Buckling and Postbuckling Structures: Experimental, Analytical and Numerical Studies

This book provides an in-depth treatment of the study of the stability of engineering structures. Contributions from internationally recognized leaders in the field ensure a wide coverage of engineering disciplines in which structural stability is of importance, in particular the analytical and numerical modelling of structural stability applied to aeronautical, civil, marine and offshore structures. The results from a number of comprehensive experimental test programs are also presented, thus enhancing our understanding of stability phenomena as well as validating the analytical and computational solution schemes presented. A variety of structural materials are investigated with special emphasis on carbon-fibre composites, which are being increasingly utilized in weight-critical structures. Instabilities at the meso- and micro-scales are also discussed. This book will be particularly relevant to professional engineers, graduate students and researchers interested in structural stability.

Experimental and numerical studies on rock breaking with TBM tools under high stress confinement

The understanding of rock breaking and chipping due to the TBM cutter disks mechanism in deep tunnels is considered in this paper. The interest stems from the use of TBMs for the excavation of long Trans-Alpine tunnels. Some tests that simulate the disk cutter action at the tunnel face by means of an indenter, acting on a rock specimen are proposed. The rock specimen is confined through a flat-jack and a confinement-free area on one side of the specimen simulates the formation of a groove near the indenter, like it occurs in TBM excavation conditions. Results show a limited influence of the confinement stress versus the thrust increment required for breaking the rock between the indenter and the free side of the specimen. Numerical modelling of the cutter disk action on confined material has also been carried out in order to investigate further aspects of the fracture initiation. Also in this case the importance of the relative position between disk cutter and groove is pointed out. © 2006 Springer-Verlag.

Numerical Study of Wave Slamming on an Oscillating Flap

Bottom hinged Oscillating Wave Surge Converters (OWSCs) are efficient devices for extracting power from ocean waves. There is limited knowledge about wave slamming on such devices. This paper deals with numerical studies of wave slamming on an oscillating flap to investigate the mechanism of slamming events. In our model, the Navier–Stokes equations are discretized using the Finite Volume method with the Volume of Fluid (VOF) approach for interface capturing. Waves are generated by a flaptype wave maker in the numerical wave tank, and the dynamic mesh method is applied to model the motion of the oscillating flap. Basic mesh and time step refinement studies are performed. The flow characteristics in a slamming event are analysed based on numerical results. Various simulations with different flap densities, water depths and wave amplitudes are performed for a better understanding of the slamming.

Modulational instability and localized excitations of dust-ion acoustic waves

Theoretical and numerical studies are carried out of the nonlinear amplitude modulation of dust-ion acoustic waves propagating in an unmagnetized weakly coupled plasma comprised of electrons, positive ions, and charged dust grains, considering perturbations oblique to the carrier wave propagation direction. The stability analysis, based on a nonlinear Schrodinger-type equation, exhibits a wide instability region, which depends on both the angle theta between the modulation and propagation directions and the dust number density n(d). Explicit expressions for the instability increment and threshold are obtained. The possibility and conditions for the existence of different types of localized excitations are also discussed. (C) 2003 American Institute of Physics.

Oblique amplitude modulation of dust-acoustic plasma waves

Theoretical and numerical studies are presented of the nonlinear amplitude modulation of dust-acoustic (DA) waves propagating in an unmagnetized three component, weakly-coupled, fully ionized plasma consisting of electrons, positive ions and charged dust particles, considering perturbations oblique to the carrier wave propagation direction. The stability analysis, based on a nonlinear Schrodinger-type equation (NLSE), shows that the wave may become unstable; the stability criteria depend on the angle theta between the modulation and propagation directions. Explicit expressions for the instability rate and threshold have been obtained in terms of the dispersion laws of the system. The possibility and conditions for the existence of different types of localized excitations have also been discussed.

Ion-acoustic waves in a two-electron-temperatute plasma: oblique modulation and envelope excitations

Theoretical and numerical studies are carried out for the nonlinear amplitude modulation of ion-acoustic waves propagating in an unmagnetized, collisionless, three-component plasma composed of inertial positive ions moving in a background of two thermalized electron populations. Perturbations oblique to the carrier wave propagation direction have been considered. The stability analysis, based on a nonlinear Schrodinger-type equation, shows that the wave may become unstable; the stability criteria depend on the angle theta between the modulation and propagation directions. Different types of localized excitations (envelope solitary waves) are shown to exist in qualitative agreement with satellite observations in the magnetosphere.

Electron-acoustic plasma waves: Oblique modulation and envelope solitons

Theoretical and numerical studies are presented of the amplitude modulation of electron-acoustic waves (EAWs) propagating in space plasmas whose constituents are inertial cold electrons, Boltzmann distributed hot electrons, and stationary ions. Perturbations oblique to the carrier EAW propagation direction have been considered. The stability analysis, based on a nonlinear Schrodinger equation, reveals that the EAW may become unstable; the stability criteria depend on the angle theta between the modulation and propagation directions. Different types of localized EA excitations are shown to exist.

