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The issue of forest trees biologically designed to withstand wind and slope stresses for improving tree anchorage and stability has been of interest to many forest ecologists for over 200 years. Asymmetry in architecture and function is a typical effect of biomechanical design. This review tried to find the architectural connectivity between belowground and aboveground organs of a tree based on a summary of observations of the asymmetric growth of crown, trunk, and root system. The asymmetrical aboveground growth is influenced by a complex interaction of tree species, age, neighborhood competition, wind, lighting, slope, and elevation. The asymmetrical belowground development is dependent on tree species, age, trunk leaning, wind, soil, and slope. Uneven water conduction, nutrient allocation, hormone content, and photosynthesis rate can influence the relationship of architectural mechanisms between the belowground and aboveground organs. The contradictory observations on the directional deformations of the root system (buttresses) reveal the particular prominence of combined effects of multiple factors. Future research should focus on the comprehensive understanding of the belowground and aboveground architectural relationships of different tree species. Our review provides novel insights into the connotations of root-shoot balance in biomass distribution of the individual plant organs. Key words: asymmetry of belowground and aboveground organs, belowground and aboveground architectural relationship, biomechanical design, distribution of biomass within organs, root-shoot balance, tree anchorage and stability, 1. Introduction The issue of forest tree stability under the wind stress has been of interest to many forest ecologists for over 200 years (Peltola 2006; Telewski 2012). The belowground [...]

Determination of the critical state of tailings is essential to the stability analysis of tailings dams, especially to assess the susceptibility to flow liquefaction failures. In contrast to the peak resistance, which can be determined in triaxial tests at relatively small strains, the critical state is often reached at higher strains. Therefore, determining the critical state in triaxial extension tests is more difficult than in compression tests because the concentration of stresses and strains in extension tests is more severe and usually inevitable, resulting in necking that may greatly affect the computed deviatoric stress and void ratio (if the test is drained). This paper presents a simple and cost-effective method of necking correction using photography. The method was used in the determination of the critical state in drained triaxial extension tests. A sample of silty sand tailings obtained from the reservoir of the Fundao dam two years prior to its collapse was used. The critical state friction angle was approximately the same in compression and extension, showing that the proposed method is promising as many authors consider that the critical state friction angle is the same in these two conditions. Key words: extension test, tailings, sand, critical state, Introduction Critical state soil mechanics has been used to study the mechanical behavior of sands and their susceptibility to liquefaction for several decades (Castro and Poulos 1977; Ishihara 1993; Carrera [...]

To solve the problem of poor waterproof and moisture resistance caused by the high open-cell ratio of polyacrylate foam, a special foam stabilizer (MMA-g-SI) for acrylate was synthesized with polyacrylate as the main chain and aminosiloxane as the side chain. MMA-g-SI has good foam stability in the process of acrylate foaming, and can significantly reduce the surface tension and further increase the closed-cell content. When the amount of MMA-g-SI was 1.0 wt%, the water absorption of foam decreased from 64.3% to 2.1%. The point impact energy absorption increased from 50% to 72%, and the surface impact energy absorption increased from 28% to 35%. Moreover, with the increase of density, the point impact energy absorption of the foam increased, while the surface impact energy absorption decreased. The special foam stabilizer has a good application prospect in the field of acrylate foaming. KEYWORDS energy absorption, foam stabilizer, polyacrylate, poor waterproof, surface tension, 1 | INTRODUCTION With the development of technology, electronic devices, such as mobile phones and computers, have become a part of people's life. However, the screens of these electronic devices [...]

Physical building vulnerability to debris flows is defined as the potential damage degree of buildings for a given debris flow intensity. In this paper, the physical characteristics of both debris flow intensity and building response are considered. Uncertainties in building capacity and debris flow intensity are explicitly quantified to evaluate the damage probability of a typical reinforced concrete building subjected to debris flow impact. Four damage states with clear failure mechanisms are defined using multi-source information from field observations, numerical simulation, and expert experience. Two series of fragility models are proposed based on practical debris flow impact pressure models. Several debris flow intensity measures are investigated. A better indication can be provided using the intensity measure that represents a specific failure mechanism; e.g., impact force (h[v.sup.2]) for force-dominated failures or overturning moment ([h.sup.2][v.sup.2]) for moment-dominated failures, where h and v are debris flow depth and velocity, respectively. The corresponding fragility surfaces best express potential building damage. The intensity thresholds in the proposed fragility curves are consistent with those in empirical vulnerability curves. The methodology presented in this paper promotes vulnerability assessment using physics-based modeling, leading to a more reliable evaluation of building damage caused by debris flows. Key words: debris flow, building, reliability analysis, physical vulnerability, fragility analysis. La vulnerabilite physique des batiments aux coulees de debris est definie comme le degre de dommage potentiel des batiments pour une intensite de coulee de debris donnee. Dans ce rapport, les caracteristiques physiques de l'intensite des laves torrentielles et de la reaction des batiments sont prises en compte. Les incertitudes relatives a la capacite du batiment et a l'intensite de la coulee de debris sont quantifiees de maniere explicite afin d'evaluer la probabilite de dommages d'un batiment typique en beton arme soumis a l'impact d'une coulee de debris. Quatre etats d'endommagement avec des mecanismes de defaillance clairs sont definis a l'aide d'informations provenant de sources multiples telles que les observations sur le terrain, la simulation numerique et l'experience des experts. Deux series de modeles de fragilite sont proposees sur la base de modeles pratiques de pression d'impact des coulees de debris. Plusieurs mesures de l'intensite des coulees de debris sont etudiees. Une bonne indication peut etre fournie en utilisant la mesure d'intensite qui represente le mecanisme de defaillance specifique, par exemple, la force d'impact (h[v.sup.2]) pour les defaillances dominees par la force ou le moment de renversement ([h.sup.2][v.sup.2]) pour les defaillances dominees par le moment, ou h et v sont respectivement la profondeur et la vitesse de la coulee de debris. Les surfaces de fragilite correspondantes expriment au mieux les dommages potentiels des batiments. Les seuils d'intensite des courbes de fragilite proposees sont coherents avec ceux des courbes de vulnerabilite empiriques. La methodologie presentee dans cet article favorise l'evaluation de la vulnerabilite a l'aide d'une modelisation basee sur la physique, ce qui permet une evaluation plus fiable des dommages causes aux batiments par les coulees de debris. [Traduit par la Redaction] Mots-cles: coulee de debris, batiment, analyse de fiabilite, vulnerabilite physique, analyse de fragilite., 1. Introduction Debris flows pose threats to human lives, buildings, and critical infrastructures in debris-flow-prone mountain areas. Quantitative risk assessment has been used to provide an objective measure of risk. [...]

