Fast approximations of dynamic stability boundaries of slender curved structures

Curved beams and panels can often be found as structural components in aerospace, mechanical and civil engineering systems. When curved structures are subjected to dynamic loads, they are susceptible to dynamic instabilities especially dynamic snap-through buckling. The identification of the dynamic stability boundary that separate the non-snap and post-snap responses is hence necessary for the safe design of such structures, but typically requires extensive transient simulations that may lead to high computation cost. This paper proposes a scaling approach that reveals the similarities between dynamic snap-through boundaries of different structures. Such identified features can be directly used for fast approximations of dynamic stability boundaries of slender curved structures when their geometric parameters or boundary conditions are varied. The scaled dynamic stability boundaries of half-sine arches, parabolic arches and cylindrical panels are studied.     

Bayesian parameter identification in dynamic state space models using modified measurement equations

When Markov chain Monte Carlo (MCMC) samplers are used in problems of system parameter identification, one would face computational difficulties in dealing with large amount of measurement data and (or) low levels of measurement noise. Such exigencies are likely to occur in problems of parameter identification in dynamical systems when amount of vibratory measurement data and number of parameters to be identified could be large. In such cases, the posterior probability density function of the system parameters tends to have regions of narrow supports and a finite length MCMC chain is unlikely to cover pertinent regions. The present study proposes strategies based on modification of measurement equations and subsequent corrections, to alleviate this difficulty. This involves artificial enhancement of measurement noise, assimilation of transformed packets of measurements, and a global iteration strategy to improve the choice of prior models. Illustrative examples cover laboratory studies on a time variant dynamical system and a bending–torsion coupled, geometrically non-linear building frame under earthquake support motions.     

A mathematical approach to study fluid-coupled vibration of eccentric annular plates

A mathematical method is proposed to study fluid-coupled vibration of axisymmetric plate structures with asymmetries due to either imperfection or practical reasons, e.g. the weight reduction of structure, natural frequency shifting, and accessibility. The suggested approach makes use of the separation of variables to determine general solutions of the partial differential equation of the plate transverse displacement, whilst defining multiple polar coordinate systems, each of which offers its own formulation of the plate deformation with respect to its coordinate system. Moreover, closed-form geometric equations and the chain rule for determining derivatives are implemented to move from one coordinate system to the other in order to satisfy boundary conditions. The mode shapes of the vibrating plate in the dry condition are determined and in turn used in the Rayleigh–Ritz method to characterize vibrational properties of the fluid-coupled plate structure. While implementing such an energy method, the fluid motion is formulated employing the velocity potential and solved using the separation of variables. Fluid–structure interaction is also taken into account satisfying the compatibility condition on the fluid–plate​ interface. The developed methodology to predict natural frequencies has been validated by comparison with results obtained by a commercial finite element program. It is also found that the eccentricity tends to reduce natural frequencies of the fluid-coupled system for the lower serial mode, but increases them for the higher serial modes regardless of the presence of liquid.     

Parameter identification of dynamic models using a Bayes approach

ＩｎｔｒｏｄｕｃｔｉｏｎＡｃｏｍｍｏｎｐｒｏｂｌｅｍｉｎｍａｔｈｅｍａｔｉｃａｌｍｏｄｅｌｓｔｏｐｈｙｓｉｃａｌｐｈｅｎｏｍｅｎａｉｓｔｈｅｅｓｔｉｍａｔｉｏｎｏｆｍｏｄｅｌｐａｒａｍｅｔｅｒｓｆｒｏｍｏｂｓｅｒｖｅｄｄａｔａ .Ｉｎｒｅｃｅｎｔｙｅａｒｓｐａｒａｍｅｔｅｒｉｄｅｎｔｉｆｉｃａｔｉｏｎｍｅｔｈｏｄｓｈａｖｅｂｅｇｕｎｔｏｏｆｆｅｒａｖｅｒｙｐｏｗｅｒｆｕｌｂｒｉｄｇｅｂｅｔｗｅｅｎｅｘｐｅｒｉｍｅｎｔａｌａｎｄａｎａｌｙｔｉｃａｌｗｏｒｋ .Ｂｅｆｏｒｅｐｒｏｃｅｅｄｉｎｇｏｎｔｈｅ…     

