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1

Drosophila Circadian Rhythms: Stability Robustness Analysis and Model Reduction

Drosophila Circadian Rhythms: Stability Robustness Analysis and Model Reduction Jorge M. Gonc investigates two models of circadian rhythms in Drosophila: one by Gonze et al. and a more generic model the control and regulation of circadian rhythms and other limit cycle behaviours. 1 Introduction Many aspects

GonÃ§alves, Jorge

2

Settlement Prediction, Gas Modeling and Slope Stability Analysis

Settlement Prediction, Gas Modeling and Slope Stability Analysis in Coll CardÃºs Landfill Li Yu using mechanical models Simulation of gas generation, transport and extraction in MSW landfill 1 models Simulation of gas generation, transport and extraction in MSW landfill 1) Analytical solution

PolitÃ¨cnica de Catalunya, Universitat

3

A Coupled Aeroelastic Model for Launch Vehicle Stability Analysis

NASA Technical Reports Server (NTRS)

A technique for incorporating distributed aerodynamic normal forces and aeroelastic coupling effects into a stability analysis model of a launch vehicle is presented. The formulation augments the linear state-space launch vehicle plant dynamics that are compactly derived as a system of coupled linear differential equations representing small angular and translational perturbations of the rigid body, nozzle, and sloshing propellant coupled with normal vibration of a set of orthogonal modes. The interaction of generalized forces due to aeroelastic coupling and thrust can be expressed as a set of augmenting non-diagonal stiffness and damping matrices in modal coordinates with no penalty on system order. While the eigenvalues of the structural response in the presence of thrust and aeroelastic forcing can be predicted at a given flight condition independent of the remaining degrees of freedom, the coupled model provides confidence in closed-loop stability in the presence of rigid-body, slosh, and actuator dynamics. Simulation results are presented that characterize the coupled dynamic response of the Ares I launch vehicle and the impact of aeroelasticity on control system stability margins.

Orr, Jeb S.

2010-01-01

4

Models and Stability Analysis of Boiling Water Reactors

We have studied the nuclear-coupled thermal-hydraulic stability of boiling water reactors (BWRs) using a model that includes: space-time modal neutron kinetics based on spatial w-modes; single- and two-phase flow in parallel boiling channels; fuel rod heat conduction dynamics; and a simple model of the recirculation loop. The BR model is represented by a set of time-dependent nonlinear ordinary differential equations, and is studied as a dynamical system using the modern bifurcation theory and nonlinear dynamical systems analysis. We first determine the stability boundary (SB) - or Hopf bifurcation set- in the most relevant parameter plane, the inlet-subcooling-number/external-pressure-drop plane, for a fixed control rod induced external reactivity equal to the 100% rod line value; then we transform the SB to the practical power-flow map used by BWR operating engineers and regulatory agencies. Using this SB, we show that the normal operating point at 100% power is very stable, that stability of points on the 100% rod line decreases as the flow rate is reduced, and that operating points in the low-flow/high-power region are least stable. We also determine the SB that results when the modal kinetics is replaced by simple point reactor kinetics, and we thereby show that the first harmonic mode does not have a significant effect on the SB. However, we later show that it nevertheless has a significant effect on stability because it affects the basin of attraction of stable operating points. Using numerical simulations we show that, in the important low-flow/high-power region, the Hopf bifurcation that occurs as the SB is crossed is subcritical; hence, growing oscillations can result following small finite perturbations of stable steady-states on the 100% rod line at points in the low-flow/high-power region. Numerical simulations are also performed to calculate the decay ratios (DRs) and frequencies of oscillations for various points on the 100% rod line. It is determined that the U.S. NRC requirement of DR is not rigorously satisfied in the low-flow/high-power region; hence, this region should be avoided during normal startup and shutdown operations. The frequency of oscillation is shown to decrease as the flow rate is reduced. Moreover, the simulation frequency of 0.5Hz determined in the low-flow/high-power region is consistent with those observed during actual instability incidents. Additional numerical simulations show that in the low-flow/high-power region, for the same initial conditions, the use of point kinetics leads to damped oscillations, whereas the model that includes the modal neutron kinetics equations results in growing nonlinear oscillations.

John Dorning

2002-04-15

5

Stability analysis of an implicitly defined labor market model

NASA Astrophysics Data System (ADS)

Until very recently, the pervasive existence of models exhibiting well-defined backward dynamics but ill-defined forward dynamics in economics and finance has apparently posed no serious obstacles to the analysis of their dynamics and stability, despite the problems that may arise from possible erroneous conclusions regarding theoretical considerations and policy prescriptions from such models. A large number of papers have dealt with this problem in the past by assuming the existence of symmetry between forward and backward dynamics, even in the case when the map cannot be invertible either forward or backwards. However, this procedure has been seriously questioned over the last few years in a series of papers dealing with implicit difference equations and inverse limit spaces. This paper explores the search and matching labor market model developed by Bhattacharya and Bunzel [J. Bhattacharya, H. Bunzel, Chaotic Planning Solution in the Textbook Model of Equilibrium Labor Market Search and Matching, Mimeo, Iowa State University, 2002; J. Bhattacharya, H. Bunzel, Economics Bulletin 5 (19) (2003) 1-10], with the following objectives in mind: (i) to show that chaotic dynamics may still be present in the model for acceptable parameter values, (ii) to clarify some open questions related with the admissible dynamics in the forward looking setting, by providing a rigorous proof of the existence of cyclic and chaotic dynamics through the application of tools from symbolic dynamics and inverse limit theory.

Mendes, Diana A.; Mendes, Vivaldo M.

2008-06-01

6

Linear Stability Analysis of Thermo-Lattice Boltzmann Models

The numerical stability of thermo-lattice Boltzmann (TLBE) models is presented.The TLBE algorithm is linearized and represented in matrix form. The spectral radius of the resulting matrix is obtained by the method of powers. In particular, the numerical stability of two 2-speed 13-bit TLBE models—one based on the hexagonal lattice, and the other on a square lattice—is examined. For these two

Pavol Pavlo; George Vahala; Linda Vahala; Min Soe

1998-01-01

7

Stability analysis of the Euler discretization for SIR epidemic model

NASA Astrophysics Data System (ADS)

In this paper we consider a discrete SIR epidemic model obtained by the Euler method. For that discrete model, existence of disease free equilibrium and endemic equilibrium is established. Sufficient conditions on the local asymptotical stability of both disease free equilibrium and endemic equilibrium are also derived. It is found that the local asymptotical stability of the existing equilibrium is achieved only for a small time step size h. If h is further increased and passes the critical value, then both equilibriums will lose their stability. Our numerical simulations show that a complex dynamical behavior such as bifurcation or chaos phenomenon will appear for relatively large h. Both analytical and numerical results show that the discrete SIR model has a richer dynamical behavior than its continuous counterpart.

Suryanto, Agus

2014-06-01

8

Verified Analysis of a Model for Stance Stabilization

\\u000a The stabilization of stance is a subject of continuing research in biology, biomechanics and robotics. It plays an important\\u000a role in many clinical applications as well as in forward dynamical gait simulation. In this paper, we propose a new model\\u000a relying on a two cylinder foot contact scheme. This contact model has the advantage of simple and smooth dynamic behavior

Ekaterina Auer; Haider Albassam; Andrés Kecskeméthy; Wolfram Luther

9

Performance Evaluation 40 (2000) 2746 A novel approach to queue stability analysis of polling models

in modeling many real-world systems. For example, token-passing LAN, polling scheme, and processor sharing on stability of the whole system. This system stability analysis, however, fails to model many real-world systems for which some queues may continue to operate under an unstable system. In this paper we address

Chang, Rocky Kow-Chuen

10

A Versatile System Model for the Dynamic Stability Analysis of Power Systems Including HVDC Links

A general and novel approach for the development of the linearized model for the dynamic stability analysis of multimachine power systems including HVDC links is presented in this paper. The modelling considers detailed representation of the generators and DC links with their associated controllers and nonlinear voltage dependent loads. The component models are formulated separately and are interconnected by the

K. R. Padiyar; M. A. Pal; C. Radhakrishna

1981-01-01

11

Floquet stability analysis of the longitudinal dynamics of two hovering model insects