Slope stability is generally analyzed by a two-dimensional (2D) method, ignoring the spatial effect (SE), which no longer meets the evaluation accuracy requirement. This problem is particularly prominent for embankments built on double V-shaped gullies (E-DVSG), which is a typical but unique topography in mountainous areas. Moreover, the technical requirements for engineers will increase sharply if a three-dimensional (3D) slope stability calculation method is adopted. To tackle this issue, pragmatically, a dimension-reduced slope stability analysis method (DR-SSAM) considering the SE of the E-DVSG was proposed and verified by a case study. Specifically, the DR-SSAM obtained the 3D safety factor of the E-DVSG by using the spatial effect curves and the safety factor from the 2D model. The results showed that this method was rational and efficient for engineering applications. In addition, the SE generation mechanism and condition, along with the influences resulting from the major factors, were addressed thoroughly. Key words: double V-shaped gully, embankment, slope stability, dimension-reduced, spatial effect, finite element method. La stabilite des pentes est generalement analysee par une methode en deux dimensions (2D), sans tenir compte de l'effet spatial (ES), qui ne repond plus a l'exigence de precision de l'evaluation. Ce probleme est particulierement important pour le remblai construit sur un ravin en forme de V double (R-RFVD), qui est une topographie typique mais unique en region montagneuse. En outre, les exigences techniques pour les ingenieurs augmenteront grandement si une methode de calcul de stabilite des pentes en trois dimensions (3D) est adoptee. Pour aborder cette question de facon pragmatique, une methode d'analyse de la stabilite des pentes a dimension reduite (MASP-DR) tenant compte de l'ES du R-RFVD a ete proposee et verifiee par une etude de cas. Plus precisement, la MASP-DR a obtenu le facteur de securite 3D du R-RFVD en utilisant les courbes d'ES et le facteur de securite du modele 2D. Les resultats ont montre que la methode etait rationnelle et efficace pour les applications d'ingenierie. En outre, le mecanisme et l'etat de la generation de l'ES ainsi que ses effets decoulant du facteur majeur ont ete examines attentivement. [Traduit par la Redaction] Mots-cles: ravin en forme de V double, remblai, stabilite de la pente, dimension reduite, effet spatial, methode par elements finis., Introduction Highway construction in mountainous areas involves plenty of tunnel excavation, cutting, and digging, which always results in abundant earth and stone wastes. Therefore, high-filled embankments have been extensively implemented [...]

Subsurface stratigraphy of multi-layered slopes is essential and crucial for slope stability analysis. It is usual practice for engineers to interpret stratigraphic boundaries separating different soil layers using both site investigation data and prior knowledge of local geology, but such practice might encounter significant challenge when the site data are very limited. In addition, uncertainty in stratigraphic boundaries has not been explicitly or quantitatively considered in planning of site investigation (e.g., determination of borehole number and locations). There lacks a quantitative and objective tool to determine the optimal locations and number of boreholes for slope stability analysis while accounting for stratigraphic uncertainty. In this study, a smart sampling strategy based on multiple point statistics and information entropy is proposed for delineation of slope subsurface stratigraphy and planning of geotechnical boreholes. It is a data-driven approach that enables an ensemble of prior knowledge within a training image using multiple point statistics. The proposed method not only provides evolution of the most probable interpolation from sparse measurements and the associated interpolation uncertainties, but also adaptively determines the optimal locations of boreholes. Effectiveness of the proposed method is illustrated and validated through both a simulation example and a real case. It is found that the data-driven framework can automatically identify locations of largest interpolation uncertainty within a multi-layered slope conditional on its outcrops, and that the associated stratigraphic uncertainty gradually reduces as borehole number increases. More importantly, the optimal number and locations of boreholes required for slope stability analysis are adaptively determined by the proposed method. Key words: multiple point statistics, slope stratigraphy, sparse measurements, information entropy, data-driven model. Resume : La stratigraphie souterraine des pentes multicouches est essentielle et cruciale pour l'analyse de la stabilite des pentes. Il est d'usage que les ingenieurs interpretent les limites stratigraphiques separant differentes couches de sol en utilisant a la fois les donnees d'investigation du site et les connaissances prealables de la geologie locale, mais cette pratique peut se heurter a des difficultes importantes lorsque les donnees du site sont tres limitees. En outre, l'incertitude des limites stratigraphiques n'a pas ete explicitement ou quantitativement prise en compte dans la planification de l'etude du site (par exemple, determination du nombre et de l'emplacement des trous de forage). Il manque un outil quantitatif et objectif pour determiner les emplacements et le nombre optimal de trous de forage pour l'analyse de la stabilite des pentes tout en tenant compte de l'incertitude stratigraphique. Dans cette etude, une strategie d'echantillonnage intelligente basee sur des statistiques a points multiples et l'entropie de l'information est proposee pour la delimitation de la stratigraphie souterraine des pentes et la planification des forages geotechniques. Il s'agit d'une approche basee sur les donnees qui permet d'integrer un ensemble de connaissances prealables dans une image d'entrainement en utilisant des statistiques a points multiples. La methode proposee permet non seulement de determiner l'evolution de l'interpolation la plus probable a partir de mesures eparses et les incertitudes d'interpolation associees, mais aussi de determiner de maniere adaptative l'emplacement optimal des forages. L'efficacite de la methode proposee est illustree et validee par un exemple de simulation et un cas reel. On constate que le cadre fonde sur les donnees peut identifier automatiquement l'emplacement de la plus grande incertitude d'interpolation dans une pente multicouche en fonction de ses affleurements, et que l'incertitude stratigraphique associee diminue progressivement a mesure que le nombre de trous de forage augmente. Plus important encore, le nombre et l'emplacement optimaux des trous de forage necessaires a l'analyse de la stabilite des pentes sont determines de maniere adaptative par la methode proposee. [Traduit par la Redaction] Mots-cles : statistiques de points multiples, stratigraphie des pentes, mesures eparses, entropie de l'information, modele fonde sur les donnees., 1. Introduction An accurate geotechnical site characterization is essential and crucial for evaluation of slope stability. It is well acknowledged that slope stability can be significantly affected by soil heterogeneities, [...]

In this study, a theoretical analysis is presented for estimating the in-plane geometric nonlinear elastic stability behavior of steel members with tapered elements under dynamic loads. Beam-column approach is adopted for modeling the structural members as beam-column elements. The formulation is based on the Eulerian description taking into consideration the influence of axial force on bending stiffness. The changes in member chord length due to axial deformation and flexural bowing are also considered. In the dynamic analysis, the system mass properties have been represented using both lumped and consistent mass matrices. The consistent mass matrix is derived in three components: translational, rotational, and axial inertia. The formulation of the mass matrices in local and global coordinate systems of tapered members, which incorporates geometric nonlinearity, has been presented. A parametric study is conducted to examine the effects of number of tapered elements, tapering the prismatic members, time step size, and tapering ratio. The dynamic responses of tapered members have been shown to be significantly affected by these parameters. Key words: stability, beam-column, bowing, translational inertia, rotational inertia, axial inertia. Dans cette etude, une analyse theorique est presentee dans le but d'estimer le comportement de stabilite elastique geometrique non lineaire dans le plan d'elements en acier avec des elements effiles sous des charges dynamiques. L'approche poutre-poteau est adoptee pour modeliser les elements structuraux comme des elements poutre-poteau. La formulation est etablie selon la description eulerienne en tenant compte de l'effet de la force axiale sur la rigidite en flexion. Les changements de la longueur de corde des elements en raison de la deformation axiale et de la courbure de flexion sont egalement consideres. Dans l'analyse dynamique, les proprietes de masse du systeme ont ete representees a l'aide de matrices de masse localisee et uniforme. La matrice de masse uniforme est derivee en trois composantes : inertie translationnelle, rotationnelle et axiale. La formulation des matrices de masse dans les systemes de coordonnees locaux et globaux des elements effiles, qui incorpore la non-linearite geometrique, a ete presentee. Une etude parametrique est realisee afin d'examiner les effets du nombre d'elements effiles, de l'effilement des elements prismatiques, du pas de temps et du rapport d'effilement. Il a ete demontre que ces parametres avaient une grande incidence sur les reactions dynamiques des elements effiles. Mots-cles : stabilite, poutre-poteau, courbure, inertie translationnelle, inertie rotationnelle, inertie axiale., Introduction In recent years, the nonlinear behavior of structures has been given much attention. There are different types of nonlinearity such as geometric and material nonlinearity. Another trend that has [...]

