Dynamic elastoplastic analysis of 3-D structures by the domain/boundary element method

The main objective of this paper is to present a general three-dimensional boundary element methodology for solving transient dynamic elastoplastic problems. The elastostatic fundamental solution is used in writing the integral representation and this creates in addition to the surface integrals, volume integrals due to inertia and inelasticity. Thus, an interior discretization in addition to the usual surface discretization is necessary. Isoparametric linear quadrilateral elements are used for the surface discretization and isoparametric linear hexahedra for the interior discretization. Advanced numerical integration techniques for singular and nearly singular integrals are employed. Houbolt's step-by-step numerical time integration algorithm is used to provide the dynamic response. Numerical examples are presented to illustrate the method and demonstrate its accuracy.     

Computations of Explosive Boiling in Microgravity

Dynamics of the explosive growth of a vapor bubble in microgravity is investigated by direct numerical simulation. A front tracking/finite difference technique is used to solve for the velocity and the temperature field in both phases and to account for inertia, viscosity, and surface deformation. The method is validated by comparison of the numerical results with the available analytical formulations such as the evaporation of a one-dimensional liquid/vapor interface, frequency of oscillations of capillary waves, and other numerical results. Evolution of a three-dimensional vapor nucleus during explosive boiling is followed and a fine scale structure similar to experimental results is observed. Two-dimensional simulations yield a similar qualitative instability growth.     

A Lattice BGK Scheme with General Propagation

The standard Lattice BGK (LBGK) scheme often encounters numerical instability in simulation of fluid flow with small kinematic viscosity or as the nondimensional relaxation time is close to 0.5. In this paper, based on a time-splitting scheme for the Boltzmann equation with discrete velocities, a new LBGK scheme with general propagation step is proposed to address this problem. In this model, two free parameters are introduced into the propagation step, which can be adjusted to obtain a small kinematic viscosity and improved numerical stability as well. Numerical simulations of the two-dimensional Taylor vortex and the unsteady Womersley flow are carried out to test the kinematic viscosity, numerical diffusion, and numerical stability of the proposed scheme.     

Asynchronous relaxation algorithms for optimal control problems

This work deals with the optimal regulation of a thermal process for which we propose a parallel asynchronous algorithm of relaxation and we study the behaviour of this method Numerical experimentations allow on the one hand to compare sequential relaxation algorithm with other classical methods and on the other hand to present simulations results of parallel asynchronous relaxation runnings.     

Spectral element predictions of die-swell for Oldroyd-B fluids

This paper is concerned with two dimensional numerical simulations of plane extrusion of a viscoelastic fluid. The fluid is modelled using the Oldroyd-B and UCM constitutive equations. The problem is discretized using the spectral element method and the free surface is evolved using a Arbitrary Lagrangian Eulerian (ALE) technique. Numerical simulations are performed over an wide range of parameters. The influence of Weissenberg number and viscosity ratio on the swelling ratio and exit pressure loss are investigated and comparisons with previous results in the literature are presented. The range of validity of Tanner’s theory is investigated.     

Adaptive variable structure set-point control of underactuatedrobots

Control of underactuated mechanical systems (robots) represents an important class of control problem. In this correspondence, a model-based adaptive variable structure control scheme is introduced, where the uncertainty bounds only depend on the inertia parameters of the system. Global asymptotic stability is established in the Lyapunov sense. Numerical simulations are conducted to validate the theoretical analysis     

Microfluidic two-phase interactions under variable liquid to cross-flow gas momentum flux ratios

Volume of fluid (VOF) and large eddy simulations (LES) are coupled to investigate the microfluidic two-phase interactions during the liquid emergence into the cross-flow gas in a super-hydrophobic micro-channel. Spatio-temporal evolution of the gas/liquid interface is presented for nine different cases of the liquid to gas momentum flux ratios, gas/liquid Reynolds numbers and gas/liquid Weber numbers. With increased momentum of the gas flow, the liquid topology is found deflected towards the downstream. Under variable gas resistance effects, the liquid flow emerging through the square pore may or may not develop a circular cross-section governed by the axis-switching phenomenon. At strong gas inertia, vortex shedding in the downstream of the liquid generates vorticular ligaments in the wake region. Shearing effects on the liquid surface are increased at higher liquid injection velocities and/or gas densities. Depending on the competing effects of the viscous diffusion versus gas/liquid inertia, different combinations of the interactions among the three building blocks of the fluid flow problems (boundary layer, shear layer and wake) are described in microfluidics scales. The complexity of the liquid topology is found correlated with the occurrence of the phenomena such as the Kelvin–Helmholtz (KH) instability, the horseshoe vortex system, stationary/shedding vortices in the wake of the liquid topology as well as their interaction with the micro-channel wall boundary layers.     

