1,720,999 research outputs found
Sudden relaminarisation and lifetimes in forced isotropic turbulence
No full textWe demonstrate an unexpected connection between isotropic turbulence and wall-bounded shear flows. We perform direct numerical simulations of isotropic turbulence forced at large scales at moderate Reynolds numbers and observe sudden transitions from chaotic dynamics to a spatially simple flow, analogous to the laminar state in wall bounded shear flows. We find that the survival probabilities of turbulence are exponential and the typical lifetimes increase super-exponentially with the Reynolds number. Our results suggest that both isotropic turbulence and wall-bounded shear flows qualitatively share the same phase-space dynamics.For a description of the data please see the README file
Nonuniversality and finite dissipation in magnetohydrodynamic turbulence
No full textThis dataset contains the results of the study discussed in the paper 'Nonuniversality and finite dissipation in decaying magnetohydrodynamic turbulence': "A model equation for the Reynolds number dependence of the dimensionless dissipation rate in freely decaying homogeneous magnetohydrodynamic turbulence in the absence of a mean magnetic field is derived from the real-space energy balance equation, leading to Cε=Cε,∞+C/Rz−+O(1/R2z−)), where Rz− is a generalized Reynolds number. The constant Cε,∞ describes the total energy transfer flux. This flux depends on magnetic and cross helicities, because these affect the nonlinear transfer of energy, suggesting that the value of Cε,∞ is not universal. Direct numerical simulations were conducted on up to 20483 grid points, showing good agreement between data and the model. The model suggests that the magnitude of cosmological-scale magnetic fields is controlled by the values of the vector field correlations. The ideas introduced here can be used to derive similar model equations for other turbulent systems." [paper abstract]This dataset contains: six .txt files with the results of the study, and one README.txt file
Self-organization and transition to turbulence in isotropic fluid motion driven by negative damping at low wavenumbers
No full textWe observe a symmetry-breaking transition from a turbulent to a self-organized state in direct numerical simulation of the Navier-Stokes equation at very low Reynolds number. In this self-organised state the kinetic energy is contained only in modes at the lowest resolved wavenumber, the skewness vanishes, and visualization of the flows shows a lack of small-scale structure, with the vorticity and velocity vectors becoming aligned (a Beltrami flow).This item contains: 14 tar.gz zipped folders containing the core data and 1 README.txt file
Comparison of forcing functions in magnetohydrodynamics
No full textResults are presented of direct numerical simulations of incompressible, homogeneous magnetohydrodynamic turbulence without a mean magnetic field, subject to different kinetic forcing functions commonly used in the literature. Specifically, the forces are negative damping (which uses the large-scale field as a forcing function), a nonhelical random force, and a nonhelical static sinusoidal force (analogous to helical ABC forcing). The time evolution of the three ideal invariants (energy, magnetic helicity and cross helicity), the time-averaged energy spectra, the energy ratios and the dissipation ratios are examined. The effect of the number of grid points and Reynolds number on the performance of the forces is also considered. All three forces produce qualitatively similar steady states with some differences. In particular, the magnetic helicity is well-conserved in all cases but the sinusoidal method of energy injection has a tendency to introduce cross helicity into the system. Indeed, an ensemble of sinusoidally-forced simulations with identical parameters shows large variations in the cross helicity over long time periods, casting some doubt on the validity of the principle of ergodicity in systems where the injection of helicity cannot be controlled. Cross helicity can unexpectedly enter the system through the forcing function and must be carefully monitored.McKay, Mairi E.; Linkmann, Moritz F.; Clark, Daniel; Chalupa, Adam A.; Berera, Arjun. (2017). Comparison of forcing functions in magnetohydrodynamics, [dataset]. University of Edinburgh. School of Physics and Astronomy. http://dx.doi.org/10.7488/ds/1999
Self-organization and transition to turbulence in isotropic fluid motion driven by negative damping at low wavenumbers
No full textWe observe a symmetry-breaking transition from a turbulent to a self-organized state in direct numerical simulation of the Navier-Stokes equation at very low Reynolds number. In this self-organised state the kinetic energy is contained only in modes at the lowest resolved wavenumber, the skewness vanishes, and visualization of the flows shows a lack of small-scale structure, with the vorticity and velocity vectors becoming aligned (a Beltrami flow).McComb, W. David; Linkmann, Moritz F.; Berera, Arjun; Yoffe, Samuel R.; Jankauskas, Bernardas. (2015). Self-organization and transition to turbulence in isotropic fluid motion driven by negative damping at low wavenumbers, [dataset]. University of Edinburgh. School of Physics and Astronomy. http://dx.doi.org/10.7488/ds/250
Nonlinear waves In nematic liquid crystals
Full text linkIn the last few decades, nonlinear, nonlocal optical media have emerged as an ideal setting for experimentally observing and studying nonlinear optical phenomena such as modulation instability, random lasing, spatial solitons and shock waves. In particular, liquid crystals in the nematic mesophase (NLC) support self-confined optical spatial solitons, named in this context as nematicons, i.e. stable and robust self-confined beams which can propagate without diffraction within the self-induced channel waveguide. They have become the focus of several studies following their demonstration in planar nematic liquid crystal cells and hold special interest due to their potential use in the design of all-optical devices such as diodes, isolators and optical switches. From a theoretical perspective, nematicon propagation is described by a system of nonlinear dispersive-wave equations constituted by a nonlinear Schrödinger-like equation for the optical beam and an elliptic Poisson equation for the response of the liquid crystal. This system of equations has no exact solutions, therefore most effort is devoted to improving numerical methods, although modulation theories can also give insight into the mechanisms behind the optical beam evolution.
