equilibrium configuration
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2021 ◽  
Author(s):  
Niccolò Ragno ◽  
Nicoletta Tambroni ◽  
Michele Bolla Pittaluga

The morphodynamic evolution of river deltas is intimately tied to flow and sediment partitioning at bifurcations. In this work, we investigate the long-term equilibrium configuration of a simple delta network using an analytical model, which accounts for the effect of small tidal oscillations.Differently from individual bifurcations, where tidal action is always a stabilizing factor, in the case of a tree-like delta with multiple bifurcations a dual response emerges.Specifically, depending on the values of four reference parameters function of tidal amplitude, upstream flow conditions, and on the geometry of the channels, tides can either promote or discourage an unbalanced discharge distribution. This behavior primarily concerns the apex bifurcation, which is affected by the variations of the relative tidal amplitude at the internal nodes. In turn these variations depend on how flow and sediment are diverted upstream. Finally, we discuss the outcomes of the model performing a qualitative comparison with field and experimental tide-influenced deltas. Results highlight the need of including in a unified scheme river-influenced (i.e. depositional) and tide-influenced (i.e. erosional) effects.


2021 ◽  
Vol 152 ◽  
pp. 111415
Author(s):  
Javier Rodríguez-Cuadrado ◽  
Jesús San Martín

2021 ◽  
Author(s):  
Karen Gellman ◽  
Andy Ruina

What is the effect of posture on the stability of a standing horse? We address this with a 2D quasi-static model. The model horse has 3 rigid parts: a trunk, a massless fore-limb and a massless rear limb, and has hinges at the shoulder, hip, and hooves. The postural parameter lg is the distance between the hooves. For a given lg, statics finds an equilibrium configuration which, with no muscle stabilization, is unstable. To measure the neuro-muscular effort to maintain stability, we add springs at the shoulder and hip; the larger the springs needed to stabilize the model, the more the neuro-muscular effort needed for stabilization. We find that a canted-in posture (small lg), observed in some pathological domestic horses, requires about twice the spring stiffness (representing twice the neuromuscular effort) as is needed for postures with vertical or slightly splayed-out (large lg ) legs.


2021 ◽  
Vol 2021 (9) ◽  
Author(s):  
Sayantani Bhattacharyya ◽  
Prateksh Dhivakar ◽  
Anirban Dinda ◽  
Nilay Kundu ◽  
Milan Patra ◽  
...  

Abstract We construct a proof of the second law of thermodynamics in an arbitrary diffeomorphism invariant theory of gravity working within the approximation of linearized dynamical fluctuations around stationary black holes. We achieve this by establishing the existence of an entropy current defined on the horizon of the dynamically perturbed black hole in such theories. By construction, this entropy current has non-negative divergence, suggestive of a mechanism for the dynamical black hole to approach a final equilibrium configuration via entropy production as well as the spatial flow of it on the null horizon. This enables us to argue for the second law in its strongest possible form, which has a manifest locality at each space-time point. We explicitly check that the form of the entropy current that we construct in this paper exactly matches with previously reported expressions computed considering specific four derivative theories of higher curvature gravity. Using the same set up we also provide an alternative proof of the physical process version of the first law applicable to arbitrary higher derivative theories of gravity.


Author(s):  
H. Rodrigues ◽  
J. A. Rosero-Gil ◽  
A. M. Endler ◽  
S. B. Duarte ◽  
M. Chiapparini

We describe the dynamical behavior of newborn neutron stars modelled as homogeneous rotating spheroids. The dynamical evolution is triggered by the escape of trapped neutrinos, providing the initial equilibrium configuration. It is shown that for a given set of values of the initial angular momentum, a shape transition to a triaxial ellipsoid configuration occurs. Gravitational waves are then generated by the breaking of the axial symmetry, and some aspects of their observation are discussed. We found a narrow window for both, the initial values of the angular frequency and the eccentricity, able to enable a dynamical shape transition, with their upper bound determined by the Kepler frequency. The energy and angular momentum carried away by the gravitational wave are treated consistently with the solution of the equations of motion of the system.


