scholarly journals Transient growth of perturbations in Stokes oscillatory flows

2016 ◽  
Vol 794 ◽  
Author(s):  
Damien Biau
Keyword(s):  
Maximum Energy ◽  
Base Flow ◽  
Low Energy ◽  
Transition To Turbulence ◽  
Two Dimensional ◽  
Oscillatory Flows ◽  
Energy Growth ◽  
Subcritical Regime ◽  
Nonlinear Simulations ◽  
Orr Mechanism

Oscillatory Stokes flows, with zero mean, are subjected to subcritical transition to turbulence. The maximal energy growth of perturbations is computed in the subcritical regime through an optimisation method. The results show strong amplifications during half a period. The energy transfer from the base flow involves an Orr mechanism with two-dimensional vorticity waves, and the maximum energy scales exponentially with the Reynolds number. Nonlinear simulations show that low-energy perturbations are sufficient to trigger turbulent flow.

Non-normal dynamics of time-evolving co-rotating vortex pairs

2012 ◽  
Vol 701 ◽  
pp. 430-459 ◽  
Author(s):  
X. Mao ◽  
S. J. Sherwin ◽  
H. M. Blackburn
Keyword(s):  
Base Flow ◽  
Asymptotically Stable ◽  
Two Dimensional ◽  
Stable Case ◽  
Energy Growth ◽  
Time Horizons ◽  
Vortex Merging ◽  
Transient Energy ◽  
The Individual ◽  
Optimal Disturbances

AbstractTransient energy growth of disturbances to co-rotating pairs of vortices with axial core flows is investigated in an analysis where vortex core expansion and vortex merging are included by adopting a time-evolving base flow. The dynamics of pairs are compared with those of individual vortices in order to highlight the effect of vortex interaction. Three typical vortex pair cases are studied, with the pairs comprised respectively of individually inviscidly unstable vortices at the streamwise wavenumber that maximizes the individual instabilities, viscously unstable vortices also at the streamwise wavenumber maximizing the individual instabilities and asymptotically stable vortices at streamwise wavenumber zero. For the inviscidly unstable case, the optimal perturbation takes the form of a superposition of two individual helical unstable modes and the optimal energy growth is similar to that predicted for an individual inviscid unstable vortex, while where the individual vortices are viscously unstable, the optimal disturbances within each core have similar spatial distributions to the individually stable case. For both of these cases, time horizons considered are much lower than those required for the merger of the undisturbed vortices. However, for the asymptotically stable case, large linear transient energy growth of optimal perturbations occurs for time horizons corresponding to vortex merging. Linear transient disturbance energy growth exhibited by pairs in this stable case is two to three orders of magnitude larger than that for a corresponding individual vortex. The superposition of the perturbation and the base flow shows that the perturbation has a displacement effect on the vortices in the base flow. Direct numerical simulations of stable pairs seeded by optimal initial perturbations have been carried out and acceleration/delay of vortex merging associated with a dual vortex meandering and vortex breakup related to axially periodic acceleration and delay of vortex merging are observed. For axially invariant cases, the sign of perturbation has an effect, as well as magnitude; the sign dependence relates to whether or not the perturbation adds to or subtracts from the swirl of the base flow. For a two-dimensional perturbation that adds to the swirl of the base flow, seeding with the linear optimal disturbance at a relative energy level $1\ensuremath{\times} 1{0}^{\ensuremath{-} 4} $ induces the pair to move towards each other and approximately halves the time required for merger. Direct numerical simulation shows that the optimal three-dimensional perturbation can induce the vortex system to break up before merging occurs, since the two-dimensional nature of vortex merging is broken by the development of axially periodic perturbations.

scholarly journals Complex magnetism of the two-dimensional antiferromagnetic Ge2F: from a Néel spin-texture to a potential antiferromagnetic skyrmion

RSC Advances ◽  
2021 ◽  
Vol 11 (15) ◽  
pp. 8654-8663
Author(s):  
Fatima Zahra Ramadan ◽  
Flaviano José dos Santos ◽  
Lalla Btissam Drissi ◽  
Samir Lounis
Keyword(s):  
Density Functional Theory ◽  
Magnetic Properties ◽  
Density Functional ◽  
Low Energy ◽  
Two Dimensional ◽  
Functional Theory ◽  
2D Material ◽  
Energy Models ◽  
Spin Texture

Based on density functional theory combined with low-energy models, we explore the magnetic properties of a hybrid atomic-thick two-dimensional (2D) material made of germanene doped with fluorine atoms in a half-fluorinated configuration (Ge2F).

scholarly journals Transition to turbulence in quasi-two-dimensional MHD flow driven by lateral walls

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Christopher J. Camobreco ◽  
Alban Pothérat ◽  
Gregory J. Sheard
Keyword(s):  
Mhd Flow ◽  
Transition To Turbulence ◽  
Two Dimensional ◽  
Lateral Walls

scholarly journals Bra-ket wormholes in gravitationally prepared states

2021 ◽  
Vol 2021 (2) ◽  
Author(s):  
Yiming Chen ◽  
Victor Gorbenko ◽  
Juan Maldacena
Keyword(s):  
Hyperbolic Space ◽  
Path Integral ◽  
Flat Space ◽  
Low Energy ◽  
Two Dimensional ◽  
De Sitter ◽  
Euclidean Case ◽  
Fine Grained ◽  
Entropy Formula ◽  
First Case

Abstract We consider two dimensional CFT states that are produced by a gravitational path integral.As a first case, we consider a state produced by Euclidean AdS2 evolution followed by flat space evolution. We use the fine grained entropy formula to explore the nature of the state. We find that the naive hyperbolic space geometry leads to a paradox. This is solved if we include a geometry that connects the bra with the ket, a bra-ket wormhole. The semiclassical Lorentzian interpretation leads to CFT state entangled with an expanding and collapsing Friedmann cosmology.As a second case, we consider a state produced by Lorentzian dS2 evolution, again followed by flat space evolution. The most naive geometry also leads to a similar paradox. We explore several possible bra-ket wormholes. The most obvious one leads to a badly divergent temperature. The most promising one also leads to a divergent temperature but by making a projection onto low energy states we find that it has features that look similar to the previous Euclidean case. In particular, the maximum entropy of an interval in the future is set by the de Sitter entropy.

