Keith Stein

Mesh Moving Techniques for Fluid-Structure Interactions With Large Displacements

In computation of fluid-structure interactions, we use mesh update methods consisting of mesh-moving and remeshing-as-needed. When the geometries lire complex and the structural displacements are large, it becomes even more important that the mesh moving techniques lire designed with the objective to reduce the frequency of remeshing. To that end, we present here mesh moving techniques where the motion of the nodes is governed by the equations of elasticity, with selective treatment of mesh deformation based on element sizes as well as deformation modes in terms of shape and volume changes. We also present results from application of these techniques to a set of two-dimensional test cases.

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.

Fluid-structure interactions of a cross parachute: numerical simulation

Abstract The dynamics of parachutes involves complex interaction between the parachute structure and the surrounding flow field. Accurate representation of parachute systems requires treatment of the problem as a fluid–structure interaction (FSI). In this paper we present the numerical simulations we performed for the purpose of comparison to a series of cross-parachute wind tunnel experiments. The FSI model consists of a 3-D fluid dynamics (FD) solver based on the Deforming-Spatial-Domain/Stabilized Space–Time (DSD/SST) procedure, a structural dynamics (SD) solver, and a method of coupling the two solvers. These FSI simulations include the prediction of the coupled FD and SD behavior, drag histories, flow fields, structural behavior, and equilibrium geometries for the structure. Comparisons between the numerical results and the wind tunnel data are conducted for three cross-parachute models and at three different wind tunnel flow speeds.

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.

Computational fluid-structure interaction model for parachute inflation

In parachute research, the canopy inflation process is the least understood and the most complex to model. Unfortunately it is during the opening process that the canopy often experiences the largest deformations and loadings. The complexity of modeling the opening process stems from the coupling between the structural dynamics of the canopy, lines, and payload with the aerodynamics of the surrounding fluid medium. The addition of a computational capability to model the coupled opening behavior would greatly assist in the understanding of the canopy inflation process. This article describes research that involves coupling a computational fluid dynamics code to a mass spring damper parachute structural code. The axisymmetric codes are coupled with an explicit marching method. The current model is described and results for a round parachute are presented. A comparison of the numerical results to experimental data will be presented. The successful solution of these problems gives us confidence that the computational aeroelastic problem for parachute openings can be solved. This solution allows moving the parachute design process from one of cut and try to one based on experimentally verified computational tools and reduces the reliance on costly and time-consuming testing during development.

Aerodynamic Interactions Between Parachute Canopies

Aerodynamic interactions between parachute canopies can occur when two separate parachutes come close to each other or in a cluster of parachutes. For the case of two separate parachutes, our computational study focuses on the effect of the separation distance on the aerodynamic interactions, and also focuses on the fluid-structure interactions with given initial relative positions. For the aerodynamic interactions between the canopies of a cluster of parachutes, we focus on the effect of varying the number and arrangement of the canopies.

Modeling of Fluid-Structure Interactions with the Space-Time Techniques

We provide an overview of the space-time finite element techniques developed by the Team for Advanced Flow Simulation and Modeling (T AFSM) for modeling of fluid–structure interaction problems. The core method is the DeformingSpatial-Domain/Stabilized Space-Time formulation, complemented with the mesh update methods, including the Solid-Extension Mesh Moving Technique and MoveReconnect-Renode Mesh Update Method. Also complementing the core method are the block-iterative, quasi-direct and direct coupling methods for the solution of the fully-discretized, coupled fluid and structural mechanics equations. Additionally, the Surface-Edge-Node Contact Tracking technique is introduced as a contact algorithm for the purpose of protecting the quality of the fluid mechanics mesh between the structural surfaces coming into contact. We present mesh-moving tests and numerical examples with incompressible flows and membrane and cable structures.

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.

Computational fluid-structure interaction methods for simulation of inflatable aerodynamic decelerators

Inflatable aerodynamic decelerators have potential advantages for planetary re-entry in robotic and human exploration missions. It is theorized that volume-mass characteristics of these decelerators are superior to those of common supersonic/subsonic parachutes and after deployment they may suffer no instabilities at high Mach numbers. A high fidelity computational fluid-structure interaction model is employed to investigate the behavior of tension cone inflatable aeroshells at supersonic speeds up to Mach 2.0. The computational framework targets the large displacements regime encountered during the inflation of the decelerator using fast level set techniques to incorporate boundary conditions of the moving structure. The preliminary results indicate large but steady aeroshell displacement with rich dynamics, including buckling of the inflatable torus that maintains the decelerator open under normal operational conditions, owing to interactions with the turbulent wake. Copyright