Lagrangian block diagonalization

Relative equilibria, i.e., steady motions associated to specified group motions, are an important class of steady motions of Hamiltonian and Lagrangian systems with symmetry. Relative equilibria can be identified by means of a variational principle on the tangent space of the configuration manifold. We show that relative equilibria can also be found by means of a variational principle on the configuration manifold itself. Formal stability of a relative equilibrium corresponds to definiteness of the second variation of the energymomentum functional, which is a specified combination of the total energy and the group momentum, on an appropriate subspace. We decompose this subspace into three subspaces by means of the Legendre transformation and the group action and show that the second variation block diagonalizes with respect to these subspaces. The techniques employed here are a generalization of the reduced energy-momentum method of Simoet al. (1991), which applies only to simple mechanical systems, to a more general class of conservative systems, including systems on which the symmetry group does not act freely. We briefly discuss a generalization of a result due to Patrick (1990) that provides conditions under which formal stability implies nonlinear orbital stability. Several simple examples, including natural mechanical systems, are used to illustrate the block diagonalization procedure.     

A multifractal model for linking Lagrangian and Eulerian velocity structure functions

A multifractal model is developed to connect the Lagrangian multifractal dimensions with their Eulerian counterparts. We propose that the characteristic time scale of a Lagrangian quantity should be the Lagrangian time scale, and it should not be the Eulerian time scale which was widely used in previous studies on Lagrangian statistics. Using the present model, we can obtain the scaling exponents of Lagrangian velocity structure functions from the existing data or models of scaling exponents of Eulerian velocity structure functions. This model is validated by comparing its prediction with the results of experiments, direct numerical simulations, and the previous theoretical models. The comparison shows that the proposed model can better predict the scaling exponents of Lagrangian velocity structure functions, especially for orders larger than 6.     

A Lagrangian Discontinuous Galerkin‐type method on unstructured meshes to solve hydrodynamics problems

This paper concerns a new Lagrangian Discontinuous Galerkin‐type method to solve 2D fluid flows on unstructured meshes. By using a basis of Bernstein polynomials of degree m in each triangle, we define a diffusion process which ensures positivity and stability of the scheme. The discontinuities of the physical variables at the interfaces between cells are solved with an acoustic Riemann solver. A remeshing/remapping process is performed with a particle method: the remapping is locally conservative and its accuracy can be adapted to the accuracy of the numerical method. Copyright © 2004 John Wiley & Sons, Ltd.     

Singularity-free augmented Lagrangian algorithms for constrained multibody dynamics

After a general review of the methods currently available for the dynamics of constrained multibody systems in the context of numerical efficiency and ability to solve the differential equations of motion in singular positions, we examine the acceleration based augmented Lagrangian formulations, and propose a new one for holonomic and non-holonomic systems that is based on the canonical equations of Hamilton. This new one proves to be more stable and accurate that the acceleration based counterpart under repetitive singular positions. The proposed algorithms are numerically efficient, can use standard conditionally stable numerical integrators and do not fail in singular positions, as the classical formulations do. The reason for the numerical efficiency and better behavior under singularities relies on the fact that the leading matrix of the resultant system of ODEs is sparse, symmetric, positive definite, and its rank is independent of that of the Jacobian of the constraint equations. The latter fact makes the proposed method particularly suitable for singular configurations.     

Kronecker product and a new matrix form of Lagrangian equations with multipliers for constrained multibody systems

The automatic derivation of motion equations is an important problem of multibody system dynamics. Firstly, an overview of the matrix calculus related to Kronecker product of two matrices is presented. A new matrix form of Lagrangian equations with multipliers for constrained multibody systems is then developed to demonstrate the usefulness of Kronecker product of two matrices in the study of dynamics of multibody systems. Finally, the equations of motion of mechanisms are derived using the proposed matrix form of Lagrangian equations as application examples.     