This paper demonstrates reliability-based design (RBD) and analysis for tensile strength (rupture) and pullout limit states for mechanically stabilized earth (MSE) walls constructed with steel grid reinforcement in combination with frictional soils. Five different reinforcement tensile load models for walls under operational conditions are considered in combination with six different pullout models and one tensile strength model. The general approach considers the accuracy of the load and resistance models that appear in each limit state equation plus uncertainty in the choice of nominal values at the time of design that is linked to the concept of 'level of understanding' used in Canadian load and resistance factor design (LRFD) foundation engineering practice. The effect of potential steel corrosion on reliability index for the tensile strength limit state is considered in calculations. A well-documented MSE wall case study is used to demonstrate the general approach. The relationship between nominal factor of safety and reliability index is used to demonstrate how to optimize steel grid member diameters and arrangement to achieve a target reliability index of [beta] = 2.33. The approach described in this paper is an important contribution to next-generation analysis and design using modern concepts of RBD for MSE walls. Key words: reliability-based design (RBD), mechanically stabilized earth (MSE) walls, steel grid reinforcement, internal stability limit states. Cet document demontre la conception basee sur la fiabilite (CBF) et l'analyse de la resistance a la traction (rupture) et des etats limites d'arrachement pour les murs en terre stabilisee mecaniquement (TSM) construits avec un renforcement de grille en acier en combinaison avec des sols de friction. Cinq modeles differents de charge de traction des armatures pour les murs dans des conditions operationnelles sont consideres en combinaison avec six modeles d'arrachage differents et un modele de resistance a la traction. L'approche generale tient compte de la precision des modeles de charge et de resistance qui apparaissent dans chaque equation d'etat limite plus l'incertitude dans le choix des valeurs nominales au moment de la conception qui est liee au concept de << niveau de comprehension >> qui est utilise dans la pratique canadienne d'ingenierie des fondations en matiere de conception des facteurs de charge et de resistance (CFCR). L'effet de la corrosion potentielle de l'acier sur l'indice de fiabilite pour l'etat limite de resistance a la traction est pris en compte dans les calculs. Une etude de cas bien documentee sur le mur des TSM est utilisee pour demontrer l'approche generale. La relation entre le facteur nominal de securite et l'indice de fiabilite est utilisee pour demontrer comment optimiser le diametre et la disposition des elements de la grille en acier afin d'atteindre un indice de fiabilite cible de [beta] = 2,33. L'approche decrite dans ce document est une contribution importante a l'analyse et a la conception de la prochaine generation utilisant les concepts modernes de CBF pour les murs des TSM. [Traduit par la Redaction] Mots-cles : conception basee sur la fiabilite (CBF), murs en terre stabilisee mecaniquement (TSM), renforcement de la grille en acier, etats limites de stabilite interne., 1. Introduction This paper is focused on reliability-based design (RBD) and analysis for tensile strength and pullout limit states of steel grid-reinforced mechanically stabilized earth (MSE) walls constructed with frictional [...]

To investigate the effects of the height-thickness ratio ([beta]) on the mechanical properties and stability coefficients ([[phi].sub.s]) of interlocking compressed earth block (ICEB) masonry members under axial compression, four groups of specimens with different [beta] of 3.75, 6.75, 11.25, and 14.25 were tested, thereby assessing their stress process, failure mode, compressive strength, and in- and out-of-plane deformations. All the specimens underwent brittle failure under axial compression: the compressive strength was found to decrease in a range from 5.6% to 43% with increasing [beta], whereas the initial stacking defects and the in- and out-of-plane deformations increased. The specimens became less stable, and we noticed that the overall damage was caused by strength failure and not instability failures. Because the stability coefficient of ICEB-based masonry components cannot be calculated as those of more conventional brickwork, we combined our results with well-established masonry design guidelines and derived an interlocking improvement coefficient. Finally, we proposed a formula for calculating the axial compression stability coefficient of ICEB masonry that can provide a useful reference for subsequent research on ICEB walls. Key words: interlocking compressed earth block masonry, axial compression, height-thickness ratio, deformation performance, stability coefficient. Dans le but d'etudier les effets du rapport hauteur-epaisseur (b) sur les proprietes mecaniques et les coefficients de stabilite (ws) des elements de maconnerie de blocs a emboitement en terre comprimee (BETC) sous compression axiale, quatre groupes de pieces d'essai avec differents [beta] de 3,75, 6,75,11,25 et 14,25 ont ete mis a l'essai, evaluant ainsi leur processus de contrainte, leur mode de defaillance, leur resistance a la compression et leurs deformations dans le plan et hors plan. Toutes les pieces d'essai ont subi une rupture fragile sous compression axiale : la resistance a la compression a diminue dans une fourchette de 5,6 a 43% avec l'augmentation de b, tandis que les defauts d'empilage initiaux et les deformations dans le plan et hors plan ont augmente. Les pieces d'essais sont devenues moins stables, et nous avons remarque que les dommages ont ete causes par une defaillance de la resistance et non par des defaillances d'instabilite. Etant donne que le coefficient de stabilite des elements de maconnerie a base de BETC ne peut pas etre calcule comme celui de la maconnerie de briques plus conventionnelle, nous avons combine nos resultats avec des lignes directrices bien etablies en matiere de conception de maconnerie et avons obtenu un coefficient d'amelioration de l'emboitement. Enfin, nous avons propose une formule pour calculer le coefficient de stabilite de la compression axiale de la maconnerie de BETC qui peut servir de reference pour des recherches ulterieures sur les murs de BETC. [Traduit par la Redaction] Mots-cles : maconnerie de blocs a emboitement en terre comprimee (BETC), compression axiale, rapport hauteur-epaisseur, resultats de deformation, coefficient de stabilite., Introduction An interlocking compressed earth block (ICEB) is a new type of compressed earth block (CEB) produced from fine-grained raw soil and small amounts of cement admixtures; it is shaped [...]