Two-dimensional model variability in thermal inertia surveys

Calibration of thermal inertia surveys requires the production of a numerical model where thermal inertia is expressed as a function of the diurnal ground surface temperature range and albedo. The model is calculated for a given set of local meteorological and surface conditions which must be assumed constant over the whole of the surveyed area. This paper reports the results of an investigation which estimates the magnitudes of thermal inertia errors as a function of the departure of the local conditions from those assumed. The parameters under investigation are wind speed, average air temperature, air-temperature fluctuations, surface roughness, slope, and changes in the thermal inertia profile.     

Local penalty methods for flows interacting with moving solids at high Reynolds numbers

An original numerical modelling of multiphase flows interacting with solids in unsteady regimes is presented. Based on the generalized Navier-Stokes equations for multiphase flows and Volume of Fluid (VOF) formulations, an Uzawa minimization algorithm is implemented for the treatment of incompressibility and solid constraints. Augmented Lagrangian terms are added in the momentum equations to speed the convergence of the iterative solver. Defining a priori the penalty parameters which are dedicated to incompressibility and solid constraints is difficult, or impossible, as soon as the flow involves more than one phase and inertia becomes predominant compared to viscous and gravity forces. The Lagrangian penalty terms are calculated automatically according to an original local estimate of the various physical contributions. Numerical validations have been carried out for single particle settling in confined media and viscous flow through a fixed Cubic Faced Centered array. A very good agreement is obtained between experimental, theoretical and numerical results. Extension to unsteady free surface flow interacting with particles is illustrated with the simulation of a dam break flow over moving obstacles.     

Nonsingular terminal sliding mode based finite-time control for spacecraft attitude tracking

This paper studies finite-time attitude tracking control problem of a rigid spacecraft system with external disturbances and inertia uncertainties. Firstly, a new finite-time attitude tracking control law is designed using nonsingular terminal sliding mode concepts. In the absence and presence of external disturbances and inertia uncertainties, this controller can drive the attitude and angular velocity tracking errors to reach zero in finite time. Secondly, a finite-time disturbance observer is introduced to estimate the disturbance, and a composite controller is developed which consists of a feedback control based on nonsingular terminal sliding mode method and compensation term based on finite-time disturbance observer. Finite-time convergence of attitude tracking errors and the stability of the closed-loop system is ensured by the Lyapunov approach. Numerical simulations on attitude control of spacecraft are also given to demonstrate the performance of the proposed controllers.     

小型无人直升机的姿态与高度自适应反步控制

针对小型无人直升机的姿态与高度控制问题, 本文提出了一种基于反步法的自适应控制策略. 具体而言, 首先对小型无人直升机的运动学模型进行了等效变换, 使系统中未知参数满足线性参数化条件, 然后应用反步法设计了包含主旋翼挥舞模型的姿态与高度自适应控制器, 并借助Lyapunov方法和芭芭拉定理对闭环系统的稳定性进行了严格的数学分析. 最后, 对该控制器的性能进行了仿真验证, 结果表明在直升机质量和惯性矩阵存在不确定性(未知)的情况下, 该控制算法依然能够取得良好的控制效果.     

Numerical Simulation of growth and collapse of a bubble induced by a pulsed microheater

A three-dimensional numerical analysis of the growth and collapse of a bubble on a microheater is presented. SIMULENT code, which solves the full Navier-Stokes equations with surface tension effects, is used in these simulations. A volume of fluid (VOF) interface tracking algorithm is used to track the evolution of the free surface flow. A one-dimensional heat conduction model is used to consider the energy transfer between the bubble and the surrounding liquid, as well as the temperature distribution in the liquid layer. Details of the velocity and pressure distribution in the liquid during the growth and collapse of the vapor bubble are obtained. Numerical results for the growth and the collapse of the bubbles are compared with those of experiments under similar conditions. Comparisons show that the volume evolution of the vapor bubble is well predicted by the numerical model.     

Stability of numerical procedures for a linear barotropic vorticity equation

The stability analysis of some commonly used numerical procedures for a finite difference linear barotropic vorticity equation is performed. Upper bounds of the time step to be used are given to maintain stable the numerical processes. In some special cases it is verified that these iterative procedures are block successive relaxation methods. The optimum process is an underrelaxation process. Numerical calculations support the analytical results.     

