Computers & Structures

Right ventricular dysfunction is one of the more common causes of heart failure in patients with congenital heart defects. Use of computer-assisted procedures is becoming more popular in clinical decision making process and computer-aided surgeries. A 3D in vivo MRI-based RV/LV combination model with fluid-structure interaction (FSI), RV-LV interaction, and RV-patch interaction was introduced to perform mechanical analysis for human right ventricle with potential clinical applications. Patient-specific RV/LV morphologies were acquired by using planar tagged MRI. The 3D RV/LV FSI model was solved using a commercial finite element package ADINA. Our results indicated that flow and stress/strain distributions in the right ventricle are closely related to RV morphology, material properties and blood pressure conditions. Patches with material properties better matching RV tissue properties and smaller size lead to better RV function recoveries. Computational RV volumes showed very good agreement with MRI data (error < 3%). More patient studies are needed to establish baseline database so that computational simulations can be used to replace empirical and often risky clinical experimentation to examine the efficiency and suitability of various reconstructive procedures in diseased hearts and optimal design can be found.

The flow-induced vibration of synthetic vocal fold models has been previously observed to be acoustically-coupled with upstream flow supply tubes. This phenomenon was investigated using a finite element model that included flow-structure-acoustic interactions. The length of the upstream duct was varied to explore the coupling between model vibration and subglottal acoustics. Incompressible and slightly compressible flow models were tested. The slightly compressible model exhibited acoustic coupling between fluid and solid domains in a manner consistent with experimental observations, whereas the incompressible model did not, showing the slightly compressible approach to be suitable for simulating acoustically-coupled vocal fold model flow-induced vibration.

Multi-physics right and left ventricle (RV/LV) fluid-structure interaction (FSI) models were introduced to perform mechanical stress analysis and evaluate the effect of patch materials on RV function. The FSI models included three different patch materials (Dacron scaffold, treated pericardium, and contracting myocardium), two-layer construction, fiber orientation, and active anisotropic material properties. The models were constructed based on cardiac magnetic resonance (CMR) images acquired from a patient with severe RV dilatation and solved by ADINA. Our results indicate that the patch model with contracting myocardium leads to decreased stress level in the patch area, improved RV function and patch area contractility.

Patients with repaired tetralogy of Fallot account for the majority of cases with late onset right ventricle (RV) failure. A new surgical procedure placing an elastic band in the right ventricle is proposed to improve RV function measured by ejection fraction. A multiphysics modeling approach is developed to combine cardiac magnetic resonance imaging, modeling, tissue engineering and mechanical testing to demonstrate feasibility of the new surgical procedure. Our modeling results indicated that the new surgical procedure has the potential to improve right ventricle ejection fraction by 2-7% which compared favorably with recently published drug trials to treat LV heart failure.

In the serosal cavities (e.g. pleural, pericardial) soft tissues slide against each other, lubricated by thin fluid. We used rotational devices to study the tribology of such tissues, which appear to exhibit mixed and hydrodynamic lubrication. To explore mechanism, we modeled the interaction of fluid and soft material in 3D using a simple cylindrical geometry with an uneven solid-fluid interface in rotation. Deformation of the solid, frictional force, and fluid thickness are presented as a function of applied rotational velocity, applied normal load and material properties. The results suggest that the deformation caused by hydrodynamic pressure leads to load-supporting behavior.

The flow-induced response of a membrane covering a fluid-filled cavity located in a section of a rigid-walled channel was explored using finite element analysis. The membrane was initially aligned with the channel wall and separated the channel fluid from the cavity fluid. As fluid flowed over the membrane-covered cavity, a streamwise-dependent transmural pressure gradient caused membrane deformation. This model has application to synthetic models of the vocal fold cover layer used in voice production research. In this paper, the model is introduced and responses of the channel flow, the membrane, and the cavity flow are summarized for a range of flow and membrane parameters. It is shown that for high values of cavity fluid viscosity, the intracavity pressure and the beam deflection both reached steady values. For combinations of low cavity viscosity and sufficiently large upstream pressures, large-amplitude membrane vibrations resulted. Asymmetric conditions were introduced by creating cavities on opposing sides of the channel and assigning different stiffness values to the two membranes. The asymmetry resulted in reduction in or cessation of vibration amplitude, depending on the degree of asymmetry, and in significant skewing of the downstream flow field.

A three-dimensional simulation of the formation of metachronal waves in rows of pulmonary cilia is presented. The cilia move in a two-layer fluid model. The fluid layer adjacent to the cilia bases is purely viscous while the tips of the cilia move through a viscoelastic fluid. An overlapping fixed-moving grid formulation is employed to capture the effect of the cilia on the surrounding fluid. In contrast with immersed boundary methods, this technique allows a natural enforcement of boundary conditions without the need for smoothing of singular force distributions. The fluid domains are discretized using a finite volume method. The 9 + 2 internal microtubule structure of an individual cilium is modeled using large-deflection, curved, finite-element beams. The microtubule skeleton is cross-linked to itself and to the cilium membrane through spring elements which model nexin links. The cilium membrane itself is considered to be elastic and subject to fluid stresses computed from the moving grid formulation as well as internal forces transmitted from the microtubule skeleton. A cilium is set into motion by the action of dynein molecules exerting forces between adjacent microtubules. Realistic models of the forces exerted by dynein molecules are extracted from measurements of observed cilia shapes.