Nonlinear propagation of modulated ion-acoustic plasma waves in the presence of an electron beam

Theoretical and numerical studies are presented of the amplitude modulation of ion-acoustic waves (IAWs) in a plasma consisting of warm ions, Maxwellian electrons, and a cold electron beam. Perturbations parallel to the carrier IAW propagation direction have been investigated. The existence of four distinct linear ion acoustic modes is shown, each of which possesses a different behavior from the modulational stability point of view. The stability analysis, based on a nonlinear Schrodinger equation (NLSE) reveals that the IAW may become unstable. The stability criteria depend on the IAW carrier wave number, and also on the ion temperature, the beam velocity and the beam electron density. Furthermore, the occurrence of localized envelope structures (solitons) is investigated, from first principles. The numerical analysis shows that the two first modes (essentially IAWs, modified due to the beam) present a complex behavior, essentially characterized by modulational stability for large wavelengths and instability for shorter ones. Dark-type envelope excitations (voids, holes) occur in the former case, while bright-type ones (pulses) appear in the latter. The latter two modes are characterized by an intrinsic instability, as the frequency develops a finite imaginary part for small ionic temperature values. At intermediate temperatures, both bright- and dark-type excitations may exist, although the numerical landscape is intertwined between stability and instability regions.(c) 2006 American Institute of Physics.

Lay-up optimization of composite plates to delay mode-jump instabilities

Experimental and numerical studies have shown that the occurrence of abrupt secondary instabilities, or mode-jumps, in a postbuckling stiffened composite panel may initiate structural failure. This study presents an optimisation methodology, using a genetic algorithm and finite element analysis for the lay-up optimisation of postbuckling composite plates to delay the onset of mode-jump instabilities. A simple and novel approach for detecting modejumps is proposed, based on the RMS value of out-of-plane pseudo-velocities at a number of locations distributed over the postbuckling structure

Macroscopic limit of time-dependent density-functional theory for adiabatic local approximations of the exchange-correlation kernel

Time-dependent density-functional theory is a rather accurate and efficient way to compute electronic excitations for finite systems. However, in the macroscopic limit (systems of increasing size), for the usual adiabatic random-phase, local-density, or generalized-gradient approximations, one recovers the Kohn-Sham independent-particle picture, and thus the incorrect band gap. To clarify this trend, we investigate the macroscopic limit of the exchange-correlation kernel in such approximations by means of an algebraical analysis complemented with numerical studies of a one-dimensional tight-binding model. We link the failure to shift the Kohn-Sham spectrum of these approximate kernels to the fact that the corresponding operators in the transition space act only on a finite subspace.

Assessment of rolling resistance models in discrete element simulations

Particulate systems are of interest in many disciplines. They are often investigated using the discrete element method because of its capability to investigate particulate systems at the individual particle scale. To model the interaction between two particles and between a particle and a boundary, conventional discrete element models use springs and dampers in both the normal and tangential directions. The significance of particle rotation has been highlighted in both numerical studies and physical experiments. Several researchers have attempted to incorporate a rotational torque to account for the rolling resistance or rolling friction by developing different models. This paper presents a review of the commonly used models for rolling resistance and proposes a more general model. These models are classified into four categories according to their key characteristics. The robustness of these models in reproducing rolling resistance effects arising from different physical situations was assessed by using several benchmarking test cases. The proposed model can be seen to be more general and suitable for modelling problems involving both dynamic and pseudo-static regimes. An example simulation of the formation of a 2D sandpile is also shown. For simplicity, all formulations and examples are presented in 2D form, though the general conclusions are also applicable to 3D systems.

Wave Interaction with an Oscillating Wave Surge Converter, Part 1: Viscous Effects

Bottom hinged oscillating wave surge converters are known to be an efficient method of extracting power from ocean waves. The present work deals with experimental and numerical studies of wave interactions with an oscillating wave surge converter. It focuses on two aspects: (1) viscous effects on device performance under normal operating conditions; and (2) effects of slamming on device survivability under extreme conditions. Part I deals with the viscous effects while the extreme sea conditions will be presented in Part II. The numerical simulations are performed using the commercial CFD package ANSYS FLUENT. The comparison between numerical results and experimental measurements shows excellent agreement in terms of capturing local features of the flow as well as the dynamics of the device. A series of simulations is conducted with various wave conditions, flap configurations and model scales to investigate the viscous and scaling effects on the device. It is found that the diffraction/radiation effects dominate the device motion and that the viscous effects are negligible for wide flaps.

Hydrodynamic Loading on a Bottom Hinged Oscillating Wave Surge Converter

This paper presents experimental and numerical studies into the hydrodynamic loading of a bottom-hinged large buoyant flap held rigidly upright in waves. Possible applications and limitations of physical experiments, a linear potential analytical method, a linear potential numerical method, a weakly non-linear tool and RANS CFD simulations are discussed. Different domains of applicability of these research techniques are highlighted considering the validity of underlying assumptions, complexity of application and feasibility in terms of resources like time and computing power needed to obtain results. Conclusions are drawn regarding the future extension of the numerical methods to the case of a moving flap.

Hierarchical energy management system for multi-source multi-product microgrids

This paper proposes a hierarchical energy management system for multi-source multi-product (MSMP) microgrids. Traditional energy hub based scheduling method is combined with a hierarchical control structure to incorporate transient characteristics of natural gas flow and dynamics of energy converters in microgrids. The hierarchical EMS includes a supervisory control layer, an optimizing control layer, and an execution control layer. In order to efficiently accommodate the systems multi time-scale characteristics, the optimizing control layer is decomposed into three sub-layers: slow, medium and fast. Thermal, gas and electrical management systems are integrated into the slow, medium, and fast control layer, respectively. Compared with wind energy, solar energy is easier to integrate and more suitable for the microgrid environment, therefore, potential impacts of the hierarchical EMS on MSMP microgrids is investigated based on a building energy system integrating photovoltaic and microturbines. Numerical studies indicate that by using a hierarchical EMS, MSMP microgrids can be economically operated. Also, interactions among thermal, gas, and electrical system can be effectively managed.