One of the leading causes of dam failure is internal erosion. The impact of erosion of non-plastic fine particles, known as suffusion, on the soil structure and strength has been studied experimentally. However, influences including sample size have not been thoroughly investigated. Internally unstable gap-graded cohesionless soil samples with various sizes were investigated using an erosion-triaxial apparatus. Samples were subjected to downward inflows of different seepage velocities. The results indicated that the potential for clogging increased with an increase in specimen length, leading to less fine particle erosion. Internal erosion changed the mechanical soil behaviour even after the loss of fines equal to 5% of the overall sample volume. Eroded specimens with similar intergranular void ratios showed similar undrained post-erosion behaviour. However, the magnitude of the post-erosion initial undrained peak shear strength is a function of coarse particle interlocking, residual fine content, and equivalent intergranular contact index. It was also found that the steady state line remained unchanged after erosion of fine particles and the mobilized friction angle at the steady state line is independent of the residual fine content. Key words: internal erosion, post-erosion behaviour, flow regime, internal stability, suffusion. L'une des principales causes de rupture des barrages est l'erosion interne. L'impact de l'erosion des particules fines non plastiques, connue sous le nom de suffusion, sur la structure et la resistance du sol a ete etudie experimentalement. Cependant, les influences, y compris la taille de l'echantillon, n'ont pas ete etudiees en profondeur. Des echantillons de sol sans cohesion, instables a l'interieur et presentant des lacunes de differentes tailles ont ete etudies a l'aide d'un appareil d'erosion-triaxial. Les echantillons ont ete soumis a des flux descendants de differentes vitesses d'infiltration. Les resultats ont indique que le potentiel de colmatage augmentait avec l'augmentation de la longueur de l'echantillon, entrainant une erosion par particules moins fines. L'erosion interne a modifie le comportement mecanique du sol meme apres la perte de fines egales a 5 % du volume total de l'echantillon. Des echantillons erodes presentant des rapports de vides intergranulaires similaires ont montre un comportement post-erosion non draine similaire. Cependant, l'ampleur du pic initial de resistance au cisaillement non draine apres l'erosion est fonction de l'emboitement des particules grossieres, de la teneur residuelle en fines et de l'indice de contact intergranulaire equivalent. Il a egalement ete constate que la ligne de regime permanent restait inchangee apres l'erosion des particules fines et que l'angle de frottement mobilise a la ligne de regime permanent est independant de la teneur residuelle en particules fines. [Traduit par la Redaction] Mots-cles : erosion interne, comportement post-erosion, regime d'ecoulement, stabilite interne, suffusion., Introduction Internal erosion is one of the major causes of hydraulic structure failure (ICOLD 2015). According to ICOLD (2015), internal erosion is divided into four main mechanisms: concentrated leaks, backward [...]

A particularly challenging aspect in gravity dam stability assessment is the estimation of the induced hydrodynamic water pressure when water with significant velocity is overtopping gravity dams and flowing in or over spillway components. The water flow conditions, including the related pressure fields and resultant forces, are difficult to quantify accurately. Herein, existing dam safety guidelines to estimate the weight of the overflowing water nappe on gravity dams with rectangular crests are first reviewed. Then, a computational fluid dynamics (CFD) methodology is developed to improve the simplified estimation of hydrodynamic pressure fields acting on the rectangular crests of submerged gravity dams. The CFD pressures are used as input data for the classical structural stability analyses based on the gravity method to more adequately quantify the dam stability during overtopping. A back analysis is also performed on the stability of an existing gated spillway that was overtopped during the 1996 Saguenay flood in Quebec. Key words: computational fluid dynamics (CFD), overtopping, stability, gravity dam, gated spillways. L'estimation des pressions hydrodynamiques induites lorsque l'eau a grande vitesse submerge les barrages-poids et les evacuateurs de crues constitue un aspect particulierement difficile de l'evaluation de la stabilite de ces ouvrages. La dynamique des fluides numerique (CFD) est une alternative attrayante aux modeles physiques pour quantifier les forces hydrodynamiques agissant sur les ouvrages-poids. Dans cet article, les ligne directrices sur la securite des barrages pour estimer le poids de la nappe d'eau submergeant les barrages-poids a crete rectangulaire sont tout d'abord examinees. Ensuite, une methodologie CFD est developpee pour ameliorer l'estimation simplifiee des champs de pression hydrodynamiques agissant sur les cretes rectangulaires des ouvrages soumis a la submersion. Les pressions CFD sont utilisees comme donnees d'entree dans les analyses de stabilite structurales classiques, basees sur la methode de la gravite, afin de quantifier de maniere plus adequate la stabilite au glissement des barrages lors d'une submersion. Un retour d'experience est egalement effectue sur la stabilite d'un evacuateur vanne, qui a ete submerge lors de la crue du Saguenay en 1996. Mots-cles : dynamique numerique des fluides (<< CFD >>), submersion, stabilite, barrage-poids, evacuateurs de crues., Introduction Extreme floods are one of the most significant threats to dam structural stability. During a major flood or an unexpected hydrologic event, dam overtopping may occur (Fig. 1). The [...]

Microbially induced calcite precipitation (MICP) is a naturally driven biological process that harnesses the natural metabolic action of bacteria to induce the precipitation of calcium carbonate and alter soil engineering properties. This paper presents the results of using MICP to improve the monotonic undrained yield and critical strengths of Fraser River sand specimens. Bacteria called 'Sporosarcina ureae' are employed as a ureolytic organism to achieve MICP. The formation of calcite cementation among sand particles is confirmed using scanning electron microscopic images and X-ray compositional analysis of cemented sand clusters. The progress of MICP cementation is assessed by measuring the velocity of a shear wave (VS) traveling through the specimen. The results show that VS starts to increase just as the calcium solution is introduced into each specimen after soaking the samples with the bacterial solution. Improvement in monotonic strength of sand samples is subsequently measured in a series of direct simple shear tests. Due to the combined effects of particle cementation and densification, the sand's undrained and drained monotonic shearing strengths are significantly enhanced. Key words: instability, microbially induced calcite precipitation, direct simple shear test, shear wave velocity, critical state, undrained strength. La precipitation de calcite induite par les microbes (MICP) est un processus biologique naturel qui exploite l'action metabolique naturelle des bacteries pour induire la precipitation de carbonate de calcium et modifier les proprietes techniques du sol. Cet article presente les resultats de l'utilisation de la MICP pour ameliorer le rendement monotone non draine et les resistances critiques des specimens de sable du fleuve Fraser. Des bacteries appelees 'Sporosarcina ureae' sont employees comme organisme ureolytique pour atteindre la MICP. La formation de calcite cementee parmi les particules de sable est confirmee par des images microscopiques obtenues au microscopie electronique a balayage et par l'analyse de la composition des grappes de sable cemente aux rayons X. La progression de la cimentation de la MICP est evaluee en mesurant la vitesse d'une onde de cisaillement (VS) se propageant a travers l'echantillon. Les resultats montrent que la VS commence a augmenter juste au moment ou la solution de calcium est introduite dans chaque echantillon apres avoir trempe les echantillons dans la solution bacterienne. L'amelioration de la resistance monotone des echantillons de sable est ensuite mesuree par une serie de tests de cisaillement simples et directs. Grace aux effets combines de la cementation et de la densification des particules, les forces de cisaillement monotones du sable non draine et draine sont considerablement ameliorees. Mots-cles : instabilite, precipitation de calcite induite par des micro- organismes, test de cisaillement simple et direct, vitesse des ondes de cisaillement, etat critique, resistance non drainee., Introduction Microbially induced calcite precipitation (MICP) is an environmentally friendly process that uses natural microorganisms commonly abundant in soil to precipitate calcite (CaC[O.sub.3]) in environments with a sufficient concentration of [...]

We present a novel approach to represent ecological systems using reaction networks, and show how a particular framework called chemical organization theory (COT) sheds new light on the longstanding complexity-stability debate. Namely, COT provides a novel conceptual landscape plenty of analytic tools to explore the interplay between structure and stability of ecological systems. Given a large set of species and their interactions, COT identifies, in a computationally feasible way, each and every sub-collection of species that is closed and self-maintaining. These sub-collections, called organizations, correspond to the groups of species that can survive together (co-exist) in the long-term. Thus, the set of organizations contains all the stable regimes that can possibly happen in the dynamics of the ecological system. From here, we propose to conceive the notion of stability from the properties of the organizations, and thus apply the vast knowledge on the stability of reaction networks to the complexity-stability debate. As an example of the potential of COT to introduce new mathematical tools, we show that the set of organizations can be equipped with suitable joint and meet operators, and that for certain ecological systems the organizational structure is a non-boolean lattice, providing in this way an unexpected connection between logico-algebraic structures, popular in the foundations of quantum theory, and ecology., Author(s): Tomas Veloz [sup.1] [sup.2] [sup.3] Author Affiliations: (1) grid.8767.e, 0000 0001 2290 8069, Center Leo Apostel for Interdisciplinary Studies, Vrije Universiteit Brussel, , Brussels, Belgium (2) Instituto de Filosofía [...]

