John W. Leonard

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

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.

Dynamics of ocean cables with local low-tension regions

Abstract A general set of 3-D dynamic field equations for a cable segment is derived based on the classical Euler–Kirchhoff theory of an elastica. The model includes flexural stiffness to remove the potential singularity when cable tension vanishes and can be reduced to the equations for a perfectly flexible cable. A hybrid model and a solution scheme by direct integration are then proposed to solve the oceanic cable/body system with a localized low-tension region. Numerical examples demonstrate the capability and validity of the formulation and the numerical algorithm.

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.

Simulation of unsteady oceanic cable deployment by direct integration with suppression

Abstract An improved algorithm is developed for predicting the transient response of a system of serially connected cables and bodies during unsteady deployment from a surface vessel. The governing equations of a cable-body system are derived with dependent variables of cable velocities, direction cosines and tension magnitude to form a nonlinear combined initial-value and boundary-value problem. The problem is then solved by introducing a stable Newmark-like implicit integration scheme in time and by a direct integration method with suppression of extraneous erroneous solutions. Special boundary conditions simulating actively controlled payout and slack-cable/ocean-bottom contact boundary conditions are included in the present model.

Experimental and numerical investigations of tethered spar and sphere buoys in irregular waves

A series of large-scale experiments were conducted to examine the motions of buoys in a variety of wave climates. In conjunction with these experiments, numerical simulations of selected tests were conducted to test the present ability of a particular computer program to model buoy responses. This paper considers individual wave tests for two of the buoy models used in the experiments, a spar buoy and a sphere buoy. The wave field is generated using a JONSWAP spectrum. A description of the experiments shows that each buoy is subject to significant resonant responses, in heave for the spar and surge for the sphere, even though wave forcing is not present at or immediately adjacent to the resonant frequencies. The numerical simulations show generally good comparisons with the experimental data.

Computer Simulation of Parafoil Dynamics

Abstract : Three-dimensional transient simulations of unconstrained flight of a parafoil system are performed using a structural finite element model with prescribed pressure and drag forces. In general, simulation of parafoil flight requires coupling of a structural model with a fluid dynamics model. The approach presented here is an intermediate step towards a fully-coupled simulation. Since the structural model is considerably smaller than the required fluid dynamics model, these intermediate simulations are considerably easier to perform and provide a relatively simple method to evaluate parafoil flight. Simulations of the inflation from the initial cut pattern, steady glide, and turning and flared landing maneuvers are presented.

Experimental investigation of moored-buoys using advanced video techniques

Abstract Physical model tests were conducted to validate numerical models of moored-buoy systems. Three buoy types (sphere, spar and discus) were tested for intrinsic properties, three-dimensional impulse response and three-dimensional dynamic response to two-dimensional regular and random wave excitation. Buoy kinematics were measured using advanced video imaging techniques. Other data collected included upper and lower mooring line tension and mooring line inclination. Physical model development, test and measurement procedures and data collected are discussed.