Because of the periodically varying aerodynamic and inertial forces of the flapping wings, a hovering or constant-speed flying insect is a cyclically forcing system, and, generally, the flight is not in a fixed-point equilibrium, but in a cyclic-motion equilibrium. Current stability theory of insect flight is based on the averaged model and treats the flight as a fixed-point equilibrium. In the present study, we treated the flight as a cyclic-motion equilibrium and used the Floquet theory to analyse the longitudinal stability of insect flight. Two hovering model insects were considered—a dronefly and a hawkmoth. The former had relatively high wingbeat frequency and small wing-mass to body-mass ratio, and hence very small amplitude of body oscillation; while the latter had relatively low wingbeat frequency and large wing-mass to body-mass ratio, and hence relatively large amplitude of body oscillation. For comparison, analysis using the averaged-model theory (fixed-point stability analysis) was also made. Results of both the cyclic-motion stability analysis and the fixed-point stability analysis were tested by numerical simulation using complete equations of motion coupled with the Navier–Stokes equations. The Floquet theory (cyclic-motion stability analysis) agreed well with the simulation for both the model dronefly and the model hawkmoth; but the averaged-model theory gave good results only for the dronefly. Thus, for an insect with relatively large body oscillation at wingbeat frequency, cyclic-motion stability analysis is required, and for their control analysis, the existing well-developed control theories for systems of fixed-point equilibrium are no longer applicable and new methods that take the cyclic variation of the flight dynamics into account are needed. PMID:22491980

Wu, Jiang Hao; Sun, Mao

2012-01-01

12

Floquet stability analysis of the longitudinal dynamics of two hovering model insects.

Because of the periodically varying aerodynamic and inertial forces of the flapping wings, a hovering or constant-speed flying insect is a cyclically forcing system, and, generally, the flight is not in a fixed-point equilibrium, but in a cyclic-motion equilibrium. Current stability theory of insect flight is based on the averaged model and treats the flight as a fixed-point equilibrium. In the present study, we treated the flight as a cyclic-motion equilibrium and used the Floquet theory to analyse the longitudinal stability of insect flight. Two hovering model insects were considered-a dronefly and a hawkmoth. The former had relatively high wingbeat frequency and small wing-mass to body-mass ratio, and hence very small amplitude of body oscillation; while the latter had relatively low wingbeat frequency and large wing-mass to body-mass ratio, and hence relatively large amplitude of body oscillation. For comparison, analysis using the averaged-model theory (fixed-point stability analysis) was also made. Results of both the cyclic-motion stability analysis and the fixed-point stability analysis were tested by numerical simulation using complete equations of motion coupled with the Navier-Stokes equations. The Floquet theory (cyclic-motion stability analysis) agreed well with the simulation for both the model dronefly and the model hawkmoth; but the averaged-model theory gave good results only for the dronefly. Thus, for an insect with relatively large body oscillation at wingbeat frequency, cyclic-motion stability analysis is required, and for their control analysis, the existing well-developed control theories for systems of fixed-point equilibrium are no longer applicable and new methods that take the cyclic variation of the flight dynamics into account are needed. PMID:22491980

Wu, Jiang Hao; Sun, Mao

2012-09-01

13

Stability analysis of dynamic collaboration model with control signals on two lanes

NASA Astrophysics Data System (ADS)

In this paper, the influence of control signals on the stability of two-lane traffic flow is mainly studied by applying control theory with lane changing behaviors. We present the two-lane dynamic collaboration model with lateral friction and the expressions of feedback control signals. What is more, utilizing the delayed feedback control theory to the two-lane dynamic collaboration model with control signals, we investigate the stability of traffic flow theoretically and the stability conditions for both lanes are derived with finding that the forward and lateral feedback signals can improve the stability of traffic flow while the backward feedback signals cannot achieve it. Besides, direct simulations are conducted to verify the results of theoretical analysis, which shows that the feedback signals have a significant effect on the running state of two vehicle groups, and the results are same with the theoretical analysis.

Li, Zhipeng; Zhang, Run; Xu, Shangzhi; Qian, Yeqing; Xu, Juan

2014-12-01

14

Stability analysis of interval type-2 fuzzy-model-based control systems.

This paper presents the stability analysis of interval type-2 fuzzy-model-based (FMB) control systems. To investigate the system stability, an interval type-2 Takagi-Sugeno (T-S) fuzzy model, which can be regarded as a collection of a number of type-1 T-S fuzzy models, is proposed to represent the nonlinear plant subject to parameter uncertainties. With the lower and upper membership functions, the parameter uncertainties can be effectively captured. Based on the interval type-2 T-S fuzzy model, an interval type-2 fuzzy controller is proposed to close the feedback loop. To facilitate the stability analysis, the information of the footprint of uncertainty is used to develop some membership function conditions, which allow the introduction of slack matrices to handle the parameter uncertainties in the stability analysis. Stability conditions in terms of linear matrix inequalities are derived using a Lyapunov-based approach. Simulation examples are given to illustrate the effectiveness of the proposed interval type-2 FMB control approach. PMID:18558528

Lam, H K; Seneviratne, Lakmal D

2008-06-01

15

A synergetic model describing the state of an ultrathin lubricant layer squeezed between two atomically smooth solid surfaces operating in the boundary friction mode has been developed further. To explain the presence of different operation modes of the system for various sets of its main parameters, the mathematical analysis of the synergetic model is carried out. The type of functioning a tribological system is described in accordance with the stability character of singular points, and the diagrams distinguishing various operation modes are obtained. Phase portraits corresponding to different stability types are plotted for all diagram areas. A stick-slip mode of motion that is often observed experimentally is described.

Iakov A. Lyashenko; Nataliia N. Manko

2014-01-17

16

Turbine-generator transient monitoring; Torsional stress analysis and stability model identification

A hardware and software system has been developed under EPRI sponsorship to monitor turbine-generator transient events, perform shaft torsional stress analysis for each event, and derive the best fit stability model parameters for each event. As the acquisition hardware is built around commercial digital transient recorder equipment, standard fault recorder type analysis of event data is also possible. The off-line analysis workstation is an 80386 personal computer running 32 bit FORTRAN under the XENIX operating system. The EPRI project is currently underway with two turbine-generators being monitored and events analyzed. Typical analysis results are presented in this paper along with implementation details.

Wegner, C.A.; Bowler, C.E.J.; Rubino, J.C.P. (Laboratoires d'Electronique et de Physique Appliquee (LEP), 75 - Paris (France)); Edmonds, J.S. (Electric Power Research Inst., Palo Alto, CA (United States))

1991-01-01

17

We present a general approach to generate a linear parametric state-space model, which approximates a nonlinear system with high accuracy. It is optimally suited for LFT-based robust stability analysis and control design. At the beginning a Jacobian-based linearization is applied to generate a set of linearized state-space systems describing the local behavior of the nonlinear plant about the corresponding equilibrium

Harald Pfifer; Simon Hecker

2008-01-01

18

NASA Technical Reports Server (NTRS)

Results of dynamic response and stability wind tunnel tests of three 62.2 cm (24.5 in) diameter models of the Prop-Fan, advanced turboprop, are presented. Measurements of dynamic response were made with the rotors mounted on an isolated nacelle, with varying tilt for nonuniform inflow. One model was also tested using a semi-span wing and fuselage configuration for response to realistic aircraft inflow. Stability tests were performed using tunnel turbulence or a nitrogen jet for excitation. Measurements are compared with predictions made using beam analysis methods for the model with straight blades, and finite element analysis methods for the models with swept blades. Correlations between measured and predicted rotating blade natural frequencies for all the models are very good. The IP dynamic response of the straight blade model is reasonably well predicted. The IP response of the swept blades is underpredicted and the wing induced response of the straight blade is overpredicted. Two models did not flutter, as predicted. One swept blade model encountered an instability at a higher RPM than predicted, showing predictions to be conservative.

Bansal, P. N.; Arseneaux, P. J.; Smith, A. F.; Turnberg, J. E.; Brooks, B. M.