An analysis is developed for a compressible bulk-flow model of the leakage path between a centrifugal-compressor impeller's shroud and its housing along the impeller's front and back sides. This development is an extension of analyses performed first by Childs (1989, ASME J. Vib. Acoust., Stress, Reliab. Des., 111, pp. 216-225) for pump impellers. The bulk-flow model is used to predict reaction force and moment coefficients for the impeller shroud. A labyrinth seal code developed by Childs and Scharrer (1986, ASME Trans. J. Eng. Gas Turbines Power, 108, pp. 325-331) is used to calculate the rotordynamic coefficients developed by the labyrinth seals in the compressor stage and also provides a boundary condition for the shroud calculations. Comparisons between the measured shroud moment coefficients by Yoshida et al. (1996, Proceedings of the 6th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, 2, pp. 151-160) and model predictions show reasonable agreements for the clearance flow and reaction moments. For the conditions considered, low Mach number flow existed in the shroud clearance areas and compressible-flow and incompressible-flow models produced similar predictions. Childs' model predictions for the direct damping and cross-coupled stiffness coefficients of a pump impeller produced reasonable agreement; hence the present model was validated to the extent possible. A rotor model consisting of an overhung impeller stage supported by a nominally cantilevered rotor was analyzed for stability using the present bulk-flow model and an API standard Wachel--von Nimitz formula model (1981, J. Petrol. Technol., pp. 2252-2260). The bulk-flow model predicted significantly higher onset speeds of instability. Given that some compressors have been predicted to be comfortably stable using API standard Wachel--von Nimitz formula but have been unstable on the test stand, these results suggest that unidentified destabilizing forces and or moments are present in compressors. Seal rub conditions that arise from surge events and increase the seal clearances are simulated, showing that enlarged clearances increase the preswirl at the seals, thus increasing these seal's destabilizing forces and reducing stability margins. These results are consistent with field experience. Predictions concerning the back shroud indicate that shunt-hole injection mainly acts to enhance stability by changing the flow field of the division wall or balance piston seals, not by influencing the back-shroud's forces or moments. Effective swirl brakes at these seals also serve this purpose. [DOI: 10.1115/1.2720519] Keywords: rotordynamic coefficients, impeller-shroud forces

This paper addresses the rotordynamic instability of an overhung rotor caused by a hydrodynamic moment due to whirling motion through the structural coupling between whirl and precession modes. First, the possibility of instability is discussed based on a vibration model in which the hydrodynamic forces and moments are assumed to be smaller than structural forces with the structural coupling being represented by a structural influence factor. Then, the fundamental characteristics of rotordynamic moment on the backshroud of a Francis turbine runner under whirling motion were studied using model tests and numerical calculations. The runner is modeled by a disk positioned close to a casing with a small radial clearance at the outer periphery. The moment is caused by an inward leakage flow that is produced by an external pump in the model test. The experiments were designed to measure the rotordynamic fluid force moments under various leakage flow rates with various preswirl velocities and various axial clearances between the backshroud and casing. The computation was carried out based on a bulk flow model. It was found that the fluid force moment is generally destabilizing, except for a small region of positive whirling speed ratios. [DOI: 10.1115/1.4001802]

This paper deals with the output feedback [H.sub.[infinity]] control design problem for continuous-time switched linear systems. More specifically, the main goal is to design a switching rule together with a dynamic full order linear controller to satisfy a prespecified [H.sub.[infinity]] level defined by the [L.sub.2] gain from the input to the output signal. Initially, the state feedback version of this problem is solved in order to put in evidence the main difficulties we have to face toward the solution of the output feedback control design problem. The results reported in this paper are based on the so called Lyapunov--Metzler inequalities, which express a sufficient condition for switched linear systems global stability. The solution of the previously mentioned output feedback control design problem through a linear matrix inequality based method is the main contribution of the present paper. An academic example borrowed from literature is used for illustration. [DOI: 10.1115/1.4001711]

The authors present a model-based analysis of a position-velocity-acceleration-controlled pneumatic actuator that indicates that supplementing the pneumatic actuator with mechanical damping can significantly increase the gain margin, tracking accuracy, and disturbance rejection of a closed-loop-controlled pneumatic servoactuator. In order to validate the model-based analysis and purported performance and stability benefits provided by supplemental damping, experiments were performed on a single-degree-of-freedom pneumatic servosystem. Measurements conducted on the experimental setup, which validate the respective improvements in stability margin, tracking accuracy, and disturbance rejection, are described. [DOI: 10.1115/1.4001796]

The article addresses the problem of stabilization for uncertain discrete input-shaped systems. The uncertainty affects the autoregressive portion of the transfer function of the system. A discrete input shaper compensator is designed in order to reduce the oscillations of the plant's response. The input-shaped system's dynamics are appropriately reformulated for robust controller synthesis, and a robust [H.sub.[infinity]]-controller is used in an outer-loop, in order to guarantee stability of the uncertain input-shaped plant. Simulation results confirm the efficacy of the proposed combined scheme in comparison with open-loop input shaping and closed-loop linear quadratic control. [DOI: 10.1115/1.4001704]

Flow fields in an annulus between two rotating cylinders with a porous lining have been numerically examined in this study. While the outer cylinder is stationary, the inner cylinder is rotating with a constant angular speed. A homogeneous and isotropic porous layer is press-fit to the inner surface of the outer cylinder. The porous sleeve is saturated with the fluid that fills the annulus. The effects of porous sleeve thickness and its properties on the flows and their stability in the annulus are numerically investigated. Three-dimensional momentum equations for the porous and fluid layers are formulated separately and solved simultaneously in terms of velocity and vorticity. The solutions have covered a wide range of the governing parameters ([10.sup.-5] [less than or equal to] Da [less than or equal to] [10.sup.-2], Ta [less than or equal to]5000, 0.8 [less than or equal to] [bar.b] [less than or equal to] 0.95). The results obtained show that the presence of a porous sleeve generally has a stabilizing effect on the flows in the annulus. [DOI: 10.1115/1.4001541]

This article extends the perturbation method introduced by Wang (2008, 'A Novel Method for Analyzing the Global Stability of Inviscid Columnar Swirling Flow in a Finite Pipe,' Phys. Fluids, 20(7), p. 074101) to determine the global stability of a swirling flow in a straight circular pipe with specified inlet and outlet boundary conditions. To accurately compute the flow stability characteristics, a general procedure to treat the complexity arising from high-order terms is developed. It extends the previous fourth-order method to an eighth-order method. The technique is first applied to the benchmark case of a solid-body rotation flow with a uniform axial speed. It is demonstrated that the eighth-order method is sufficient to construct the growth rate curve between the first and second critical swirls of this flow. Note that this range of swirl is crucial for the study of the vortex breakdown phenomenon since the base flow is unstable and starts the initial stage of transition to a breakdown state. The method is then applied to the Lamb--Oseen vortex to construct the growth rate curve between the first and second critical swirls of this flow. Calculated results are compared with the growth rate curve computed from direct numerical simulations and an overall agreement between the two computations is found. This demonstrates that the Wang and Rusak (1996, 'On the Stability of an Axisymmetric Rotating Flow in a Pipe,' Phys. Fluids, 8(4), pp. 1007-1076) instability mechanism captures quantitatively the initial growth of disturbance, which eventually evolves into a breakdown state. [DOI: 10.1115/1.4001106]