Inertia‐free attitude stabilization for flexible spacecraft with active vibration suppression

This paper addresses the inertia‐free attitude control problem for flexible spacecraft in the presence of inertia uncertainties, external disturbances, actuator faults, measurement errors, and input magnitude and rate constraints (MRCs). By analyzing the influence of external disturbances, faulty signals, and actual inertial matrix, a lumped disturbance is reconstructed to facilitate the controller design. Then, a new intermediate observer is developed to estimate the attitude and modal information and the lumped disturbance. The Lyapunov stability analysis shows that the developed controller can achieve the objectives of the attitude stabilization and vibration suppression with input MRCs. Finally, numerical simulations are performed to demonstrate the effectiveness and superiority of the proposed control method.     

Asymptotic-preserving schemes for unsteady flow simulations

Asymptotic preserving schemes are proposed for numerical simulation of unsteady flows in the relaxation framework. Using a splitting operator to treat the transport and the collision terms separately, we reconstruct a stable and accurate method that converges uniformly to the correct equilibrium. This convergence is also ensured when the relaxation time is unresolved (asymptotic preserving). Numerical results and comparisons are shown for the multidimensional Euler system of gas dynamics.     

Maximization of the fundamental eigenfrequency of micropolar solids through topology optimization

Aim of this work is the maximization of the fundamental eigenfrequency of 2D bodies made of micropolar (or Cosserat) materials using a topology optimization approach. A classical SIMP–like model is used to approximate the constitutive parameters of the micropolar medium. A suitable penalization is introduced for both the linear and the spin inertia of the material, to avoid the occurrence of undesired local modes. The robustness of the proposed procedure is investigated through numerical examples; the influence of the material parameters on the optimal material layouts is also discussed. The optimal layouts for Cosserat solids may differ significantly from the truss–like solutions typical of Cauchy solids, as the intrinsic flexural stiffness of the material can lead to curved beam-like material distributions. The numerical simulations show that the results are quite sensitive to the material characteristic length and the spin inertia.     

Starting jet flows in a three-dimensional channel with larynx-shaped constriction

A numerical model for the three-dimensional starting jet flow in a channel with a static larynx-shaped constriction is presented. Detailed resolution of this kind of jet flow is necessary in order to understand the complex coupling between flow and acoustics in the process of human phonation. The numerical model is based on the equation of continuity and the Navier–Stokes equations. The investigations are done with the open source CFD package OpenFOAM. Numerical simulations are performed for a square-sectioned channel geometry, which is constricted with a fixed shape conforming to the fully opened human glottis. Time-dependent inflow boundary conditions are applied in order to model transient glottal flow rates. The setup of the numerical simulations corresponds to the configuration of a model experiment in order to allow detailed validation. The numerical results are in good agreement with the experimental data, when the near-wall region in the glottal gap is adequately resolved by the numerical grid. The results illustrate the complex interactions between the jet flow and the surrounding vortices.     

基于改进粒子群算法的PID参数优化与仿真

提出了一种基于改进的粒子群优化（PSO）算法的PID控制器参数整定方法。该方法采用了PSO的惯性权值自适应调整机制和粒子种群的动态更新策略,用以加速优化算法的收敛和维持群体的多样性。与常规的PSO算法相比,该方法简单易行,更容易找到全局最优解,优化效率和性能明显提高。将该算法应用非最小相位、一阶滞后等系统的PID控制器参数的优化,能够使控制系统获得较好的动态特性和很强的鲁棒性。仿真实验表明了所提出算法的有效性和优越性。     

基于混沌序列的自适应粒子群优化算法

提出一种改进粒子群局部搜索能力的自适应优化算法。通过大量仿真试验,考察粒子平均速度和收敛性之间的关系,给出一种新的自适应调整权重策略。以粒子平均速度作为反馈信息,动态调整权重因子,控制粒子速度并使其沿理想速度曲线下降。在搜索过程中引入混沌序列以改进算法的局部搜索能力。对经典函数的测试结果表明,改进的混合算法通过微粒自适应更新机制确保了全局搜索性能和局部搜索性能的动态平衡,在稳定性和精度上均优于普通PSO算法。     

Composite scattering of an electrically large ship on a two-dimensional time-evolved sea surface

A time-varying composite scattering of a moving ship on a two-dimensional non-linear sea surface is modelled with the help of the Doppler spectrum in high-frequency bands. A non-linear sea surface is simulated to approximate a real sea environment with high-order terms of the solution to hydrodynamic equations. Numerical simulations are based on the principle of a quasi-stationary algorithm (QSA), which solves time domain problems with frequency domain methods. A four-path model is modified to calculate the composite scattering field, which is utilized for the analysis of the Doppler spectrum. With depth-buffer technology, self-shadowing of the complex physical model is eliminated. Correspondence of the numerical results with experimental data proves that the inclusion of the non-linear interaction term and wind-speed influence is reasonable. The simulated Doppler spectrum of the composite scattering shows a functional dependence of time-evolved scattering components on incidence direction and sea state.