2021 ◽  
Author(s):  
Shibo Liu ◽  
Jiangping Mei ◽  
Panfeng Wang ◽  
Fan Guo

Abstract In recent decades, researchers have successfully applied tensegrity robots in wilderness exploration, aerospace and biomimicry, based on the strong adaptability and high stiffness to mass ratio. In this paper, a fusiform tensegrity robot driven by cable and telescopic strut is proposed, which is intended to be used as a foot module of a multi-legged robot. The numerical kinematic and static solution of the dual drive fusiform tensegrity robot is derived using the principle of minimum energy. Then, its force space, which is a set of external forces applied to the robot in a certain equilibrium configuration, is calculated. Next, the workspace of one reference point is derived by calculating an equivalent four bar mechanism. Meanwhile the workspace of the other end of the strut is calculated numerically. Finally, the theoretical analysis is verified by a simulation, and the dual drive fusiform tensegrity robot module is proved to be feasible as one foot of a multi-legged robot.


Computation ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 41
Author(s):  
Mario Versaci ◽  
Francesco Carlo Morabito

The recovery of the membrane profile of an electrostatic micro-electro-mechanical system (MEMS) is an important issue, because, when an external electrical voltage is applied, the membrane deforms with the risk of touching the upper plate of the device producing an unwanted electrostatic effect. Therefore, it is important to know whether the movement admits stable equilibrium configurations especially when the membrane is closed to the upper plate. In this framework, this work analyzes the behavior of a two-dimensional (2D) electrostatic circular membrane MEMS device subjected to an external voltage. Specifically, starting from a well-known 2D non-linear second-order differential model in which the electrostatic field in the device is proportional to the mean curvature of the membrane, the stability of the only possible equilibrium configuration is studied. Furthermore, when considering that the membrane is equipped with mechanical inertia and that it must not touch the upper plate of the device, a useful range of possible values has been obtained for the applied voltage. Finally, the paper concludes with some computations regarding the variation of potential energy, identifying some optimal control conditions.


PLoS ONE ◽  
2021 ◽  
Vol 16 (2) ◽  
pp. e0246588
Author(s):  
John M. McNamara ◽  
Alasdair I. Houston ◽  
Olof Leimar

We focus on learning during development in a group of individuals that play a competitive game with each other. The game has two actions and there is negative frequency dependence. We define the distribution of actions by group members to be an equilibrium configuration if no individual can improve its payoff by unilaterally changing its action. We show that at this equilibrium, one action is preferred in the sense that those taking the preferred action have a higher payoff than those taking the other, more prosocial, action. We explore the consequences of a simple ‘unbiased’ reinforcement learning rule during development, showing that groups reach an approximate equilibrium distribution, so that some achieve a higher payoff than others. Because there is learning, an individual’s behaviour can influence the future behaviour of others. We show that, as a consequence, there is the potential for an individual to exploit others by influencing them to be the ones to take the non-preferred action. Using an evolutionary simulation, we show that population members can avoid being exploited by over-valuing rewards obtained from the preferred option during learning, an example of a bias that is ‘rational’.


2021 ◽  
Vol 0 (0) ◽  
pp. 0
Author(s):  
Jianxing Du ◽  
Xifeng Su

<p style='text-indent:20px;'>This article focuses on recent investigations on equilibria of the Frenkel-Kontorova models subjected to potentials generated by quasi-crystals.</p><p style='text-indent:20px;'>We present a specific one-dimensional model with an explicit potential driven by the Fibonacci quasi-crystal. For a given positive number <inline-formula><tex-math id="M1">\begin{document}$ \theta $\end{document}</tex-math></inline-formula>, we show that there are multiple equilibria with rotation number <inline-formula><tex-math id="M2">\begin{document}$ \theta $\end{document}</tex-math></inline-formula>, e.g., a minimal configuration and a non-minimal equilibrium configuration. Some numerical experiments verifying the existence of such equilibria are provided.</p>


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