The aim of this paper is to determine the face collapse pressure of a circular tunnel driven by a pressurized shield. The analysis is performed in the framework of the kinematical approach of limit analysis theory. It is based on a translational three-dimensional multiblock failure mechanism. The present failure mechanism has a significant advantage with respect to the existing limit analysis mechanisms developed in the case of a frictional soil: it takes into account the entire circular tunnel face and not only an inscribed ellipse to this circular area. This was made possible by the use of a spatial discretization technique. Hence, the three-dimensional failure surface was generated 'point by point' instead of simple use of existing standard geometric shapes such as cones or cylinders. The numerical results have shown that a multiblock mechanism composed of three blocks is a good compromise between computation time and results accuracy. The present method significantly improves the best available solutions of the collapse pressure given by other kinematical approaches. Design charts are given in the case of a frictional and cohesive soil for practical use in geotechnical engineering. DOI: 10.1061/(ASCE)GT.1943-5606.0000194 CE Database subject headings: Tunnels; Limit analysis; Pressure; Kinematics. Author keywords: Tunnel; Limit analysis; Tunnel face stability; Pressurized shield; Upper-bound method.

For determining the stochastic response and stability of a strongly nonlinear single-degree-of-freedom system using the stochastic averaging technique, the size of excitations should be small such that the response of the system converges weakly to a Markov process. This condition is not often met with practical problems, and therefore, application of this method for obtaining their responses becomes difficult. Further, for systems with nonlinearities that cannot be integrated in closed form, stability analysis by examining the conditions of the two boundaries of the problem is not possible. A semianalytical method along with a weighted residual technique is presented here to circumvent these difficulties and to determine the response and stability of a strongly nonlinear system subjected to sizable stochastic excitation. The weighted residual technique is employed to correct the errors in averaged drift and diffusion coefficients resulting due to the size of the stochastic excitation. Two example problems are solved as illustrations of the method. DOI: 10.1061/(ASCE)0733-9399(2008)134:8(698) CE Database subject headings: Stochastic processes; Stability; Nonlinear systems; Markov chains.

This paper investigates the stability and boundary of a swarm with a general directed and weighted topology. The stability of a swarm is generally considered as cohesiveness. We construct a symmetric eigenmatrix and define an orthogonal eigenparameter which reflects the degree of orthogonality between the left eigenvector of the coupling matrix corresponding to its zero eigenvalue and the eigenvectors of the eigenmatrix corresponding to its nonzero eigenvalues. We prove that, if the topology of the underlying swarm is strongly connected, the swarm is then stable in the sense that all agents will globally and exponentially converge to a hyperellipsoid in finite time, both in open space and profiles, whether the center of the hyperellipsoid is moving or not. The swarm boundary and convergence rate are characterized by the eigenparameters of the swarm, which reveals the quantitative relationship between the swarming behavior and characteristics of the coupling topology. Index Terms--Eigenstructure, multi-agent, stability, swarm boundary.

Input impedance models of boost single-phase power factor correction converters suitable for predicting small-signal input characteristics below the line fundamental frequency are presented in this paper. Existing low-frequency input impedance models based on double averaging predicts input current responses to perturbations in the amplitude of the input voltage. It is shown in this paper that such impedance models do not conform to the standard definition of impedance, and, hence, cannot be used in conjunction with the source output impedance to study system interactions based on the Nyquist stability criterion, as traditionally done in dc power systems. A new input impedance model that overcomes this problem is developed by using the method of harmonic balance. The new model conforms to the standard definition of impedance and predicts input current responses to superimposed input harmonic voltages. Numerical simulation and experimental results are presented to validate the proposed model. Mathematical relationship between the two types of models are also presented. Index Terms--Input impedance, power factor correction (PFC), stability.

Long resistive cables used in the operation of remote instrumentation impose fundamental limits on the amount of power delivered. Due to their constant-power load profile, switching regulators pose unique problems when operated at the end of long resistive cables. Some of the problems associated with the operation of switching regulators in these systems include limitations on power delivered at the remote-end, voltage jumps at startup and startup load cycling. Analytical calculations and experimental results are provided to validate the previously mentioned phenomena. Two new methods of power delivery based on state feedback control and parallel operation of switching and linear regulators to enhance stability and increase the power delivered at the remote-end are proposed and validated experimentally. Index Terms--Distributed systems, linear regulators, remote instrumentation, stability, switching regulators.

We present the results of spin transfer torque magnetization switching results for some MgO-based magnetic tunnel junctions (MTJs) with tunneling magnetoresistance (TMR) ratios of 38.6% to 200% and critical current densities of 1.032 x [10.sup.7] A/[cm.sup.2] down to 1.68 x [10.sup.6] A/[cm.sup.2]. We observed 2.2 x [10.sup.4] stable read/write (R/W) cycles before failure under the spin transfer torque magnetization switching. The synchronized variations in both high and low magnetoresistance (MR) over the stable R/W cycles indicate that magnetization reversal into either high or low state occurs in parallel. We made a stability analysis for high and low MR distribution matching to a normalized fraction of stable cycles. Index Terms--MgO, MRAM, MTJ, read/write cycle, spin transfer torque, stability analysis, TMR.

Magnetic bearings are electromechanical systems. From a mechanical point of view, their modeling is plainly related to general rotor dynamics modeling. But the forces acting between the rotor and the stator are of an electromagnetic nature. This paper presents the analysis of the potentially unstable dynamical behavior of magnetic bearings. It adopts an electromechanical point of view and represents the forces acting on the system by dampers, springs, and an electrical phase shift. The coefficients of the model are found by parameter identification with a finite-element model of the system. Index Terms--Dynamic stability, eddy currents, magnetic bearing, rotating parts.

The stabilization of tidal inlets by jetties interrupts the littoral drift and has negative visual impacts. This paper proposes an innovative solution to improve the stability of a tidal inlet (the Obidos Lagoon, Portugal) and minimize maintenance dredging, while avoiding the disadvantages of the traditional solution. The new solution has two key features. First, transverse channels are dredged over the tidal flats to promote ebb dominance, thereby improving the ability of the lagoon to flush out incoming sediments. Second, a partially submerged guiding wall near the southern shoreline, together with the existing guiding wall in the northern shore, prevents the movement of the inlet channel and concentrates the ebb flow in the channel. Analyses with empirical, numerical, and analytical models indicate that the solution improves the stability of the tidal inlet and reduces the accretion in the system. DOI: 10.1061/(ASCE)0733-9429(2007)133:7(816) CE Database subject headings: Morphology; Sand; Portugal; Inlets, waterways; Lagoons; Case reports.