1985-01-01

19

This paper concerns with the stability analysis and synthesis of nonlinear retarded systems via linear Takagi–Sugeno (TS) fuzzy model approach. First, the TS fuzzy models with time-delay are present and the stability conditions are derived using Lyapunov–Razumikhin functional approach. Then a stabilization approach for nonlinear retarded systems through fuzzy state feedback and fuzzy observer-based controller is proposed. It shows that

Yong-yan Cao; P. M. Frank

2001-01-01

20

NASA Astrophysics Data System (ADS)

Most of the slopes of the hilly areas of the Apennines are composed of colluvial soils originating from the weathering of the bedrock and down slope transportation. Shallow slides affect this superficial cover, depend largely on the surface topography and are a recurrent problem. SINMAP and SHALSTAB are terrain stability models that combine steady state hydrology assumptions with the infinite slope stability model to quantify shallow slope stability. They have a similar physical basis but they use different indices to quantify instability. The purposes of this study are to test and compare the approaches of SINMAP and SHALSTAB models for slope stability analysis and to compare the results of these analyses with the locations of the shallow landslides that occurred on November 2002 in an area of the Oltrepo Pavese (Northern Apennines). The territory of S. Giuletta, characterized by clayey-silty colluvial soils, represents the test site. The Digital Elevation Model was constructed from a 1:5000 scale contour map and was used to estimate the slope of the terrain as well as the potential soil moisture conditions. In situ and laboratory tests provided the basis for measuring values for soil hydraulic and geotechnical parameters (moisture content, soil suction, Atterberg limits, methylene blue dye adsorption, hydraulic conductivity). Soil thickness was extracted from a soil database. An inventory of landslide from interpretation of aerial photographs and field surveys was used to document sites of instability (mostly soil slips) and to provide a test of model performance by comparing observed landslide locations with model predictions. The study discusses the practical advantages and limitations of the two models in connection with the geological characteristics of the studied area, which could be representative of similar geological contexts in the Apennines.

Meisina, C.; Scarabelli, S.

2007-06-01

21

The stability of the vertical position control system in TCV is analysed using two different plasma models, a deformable plasma model (DPM) and a rigid plasma model (RPM). The open loop growth rates and closed loop stability are computed, as functions of various plasma parameters, for both models. It is found that the difference between the open loop growth rates

F. Hofmann; J.-M. Moret; D. J. Ward

1998-01-01

22

Stability analysis of an epidemic model with diffusion and stochastic perturbation

NASA Astrophysics Data System (ADS)

In this paper, we investigate the stability of an epidemic model with diffusion and stochastic perturbation. We first show both the local and global stability of the endemic equilibrium of the deterministic epidemic model by analyzing corresponding characteristic equation and Lyapunov function. Second, for the corresponding reaction-diffusion epidemic model, we present the conditions of the globally asymptotical stability of the endemic equilibrium. And we carry out the analytical study for the stochastic model in details and find out the conditions for asymptotic stability of the endemic equilibrium in the mean sense. Furthermore, we perform a series of numerical simulations to illustrate our mathematical findings.

Rao, Feng; Wang, Weiming; Li, Zhibin

2012-06-01

23

Stability Analysis and Stability Chart for Unsaturated Residual Soil Slope

In tropical residual soils most hill slope failures are caused by rainfall. It is therefore important to consider dynamic hydrological conditions when attempting to analyze the stability of residual soil slopes. This paper describes a coupled hydrology\\/stability model that has been developed to overcome the limitations of the standard method of analysis used to investigate stability of tropical soil slopes.

Bujang B. K. Huat; Faisal Hj; R. S. K. Rajoo

2006-01-01

24

Slope Stability Analysis Program

NSDL National Science Digital Library

This site provides information as well as a downloadable demo version of a slope stability and reinforced soil analysis and design software. The software includes an analysis option which analyzes strata profile and groundwater conditions, surcharge loads and earthquake forces, slip surfaces, and reinforced soil options. Data input and editing can also be performed as well as output from the program and graphics.

2008-07-17

25

Stability and Folding Behavior Analysis of Zinc-Finger Using Simple Models

Zinc-fingers play crucial roles in regulating gene expression and mediating protein-protein interactions. In this article, two different proteins (Sp1f2 and FSD-1) are investigated using the Gaussian network model and anisotropy elastic network model. By using these simple coarse-grained methods, we analyze the structural stabilization and establish the unfolding pathway of the two different proteins, in good agreement with related experimental and molecular dynamics simulation data. From the analysis, it is also found that the folding process of the zinc-finger motif is predominated by several factors. Both the zinc ion and C-terminal loop affect the folding pathway of the zinc-finger motif. Knowledge about the stability and folding behavior of zinc-fingers may help in understanding the folding mechanisms of the zinc-finger motif and in designing new zinc-fingers. Meanwhile, these simple coarse-grained analyses can be used as a general and quick method for mechanistic studies of metalloproteins. PMID:21152317

Chang, Shan; Jiao, Xiong; Hu, Jian-Ping; Chen, Yan; Tian, Xu-Hong

2010-01-01

26

A study of the compressibility behavior of peat stabilized by DMM: Lab model and FE analysis

Peats are considered as extremely soft, unconsolidated deposits. These soils are geotechnically problematic, due to their high compressibility and low shear strength. Cement is widely used for the stabilization of peat by deep mixing method (DMM). This paper presents the results of the model study of compressibility properties of fibrous, hemic and sapric peats, stabilized with columns formed by DMM.

B B K Huat; S Kazemian; A Prasad; M Barghchi

2011-01-01

27

Stability and bifurcation analysis of a preypredator model with age based predation

naturally. In light of this, a mathematical model is proposed to study the stability and bifurcation of the predators in forest in higher age-groups are large in size so it becomes essential to consider age

Anderson, Douglas R.

28

The improvement of OPC accuracy and stability by the model parameters' analysis and optimization

NASA Astrophysics Data System (ADS)

The OPC model is very critical in the sub 45nm device because the Critical Dimension Uniformity (CDU) is so tight to meet the device performance and the process window latitude for the production level. The OPC model is generally composed of an optical model and a resist model. Each of them has physical terms to be calculated without any wafer data and empirical terms to be fitted with real wafer data to make the optical modeling and the resist modeling. Empirical terms are usually related to the OPC accuracy, but are likely to be overestimated with the wafer data and so those terms can deteriorate OPC stability in case of being overestimated by a small cost function. Several physical terms have been used with ideal value in the optical property and even weren't be considered because those parameters didn't give a critical impact on the OPC accuracy, but these parameters become necessary to be applied to the OPC modeling at the low k1 process. Currently, real optic parameter instead of ideal optical parameter like the laser bandwidth, source map, pupil polarization including the phase and intensity difference start to be measured and those real measured value are used for the OPC modeling. These measured values can improve the model accuracy and stability. In the other hand these parameters can make the OPC model to overcorrect the process proximity errors without careful handling. The laser bandwidth, source map, pupil polarization, and focus centering for the optical modeling are analyzed and the sample data weight scheme and resist model terms are investigated, too. The image blurring by actual laser bandwidth in the exposure system is modeled and the modeling result shows that the extraction of the 2D patterns is necessary to get a reasonable result due to the 2D patterns' measurement noise in the SEM. The source map data from the exposure machine shows lots of horizontal and vertical intensity difference and this phenomenon must come from the measurement noise because this huge intensity difference can't be caused by the scanner system with respect to the X-Y intensity difference specification in the scanner. Therefore this source map should be well organized for the OPC modeling and a manipulated source map improves the horizontal and vertical mask bias and even OPC convergence. The focus parameter which is critical for the process window OPC and ORC should be matched to the tilted Bossung plot which is caused by uncorrectable aberration to predict the CD change in the through focus with a new devised method. Pupil polarization data can be applied into the OPC modeling and this parameter is also used for the unpolarized source and the polarized source and specially this parameter helps Apodization loss to be 0 and is evaluated for the effect into the modeling. With the analysis and optimization about the model parameters the robust model is achieved in the sub 45nm device node.