A Lagrangian approach for the coupled simulation of fixed net structures in a Eulerian fluid model

A Lagrangian approach for the coupled numerical simulation of fixed net structures and fluid flow is derived. The model is based on solving the Reynolds-averaged Navier–Stokes equations in a Eulerian fluid domain. The equations include disturbances to account for the presence of the net. For this purpose, forces on the net are calculated using a screen force model and are distributed on Lagrangian points to represent the geometry of the net. In comparison to previous approaches based on porous media representations, the new model includes a more physical derivation and simplifies the necessary numerical procedure. Hence, it is also suitable for arbitrary geometries and large scale simulations. An extensive validation section provides insight into the performance of the new model. It includes the simulation of steady currents through single and multiple fixed net panels and cages, and wave propagation through a net panel. Different solidities, inflow velocities and angles of attack are considered. The comparison of loads on and velocity reductions behind the net with available measurements indicates superior performance of the proposed model over existing approaches for a wide range of applications.     

A mixed lumped stress–displacement approach to the elastic problem of masonry walls

We present a novel approach to the elastic problem of masonry walls, which generalizes the lumped stress method presented in [Fraternali, 2001], [Fraternali, 2007] and [Fraternali, 2010] and Fraternali et al. (2002). The generalization consists of a mixed lumped stress-displacement approach to the elastic problem of a wall that incorporates no-tension elements. Such an approach depends on the nodal values of the Airy stress function and the displacements of selected (“pivot”) nodes. The latter coincide with inter-element and boundary nodes. The mixed lumped stress-displacement method can be conveniently coupled with standard finite element and boundary element approaches. Numerical applications dealing with recurrent structural elements are given, showing that such a method is able to capture some essential features of the actual response of masonry constructions.     

Measurements of fluid fluctuations around an oscillating nuclear fuel assembly

In this paper, dynamic measurements of fluid velocity in the by-passes of a test-section representing a nuclear fuel assembly are presented. The test-section was designed to identify stiffness, damping and mass coefficients of a fuel assembly under axial flow, and previous studies have shown that the by-passes have an influence on the identified coefficients. The results presented in this paper show that the motion of the fuel assembly induces fluctuations in the axial fluid velocity in the by-passes. These fluctuations depend on the excitation frequency and position. A delay has been observed between the fuel assembly displacement and the fluid velocity fluctuations. The delay decreases when the axial velocity increases which means that it is a convection driven phenomenon.     

Aeroelastic study of flexible flapping wings by a coupled lattice Boltzmann-finite element approach with immersed boundary method

In this paper, the behavior of two-dimensional symmetric flapping wings moving in a viscous fluid is investigated. Harmonic motion is applied to idealize flying organisms with flexible wings and extensive testing is carried out to investigate the resultant flight behavior related to the ability to take-off or accelerate the flapping wing system away from a starting location. Special attention is paid to analyze the effect of the main mechanical parameters, as well as the effect of lateral wind on flight performances. Moreover, aiming to investigate the possible benefits of flying in flocks, a couple of synchronously flapping wings is considered in addition to the single arrangement. The numerical simulations are performed by solving the fluid–structure interaction problem through a strongly coupled partitioned approach. Fluid dynamics are modeled at the mesoscopic scale by the lattice Boltzmann method. The resulting macroscopic quantities are derived, as usual, based on the statistical molecular-level interpretation.Wings are modeled by geometrically nonlinear, elastic beam finite elements and structure dynamics is solved by the time discontinuous Galerkin method. Fluid–structure interface conditions are handled using the immersed boundary method. The resultant numerical approach combines simplicity and high computational efficiency. A Monte Carlo simulation strategy is employed to characterize the flight behavior subjected to lateral wind. Various scenarios are discussed.     

