. On the long time scale, only the modulation equation for energy is required. In this example, the results compare well with linear viscous theory, weakly nonlinear inviscid theory and experimental observations. The new results show that previous experimental observations and numerical simulations are all manifestations of a single-valued relationship between dimensionless decay rate and amplitude. Moreover, if the amplitude of the oscillations does not exceed 30% of the drop radius, this decay rate may be approximated by a quadratic. The new results also show that, when the amplitude of the oscillations exceeds 20% of the drop radius, fluid in the inviscid bulk of the drop is undergoing abrupt changes in its acceleration in comparison to the acceleration during small-amplitude deformations.

scholarly journals A Spatiotemporal Water Vapor–Deep Convection Correlation Metric Derived from the Amazon Dense GNSS Meteorological Network

2017 ◽  
Vol 145 (1) ◽  
pp. 279-288 ◽  
Author(s):  
David K. Adams ◽  
Henrique M. J. Barbosa ◽  
Karen Patricia Gaitán De Los Ríos
Keyword(s):  
High Resolution ◽  
Decay Time ◽  
Time Scale ◽  
Numerical Models ◽  
Spatial Scales ◽  
Deep Convection ◽  
Model Verification ◽  
Correlation Decay ◽  
Convection Correlation ◽  
The Tropics

Abstract Deep atmospheric convection, which covers a large range of spatial scales during its evolution, continues to be a challenge for models to replicate, particularly over land in the tropics. Specifically, the shallow-to-deep convective transition and organization on the mesoscale are often not properly represented in coarse-resolution models. High-resolution models offer insights on physical mechanisms responsible for the shallow-to-deep transition. Model verification, however, at both coarse and high resolution requires validation and, hence, observational metrics, which are lacking in the tropics. Here a straightforward metric derived from the Amazon Dense GNSS Meteorological Network (~100 km × 100 km) is presented based on a spatial correlation decay time scale during convective evolution on the mesoscale. For the shallow-to-deep transition, the correlation decay time scale is shown to be around 3.5 h. This novel result provides a much needed metric from the deep tropics for numerical models to replicate.

Statistical Tests of Taylor’s Hypothesis: An Application to Precipitation Fields

2009 ◽  
Vol 10 (1) ◽  
pp. 254-265 ◽  
Author(s):  
Bo Li ◽  
Aditya Murthi ◽  
Kenneth P. Bowman ◽  
Gerald R. North ◽  
Marc G. Genton ◽  
...  
Keyword(s):  
Correlation Analysis ◽  
Time Scale ◽  
Statistical Tests ◽  
Spatial Scales ◽  
Statistical Significance ◽  
Time Lag ◽  
Mean Flow ◽  
Spatial Covariance ◽  
Simple Correlation ◽  
Taylor’S Hypothesis

Abstract The Taylor hypothesis (TH) as applied to rainfall is a proposition about the space–time covariance structure of the rainfall field. Specifically, it supposes that if a spatiotemporal precipitation field with a stationary covariance Cov(r, τ) in both space r and time τ moves with a constant velocity v, then the temporal covariance at time lag τ is equal to the spatial covariance at space lag r = vτ that is, Cov(0, τ) = Cov(vτ, 0). Qualitatively this means that the field evolves slowly in time relative to the advective time scale, which is often referred to as the frozen field hypothesis. Of specific interest is whether there is a cutoff or decorrelation time scale for which the TH holds for a given mean flow velocity v. In this study, the validity of the TH is tested for precipitation fields using high-resolution gridded Next Generation Weather Radar (NEXRAD) reflectivity data produced by the WSI Corporation by employing two different statistical approaches. The first method is based on rigorous hypothesis testing, while the second is based on a simple correlation analysis, which neglects possible dependencies between the correlation estimates. Radar reflectivity values are used from the southeastern United States with an approximate horizontal resolution of 4 km × 4 km and a temporal resolution of 15 min. During the 4-day period from 2 to 5 May 2002, substantial precipitation occurs in the region of interest, and the motion of the precipitation systems is approximately uniform. The results of both statistical methods suggest that the TH might hold for the shortest space and time scales resolved by the data (4 km and 15 min) but that it does not hold for longer periods or larger spatial scales. Also, the simple correlation analysis tends to overestimate the statistical significance through failing to account for correlations between the covariance estimates.

Sign in / Sign up

Export Citation Format

Share Document