This paper presents a new method for applying reliability-based design approaches to slope stability analysis. In this method the soil properties are considered to be random variables. The factor of safety of the slope is found using Bishop's simplified method for noncircular slip surfaces. By considering the variability of the soil properties, the probability of failure is determined from the reliability index ([beta]). The minimization problem (determination of the lowest [beta] value for the range of variables and possible slip surfaces considered) is solved using a genetic algorithm approach, which simultaneously locates the critical slip surface and determines the reliability index. The performance of the new method is compared to some existing reliability approaches when applied to case histories of slope failures from the geotechnical literature. The new approach is seen to provide reasonable and consistent estimates of the reliability index and critical slip surface. DOI: 10.1061/(ASCE)1090-0241(2007)133:7(878) CE Database subject headings: Slope stability; Probabilistic methods; Reliability; Algorithms; Safety factors.

So far, earthquake-induced slope instability has been evaluated by force equilibrium of soil mass in engineering practice, which cannot evaluate failure deformation once large failure occurs. An energy approach is proposed here, in which the amount of earthquake energy is evaluated in conjunction with the gravitational potential energy dissipated in slope displacement including large flow deformations. Shake table tests of dry sand slopes are carried out in which the earthquake energy used for slope failure can be successfully quantified. Measured slope displacement can be reliably evaluated by the proposed energy approach based on a rigid block model if an appropriate friction coefficient of the slope is specified. The energy approach is then applied to hypothetical slopes, indicating that even if extremely large earthquake energy is considered, slope failures with long run-out distance will not occur unless friction coefficients reduce near to or smaller than slope inclinations. DOI: 10.1061/(ASCE)1090-0241(2007)133:7(828) CE Database subject headings: Seismic waves; Shake table tests; Friction; Slope stability; Failures.

Various experiments and measurements are conducted to analyze the stability in the multifilamentary [MgB.sub.2] tapes.

Luminescent solar concentrator samples using polymethylmethacrylate (PMMA) as the matrix with phthalocyanine, nickel-phthalocyanine and hematoporphyrin IX dichloride laser dyes were prepared by a casting method. Optical absorption measurements were carried out at room temperature across the 200-900 nm wavelength region both before and after the samples were exposed to sunlight for two weeks. The degradation of PMMA--dye samples, measured by absorption, was found to obey a first-order kinetic equation. Values of the optical band gaps ([E.sub.g]) have been obtained for the direct allowed transitions before and after the samples were exposed to sunlight. The tail width of localized states in the band gap ([E.sub.u]) was calculated from Urbach's formula. Photodegradation studies revealed that the phthalocyanine dye systems are the most stable. Emission spectra and quantum efficiency were also investigated, and phthalocyanine dyes were shown to have a lower Stokes shift and a higher fluorescence quantum yield ([Q.sub.f]) than the hematoporphyrin IX dichloride. Collectively, these results suggest that PMMA doped with phthalocyanines provide the better system for use in fluorescent solar collector systems. Keywords: Polymethylmethacrylate; Phthalocyanine; Ni-phthalocyanine; Hematoporphyrin IX chloride; Optical band gap; Urbach's formula; Stokes shift; Fluorescence quantum yield; Fluorescent solar collectors

This paper presents a comprehensive dynamic model that simulates infeed centerless grinding processes and predicts their instability-related characteristics. The new model has the unique ability of accurately predicting the coupled chatter and lobing process of a multi-degree of freedom and two-dimensional centerless grinding system by considering its critical issues. First, the model considers the complete two-dimensional kinematics, dynamics, surface profiles, and the geometrical interactions of the workpiece with the grinding wheel, regulating wheel, and supporting blade. Second, a two-dimensional distributed grinding force model along the contact length is adopted and modified for centerless grinding processes as a function of normalized uncut chip thickness. The forces of the work holding system are determined by balancing the grinding force and accordingly the work holding instability can be identified as well. Third, a two-dimensional contact deformation model under the condition of general surface profiles or pressure distributions is developed for the contacts of the workpiece with the grinding wheel, regulating wheel, and supporting blade. The new model is validated by comparing the predicted chatter and lobing occurrences with experimental results. Keywords: centerless grinding, chatter, dynamic simulation model, mechanistic model [DOI: 10.1115/1.2716729]

Stability of a cemented implant, once the stem-cement interface has debonded, is reliant upon stem geometry and surface finish. There are relatively few studies addressing the effect of cross-sectional stem shape on cemented implant fixation. The purpose of this investigation was to compare the torsional stability of five different stem cross-sectional shapes--circular, oval, triangular, rectangular with rounded edges, and rectangular with sharp edges--under monotonically increasing and cyclic loading conditions. Seven samples of each stem geometry were tested. Stems were potted in bone cement and loaded to 5 deg of rotation. For monotonic loading, torque was applied at a constant rate of 2.5 deg/min. For cyclic loading, a sine wave torque pattern was applied, with a maximum magnitude that began at 4.5 Nm for 1500 cycles and then increased by 2.25 Nm every 1500 cycles until 5 deg of rotation. The rectangular stem with the sharp edges always provided the greatest resistance to torque, followed by the rectangular with rounded edges, triangular, oval, and circular. These results, including the effects of sharp corners, may differ for modes of loading other than torsion. These experimental results support the findings of earlier finite element models, indicating stem shape has a significant effect on resistance to torsional loading. [DOI: 10.1115/1.2720907] Keywords: implant fixation, implant design, cemented implant failure, torsional stability, arthroplasty

It was recently conjectured that the Hurwitz stability of the convex hull of a set of Metzler matrices is a necessary and sufficient condition for the asymptotic stability of the associated switched linear system under arbitrary switching. In this note, we show that 1) this conjecture is true for systems constructed from a pair of second-order Metzler matrices; 2) the conjecture is true for systems constructed from an arbitrary finite number of second-order Metzler matrices; and 3) the conjecture is in general false for higher order systems. The implications of our results, both for the design of switched positive linear systems, and for research directions that arise as a result of our work, are discussed toward the end of the note. Index Terms--Positive linear systems, stability theory, switched linear systems.

A model for sliding contact of a thermoelastic rod is considered and is subjected to a multiple scales analysis to uncover its nonlinear behavior near a neutrally stable state. The analysis reveals a combination of the contact resistance and frictional intensity that describes the generic unfolding of this critical state and its associated bifurcations. In particular, the system can describe how two equilibria coalesce in a saddle-node bifurcation and generalizes stability criteria that have been presented previously in the literature for this model. Moreover, this analysis describes the role of the initial deformation of the rod on its long-term dynamical behavior. [DOI: 10.1115/1.2423034]

This paper introduces an adaptive structure composed of n coupled paralleled subfilters that directly estimates frequencies of multiple sinusoids. The order of differential equations governing this algorithm is 3n, where n is the number of sinusoids. Stability analysis of this algorithm is carried out based on the application of the averaging theory under the assumption of the slow adaptation. Simulation results verify the validity of the analytical issues. Index Terms--Adaptive notch filter (ANF), frequency estimation, nonlinear systems, parallel structure, stability analysis.

