Gravity and Turbidity Currents: Numerical Simulations and Theoretical Models

2017 ◽  
pp. 129-180
Author(s):  
Eckart Meiburg ◽  
Mohamad M. Nasr-Azadani
Keyword(s):  
Numerical Simulations ◽  
Theoretical Models ◽  
Turbidity Currents

scholarly journals Numerical Simulations of Jets from Active Galactic Nuclei

Galaxies ◽  
2019 ◽  
Vol 7 (1) ◽  
pp. 24 ◽  
Author(s):  
José-María Martí
Keyword(s):  
Numerical Simulations ◽  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Theoretical Models ◽  
The Status ◽  
Hydrodynamical Description ◽  
Jet Plasma ◽  
The Impact ◽  
Continuous Injection ◽  
Double Radio Galaxies

Numerical simulations have been playing a crucial role in the understanding of jets from active galactic nuclei (AGN) since the advent of the first theoretical models for the inflation of giant double radio galaxies by continuous injection in the late 1970s. In the almost four decades of numerical jet research, the complexity and physical detail of simulations, based mainly on a hydrodynamical/magneto-hydrodynamical description of the jet plasma, have been increasing with the pace of the advance in theoretical models, computational tools and numerical methods. The present review summarizes the status of the numerical simulations of jets from AGNs, from the formation region in the neighborhood of the supermassive central black hole up to the impact point well beyond the galactic scales. Special attention is paid to discuss the achievements of present simulations in interpreting the phenomenology of jets as well as their current limitations and challenges.

scholarly journals Saga of Superfluid Solids

Physics ◽  
2020 ◽  
Vol 2 (1) ◽  
pp. 49-66 ◽  
Author(s):  
Vyacheslav I. Yukalov
Keyword(s):  
Numerical Simulations ◽  
State Of The Art ◽  
Theoretical Models ◽  
The State ◽  
Experimental Results ◽  
Standing Problem

The article presents the state of the art and reviews the literature on the long-standing problem of the possibility for a sample to be at the same time solid and superfluid. Theoretical models, numerical simulations, and experimental results are discussed.

Theoretical Modelling of Free Falling Wedges Entering the Water Surface: Part I — Wedge Motions

2013 ◽  
Author(s):  
Jingbo Wang
Keyword(s):  
Numerical Simulations ◽  
Froude Number ◽  
Penetration Depth ◽  
Water Surface ◽  
Theoretical Models ◽  
Theoretical Modelling ◽  
Rapid Variation ◽  
Drag Coefficients ◽  
Free Falling ◽  
Theoretical Results

As the first of two companion papers, theoretical models are proposed to describe the motions of free falling wedges vertically entering the water surface at Froude numbers: 1 ≤ Fn < 9 (Here, the Froude number is defined as Fn=V0/gc0). The time evolutions of the penetration depth, the velocity and the acceleration are analyzed and expressed explicitly The maximum and average accelerations are predicted. The drag (slamming) coefficients are extensively studied. It is found that for the light wedge the transient drag coefficients have slow variation in the first half stage and rapid variation in the last half stage, and for the heavy wedge the transient drag coefficients vary slowly during the whole stage and can be treated as constant. The theoretical results are compared with numerical simulations by nonlinear BEM (Wang & Faltinsen (2010, 2013)), and good agreements are obtained.

scholarly journals Numerical study of the propulsive performance of two-dimensional pitching foils at very high frequencies: scaling laws and turbulence effects

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Enrique Sanmiguel-Rojas ◽  
Ramon Fernandez-Feria
Keyword(s):  
Numerical Simulations ◽  
Scaling Laws ◽  
Theoretical Models ◽  
Linear Potential ◽  
Propulsive Efficiency ◽  
Content Type ◽  
High Frequencies ◽  
Linear Potential Theory ◽  
Very High Frequencies ◽  
Very High

Purpose This paper aims to analyze the propulsive performance of small-amplitude pitching foils at very high frequencies with double objectives: to find out scaling laws for the time-averaged thrust and propulsive efficiency at very high frequencies; and to characterize the Strouhal number above which the effect of turbulence on the mean values cannot be neglected. Design/methodology/approach The thrust force and propulsive efficiency of a pitching NACA0012 foil at high reduced frequencies (k) and a Reynolds number Re = 16 000 are analyzed using accurate numerical simulations, both assuming laminar flow and using a transition turbulence model. The time-averaged results are validated with available experimental data for k up to about 12 (Strouhal number, St, up to 0.6). This study also compares the present numerical results with the predictions of theoretical models and existing numerical results. For a foil pitching about its quarter chord with amplitude α0 = 8o, the reduced frequency is varied here up to k = 30 (St up to 2), much higher than in any previous numerical or experimental work. Findings For this pitch amplitude, turbulence effects are found negligible for St ≲ 0.8, and affecting less than 10% to the time-averaged thrust coefficient CT¯ for larger St Linear potential theory fails for very large k, even for the small pitch amplitude considered, particularly for the power coefficient, and therefore for the propulsive efficiency. It is found that CT¯ ∼ St2 for large St, in agreement with recent models, and the propulsive efficiency decays as 1/k, in disagreement with the linear potential theory. Originality/value Pitching foils are increasingly studied as efficient propellers and energy harvesting devices. Their performance at very high reduced frequencies has not been sufficiently analyzed before. The authors provide accurate numerical simulations to discern when turbulence is relevant for the computation of the time-averaged thrust and efficiency and how their scaling with the reduced frequency is affected in relation to the laminar-flow predictions. This is relevant because some small-amplitude theoretical models predict high propulsive efficiency of pitching foils at very high frequencies over certain ranges of the structural parameters, and only very accurate numerical simulations may decide on these predictions.