Computational vocal fold models are often used to study the physics of voice production. In this paper the sensitivity of predicted vocal fold flow-induced vibration and resulting airflow patterns to several modeling selections is explored. The location of contact lines used to prevent mesh collapse and assumptions of symmetry were found to influence airflow patterns. However, these variables had relatively little effect on the vibratory response of the vocal fold model itself. Model motion was very sensitive to Poisson's ratio. The importance of these parameter sensitivities in the context of vocal fold modeling is discussed.

A new cell-vortex unstructured finite volume method for structural dynamics is assessed for simulations of structural dynamics in response to fluid motions. A robust implicit dual-time stepping method is employed to obtain time accurate solutions. The resulting system of algebraic equations is matrix-free and allows solid elements to include structure thickness, inertia, and structural stresses for accurate predictions of structural responses and stress distributions. The method is coupled with a fluid dynamics solver for fluid-structure interaction, providing a viable alternative to the finite element method for structural dynamics calculations. A mesh sensitivity test indicates that the finite volume method is at least of second-order accuracy. The method is validated by the problem of vortex-induced vibration of an elastic plate with different initial conditions and material properties. The results are in good agreement with existing numerical data and analytical solutions. The method is then applied to simulate a channel flow with an elastic wall. The effects of wall inertia and structural stresses on the fluid flow are investigated.

The governing equations for constrained multibody systems are formulated in a manner suitable for their automated, numerical development and solution. The closed loop problem of multibody chain systems is addressed. The governing equations are developed by modifying dynamical equations obtained from Lagrange's form of d'Alembert's principle. The modifications is based upon a solution of the constraint equations obtained through a zero eigenvalues theorem, is a contraction of the dynamical equations. For a system with n-generalized coordinates and m-constraint equations, the coefficients in the constraint equations may be viewed as constraint vectors in n-dimensional space. In this setting the system itself is free to move in the n-m directions which are orthogonal to the constraint vectors.

A Hybrid Big Bang–Big Crunch (HBB–BC) optimization algorithm is employed for optimal design of truss structures. HBB–BC is compared to Big Bang–Big Crunch (BB–BC) method and other optimization methods including Genetic Algorithm, Ant Colony Optimization, Particle Swarm Optimization and Harmony Search. Numerical results demonstrate the efficiency and robustness of the HBB–BC method compared to other heuristic algorithms.

This paper describes procedures for design sensitivity analysis and optimization of nonlinear structural systems with the computer program ADINA. Formulation of the structural optimization problem, design sensitivity analysis with nonlinear response using incremental finite element procedures, and two strategies to use ADINA for design optimization are described. A database and a modem database management system are used to couple ADINA with design sensitivity analysis and optimization modules. Comparison of optimum designs with linear and nonlinear structural responses for trusses with material and geometric nonlincarities are given. More complex structures can be optimized with the developed procedures to fully exploit the capabilities of ADINA.

This paper describes a finite element algorithm developed for analysis of nonlinear viscoelastic materials. A single integral constitutive law proposed by Schapery is used to describe viscoelastic material behavior. Work leading to this paper focused on adhesives, but the FE formulation is general and readily extended to structural systems other than plane strain, plane stress and axisymmetric analysis as described. Cartesian strain components are written in terms of current and past stress states. Thus strains are conveniently defined by a stress operator that includes instantaneous compliance and hereditary strain which is updated by recursive computation. Equilibrium at each time step is insured with a modified Newton Raphson technique, incorporating convergence acceleration. Verification analyses show excellent agreement with experimental data for FM-73 adhesive systems. A plane strain analysis of a butt joint is included.

We consider Papcovitch–Neuber (PN) solution to the Navier equation, comprising only the vector potential, and develop a new displacement-based 14-node brick element. We assume PN solution in polynomial form. We impose constraints on unknown coefficients of the polynomials such that the element correctly represents linear stress fields. To validate the performance of the new element which we call PN5X1, we conduct several pathological tests available in the literature. PN5X1 predicts, as anticipated, both stresses and displacements accurately at every point inside the elastic continuum for linear stress fields.

In this paper, we present a new quadrilateral discrete Kirchhoff flat shell element (called DKQ16) with 16 degrees of freedom (three displacements U, V, W at each corner, a rotation θs at each mid-side) for the linear analysis of plates and shells. This new element is formulated on the basis of the so-called rational element method proposed by Zhong et al. [Zhong WX, Zeng J. J Computat Struct Mech Appl 1996;13:1–8 [in Chinese]]. In this new formulation, the rational quadrilateral plane element (RQ4) is employed for the membrane part and a new discrete Kirchhoff plate element DKQ8 proposed by the present authors [Batoz JL, Hammadi F, Zheng CL, Zhong WX, submitted for publication] is taken for the bending part. The DKQ16 element is one of the most simple quadrilateral flat shell elements. It can be combined with the DKT12 (or Morley) element. Numerical results for some typical problems demonstrate the overall good performance of the new shell element.