Chung, No-Young; Choi, Woon-Hyuk; Lee, Sung-Ho; Kim, Sung-Il; Lee, Sun-Yong

2007-10-01

29

This paper deals with modeling and stability analysis of a doubly-fed induction generator (DFIG)-based wind-power unit that is interfaced with the grid via a series-compensated transmission line. A detailed mathematical model is developed in this paper that takes into account dynamics of the flux observer, phase-locked loop (PLL), controllers of the power-electronic converter, and wind turbine. Using the model and

Amir Ostadi; Amirnaser Yazdani; Rajiv K. Varma

2009-01-01

30

Analysis of the market weights under the Volatility-Stabilized Market models

We derive the joint density of market weights, at fixed times and suitable stopping times, of the Volatility-stabilized market models introduced by Fernholz and Karatzas in 2005. The argument rests on computing the exit density of a collection of independent Bessel-square processes of possibly different dimensions from the unit simplex. We show that the law of the market weights is

Soumik Pal

2009-01-01

31

Exponential stability analysis of large-scale time-delay nonlinear systems with hybrid models

This paper presents one method to deal with the exponential stability problems for a kind of complex large-scale time-delay nonlinear systems with hybrid models. These hybrid large-scale time-delay systems are composed of considerable interconnected time-delay nonlinear subsystems, some of which are described by differential equations and others by Takagi-Sugeno fuzzy models. Novel techniques to cope with the nonlinear interconnection between

Degang Xu; Guiwei Hua; Zhifang Su

2008-01-01

32

Stability Analysis of Ecomorphodynamic Equations

Although riparian vegetation is present in or along many water courses of the world, its active role resulting from the interaction with flow and sediment processes has only recently become an active field of research. Especially, the role of vegetation in the process of river pattern formation has been explored and demonstrated mostly experimentally and numerically until now. In the present work, we shed light on this subject by performing a linear stability analysis on a simple model for riverbed vegetation dynamics coupled with the set of classical river morphodynamic equations. The vegetation model only accounts for logistic growth, local positive feedback through seeding and resprouting, and mortality by means of uprooting through flow shear stress. Due to the simplicity of the model, we can transform the set of equations into an eigenvalue problem and assess the stability of the linearized equations when slightly perturbated away from a spatially homogeneous solution. If we couple vegetation dynamics wi...

Bärenbold, Fabian; Perona, Paolo

2014-01-01

33

NASA Technical Reports Server (NTRS)

In pursuit of higher performance, the XV-15 Tiltrotor Research Aircraft was modified by the installation of new composite rotor blades. Initial flights with the Advanced Technology Blades (ATB's) revealed excessive rotor control loads that were traced to a dynamic mismatch between the blades and the aircraft control system. The analytical models of both the blades and the mechanical controls were extensively revised for use by the CAMRAD computer program to better predict aeroelastic stability and loads. This report documents the most important revisions and discusses their effects on aeroelastic stability predictions for airplane-mode flight. The ATB's may be flown in several different configurations for research, including changes in blade sweep and tip twist. The effects on stability of 1 deg and 0 deg sweep are illustrated, as are those of twisted and zero-twist tips. This report also discusses the effects of stiffening the rotor control system, which was done by locking out lateral cyclic swashplate motion with shims.

Acree, C. W., Jr.

1993-01-01

34

Thermohaline circulation stability : a box model

A thorough analysis of the stability of uncoupled and coupled versions of an inter-hemispheric 3-box model of Thermohaline Circulation (THC) is presented. The model consists of a northern high latitudes box, a tropical ...

Lucarini, Valerio

2003-01-01

35

A simplified spatial model for BWR stability

A spatial reduced order model for the study of BWR stability, based on the phenomenological model of March-Leuba et al., is presented. As one dimensional spatial dependence of the neutron flux, fuel temperature and void fraction is introduced, it is possible to describe both global and regional oscillations of the reactor power. Both linear stability analysis and numerical analysis were applied in order to describe the parameters which govern the model stability. The results were found qualitatively similar to past results. Doppler reactivity feedback was found essential for the explanation of the different regions of the flow-power stability map. (authors)

Berman, Y. [Dept. of Physics, Ben-Gurion Univ. of the Negev, Nuclear Research Center-Negev, Beer-Sheva (Israel); Lederer, Y. [Dept. of Physics, Nuclear Research Center-Negev, Beer-Sheva (Israel); Meron, E. [Dept. of Solar Energy and Environmental Physics, Dept. of Physics, Ben-Gurion Univ. of the Negev, Beer-Sheva (Israel)

2012-07-01

36

Numerical Stability Analysis of Linear Wave Propagation in Extended MHD Modeling

NASA Astrophysics Data System (ADS)

Extended MHD (2-fluid) modeling of fusion plasmas using a split semi-implicit time-advance based on a particular high-order finite element with C^1 continuity has been shown to offer significant advantages in efficiency and accuracy[1,2]. However, the method requires the introduction of several viscosity and hyper-viscosity coefficients to provide robust numerical stability. As a code-validation exercise, we report on a systematic study of the simulation of wave propagation in the linear regime. We initialize the simulation in a stable linear eigenmode of the extended MHD equations and follow the evolution to measure the numerical dispersion relation for the 3 sets of MHD waves. We present results showing how the stability, dispersion and dissipation depend on grid size, time step, and the magnitude of the viscosity and hyper-viscosity coefficients. These linear perturbation tests act as useful benchmarks and guides for numerical stability for nonlinear simulations. [1] S. Jardin, J. Comput. Phys. 200 (2004) 133 [2] S. Jardin and J. Breslau, Phys. Plasmas 12, 056101 (2005)

Gan, Yu; Jardin, Stephen

2006-10-01

37

Stability and parameter dependency analysis of a facilitation tectal column (FTC) model.

Mathematical models and computer simulations have been widely used to study the spatio-temporal characteristics of the processing of information carried out by the central nervous system. When trying to show whether or not a neural model accounts for the phenomena under study, if the number of parameters whose values need to be calculated becomes large, then computer simulations alone become very inefficient to define such values. Here, we developed stability and parameter dependency analyses of the mathematical representation of a single facilitation tectal column (FTC) model, to show how by using techniques from non-linear systems theory we can define the ranges of parameter values under which the model would explain the required performance of the neural net model. The benefits of these analyses can be grouped in two parts: first, the advantage of using non-linear systems techniques to analyze, analytically, the dynamics of neural net models; and second, we get a deeper understanding of why the hypotheses embedded in the models yield the appropriate behaviors and what are the critical situations (parametric combinations) under which these behaviors are displayed. PMID:2277234

Cervantes-Pérez, F; Arbib, M A

1990-01-01

38

Stability analysis of a degenerate hyperbolic system modelling a heat exchanger

Mathematical modelling of a heat exchanger in a carbon dioxide heat pump, an evap- orator, is considered. A reduced model, called the the zero Mach-number limit, is derived from the Euler equations of compressible fluid flow through elimination of time scales associated with sound waves. The well-posedness of the resulting partial differential- algebraic equation (PDAE) is investigated by analysis of

Michael Hanke; K. Henrik; A. Olsson; Magnus Strömgren

2007-01-01

39

Stability analysis of an HIV/AIDS epidemic model with treatment

NASA Astrophysics Data System (ADS)

An HIV/AIDS epidemic model with treatment is investigated. The model allows for some infected individuals to move from the symptomatic phase to the asymptomatic phase by all sorts of treatment methods. We first establish the ODE treatment model with two infective stages. Mathematical analyses establish that the global dynamics of the spread of the HIV infectious disease are completely determined by the basic reproduction number [real]0. If [real]0<=1, the disease-free equilibrium is globally stable, whereas the unique infected equilibrium is globally asymptotically stable if [real]0>1. Then, we introduce a discrete time delay to the model to describe the time from the start of treatment in the symptomatic stage until treatment effects become visible. The effect of the time delay on the stability of the endemically infected equilibrium is investigated. Moreover, the delay model exhibits Hopf bifurcations by using the delay as a bifurcation parameter. Finally, numerical simulations are presented to illustrate the results.

Cai, Liming; Li, Xuezhi; Ghosh, Mini; Guo, Baozhu

2009-07-01

40

Stochastic stability and instability of model ecosystems

NASA Technical Reports Server (NTRS)

In this work, we initiate a stability study of multispecies communities in stochastic environment by using Ito's differential equations as community models. By applying the direct method of Liapunov, we obtain sufficient conditions for stability and instability in the mean of the equilibrium populations. The conditions are expressed in terms of the dominant diagonal property of community matrices, which is a suitable mechanism for resolving the central problem of 'complexity vs stability' in model ecosystems. As a by-product of this analysis we exhibit important structural properties of the stochastic density-dependent models, and establish tolerance of community stability to a broad class of nonlinear time-varying perturbations.

Ladde, G. S.; Siljak, D. D.