scholarly journals Properties of Hall-MHD Turbulence at Sub-Ion Scales: Spectral Transfer Analysis

Atmosphere ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 1632
Author(s):  
Emanuele Papini ◽  
Petr Hellinger ◽  
Andrea Verdini ◽  
Simone Landi ◽  
Luca Franci ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Transfer Rate ◽  
Space Plasma ◽  
Theoretical Models ◽  
Magnetic Fluctuations ◽  
Energy Transfer Rate ◽  
Mhd Turbulence ◽  
Developed Turbulence ◽  
First Time ◽  
Hall Mhd

We present results of a multiscale study of Hall-magnetohydrodynamic (MHD) turbulence, carried out on a dataset of compressible nonlinear 2D Hall-MHD numerical simulations of decaying Alfvénic turbulence. For the first time, we identify two distinct regimes of fully developed turbulence. In the first one, the power spectrum of the turbulent magnetic fluctuations at sub-ion scales exhibits a power law with a slope of ∼−2.9, typically observed both in solar wind and in magnetosheath turbulence. The second regime, instead, shows a slope of −7/3, in agreement with classical theoretical models of Hall-MHD turbulence. A spectral-transfer analysis reveals that the latter regime occurs when the energy transfer rate at sub-ion scales is dominated by the Hall term, whereas in the former regime, the governing process is the dissipation (and the system exhibits large intermittency). Results of this work are relevant to the space plasma community, as they may potentially reconcile predictions from theoretical models with results from numerical simulations and spacecraft observations.

scholarly journals Stochastic Excitation in Solar-Type Stars

2002 ◽  
Vol 185 ◽  
pp. 447-455 ◽  
Author(s):  
G. Houdek
Keyword(s):  
Numerical Simulations ◽  
Theoretical Models ◽  
Basic Mechanism ◽  
Convincing Evidence ◽  
Turbulent Convection ◽  
The Sun ◽  
Stochastic Excitation ◽  
Current Belief ◽  
Solar Type

AbstractThe most convincing evidence to date of solar-type oscillations in other stars comes from recent observations of β Hydri (Bedding et al., 2001) and α Cen A (Bouchy & Carrier, 2001). It is the current belief that the convection dynamics in the outer layers of sun-like stars is the source for driving the intrinsically stable modes to the observed amplitudes. Comparing such observations with theoretical models will help us improve our understanding of the interaction between convection and pulsation.In this contribution I review the mechanisms responsible for mode damping in stars with convective envelopes, and the basic mechanism of stochastic driving by turbulent convection. The application of a stochastic excitation formalism to the Sun is discussed and compared with recent measurements and numerical simulations. Amplitude predictions for models of Procyon, α Cen A and β Hydri are compared with observations.

Commission 16: Physical Study of Planets and Satellites (Etude Physique Pes Planetes Et Satellites)

1988 ◽  
Vol 20 (1) ◽  
pp. 167-178
Author(s):  
G. Hunt ◽  
A. Brahic ◽  
D. Morrison ◽  
J. L. Bertaux ◽  
J. Burns ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Laboratory Experiments ◽  
Space Exploration ◽  
Theoretical Models ◽  
Physical Study ◽  
Perfect Complementarity ◽  
Planets And Satellites ◽  
Space Data

The physical study of planets and satellites is probably one of the more active fields of research of the second half of this century. This is due to space exploration by spacecraft, but also to the use of modern detectors, of large ground-based telescopes, and of powerful computers by active researchers. Planetary research (or planetology) is a pluridisciplinary domain, which requires not only the competence of astronomers, but also of geophysicists, of mineralogists, of climatologists, of biologists, of chemists, of physicists, of “pure„ mathematicians, and many other scientists. Many results are at the boundary of those of other commissions such as the 15, 20, 7, 19, 33, 40, 44, 49 and 51 ones. The study of the main results obtained during this last triennum shows a perfect complementarity between space and ground-based observations. It should be arbitrary to separate space and ground-based scientists. The have the same goal and they study the same objects. Quite often, the same individuals use both techniques, depending on the most efficient one for the problem under study. It is remarkable to see that space data collected more than ten years ago are still analysed in connection with ground-based observations. The same remarks can apply for ground-based data. In addition to that, new theoretical models, new numerical simulations and new laboratory experiments have ben recently developed. They all contribute to a better understanding of planets and satellites physics.