Recent advances in computational structural and fluid dynamics are discussed in reviews and reports. Topics addressed include fluid-structure interaction and aeroelasticity, CFD techniques for reacting flows, micromechanics, stability and eigenproblems, probabilistic methods and chaotic dynamics, and perturbation and spectral methods. Consideration is given to finite-element, finite-volume, and boundary-element methods; adaptive methods; parallel processing machines and applications; and visualization, mesh generation, and AI interfaces.

The paper describes a new four-noded quadrilateral shell element, called QUAD4, which is based on isoparametric principles with modifications which relax excessive constraints. The modifications include reduced order integration for shear terms, enforcement of curvature compatibility, and the augmentation of transverse shear flexibility to account for a deficiency in the bending strain energy. Practical features are discussed, including conversion to a nonplanar shape, coupling between bending and stretching, mass properties, and geometric stiffness. Experimental results are described which illustrate the accuracy and economy claimed for the element.

This paper presents an investigation into the hydraulics of regular ogee-profile spillways. The free-surfaces of the fluid for several flow heads as measured in the hydraulics laboratory are used as benchmarks. The finite element computational fluid dynamics software, ADINA, was used to predict the free surface over an ogee spillway and thus model the flow field. Since the actual flow is turbulent the k–ε flow model was used. For the cases considered in this paper, ADINA predicted reasonable free surface results that are consistent with general flow characteristics over spillways. The results are also in close agreement with measured free-surface profiles over the entire length of the spillway.

This paper assesses the global performance and the underlying assumptions of a recently developed one-dimensional model characterising the elastic lateral-torsional buckling behaviour of singly symmetric tapered thin-walled open beams, which is able to account for the influence of the pre-buckling deflections. A comparative study involving the critical load factors and buckling modes yielded by (i) the one-dimensional model and (ii) two-dimensional shell finite element analyses (reference results) is presented and discussed. The results concern I-section cantilevers and simply supported beams (i) with uniform or linearly tapered webs, (ii) equal or unequal uniform flanges and (iii) acted by point loads applied at the free end or mid-span sections, respectively. In general, the one-dimensional predictions are found to agree well with the shell finite element results. Some significant discrepancies are also recorded (for the shorter beams), which are due to the occurrence of relevant cross-section distortion or localised buckling phenomena.

Various levels of modelling of reinforced concrete beams are presented and compared. Their complementary character is outlined in some simple cases. Further studies are proposed.

The geometrically nonlinear free vibration of symmetrically laminated rectangular plates (1st and higher modes) and the higher mode of an isotropic plate with fully clamped boundary conditions is studied, using the hierarchical finite element method (HFEM). The relationships between the vibration amplitude ratio and nonlinear frequencies, and between the vibration amplitude ratio and nonlinear mode shapes are discussed. The mode bending stresses and membrane forces at large amplitude for laminated plates are presented. The comparison between the nonlinear frequency ratio calculated from this study and the one from a published paper is in good agreement. The large variation of in-plane membrane forces over the plate span for some of the laminated plates has been observed. This will definitely affect the application of Berger's hypothesis to the geometrically nonlinear analysis of these laminated plates. It has been found that the geometrically nonlinear dynamic properties of laminated plates varies from plate to plate depending not only on the aspect ratio and boundary conditions, but also on the lamination and material properties of the lamina considered.

The interactive multidisciplinary aircraft design code was used for numerical simulation of Tupolev-204 airplane in-flight flutter tests. Dynamic structure loading is generated by symmetric and antisymmetric harmonic excitation of spoilers with smooth frequency sweep from 1 to 5 Hz. Major features of theoretical approach are described. Calculated and experimental results are compared. The influence of unsteady aerodynamic forces and onboard control system on dynamic loading of elastic structure is shown.

Development of a nonconforming eight to 26-node hexahedron for three-dimensional (3-D) thermal-elasto-plastic finite element analysis (FEA) is presented. The nonconforming element satisfies the MacNeal and Harder patch test and does not lock in plastic deformation, eliminating the need for selective reduced integration. The enhanced displacement and strain fields are fully consistent with a linear thermal strain field. In some cases this eliminates the need for isothermal elements in thermal-elasto-plastic FEA. Several cases are presented for validation purposes. In addition, a 3-D thermal-elasto-plastic analysis of a weld is presented, using selective reduced integration and nonconforming elements with both constant and linear thermal strain fields. The nonconforming elements show improved behaviour over selective reduced integration, but are found to be about 20% more expensive on a per iteration basis when all the elements are replaced. However, for the problem considered, the nonconforming elements provide about 2.5 times more degrees of freedom (DOF). Adaptive processes could reduce this by utilizing nonconforming elements only where they are required.