1975-01-01

41

NASA Astrophysics Data System (ADS)

The longevity of ore pass systems is an important consideration in underground mines. This is controlled to a degree by the structural stability of an ore pass which can be compromised by changes in the stress regime and the degree of fracturing of the rock mass. A failure mechanism specific to ore pass systems is damage on the ore pass wall by impact load or wear by material flow. Structural, stress and material flow-induced failure mechanisms interact with severe repercussions, although in most cases one mechanism is more dominant. This paper aims to provide a better understanding of the interaction of ore pass failure mechanisms in an operating mine. This can provide an aid in the design of ore pass systems. A two-stage numerical approach was used for the back analysis of an ore pass at Brunswick mine in Canada. The first stage in the analysis relied on a 3D boundary element analysis to define the stress regime in the vicinity of the ore pass. The second stage used a synthetic rock mass (SRM) model, constructed from a discrete fracture network, generated from quantitative rock mass field data. The fracture network geometry was introduced into a bonded particle model, in a particle flow code (PFC). Subsequently, the ore pass was excavated within the SRM model. A stability analysis quantified the extent of rock mass failure around the ore pass due to the interaction of pre-existing fractures and the failure of the intact rock bridges between these fractures. The resulting asymmetric failure patterns along the length of the ore pass were controlled to a large degree by the in situ fractures. The influence of particle flow impact was integrated into the model by projecting a discrete rock fragment against the ore pass walls represented by the SRM model. The numerical results illustrated that material impact on ore pass walls resulted in localised damage and accelerated the stress-induced failure.

Esmaieli, Kamran; Hadjigeorgiou, John; Grenon, Martin

2013-11-01

42

Tailings dams stability analysis using numerical modelling of geotechnical and geophysical data

NASA Astrophysics Data System (ADS)

Methods for monitoring seepage and detecting internal erosion are essential for the safety evaluation of embankment dams. Internal erosion is one of the major reasons for embankment dam failures, and there are thousands of large tailings dams and waste-rock dumps in the world that may pe considered as hotspots for environmental impact. In this research the geophysical survey works were performed on Cetatuia 2 tailings dam. Electrical resistivity imaging (ERI) method was able to detect spatially anomalous zones inside the embankment dam. These anomalies are the results of internal erosion phenomena which may progressing inside the dam and is difficult to detect by conventional methods. Data aquired by geophysical survey together with their interpretations were used in the numerical model for slope stability assessment. The final results show us the structural weakness induced by the presence of internal erosion elements especially for seismic loading case. This research methodology may be also available for tailings dam monitoring purposes. Electrical Rezistivity Imaging (ERI) was performed on Cetatuia 2 dam at the Uranium Milling Plant Feldioara, in order to map areas with lateral and vertical changes in resistivity. The electrodes are connected to an automated computer operated switch box that selects the 4 electrodes to be used. A computer controls the switch box and the measuring device, and runs a program that selects the electrodes, makes the measurement, and stores the measurement. For inversion processing procedures was used Res2Din software. The measured resistivity were plotted by the pseudo section contouring method. There are five resistivity pseudosections obtained from the Cetatuia 2 tailings dam during the october 2007 measurements. Four transversal profiles trans1 to trans4 are perpendicular to the berms and the longitudinal one long1 is placed along dam's crest. The high resistivities near the berms surfaces corresponds to unsaturated fill materials and the low resistivities near the crest correspond to water saturated material. The resistivities values greater then 80 ohm.m may be explained by some error obtained for that inversion model. Profiles trans3 and trans4 were measured on perpendicular directions to berm alignment and show two distinct zones. The upward low resistivities zone correspond to water saturated materials especially from the compacted clay dam's core and the downward high resistivities zone belongs to unsaturated fill materials. The boundary between high and low resistivity at the depth of about 5 to 7 meters shows the groundwater level. The continuation of the high resistivity zones towards the end of the profile trans3, which is different from other profiles is probably due to the presence of dry coarse materials in shallow depth correspondingly to sandy clay. The sand fractions from the clay matrix may be affected by internal erosional phenomena, due to seepage currents that overpassed the material critical gradient. In this case the relative high resistivities values were considered as a presumptive erosional pattern. This profile was considered for the slope stability finite element modelling. The profile long1 which is placed along dam's crest is the longest profiles and extends up to nearly 420 m. The boundary between high and low resistivity at the depth of about 4 to 8 meters shows the groundwater across the dam core. The central part of the profile (about meter 200) shows the same relative high resistivities that occurred on transversal profile trans3. Resistivity data was used for building the 3D electrical resistivity model. The water saturated materials have locations very close to dam's crest (resistivity values usually lower then 10 ohm.m) and on both dam's arms. The groundwater levels were confirmed by the piezometric measurements. Electrical Rezistivity Imaging method had the possibility to show the most important disturbant elements that in certain conditions may weak the dam's state of safety. This study considered the SSR (Shear Strength Reduction) technique for sl

Mihai, S.; Zlagnean, M.; Oancea, I.; Petrescu, A.

2009-04-01

43

between the MGs on stability of these systems. This model can be also used to synthesis and study dynamics type such as photovoltaic (PV) system, fuel cell, and energy storages (e.g., secondary battery to create dc MG clusters. Modeling of power electronic systems has been addressed in several research works

Vasquez, Juan Carlos

44

Linear stability analysis for an optimum Glauert rotor modelled by an actuator disc

NASA Astrophysics Data System (ADS)

We approximate a wind turbine using the Actuator Disc methodology with loading for an optimum Glauert rotor, and vary blade length and tip speed ratio, to determine base flows for linear stability computations at a Reynolds number of 100. Results from such computations suggest that the least stable mode is axisymmetric and insensitive to changes in tip speed operation, suggesting that the stability properties in the farfield wake for an optimised rotor are independent of the chosen tip speed optimization point. Higher azimuthal modes promote greater variation in velocities and may be relevant to cases at higher Reynolds numbers.

Smith, D. M.; Blackburn, H. M.; Sheridan, J.

2014-06-01

45

An analytical model for stability analysis of rock layers over a circular opening

The objective of the study presented is to provide an analytical solution for determining the stability of a flexible rock layer overlying a circular opening. This problem is of general interest to the mining community but is particularly relevant to the oil and gas industry where drilling of deep wellbores often results in sand production around the well and hence

Hans H. Vaziri; Javad S. Jalali; Rafiq Islam

2001-01-01

46

Global stability analysis for cosmological models with nonminimally coupled scalar fields

NASA Astrophysics Data System (ADS)

We explore dynamics of cosmological models with a nonminimally coupled scalar field evolving on a spatially flat Friedmann-Lemaître-Robertson-Walker background. We consider cosmological models including the Hilbert-Einstein curvature term and the N degree monomial of the scalar field nonminimally coupled to gravity. The potential of the scalar field is the n degree monomial or polynomial. We describe several qualitatively different types of dynamics depending on values of power indices N and n. We identify that three main possible pictures correspond to n

Skugoreva, Maria A.; Toporensky, Alexey V.; Vernov, Sergey Yu.

2014-09-01

47

Energy function analysis for power system stability

Energy Function Analysis for Power System Stability presents the concept of energy function, which has found wide-spread applications for power systems in recent years. The most recent advances in five distinct areas are reviewed: Development of energy functions for structure preserving models, which can incorporate non-linear load models; energy functions which include a detailed model of the generating unit (i.e.

M. A. Pai

1989-01-01

48

Liapunov stability analysis of spinning flexible spacecraft.

NASA Technical Reports Server (NTRS)

The attitude stability of a class of spinning flexible spacecraft in a force-free environment is analyzed. The spacecraft is modeled as a rigid core having attached to it a flexible appendage idealized as a collection of elastically interconnected particles. Liapunov stability theorems are employed with the Hamiltonian of the system, constrained through the angular momentum integral so as to admit complete damping, used as a testing function. The Hamiltonian is written in terms of modal coordinates as interpreted by the hybrid coordinate formulation, thus allowing truncation to a level amenable to literal stability analysis. Testing functions are constructed for a spacecraft with an arbitrary (discretized) appendage, and closed form stability criteria are generated for the first mode of a restricted appendage model lying in a plane which contains the center of mass and is orthogonal to the spin axis. The criteria are (except for idealized cases on the stability boundary line in the parameter space) both necessary and sufficient for stability for any spacecraft characterized by the planar appendage model, such as a spacecraft containing solar panels and/or radial booms.

Barbera, F. J.; Likins, P.

1973-01-01

49

NASA Astrophysics Data System (ADS)

Stability of parametrically excited torsional vibrations of a shaft system composed of two torsionally elastic shafts interconnected through a Hooke's joint is studied. The shafts are considered to be continuous (distributed-parameter) systems and an approximate discrete model for the torsional vibrations of the shaft system is derived via a finite element scheme. The stability of the solutions of the linearized equations of motion, consisting of a set of Mathieu-Hill type equations, is examined by means of a monodromy matrix method and the results are presented in the form of a Strutt-Ince diagram visualizing the effects of the system parameters on the stability of the shaft system.