scholarly journals Onset of fast magnetic reconnection and particle energization in laboratory and space plasmas

2020 ◽  
Vol 86 (6) ◽  
Author(s):  
F. Pucci ◽  
M. Velli ◽  
C. Shi ◽  
K. A. P. Singh ◽  
A. Tenerani ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Magnetic Reconnection ◽  
Theoretical Models ◽  
Laboratory Plasma ◽  
Plasma Parameters ◽  
Astrophysical Plasmas ◽  
Plasma Dynamics ◽  
Laboratory Plasmas ◽  
Wide Range ◽  
Kinetic Effects

The onset of magnetic reconnection in space, astrophysical and laboratory plasmas is reviewed discussing results from theory, numerical simulations and observations. After a brief introduction on magnetic reconnection and approach to the question of onset, we first discuss recent theoretical models and numerical simulations, followed by observations of reconnection and its effects in space and astrophysical plasmas from satellites and ground-based detectors, as well as measurements of reconnection in laboratory plasma experiments. Mechanisms allowing reconnection spanning from collisional resistivity to kinetic effects as well as partial ionization are described, providing a description valid over a wide range of plasma parameters, and therefore applicable in principle to many different astrophysical and laboratory environments. Finally, we summarize the implications of reconnection onset physics for plasma dynamics throughout the Universe and illustrate how capturing the dynamics correctly is important to understanding particle acceleration. The goal of this review is to give a view on the present status of this topic and future interesting investigations, offering a unified approach.

scholarly journals Displacement flows under elastic membranes. Part 1. Experiments and direct numerical simulations

2015 ◽  
Vol 784 ◽  
pp. 487-511 ◽  
Author(s):  
Draga Pihler-Puzović ◽  
Anne Juel ◽  
Gunnar G. Peng ◽  
John R. Lister ◽  
Matthias Heil
Keyword(s):  
Numerical Simulations ◽  
Stokes Equations ◽  
Theoretical Models ◽  
Navier Stokes ◽  
Elastic Membrane ◽  
Two Phase ◽  
Constant Flow Rate ◽  
Viscous Forces ◽  
Elastic Membranes ◽  
Set Up

The injection of fluid into the narrow liquid-filled gap between a rigid plate and an elastic membrane drives a displacement flow that is controlled by the competition between elastic and viscous forces. We study such flows using the canonical set-up of an elastic-walled Hele-Shaw cell whose upper boundary is formed by an elastic sheet. We investigate both single- and two-phase displacement flows in which the localised injection of fluid at a constant flow rate is accommodated by the inflation of the sheet and the outward propagation of an axisymmetric front beyond which the cell remains approximately undeformed. We perform a direct comparison between quantitative experiments and numerical simulations of two theoretical models. The models couple the Föppl–von Kármán equations, which describe the deformation of the thin elastic membrane, to the equations describing the flow, which we model by (i) the Navier–Stokes equations or (ii) lubrication theory. We identify the dominant physical effects that control the behaviour of the system and critically assess modelling assumptions that were made in previous studies. The insight gained from these studies is then used in Part 2 of this work, where we formulate an improved lubrication model and develop an asymptotic description of the key phenomena.

scholarly journals Development and Parameters Analysis of Hydraulic Controlled Rotary Valve Excitation System

Energies ◽  
2020 ◽  
Vol 13 (15) ◽  
pp. 3905
Author(s):  
Wenjing Li ◽  
Guofang Gong ◽  
Yakun Zhang ◽  
Jian Liu ◽  
Yuxi Chen ◽  
...  
Keyword(s):  
Simulation Model ◽  
Numerical Simulations ◽  
Electric Motor ◽  
Experimental Studies ◽  
Theoretical Models ◽  
Rotary Valve ◽  
Excitation System ◽  
Coupling Relationship ◽  
Parameters Analysis ◽  
Relationship Of

Electro-hydraulic excitation systems are key equipment in various industries. Electric motor driving rotary valves are mostly used in existing systems. However, due to the separate design of the driving and hydraulic parts, highly compact integration cannot be achieved by these type of systems. Moreover, investigation on the influence of relevant parameters on the system has been insufficient in previous studies. To overcome these problems, a novel full electro-hydraulic excitation system scheme as well as a parameters analysis are presented in this paper. Theoretical models of the flow areas for valve orifices of different geometric shapes are obtained, based on which an AMESim® simulation model of the system is established. The effects of the main parameters are analyzed using numerical simulations, and the coupling relationship of the parameters is revealed. The results demonstrate the feasibility and effectiveness of the proposed method. Experimental studies were conducted to verify the effectiveness of the proposed system scheme and the analysis results. We found that a highly compact integration can be obtained while maintaining a high reversing frequency. We also found that the proposed system has a certain level of load adaptability, which is superior to the existing methods.

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