Five-degree-of-freedom (5-DOF) characterization of the stability of a gas-lubricated conical bearing of the spiral-groove design is presented for bearing numbers up to 500. Critical points in stability analysis are identified in impedance contour plots separately for axial, cylindrical, and conical modes. The stability thresholds with respect to each mode are graphed as functions of the bearing number. For axial and cylindrical modes, the threshold parameter is the rotor mass. For the conical mode, the threshold parameter is an equivalent mass that is dependent on both transverse and polar radii of gyration of the rotor An application example illustrates a rational procedure to specify nominal bearing clearance and its allowable tolerance range. Keywords: gas lubrication, conical bearing, stability analysis, gap tolerance effects

We study annual U.S. data from 1869 or 1900 to 1999. We find evidence for a well-specified and stable model of money demand with data from 1946 to 1999. We carry out diagnostic and stability tests, including linearity tests. A linear error-correction model with the monetary base performs better than a model with M1. A specification with M2 is not supported. We use real gross national product as the scale variable and a short-term interest rate as the opportunity cost measure. We estimate an income elasticity of 0.86 and an interest rate elasticity of -0.44 for the monetary base. (JEL E41), I. INTRODUCTION The stability of money demand has been a long-standing issue. (1) Lucas (1988) informally analyzes the stability of a log-linear M1 money demand function and concludes that real [...]

Abstract Stability is essential for teleoperation and a prerequisite for performance. This paper analyzes the the stability/performance trade-off of a teleoperator where the slave device has a built-in passive intrinsic [...]

In recent years, there have been a number of new delay-dependent stability criteria based on linear matrix inequalities published in the literature. This note aims to theoretically establish the equivalence of seven of these stability criteria. Moreover, the efficiency of these stability criteria is assessed based on the number of unknowns in the linear matrix inequalities. Index Terms--Delay-dependent condition, stability, time-delay systems.

Current design methods divide reinforced earth structures into walls and slopes by using an arbitrary face inclination of 70[degrees] as the boundary. The required maximum strength of reinforcement computed for reinforced walls are significantly higher than that computed for reinforced slopes even if the inclination is practically the same. Presented is a general analytical framework for design of flexible reinforced earth structures regardless of the slope face inclination. In fact, the framework is consistent for any structural geometry and any applicable slope stability analysis although, for demonstration purposes, the simple Culmann formulation is utilized for simple geometry with zero batter. Using an adequate slope stability formulation, the required tensile resistance of reinforcement for a given layout is calculated so as to produce the same prescribed factor of safety anywhere within the reinforced zone. That is, using the design shear strength of the soil, the required reinforcement resistance along each layer is computed to fully mobilize this shear strength for all possible slip surfaces. That is, a baseline solution is produced for an ideal long-term strength of reinforcement at any location. Consequently, the required strength of the connection between each reinforcement layer and the facing unit can also be determined. This connection strength, however, assumes small facing units with negligibly small shear and moment resistance. Parametric study is conducted to demonstrate the reasonableness of the presented framework. It is shown that the required tensile resistance and connection strength depend on factors such as: reinforcement length; intermediate reinforcement; percent coverage; and quality of fill. When compared with the current AASHTO design for walls, the required maximum long-term strength of the reinforcement as well as the required connection strength in the proposed approach are substantially smaller. DOI: 10.1061/(ASCE)1090-0241(2006)132:11(1427) CE Database subject headings: Soil stabilization; Retaining walls; Tensile strength; Connections; Limit equilibrium; Geosynthetics; Slope stability; Safety factors; Soil structure.

In this brief, a systematic method to design stable single-stage high-order sigma-delta modulators (SDMs) without the need of stability-recovery mechanisms is proposed. It is shown that if the critical root loci of an SDM transfer function are all located inside a unit circle, then the SDM is guaranteed to be stable. Instead of basing the design on noise transfer function as in previous designs, the presented method bases the design on proper choice of open-loop transfer function departure angles so as to keep the critical root loci within the unit circle. A stable fourth-order SDM with the critical root loci inside the unit circle is given as a design example. Simulation results reveal that based on the presented design method, stable single-stage high-order SDMs can be free from overload instability even with full-scale input magnitude. Tradeoffs between stability and performance of the SDMs are presented. By allowing parts of the critical root loci to be located outside the unit circle, this method can be extended for SDM designs with higher signal-to-noise ratio but smaller stable input range. Index Terms--Departure angle, root-locus, sigma-delta modulator (SDM), stability.

One major challenge for the successful operation of high-power-density micro-devices lies in the stable operation of the bearings supporting the high-speed rotating turbomachinery. Previous modeling efforts by Piekos (2000, 'Numerical Simulation of Gas-Lubricated Journal Bearings for Microfabricated Machines,' Ph.D. thesis, Department of Aeronautics and Astronautics, MIT), Liu et al. (2005, 'Hydrostatic Gas Journal Bearings for Micro-Turbo Machinery,' ASME J. Vib. Acoust., 127, pp. 157-164), and Spakovszky and Liu (2005, 'Scaling Laws for Ultra-Short Hydrostatic Gas Journal Bearings,' ASME J. Vib. Acoust. 127, pp. 254-261) have focused on the operation and stability of journal bearings. Thrust bearings play a vital role in providing axial support and stiffness, and there is a need to improve the understanding of their dynamic behavior. In this work, a rigorous theory is presented to analyze the effects of compressibility in micro-flows (characterized by low Reynolds numbers and high Mach numbers) through hydrostatic thrust bearings for application to micro-turbomachines. The analytical model, which combines a one-dimensional compressible flow model with finite-element analysis, serves as a useful tool for establishing operating protocols and assessing the stability characteristics of hydrostatic thrust bearings. The model is capable of predicting key steady-state performance indicators, such as bearing mass flow, axial stiffness, and natural frequency as a function of the hydrostatic supply pressure and thrust-bearing geometry. The model has been applied to investigate the static stability of hydrostatic thrust bearings in micro-turbine generators, where the electrostatic attraction between the stator and rotor gives rise to a negative axial stiffness contribution and may lead to device failure. Thrust-bearing operating protocols have been established for a micro-turbopump, where the bearings also serve as an annular seal preventing the leakage of pressurized liquid from the pump to the gaseous flow in the turbine. The dual role of the annular pad poses challenges in the operation of both the device and the thrust bearing. The operating protocols provide essential information on the required thrust-bearing supply pressures and axial gaps required to prevent the leakage of water into the thrust bearings. Good agreement is observed between the model predictions and experimental results. A dynamic stability analysis has been conducted, which indicates the occurrence of instabilities due to flow choking effects in both forward and aft thrust bearings. A simple criterion for the onset of axial rotor oscillations has been established and subsequently verified in a micro-turbocharger experiment. The predicted frequencies of the unstable axial oscillations compare well with the experimental measurements.

In this paper we relate the stability radius that can be achieved for the closed-loop matrix (A-BK) to the distance to unstabilizability of the pair (A,B). In the paper we show that the closed-loop matrix (A-BK) can achieve a stability radius of [gamma] with a real feedback matrix K only if the distance to unstabilizability of (A,B) is greater than [gamma]. Thus the distance to the unstabilizabilitv of (A ,B) provides an upper bound on the maximum stability radius that can be achieved by state feedback. [DOI: 10.1115/1.2234490]

A new stability criterion for time-varying systems consisting of linear and norm bounded nonlinear terms with uncertain time-varying delays is formulated. An explicit delay-independent sufficient stability condition is formulated in the terms of the transition matrix of the given linear part without delay and the bounds for the uncertain terms. The obtained condition turns out to be also necessary if the matrix of the linear part is time-invariant and symmetric; it is shown that these systems satisfy the well-known Aizerman's conjecture. The obtained criterion is contrasted by some of stability estimates available in the literature for these kinds of systems; in all cases the proposed criterion provides less conservative stability bounds. Index Terms--Aizerman's conjecture, bounded nonlinearities, delay systems, stability conditions.