Bulut, Gökhan

2014-08-01

50

DYNAMIC CHARACTERIZATION AND STABILITY OF A LARGE SIZE MULTIBODY TILTROTOR MODEL BY POD ANALYSIS

A great level of flexibility and accuracy can be gained by em- ploying a multibody approach as the modeling strategy for servo- aeroelastic analyses of rotating wing aircraft. However, the re- sulting models are often large, in terms of degrees of freedom, and fully nonlinear, making the dynamic characterization phase cumbersome and computationally demanding. Moreover, non- minimal set formulations such

Pierangelo Masarati; Giuseppe Quaranta; Massimiliano Lanz; Paolo Mantegazza

51

This paper investigates a RVF epidemic model by qualitative analysis and numerical simulations. Qualitative analysis have been used to explore the stability dynamics of the equilibrium points while visualization techniques such as bifurcation diagrams, Poincaré maps, maxima return maps and largest Lyapunov exponents are numerically computed to confirm further complexity of these dynamics induced by the seasonal forcing on the mosquitoes oviposition rates. The obtained results show that ordinary differential equation models with external forcing can have rich dynamic behaviour, ranging from bifurcation to strange attractors which may explain the observed fluctuations found in RVF empiric outbreak data, as well as the non deterministic nature of RVF inter-epidemic activities. Furthermore, the coexistence of the endemic equilibrium is subjected to existence of certain number of infected Aedes mosquitoes, suggesting that Aedes have potential to initiate RVF epidemics through transovarial transmission and to sustain low levels of the disease during post epidemic periods. Therefore we argue that locations that may serve as RVF virus reservoirs should be eliminated or kept under control to prevent multi-periodic outbreaks and consequent chains of infections. The epidemiological significance of this study is: (1) low levels of birth rate (in both Aedes and Culex) can trigger unpredictable outbreaks; (2) Aedes mosquitoes are more likely capable of inducing unpredictable behaviour compared to the Culex; (3) higher oviposition rates on mosquitoes do not in general imply manifestation of irregular behaviour on the dynamics of the disease. Finally, our model with external seasonal forcing on vector oviposition rates is able to mimic the linear increase in livestock seroprevalence during inter-epidemic period showing a constant exposure and presence of active transmission foci. This suggests that RVF outbreaks partly build upon RVF inter-epidemic activities. Therefore, active RVF surveillance in livestock is recommended. PMID:25271641

Pedro, Sansao A.; Abelman, Shirley; Ndjomatchoua, Frank T.; Sang, Rosemary; Tonnang, Henri E. Z.

2014-01-01

52

Global stability analysis of SEIR model with holling type II incidence function.

A deterministic model for the transmission dynamics of a communicable disease is developed and rigorously analysed. The model, consisting of five mutually exclusive compartments representing the human dynamics, has a globally asymptotically stable disease-free equilibrium (DFE) whenever a certain epidemiological threshold, known as the basic reproduction number (??), is less than unity; in such a case the endemic equilibrium does not exist. On the other hand, when the reproduction number is greater than unity, it is shown, using nonlinear Lyapunov function of Goh-Volterra type, in conjunction with the LaSalle's invariance principle, that the unique endemic equilibrium of the model is globally asymptotically stable under certain conditions. Furthermore, the disease is shown to be uniformly persistent whenever ?? > 1. PMID:23091562

Safi, Mohammad A; Garba, Salisu M

2012-01-01

53

Modelling and Stability Analysis of Hybrid Multiple Access in the IEEE 802.15.4 Protocol

.2 [Computer-Communication Networks]: Network Protocols General Terms: Performance, Standardization, Theory. INTRODUCTION The IEEE 802.15.4 communication standard is arguably becoming the most popular protocol for low framework for modelling the behavior of the hybrid MAC protocol of the IEEE 802.15.4 standard is proposed

Johansson, Karl Henrik

54

Complete Stability Analysis of Multifunction MMIC Circuits

This paper describes a systematic methodology for complete stability analysis of nonlinear microwave multifunction circuits. The proposed strategy has two different stages: the stability analysis of a nominal steady-state solution and the use of continuation techniques to efficiently determine the unstable operation ranges. The stability analysis is demanding due to the multiple loops contained in the large multifunction circuit. The

César Barquinero; Almudena Suárez; Amparo Herrera; José Luis García

2007-01-01

55

Stability and Transient Analysis in the Modelling of Railway Disc Brake Squeal

The paper deals with friction induced vibrations and especially with railway disc brake squeal. The first part of the paper\\u000a is devoted to the strategy used to model the general problem of self-excited vibrations of a rotating disc in frictional contact\\u000a with two pads. Unilateral contact conditions with Coulomb friction and constant friction coefficient are considered. In order\\u000a to predict

X. Lorang; O. Chiello

56

Cosmological Models and Stability

NASA Astrophysics Data System (ADS)

Principles in the form of heuristic guidelines or generally accepted dogma play an important role in the development of physical theories. In particular, philosophical considerations and principles figure prominently in the work of Albert Einstein. As mentioned in the talk by Ji?í Bi?ák at this conference, Einstein formulated the equivalence principle, an essential step on the road to general relativity, during his time in Prague 1911-1912. In this talk, I would like to discuss some aspects of cosmological models. As cosmology is an area of physics where "principles" such as the "cosmological principle" or the "Copernican principle" play a prominent role in motivating the class of models which form part of the current standard model, I will start by comparing the role of the equivalence principle to that of the principles used in cosmology. I will then briefly describe the standard model of cosmology to give a perspective on some mathematical problems and conjectures on cosmological models, which are discussed in the later part of this paper.

Andersson, Lars

57

Cosmological models and stability

Principles in the form of heuristic guidelines or generally accepted dogma play an important role in the development of physical theories. In particular, philosophical considerations and principles figure prominently in the work of Albert Einstein. As mentioned in the talk by Jiri Bicak at this conference Einstein formulated the equivalence principle, an essential step on the road to general relativity, during his time in Prague 1911-1912. In this talk, I would like to discuss some aspects of cosmological models. As cosmology is an area of physics where "principles" such as the "cosmological principle" or the "Copernican principle" play a prominent role in motivating the class of models which form part of the current standard model, I will start by comparing the role of the equivalence principle to that of the principles used in cosmology. I will then briefly describe the standard model of cosmology to give a perspective on some mathematical problems and conjectures on cosmological models, which are discussed in the later part of this paper.

Lars Andersson

2013-10-07

58

The reliability and initial postoperative stability of six widely used C1-C2 surgical constructs were evaluated by nondestructive biomechanical testing of ten fresh bovine upper cervical spine segments. The six fixation techniques were the simple midline sublaminar C1 wiring (SMW), the modified Gallie wiring (MGW), the Brook wedge arthrodesis (BWA), the bilateral lateral mass screw (LMS), and the Clark arthrodesis without (CWO) and with (CW) the adjunctive use of polymethylmethacrylate (PMMA) cement. Instability was produced by resection of the base of the dens. Intact, destabilized, and instrumented constructs were tested in unconstrained flexion, extension, torsion, anterior, and posterior shear forces. The LMS and CW techniques proved the most effective in achieving C1-C2 stability. Sublaminar C2 wiring methods tended to have greater torsional and shear stiffness. Less rigid fixation techniques frequently loosened. All non-PMMA-enhanced wired constructs failed to restore the ability to adequately resist the posterior translation of C1 on C2. Stabilization of an unstable atlantoaxial articulation is best accomplished by a lateral mass screw or sublaminar C1-C2 PMMA-enhanced technique. Other techniques provided less consistent and reliable surgical fixation. Because of the inadequacy of wired constructs to resist posterior shear in patients with this instability pattern, the usual treatment may need to be modified to include more prolonged or protective external immobilization or more rigid fixation techniques. Although less rigid, the SMW and the MGW techniques are safer because they require less-frequent passage of sublaminar wires, avoid potential problems with PMMA, and obviate the hazards of further lateral dissection and insertion of the transarticular screws. PMID:8472462

Smith, M D; Kotzar, G; Yoo, J; Bohlman, H

1993-05-01

59

NASA Astrophysics Data System (ADS)