Shock response of a system of two submerged co-axial cylindrical shells coupled by the inter-shell fluid

The shock response of a submerged system consisting of two co-axial cylindrical shells coupled with the fluid filling the inter-shell space is considered. The shock–structure interaction is modeled using a semi-analytical methodology based on the use of the classical apparatus of mathematical physics. Both the fluid and structural dynamics of the interaction is addressed, with special attention paid to the interplay between the two. It is demonstrated that the wave effects due to multiple reflections of the pressure waves travelling in the inter-shell fluid to a large degree determine the structural dynamics of the system, but have a more pronounced effect on the outer shell than on the inner one. It is also established that the effect of changing the thickness of the outer shell on the stress–strain state of the inner shell is incomparably more pronounced than vice versa. The investigation culminates with the results of a parametric study of the overall peak stress in the system, an example of utilizing the approach developed based on the introduced model and aiming at facilitating structural optimization of industrial systems at the pre-design stage in the context of shock resistance.     

Efficient methods free of irregular frequencies in wave and solid/porous structure interactions

The method of boundary integral equation is widely applied to compute and analyze wave–structure interactions in marine and offshore engineering, and the application is also seen in marine aquaculture to deal with waves and porous structure interactions. The application of the Fredholm integral equation of the second kind together with the free-surface Green function for a surface-piercing body suffers from irregular frequencies which may be confused with resonance peaks. A simple and efficient method to remove irregular frequencies in the wave–structure interactions is developed via enforcing null potential (and horizontal derivatives) on discrete points on the interior water-plane area and is referred to as overdetermined integral equations (and enhanced overdetermined integral equations), respectively. Structures with solid surface, porous surface and their blending are considered, and numerical results demonstrate the effectiveness of this method. In contrast to extended integral equations, the overdetermined integral equations are easy to implement and more time-efficient.     

A method for the identification of dynamic constraint parameters in multi-supported flexible structures

In this paper, a new method for identifying the dynamical parameters of local constraining supports such as mass, stiffness, and damping was developed through combining the measured frequency transfer functions and structural modification techniques. Since measurement noise often leads to erroneous identifications, regularization techniques have been implemented to reduce noise amplification in the inverse problem. The developed technique has been validated by numerical tests on a multi-supported flexible structure, which can be seen as an idealized electricity generator rotor shaft. The results are satisfactory for noise-free data as well as under realistic noise levels. The sensitivity of the identified support features to noise levels is asserted through a parametric study     

Dynamic response of a rotating disk submerged and confined. Influence of the axial gap

In this paper, the natural frequencies and mode shapes of a rotating disk submerged and totally confined inside a rigid casing, have been obtained. These have been calculated analytically, numerically and experimentally for different axial gaps disk-casing. A simplified analytical model to analyse the dynamic response of a rotating disk submerged and confined, that has been used and validated in previous researches, is used in this case, generalised for arbitrary axial gaps disk-casing. To use this model, it is necessary to know the averaged rotating speed of the flow with respect to the disk. This parameter is obtained after an analytical discussion of the motion of the flow inside the casing where the disk rotates, and by means of CFD simulations for different axial positions of the disk. The natural frequencies of the rotating disk for the different axial confinements can be calculated following this method. A Finite Element Model has been built up to obtain the natural frequencies by means of computational simulation. The relative velocity of the flow with respect to the disk is also introduced in the simulation model in order to estimate the natural frequencies of the rotating disk. Experimental tests have been performed with a rotating disk test rig. A thin stainless steel disk (thickness of 8 mm, (h/r<5%) and mass of 7.6 kg) rotates inside a rigid casing. The position of the disk can be adjusted at several axial gaps disk-casing. A piezoelectric patch (PZT) attached on the rotating disk is used to excite the structure. Several miniature and submergible accelerometers have measured the response from the rotating frame. Excitation and measured signals are transmitted from the rotating to the stationary frame through a slip ring system. Experimental results are contrasted with the results obtained by the analytical and numerical model. Thereby, the influence of the axial gap disk-casing on the natural frequencies of a rotating disk totally confined and surrounded by a heavy fluid is determined.     