A new method for the construction of a Lagrangian

In this paper, the necessary condition for the existence of a Lagrangian for the ordered direct analytic representation of a system of second-order ordinary differential equations on a connected domain is given. Under that condition, a new method for the construction of the Lagrangian is proposed. The domain of definition of the constructed Lagrangian might be diminished. Supported by China Doctoral Program Foundation of Institution of Higher Education and National Natural Science Foundation of China     

Control structure interaction in the nonlinear analysis of flexible mechanical systems

The effect of the control structure interaction on the feedforward control law as well as the dynamics of flexible mechanical systems is examined in this investigation. An inverse dynamics procedure is developed for the analysis of the dynamic motion of interconnected rigid and flexible bodies. This method is used to examine the effect of the elastic deformation on the driving forces in flexible mechanical systems. The driving forces are expressed in terms of the specified motion trajectories and the deformations of the elastic members. The system equations of motion are formulated using Lagrange's equation. A finite element discretization of the flexible bodies is used to define the deformation degrees of freedom. The algebraic constraint equations that describe the motion trajectories and joint constraints between adjacent bodies are adjoined to the system differential equations of motion using the vector of Lagrange multipliers. A unique displacement field is then identified by imposing an appropriate set of reference conditions. The effect of the nonlinear centrifugal and Coriolis forces that depend on the body displacements and velocities are taken into consideration. A direct numerical integration method coupled with a Newton-Raphson algorithm is used to solve the resulting nonlinear differential and algebraic equations of motion. The formulation obtained for the flexible mechanical system is compared with the rigid body dynamic formulation. The effect of the sampling time, number of vibration modes, the viscous damping, and the selection of the constrained modes are examined. The results presented in this numerical study demonstrate that the use of the driving forees obtained using the rigid body analysis can lead to a significant error when these forces are used as the feedforward control law for the flexible mechanical system. The analysis presented in this investigation differs significantly from previously published work in many ways. It includes the effect of the structural flexibility on the centrifugal and Coriolis forces, it accounts for all inertia nonlinearities resulting from the coupling between the rigid body and elastic displacements, it uses a precise definition of the equipollent systems of forces in flexible body dynamics, it demonstrates the use of general purpose multibody computer codes in the feedforward control of flexible mechanical systems, and it demonstrates numerically the effect of the selected set of constrained modes on the feedforward control law.     

Lagrangian formulation of continuum with internal long-range interactions

Based on a new definition of nonlocal variable, this paper establishes the Lagrangian formulation for continuum with internal long-range interactions. Distinguished from the existing theories, the nonlocal term in the Lagrangian formulation automatically satisfies the zero mean condition determined by the action and reaction law. By this formulation, elastic wave in a rod with the internal long-range interactions is investigated. The dispersion of the elastic wave is predicted.     

A nodal Godunov method for Lagrangian shock hydrodynamics on unstructured tetrahedral grids

We present a nodal Godunov method for Lagrangian shock hydrodynamics. The method is designed to operate on three‐dimensional unstructured grids composed of tetrahedral cells. A node‐centered finite element formulation avoids mesh stiffness, and an approximate Riemann solver in the fluid reference frame ensures a stable, upwind formulation. This choice leads to a non‐zero mass flux between control volumes, even though the mesh moves at the fluid velocity, but eliminates volume errors that arise due to the difference between the fluid velocity and the contact wave speed. A monotone piecewise linear reconstruction of primitive variables is used to compute interface unknowns and recover second‐order accuracy. The scheme has been tested on a variety of standard test problems and exhibits first‐order accuracy on shock problems and second‐order accuracy on smooth flows using meshes of up to O(10⁶) tetrahedra. Copyright © 2014 John Wiley & Sons, Ltd.     

An implicit Lagrangian lattice Boltzmann method for the compressible flows

In this paper, we propose a new Lagrangian lattice Boltzmann method (LBM) for simulating the compressible flows. The new scheme simulates fluid flows based on the displacement distribution functions. The compressible flows, such as shock waves and contact discontinuities are modelled by using Lagrangian LBM. In this model, we select the element in the Lagrangian coordinate to satisfy the basic fluid laws. This model is a simpler version than the corresponding Eulerian coordinates, because the convection term of the Euler equations disappears. The numerical simulations conform to classical results. Copyright © 2006 John Wiley & Sons, Ltd.     