Since soil moisture is an important influencing factor of the hydrological cycle, knowledge of its spatio-temporal dynamics is crucial for climate and hydrological modeling. In recent years several soil moisture data products from satellite information have become available with global coverage and sub-monthly resolution. Since the remote sensing of soil moisture is an indirect measurement method and influenced by a large number of factors (e.g. atmospheric correction, vegetation, soil roughness, etc.), a comprehensive validation of the resulting soil moisture products is required. However, the coarse spatial resolution of these products hampers the comparison with point-scale in situ measurements. Therefore, upscaling of in situ to the scale of the satellite data is needed. We present the validation results of the soil moisture products of the years 2010-2012 retrieved from the Soil Moisture and Ocean Salinity (SMOS) and the Advanced Scatterometer (ASCAT) for the Rur and Erft catchments in western Germany. For the upscaling of in situ data obtained from three test sites of the Terrestrial Environmental Observatories (TERENO) initiative we used the hydrological model WaSiM ETH. Correlation of the SMOS product to modeled and upscaled soil moisture resulted in a mean correlation coefficient of 0.28 whereas for ASCAT a correlation coefficient of 0.50 was obtained. However, for specific regions the SMOS product showed similar correlation coefficients as the ASCAT product. While for ASCAT correlation was mainly dependent on topography and vegetation, SMOS was also influenced by radiofrequency interferences in our study area. Both products show dry biases as compared to the soil moisture reference. However, while SMOS showed relatively constant bias values, ASCAT bias is variable throughout the year. As an additional validation method we performed a temporal stability analysis of the retrieved spatio-temporal soil moisture data. Through investigation of mean relative differences of soil moisture for every pixel, their standard deviations and their rankings, we analyzed the temporal persistence of spatial patterns. Our results show high standard deviations for both SMOS and ASCAT soil moisture products as compared to modeled soil moisture, indicating a lower temporal persistence. The consistence of ranks of mean relative differences was low for SMOS and relative ASCAT soil moisture compared to modeled soil moisture, while ASCAT soil moisture, converted to absolute values, showed higher rank consistence.

Rötzer, K.; Montzka, C.; Bogena, H.; Wagner, W.; Kerr, Y. H.; Kidd, R.; Vereecken, H.

2014-11-01

60

Fusion of modified LDL in the arterial wall promotes atherogenesis. Earlier we showed that thermal denaturation mimics LDL remodeling and fusion, and revealed kinetic origin of LDL stability. Here we report the first quantitative analysis of LDL thermal stability. Turbidity data show sigmoidal kinetics of LDL heat denaturation, which is unique among lipoproteins, suggesting that fusion is preceded by other structural changes. High activation energy of denaturation, Ea = 100 ± 8 kcal/mol, indicates disruption of extensive packing interactions in LDL. Size-exclusion chromatography, nondenaturing gel electrophoresis, and negative-stain electron microscopy suggest that LDL dimerization is an early step in thermally induced fusion. Monoclonal antibody binding suggests possible involvement of apoB N-terminal domain in early stages of LDL fusion. LDL fusion accelerates at pH < 7, which may contribute to LDL retention in acidic atherosclerotic lesions. Fusion also accelerates upon increasing LDL concentration in near-physiologic range, which likely contributes to atherogenesis. Thermal stability of LDL decreases with increasing particle size, indicating that the pro-atherogenic properties of small dense LDL do not result from their enhanced fusion. Our work provides the first kinetic approach to measuring LDL stability and suggests that lipid-lowering therapies that reduce LDL concentration but increase the particle size may have opposite effects on LDL fusion. PMID:22855737

Lu, Mengxiao; Gantz, Donald L.; Herscovitz, Haya; Gursky, Olga

2012-01-01

61

NASA Astrophysics Data System (ADS)

Polytype stability is very important for high quality SiC single crystal growth. However, the growth conditions for the 4H, 6H and 15R polytypes are similar, and the mechanism of polytype stability is not clear. The kinetics aspects, such as surface-step nucleation, are important. The kinetic Monte Carlo method is a common tool to study surface kinetics in crystal growth. However, the present lattice models for kinetic Monte Carlo simulations cannot solve the problem of the competitive growth of two or more lattice structures. In this study, a competitive lattice model was developed for kinetic Monte Carlo simulation of the competition growth of the 4H and 6H polytypes of SiC. The site positions are fixed at the perfect crystal lattice positions without any adjustment of the site positions. Surface steps on seeds and large ratios of diffusion/deposition have positive effects on the 4H polytype stability. The 3D polytype distribution in a physical vapor transport method grown SiC ingot showed that the facet preserved the 4H polytype even if the 6H polytype dominated the growth surface. The theoretical and experimental results of polytype growth in SiC suggest that retaining the step growth mode is an important factor to maintain a stable single 4H polytype during SiC growth.

Guo, Hui-Jun; Huang, Wei; Liu, Xi; Gao, Pan; Zhuo, Shi-Yi; Xin, Jun; Yan, Cheng-Feng; Zheng, Yan-Qing; Yang, Jian-Hua; Shi, Er-Wei

2014-09-01

62

Stability analysis and future singularity of the $m^2 R \\Box^{-2} R$ model of non-local gravity

We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term $\\sim m^2 R \\Box^{-2} R$ to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.

Yves Dirian; Ermis Mitsou

2014-08-21

63

Stability analysis and future singularity of the m2 R square-2 R model of non-local gravity

NASA Astrophysics Data System (ADS)

We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term ~ m2 R square-2 R to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding the analytic solution for the late-time behaviour of the scale factor, which leads to a big rip singularity, we argue that the linear perturbations are bounded in the future because of the domination of Hubble friction. In particular, this effect damps the scalar ghost perturbations which were responsible for destabilizing Minkowski space-time. Thus, the model remains phenomenologically viable.

Dirian, Yves; Mitsou, Ermis

2014-10-01

64

Edge stability analysis of high bgrp plasmas

An MHD stability analysis of the edge plasma shows that in highly triangular plasmas, the increasing global bgrp has a stabilizing effect on the low-n instabilities that trigger edge localized modes (ELMs). The improved stability allows the access of higher edge pressure gradients before the ELM is triggered. At the same time, the edge plasma moves closer to the high-n

S. Saarelma; S. Günter

2004-01-01

65

Stability analysis and future singularity of the $m^2 R \\Box^{-2} R$ model of non-local gravity

We analyse the classical stability of the model proposed by Maggiore and Mancarella, where gravity is modified by a term $\\sim m^2 R \\Box^{-2} R$ to produce the late-time acceleration of the expansion of the universe. Our study takes into account all excitations of the metric that can potentially drive an instability. There are some subtleties in identifying these modes, as a non-local field theory contains dynamical fields which yet do not correspond to degrees of freedom. Since some of them are ghost-like, we clarify the impact of such modes on the stability of the solutions of interest that are the flat space-time and cosmological solutions. We then find that flat space-time is unstable under scalar perturbations, but the instability manifests itself only at cosmological scales, i.e. out of the region of validity of this solution. It is therefore the stability of the FLRW solution which is relevant there, in which case the scalar perturbations are known to be well-behaved by numerical studies. By finding t...

Dirian, Yves

2014-01-01

66

The stability of colorectal cancer mathematical models

NASA Astrophysics Data System (ADS)

Colorectal cancer is one of the most common types of cancer. To better understand about the kinetics of cancer growth, mathematical models are used to provide insight into the progression of this natural process which enables physicians and oncologists to determine optimal radiation and chemotherapy schedules and develop a prognosis, both of which are indispensable for treating cancer. This thesis investigates the stability of colorectal cancer mathematical models. We found that continuous saturating feedback is the best available model of colorectal cancer growth. We also performed stability analysis. The result shows that cancer progress in sequence of genetic mutations or epigenetic which lead to a very large number of cells population until become unbounded. The cell population growth initiate and its saturating feedback is overcome when mutation changes causing the net per-capita growth rate of stem or transit cells exceed critical threshold.

Khairudin, Nur Izzati; Abdullah, Farah Aini

2013-04-01

67

Boiling water reactor stability analysis with RETRAN-03

An MOC option has been developed to eliminate the numerical diffusion associated with the time domain analysis of small perturbations. This model has been implemented as an option in RETRAN-03 and evaluated for BWR stability applications by comparing RETRAN analyses results with data from a series of stability tests from the Vermont Yankee reactor. The results indicate that the MOC

P. A. Bergeron; N. Fujita; M. P. Paulsen; J. H. McFadden; L. J. Agee

1994-01-01

68

Performance and stability analysis of a photovoltaic power system

NASA Technical Reports Server (NTRS)

The performance and stability characteristics of a 10 kVA photovoltaic power system are studied using linear Bode analysis and a nonlinear analog simulation. Power conversion efficiencies, system stability, and system transient performance results are given for system operation at various levels of solar insolation. Additionally, system operation and the modeling of system components for the purpose of computer simulation are described.