A simplified solution for piled-raft foundation analysis by using the two-phase approach

In common practice, the pile–soil–raft interaction still remains a challenging problem in the analysis of piled-raft foundations. In the present study, a simplified analytical approach is introduced to analyze a vertically-loaded piled-raft foundation by using a developed homogenization technique called the two-phase approach. In spite of classical and simplified methods in the literature, the proposed method considers the pile–soil interaction. The other major advantage is the ability to predict the axial pile load along the pile length. The problem is solved in the domain of elasticity and simple closed-form solutions are presented for the prediction of the settlement and the pile load sharing of a piled raft as well as the pile's axial force distribution along its length. The applicability of the proposed method is validated by considering case studies and field measurements. A comparison of the results indicates that the method can be utilized safely in a proper, quick, and effective manner with the least computational effort in comparison with sophisticated numerical approaches. The raft settlement can be accurately predicted while the pile load sharing might be over/under estimated. A parametric study is also carried out to investigate the response of piled-raft foundations including the influence of the parameters of the soil and the geometric characteristics of the piles.     

一种桁架结构损伤识别的柔度阵法

利用试验获得的一阶模态参数,提出了一种桁架结构损伤识别的柔度阵法,应用有限元方法柔度阵,建立结构振动特征方程,结构损伤后,引起柔度阵发生改变,从结构振动特征方程出发,对柔度矩阵做关于结构物理参数变化量的一阶泰勒展开,可以确定结构物理参数的变化量,识别结构损伤部位及损伤程度,通过一个桁架结构损伤识别的数值模拟证明了该方法的有效性。     

A model identification method of vibrating structures from incomplete modal information

ＡＭＯＤＥＬＩＤＥＮＴＩＦＩＣＡＴＩＯＮＭＥＴＨＯＤＯＦＶＩＢＲＡＴＩＮＧＳＴＲＵＣＴＵＲＥＳＦＲＯＭＩＮＣＯＭＰＬＥＴＥＭＯＤＡＬＩＮＦＯＲＭＡＴＩＯＮＺｈｅｎｇＸｉａｏｐｉｎｇ（郑小平）ＹａｏＺｈｅｎｈａｎ（姚振汉）ＱｕＳｈｉｓｈｅｎｇ（蘧时胜）...     

基于遗传退火混合算法的弹性地基上框架结构参数识别研究

对传统的简单遗传算法(GA)进行了改进,融合模拟退火技术(SA)的思想,建立了遗传模拟退火算法(GASA)的串行结构.GA采用群体并行搜索,通过概率意义下基于"优胜劣汰"思想的群体遗传操作来实现优化.SA采用串行优化结构,赋予搜索过程一种时变最终趋于零的概率突跳性,避免局部极小并最终趋于全局最优.两者的结合提高了遗传算法的全局搜索能力.本文对一实验室中弹性地基上框架结构进行了逐层模态实验研究,得到了四种工况下的模态频率和振型.首先对利用GASA算法对退火参数进行了优选,SA部分中的退温参数g和扰动幅度参数η对搜索效率及全局搜索能力具有重要的影响;然后对四种工况下混凝土的弹性模量和地基的动剪模量进行了识别,并与灵敏方法识别结果进行了对比,得到了结构物理参数随着结构浇注层数的增加而上升的规律,识别得到的弹性模量比回弹法结果偏大,与结构的静模量和动模量的区别有关.以上方法及其应用对于结构的健康监控具有现实的意义.