基于最小二乘法的反应流拉格朗日分析在冲击起爆中的研究

现有反应流拉格朗日分析方法在已知粒子速度情况下,求解方法仍有不足。本文针对这一情况,将反解法与自洽检验法相结合,提出了基于最小二乘法的反应流拉格朗日反解法,该方法的理论精度能够实现应力沿路径线的 M阶导数为零（M为迹线数）,并且该方法能够满足自洽检验法。为了验证该方法的有效性,利用该方法对一组小隔板冲击起爆试验数据进行了处理,对比了本文方法、试验处理以及传统的反解法处理结果,表明该方法不仅可以适当减小迹线上的偶然误差,还能够使得迹线函数更好地反映各物理量沿迹线的变化性态。     

Robust synchronization of nonlinear SISO systems using sliding mode control

We present a robust algorithm to synchronize two different single-input/single-output (SISO) nonlinear systems connected in a master/slave scheme, where the relative degree of the master system (r _m) is greater than or equal to the relative degree of the slave (r _s). The sliding mode control technique is used to design the coupling signal. This discontinuous controller renders the closed-loop system robust with respect to matched bounded disturbances. The synchronization objective is to match the first r _s normal coordinates. Depending on the characteristics of the involved systems, the closed loop system can display full or partial, identical or generalized synchronization. The performance of the proposed controlled synchronization is illustrated numerically and experimentally.     

Interaction between torsional and lateral vibrations in flexible rotor systems with discontinuous friction

In this paper, we analyze the interaction between friction-induced vibrations and self-sustained lateral vibrations caused by a mass-unbalance in an experimental rotor dynamic setup. This study is performed on the level of both numerical and experimental bifurcation analyses. Numerical analyses show that two types of torsional vibrations can appear: friction-induced torsional vibrations and torsional vibrations due to the coupling between torsional and lateral dynamics in the system. Moreover, both the numerical and experimental results show that a higher level of mass-unbalance, which generally increases the lateral vibrations, can have a stabilizing effect on the torsional dynamics, i.e. friction-induced limit cycling can disappear. Both types of analysis provide insight in the fundamental mechanisms causing self-sustained oscillations in rotor systems with flexibility, mass-unbalance and discontinuous friction which support the design of such flexible rotor systems.     

利用欧拉-拉格朗日方法预测层流泡状流的含气率分布

利用欧拉-拉格朗日方法，提出了用于预测竖直管道内绝热层流泡状流中含气率分布的三维模型。该模型能够跟踪单独的气泡轨迹，从而获取更多的界面力信息；同时气泡尺寸可以作为参数之一引入模型，使含气率分布的计算更为方便。模型中分析了绝热层流泡状流中气泡的各项受力表达式，建立了两种描述方法下的气液两相间的耦合关系。利用现有实验数据对模型进行的检验表明，该模型能够预测一定尺寸范围内气泡的分布；气泡径向分布主要取决于气泡所受侧向提升力。对于更大尺寸的气泡，气泡变形和气泡尾迹与当地流场间相互作用将对侧向提升力产生很大影响。     

A 3D staggered Lagrangian scheme for ideal magnetohydrodynamics on unstructured meshes

In this paper, we propose a 3D staggered Lagrangian scheme for the ideal magnetohydrodynamics (MHD) on unstructured meshes. All the thermal variables and the magnetic induction are defined in the cell centers while the fluid velocity is located at the nodes. The meshes are compatibly discretized to ensure the geometric conservation laws in Lagrangian computation by the classical subcell method, then the momentum equation is discretized using the subcell forces and the specific internal energy equation is obtained by the total energy conservation. Invoking the Galilean invariance, magnetic flux conservation, and the thermodynamic consistency, the expressions of subcell force as well as the cell-centered velocity are derived. Besides, the magnetic divergence-free constraint is fulfilled by a projection method after each time step. Various numerical tests are presented to assert the robustness and accuracy of our scheme.