Merrill, W. C.; Blaha, R. J.; Pickrell, R. L.

1978-01-01

69

NASA Astrophysics Data System (ADS)

The rigid slug method for modelling sloshing liquid fuel aboard dual-spin stabilized spacecraft has been shown to be inadequate by recent flight data. This rigid slug model and a uniform gravity model put forth by Abramson is examined in detail. The Abramson model is incorporated into a computer simulation written specifically to predict spacecraft attitude. An analysis is performed with both the modified and unmodified versions of this simulation to determine the boundaries of stability for rotor and platform asymmetries. The results show that the improved model is better able to predict spacecraft attitude stability.

Szostak, Michael J.

1991-06-01

70

NASA Astrophysics Data System (ADS)

This presentation summarizes several useful applications provided by the IMFIT integrated modeling framework to support DIII-D and EAST research. IMFIT is based on Python and utilizes modular task-flow architecture with a central manager and extensive GUI support to coordinate tasks among component modules. The kinetic-EFIT application allows multiple time-slice reconstructions by fetching pressure profile data directly from MDS+ or from ONETWO or PTRANSP. The stability application analyzes a given reference equilibrium for stability limits by performing parameter perturbation studies with MHD codes such as DCON, GATO, ELITE, or PEST3. The transport task includes construction of experimental energy and momentum fluxes from profile analysis and comparison against theoretical models such as MMM95, GLF23, or TGLF.

Collier, A.; Lao, L. L.; Abla, G.; Chu, M. S.; Prater, R.; Smith, S. P.; St. John, H. E.; Guo, W.; Li, G.; Pan, C.; Ren, Q.; Park, J. M.; Bisai, N.; Srinivasan, R.; Sun, A. P.; Liu, Y.; Worrall, M.

2010-11-01

71

The primary CD8 T cell immune response, due to a first encounter with a pathogen, happens in two phases: an expansion phase, with a fast increase of T cell count, followed by a contraction phase. This contraction phase is followed by the generation of memory cells. These latter are specific of the antigen and will allow a faster and stronger response when encountering the antigen for the second time. We propose a nonlinear mathematical model describing the T CD8 immune response to a primary infection, based on three nonlinear ordinary differential equations and one nonlinear age-structured partial differential equation, describing the evolution of CD8 T cell count and pathogen amount. We discuss in particular the roles and relevance of feedback controls that regulate the response. First we reduce our system to a system with a nonlinear differential equation with a distributed delay. We study the existence of two steady states, and we analyze the asymptotic stability of these steady states. Second we study the system with a discrete delay, and analyze global asymptotic stability of steady states. Finally, we show some simulations that we can obtain from the model and confront them to experimental data. PMID:21842166

Terry, Emmanuelle; Marvel, Jacqueline; Arpin, Christophe; Gandrillon, Olivier; Crauste, Fabien

2012-08-01

72

Stability analysis and stabilization strategies for linear supply chains

NASA Astrophysics Data System (ADS)

Due to delays in the adaptation of production or delivery rates, supply chains can be dynamically unstable with respect to perturbations in the consumption rate, which is known as “bull-whip effect”. Here, we study several conceivable production strategies to stabilize supply chains, which is expressed by different specifications of the management function controlling the production speed in dependence of the stock levels. In particular, we will investigate, whether the reaction to stock levels of other producers or suppliers has a stabilizing effect. We will also demonstrate that the anticipation of future stock levels can stabilize the supply system, given the forecast horizon ? is long enough. To show this, we derive linear stability conditions and carry out simulations for different control strategies. The results indicate that the linear stability analysis is a helpful tool for the judgement of the stabilization effect, although unexpected deviations can occur in the non-linear regime. There are also signs of phase transitions and chaotic behavior, but this remains to be investigated more thoroughly in the future.

Nagatani, Takashi; Helbing, Dirk

2004-04-01

73

A distributed slope stability model for steep forested basins

A distributed, physically based slope stability model (dSLAM), based on an infinite slope model, a kinematic wave groundwater model, and a continuous change vegetation root strength model, is presented. It is integrated with a contour line-based topographic analysis and a geographic information system (GIS) for spatial data extraction and display. The model can be run with either individual rainfall events

Weimin Wu; Roy C. Sidle

1995-01-01

74

Advanced stability analysis for laminar flow control

Five classes of problems are addressed: (1) the extension of the SALLY stability analysis code to the full eighth order compressible stability equations for three dimensional boundary layer; (2) a comparison of methods for prediction of transition using SALLY for incompressible flows; (3) a study of instability and transition in rotating disk flows in which the effects of Coriolis forces

S. A. Orszag

1981-01-01

75

Stability analysis of delayed cellular neural networks

In this paper, the problems of stability in a class of delayed cellular neural networks (DCNN) are studied; some new stability criteria are obtained by using the Lyapunov functional method and some analysis techniques. These criteria can be used to design globally stable networks and thus have important significance in both theory and application.

Jinde Cao; Dongming Zhou

1998-01-01

76

Stability of stochastic switched SIRS models

NASA Astrophysics Data System (ADS)

Stochastic stability problems of a stochastic switched SIRS model with or without distributed time delay are considered. By utilizing the Lyapunov methods, sufficient stability conditions of the disease-free equilibrium are established. Stability conditions about the subsystem of the stochastic switched SIRS systems are also obtained.

Meng, Xiaoying; Liu, Xinzhi; Deng, Feiqi

2011-11-01

77

NASA Astrophysics Data System (ADS)

During extrusion of viscoelastic fluids various flow instabilities may arise resulting in a distorted free surface. In order to investigate the factors generating these instabilities we performed a linear stability analysis at zero Reynolds number around the steady solution of the cylindrical or planar stick-slip flow for a viscoelastic fluid following the affine exponential Phan-Thien Tanner (PTT) model. Stick-slip flow is an important special case of the extrudate swell problem, since the latter reduces to it in the limit of infinite surface tension but avoids the complications of a free-boundary flow. The linear stability analysis is performed for various values of the rheological parameters of the PTT model in order to determine the effects of all material properties. It is found that the flow becomes unstable as the Weissenberg number increases above a critical value, due to a Hopf bifurcation suggesting that the flow will become periodic in time. Both the critical value of the Weissenberg number and the frequency of the instability depend strongly on the rheological parameters of the viscoelastic model. The corresponding eigenvectors indicate that the perturbed flow field has a spatially periodic structure, initiated at the rim of the die, extending for up to 5-7 die gaps downstream, but confined close to the surface of the extrudate, in qualitative agreement with existing experiments. This suggests that instability is generated by the combination of the singularity in the velocity and stress fields at the die lip and the strong extension that the extruded polymer undergoes near its surface. The elasticity alone can be responsible for the appearance of instabilities in the extrusion process of viscoelastic fluids and the often used assumptions of wall slip or compressibility, although they might be present, are not required. Finally, the mechanisms that produce these instabilities are examined through energy analysis of the disturbance flow.

Karapetsas, George; Tsamopoulos, John

2013-09-01

78

Developments in Cylindrical Shell Stability Analysis

NASA Technical Reports Server (NTRS)

Today high-performance computing systems and new analytical and numerical techniques enable engineers to explore the use of advanced materials for shell design. This paper reviews some of the historical developments of shell buckling analysis and design. The paper concludes by identifying key research directions for reliable and robust methods development in shell stability analysis and design.

Knight, Norman F., Jr.; Starnes, James H., Jr.

1998-01-01

79

The finite volume flic method and its stability analysis

In this study, a general finite volume Fluid-in-cell method (FVFLIC) for solving the Navier—Stokes equations is introduced. The stability of the numerical method is then analysed by directly using two-dimensional Euler equations instead of a linear model equation. This direct approach to the analysis of non-linear stability is based on the Taylor expansion of the discretized Euler equations and some

Y Mao; D. E Winterbone

1995-01-01

80

Convective instability in sedimentation: Linear stability analysis

NASA Astrophysics Data System (ADS)