Corneal surgery by two-dimensional scanning of a low-energy excimer laser beam

1994 ◽  
Author(s):  
Angela Unkroth ◽  
Karin Pachomis ◽  
Jens-Uwe Walther ◽  
D. Zimare
Keyword(s):  
Laser Beam ◽  
Excimer Laser ◽  
Low Energy ◽  
Two Dimensional ◽  
Corneal Surgery

Transient Flutter Stability Analysis of Structural Mistuned Blisk by Energy Growth Method

2021 ◽  
Author(s):  
Luohui Ouyang ◽  
Qingzhen Bi ◽  
Hua Chen ◽  
Hai Shang ◽  
Li-Min Zhu
Keyword(s):  
Stability Analysis ◽  
Finite Time ◽  
Transient Stability ◽  
Maximum Energy ◽  
Dynamic State ◽  
Infinite Time ◽  
Structural Dynamic ◽  
Energy Growth ◽  
Eigenvalue Method ◽  
Growth Method

Abstract Blisks suffer from flutter, a self-sustained vibration caused by aerodynamic coupled forces. This instability could cause serious damage to the blades and the machine. Flutter stability is usually analyzed based on the eigenvalue method in the aspect of the linear structural dynamic system, which transforms a dynamics stability analysis into a point of equilibrium in an infinite time scale. However, in reality, most of the blisk vibrations arise on a finite time horizon. The transient vibration amplification may cause serious damage. This paper proposes a transient flutter stability analysis method in a finite time for structural mistuned blisk based on the energy growth method. Firstly, two common blisk models coupled aerodynamic force with different complexity are built, and are all expressed in the state space representation. A novel energy growth method is then employed to analyze the transient stability and to find the maximum energy growth of the models. The optimal initial condition which leads to the maximum energy growth is obtained. A new flutter stability criterion is developed to consider the transient stability based on the energy growth method and the infinite time stability based on the eigenvalue method. The new transient stability method is verified by two numerical studies. It is found that the structural mistuned blisk model which is traditionally predicted stable still has a transient instability in a finite time due to the non-normal property of the dynamic state matrix.

PHOTOLUMINESCENCE OF IMPURITY-BOUND EXCITONS AND TRIONS IN MAGNETIC FIELDS

2007 ◽  
Vol 21 (08n09) ◽  
pp. 1558-1562
Author(s):  
ARKADIUSZ WÓJS ◽  
LESZEK BRYJA ◽  
ANNA GŁADYSIEWICZ ◽  
JAN MISIEWICZ ◽  
MAREK POTEMSKI
Keyword(s):  
Magnetic Fields ◽  
Low Energy ◽  
Numerical Calculations ◽  
Two Dimensional ◽  
Recombination Spectrum

Recombination spectrum of excitons and positive trions is studied by two-beam magneto-photoluminescence of a two-dimensional hole gas. For acceptor-bound trions a low-energy cyclotron replica is observed, corresponding to a hole shake-up process. The experiment is supplemented by realistic numerical calculations, allowing for identification of individual transitions and connecting the splitting of the shake-up line directly with the hole mass.

Stability of the Base Flow to Axisymmetric and Plane-Polar Disturbances in an Electrically Driven Flow Between Infinitely-Long, Concentric Cylinders

2000 ◽  
Vol 123 (1) ◽  
pp. 31-42
Author(s):  
J. Liu ◽  
G. Talmage ◽  
J. S. Walker
Keyword(s):  
Magnetic Field ◽  
Electric Current ◽  
Axial Magnetic Field ◽  
Normal Modes ◽  
Azimuthal Velocity ◽  
Base Flow ◽  
Transition To Turbulence ◽  
Electrically Conducting ◽  
Concentric Cylinders ◽  
The Stability

The method of normal modes is used to examine the stability of an azimuthal base flow to both axisymmetric and plane-polar disturbances for an electrically conducting fluid confined between stationary, concentric, infinitely-long cylinders. An electric potential difference exists between the two cylinder walls and drives a radial electric current. Without a magnetic field, this flow remains stationary. However, if an axial magnetic field is applied, then the interaction between the radial electric current and the magnetic field gives rise to an azimuthal electromagnetic body force which drives an azimuthal velocity. Infinitesimal axisymmetric disturbances lead to an instability in the base flow. Infinitesimal plane-polar disturbances do not appear to destabilize the base flow until shear-flow transition to turbulence.

scholarly journals ONE-ELECTRON SPECTRAL WEIGHT OF DOPED MOTT INSULATORS IN GAUGE FIELD THEORY APPROACH

1996 ◽  
Vol 10 (27) ◽  
pp. 3727-3736
Author(s):  
H.C. LEE
Keyword(s):  
Field Theory ◽  
Gauge Field ◽  
Spectral Weight ◽  
Low Energy ◽  
Vertex Correction ◽  
Gauge Field Theory ◽  
Mott Insulators ◽  
Theory Approach ◽  
Two Dimensional ◽  
Second Order Perturbation Theory

The electron spectral weight of doped Mott insulators based on the two-dimensional slave boson gauge field theory is studied. The vertex correction with static gauge field is calculated in the second order perturbation theory. The vertex correction is found to be singular at low energy and requires non-perturbative treatments.

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