In this thesis, we investigate self-induced waveguides which, by launching nematicons from the opposite ends of a sample cell, establish signal pipelines with distinguishable paths, resulting in a diode-like transmission. We specifically examine the generation and path of extraordinary-wave nematicons in planar cells of nematic liquid crystals (NLC) when launching identical beams from the opposite ends of samples with linearly modulated angle distributions of the optic axis, i.e. a varying molecular background orientation across the transverse and propagation coordinates
Data for "Interpreted machine learning in fluid dynamics" Lellep et al., Journal of Fluid Mechanics, 2022
No full textThis dataset is the supplementary data for the publication https://arxiv.org/abs/2102.05541. Machine Learning (ML) is becoming increasingly popular in fluid dynamics. Powerful ML algorithms such as neural networks or ensemble methods are notoriously difficult to interpret. Here, we introduce the novel Shapley additive explanations (SHAP) algorithm (Lundberg & Lee, Advances in Neural Information Processing Systems, 2017, pp. 4765–4774), a game-theoretic approach that explains the output of a given ML model in the fluid dynamics context. We give a proof of concept concerning SHAP as an explainable artificial intelligence method providing useful and human-interpretable insight for fluid dynamics. To show that the feature importance ranking provided by SHAP can be interpreted physically, we first consider data from an established low-dimensional model based on the self-sustaining process (SSP) in wall-bounded shear flows, where each data feature has a clear physical and dynamical interpretation in terms of known representative features of the near-wall dynamics, i.e. streamwise vortices, streaks and linear streak instabilities. SHAP determines consistently that only the laminar profile, the streamwise vortex and a specific streak instability play a major role in the prediction. We demonstrate that the method can be applied to larger fluid dynamics datasets by a SHAP evaluation on plane Couette flow in a minimal flow unit focussing on the relevance of streaks and their instabilities for the prediction of relaminarisation events. Here, we find that the prediction is based on proxies for streak modulations corresponding to linear streak instabilities within the SSP. That is, the SHAP analysis suggests that the break-up of the self-sustaining cycle is connected with a suppression of streak instabilities.Lellep, Martin; Linkmann, Moritz. (2022). Data for "Interpreted machine learning in fluid dynamics" Lellep et al., Journal of Fluid Mechanics, 2022, [dataset]. https://doi.org/10.7488/ds/3450
Effects of helicity on dissipation in homogeneous box turbulence
Full text linkThe dimensionless dissipation coefficient β=εL/U3, whereεis the dissipation rate, U the root-mean-square velocity andLthe characteristic scale of the largest flow structures, isan important characteristic of statistically stationary homogeneous turbulence. In studies of β, the external force is typically isotropic and large scale, and its helicity Hf either zero or not measured. Here, we study the dependence of β on Hf and find that it decreases β by up to 10% for both isotropic forces and shear flows. The numerical finding is supportedby static and dynamical upper bound theory. Both show a relative reduction similar tothe numerical results. That is, the qualitative and quantitative dependence ofβon thehelicity of the force is well captured by upper bound theory. Consequences for the valueof the Kolmogorov constant and theoretical aspects of turbulence control and modellingare discussed in connection with the properties of the external force. In particular, the eddy viscosity in large eddy-simulations of homogeneous turbulence should be decreased by at least 10% in the case of strongly helical forcing
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
