Michael L. Accorsi

Parachute fluid-structure interactions: 3-D computation

Abstract We present a parallel computational strategy for carrying out 3-D simulations of parachute fluid–structure interaction (FSI), and apply this strategy to a round parachute. The strategy uses a stabilized space-time finite element formulation for the fluid dynamics (FD), and a finite element formulation derived from the principle of virtual work for the structural dynamics (SD). The fluid–structure coupling is implemented over compatible surface meshes in the SD and FD meshes. Large deformations of the structure are handled in the FD mesh by using an automatic mesh moving scheme with remeshing as needed.

Finite element analysis of membrane wrinkling

New results are presented for the nite element analysis of wrinkling in curved elastic membranes undergoing large deformation. Concise continuum level governing equations are derived in which singularities are eliminated. A simple and e cient algorithm with robust convergence properties is established to nd the real strain and stress of the wrinkled membrane for Hookean materials. The continuum theory is implemented into a nite element code. Explicit formulas for the internal forces and the tangent sti ness matrix are derived. Numerical examples are presented that demonstrate the e ectiveness of the new theory for predicting wrinkling in membranes undergoing large deformation. Copyright ? 2001 John Wiley & Sons, Ltd.

Fluid-Structure Interactions of a Round Parachute: Modeling and Simulation Techniques

A parallel computational technique is presented for carrying out three-dimensional simulations of parachute fluid-structure interactions, and this technique is applied to simulations of airdrop performance and control phenomena in terminal descent. The technique uses a stabilized space-time formulation of the time-dependent, three-dimensional Navier-Stokes equations of incompressible flows for the fluid dynamics part. Turbulent features of the flow are accounted for by using a zero-equation turbulence model. A finite element formulation derived from the principle of virtual work is used for the parachute structural dynamics. The parachute is represented as a cable-membrane tension structure. Coupling of the fluid dynamics with the structural dynamics is implemented over the fluid-structure interface, which is the parachute canopy surface. Large deformations of the structure require that the fluid dynamics mesh is updated at every time step, and this is accomplished with an automatic mesh-moving method. The parachute used in the application presented here is a standard U.S. Army personnel parachute

Bounds on the overall elastic and instantaneous elastoplastic moduli of periodic composites

Abstract Bounds on the overall elastic and instantaneous elastoplastic moduli of composites with periodic microstructures are found using the extremum principles of Hashin and Shtrikman (1962) and the analytic solution of Nemat-Nasser et al. (1982). The bounds contain terms which depend on the geometric properties of the constituent materials and the corresponding interaction effects. Examples are presented for composites whose constituents are elastically isotropic having isotropic and kinematic plastic hardening responses.

Structural Modeling of Parachute Dynamics

The dynamic behavior of parachute systems is an extremely complex phenomenon characterized by nonlinear, time-dependent coupling between the parachute and surrounding airflow, large shape changes in the parachute, and three-dimensional unconstrained motion of the parachute through the fluid medium. Because of these complexities, the design of parachutes has traditionally been performed using a semi-empirical approach. This approach to design is time consuming and expensive. The ability to perform computer simulations of parachute dynamics would significantly improve the design process and ultimately reduce the cost of parachute system development. The finite element formulation for a structural model capable of simulating parachute dynamics is presented. Explicit expressions are given for structural mass and stiffness matrices and internal and external force vectors. Algorithms for solution of the nonlinear dynamic response are also given. The capabilities of the structural model are demonstrated by three example problems. In these examples, the effect of the surrounding airflow is approximated by prescribing the canopy pressure and by applying cable and payload drag forces on the structural model. The examples demonstrate the ability to simulate three-dimensional unconstrained dynamics beginning with an unstressed folded configuration corresponding to the parachute cut pattern. The examples include simulations of the inflation, terminal descent, and control phases.

Automatic shape optimization of three-dimensional shell structures with large shape changes

Abstract A finite element-based shape optimization program has been developed for three-dimensional shell structures which allows for large shape changes. The new shape optimization program has been achieved by linking together adaptive mesh generation, substructuring, and linear and nonlinear optimization techniques to a commercial finite element analysis program (MSC/NASTRAN). The program has the capability of optimizing shapes by allowing multiple edges to move. A new procedure was developed to determine the best edge to move at each iteration of the shape optimization. The position of this edge is then determined using nonlinear optimization techniques. This process is incrementally continued until the edges are at their minimum positions, or the change in objective function is less than a tolerance value. This program is presently being applied to weight minimization.

CURRENT 3-D STRUCTURAL DYNAMIC FINITE ELEMENT MODELING CAPABILITIES

A joint research effort between the U.S. Army Soldier Systems Command (SSCOM), Natick Research, Development and Engineering Center and the University of Connecticut has further enhanced a 3-D Structural Dynamic Finite Element Code (SD) to predict the behavior of parachute systems. The code is being modified and coupled to Computational Fluid Dynamics (CFD) codes by SSCOM, UConn and Army High Performance Computing Research Center (AHPCRC) researchers. This paper will discuss the current state of development of the code and present examples. 3-D dynamic simulations to be presented include, 1) the inflation and spin control of a cross canopy, 2) the prediction of a ram-air parafoils shape and steady state flight, and 3) the opening of a round canopy initially near a line stretch configuration. The approximations and assumptions used in the model and detailed results of the predicted time-dependent motions, orientations and stresses will be presented. Other modeling capabilities of the SD code will also be discussed which include its preparation for numerical coupling to CFD software.

A finite element based method for the analysis of free wave propagation in stiffened cylinders

Abstract A finite element based method is developed to evaluate elastic wave propagation characteristics in orthogonally stiffened cylindrical shells. The method is general and allows for detailed modelling of the shell and stiffners. It is assumed that identical stiffeners are spaced at regular intervals in both the axial and circumferential directions, so that the structure is periodic in these two dimensions. Wave propagation techniques are applied to a finite element model of a single periodic unit to determine propagation constants. The shell variables are expanded in a Fourier series in the circumferential direction, and the axial propagation constants are presented for different values of the circumferential wavenumber as a function of frequency. The method is verified for unstiffened cylinders by comparing results with exact solutions.

Cavity growth and grain boundary sliding in polycrystalline solids

Abstract A basic method is presented for the estimate of the overall mechanical response of solids which contain periodically distributed defects (inhomogeneities, regions undergoing inelastic flow, voids, and inclusions). This method is then applied to estimate the shape and growth pattern of voids that are periodically distributed over the grain boundaries in a viscous matrix. The interaction effects are fully accounted for, and the results are compared with calculations for a single void in an infinitely extended viscous solid, by Budiansky, Hutchinson, and Slutsky. Then, for a polycrystalline solid that undergoes relaxation by grain boundary sliding, the relaxed moduli are obtained, again fully accounting for the interaction effects. Finally, the overall inelastic nonlinear response at elevated temperatures is discussed in terms of a model which considers nonlinear power law creep within the grains, and linear viscous flow in the grain boundaries.

Generalization of the equations for frame-type structures; a variational approach

SummaryThe differential equations for frame-type structures with elastically deformable joints, derived recently by A. D. Kerr and A. M. Zarembski [1], are genealized first by including the translational inertia terms. The corresponding variational principle is then derived formally, and the mechanical meaning of each term is established. The variational principle is then generalized by including a geometrical non-linearity, the effect of thermal and variable axial forces, and the variation of sectional properties. The corresponding differential equations are derived and the admissible boundary and matching conditions are discussed. As examples, formulations for two problems are presented.