Improvements in Fluid Structure Interaction simulations of LS-Dyna(r)

International audienceThe French Ministry of Defence’s procurement agency, the Direction Générale de l'Armement (DGA), is in charge of assessing and testing armament systems in order to equip the armed forces and prepare for the future. DGA Aeronautical Systems, the technical centre dedicated to evaluate and test aircraft, combines test and evaluation to clear, among others, parachute systems. The parachute evaluation is historically based on experimental data and so requires numerous flight tests which can prove expensive and time consuming. In order to have a greater understanding of the parachute dynamic behavior and to optimize the parachute systems flight tests, DGA Aeronautical Systems developed a modeling and simulation capability as a support to evaluation. For this purpose, DGA Aeronautical Systems, with the help of ISAE, developed Fluid Structure Interaction (FSI) simulations of parachutes using the LS-Dyna commercial Finite Element Analysis (FEA) tool. This tool is largely used for solving highly nonlinear transient problems and enables doing coupled multi-physics simulations such as FSI simulations. DGA Aeronautical Systems has been using the software since 2003. In the recent years, the parachute simulation has been much improved thanks to the implementation of a porosity algorithm in LS-Dyna at the common request of DGA and parachute industry. The paper presents recent improvements in Arbitrary Lagrangian Eulerian (ALE) techniques used to analyze the canopy inflation and the quasi-steady state descent phases characteristics. Up to now, only infinite mass type simulations were developed by constraining the parachute confluence point and applying a prescribed airflow to the fluid. The applied airflow velocity came from real in-flight measurements of paratrooper or load trajectory determinations. This simulation type is representative to wind tunnel tests. From now on, thanks to considerable computational resources, finite mass type simulations are also possible. It consists in applying the force of gravity to the parachute system. This allows simulating both the inflation phase (from vertical packed parachute geometry) and the quasi-steady state descent. Among others, the static line parachute of the new French Army troop parachute system called EPC (Ensemble de Parachutage du Combattant) was modeled at real scale. Modeling techniques are presented and results of the EPC static line parachute simulation are compared with real inflight measurements. The benefits of FSI simulations prior to parachute testing are presented. In a near future, incompressible and compressible Navier-Stokes solvers will be available in the next version of LS-Dyna. These code enhancements will be tested to simulate the parachute flight and hopefully will bring the ability to analyze more accurately the aerodynamics of the canopy and the structural behavior of the fabrics. These future capabilities are also discussed

Nonlinear analysis of orthotropic membrane and shell structures including fluid-structure interaction

In this work, membrane and shell structures with large deformations are studied. In the structural part of this work, a new methodology for the analysis of geometrically nonlinear orthotropic membrane and rotation-free shell elements is developed based on the principal fiber orientation of the material. A direct consequence of the fiber orientation strategy is the possibility to analyze initially out-of lane prestressed membrane and shell structures. Additionally, since conventional membrane theory allows compression stresses, a wrinkling algorithm based on modifying the constitutive equation is presented. The structure is modeled with finite elements emerging from the governing equations of elastodynamics. the fluid part of this work is governed by the incompressible Navier-Stokes equations, which are modeled by stabilized equal-order interpolation finite elements. Since the monolithic solution for these equations has the disadvantage that take great computer effort to solve large algebraic system of equations, the fractional step methodology is used to take advantage of the computational efficiency given by the uncoupling of the pressure from the velocity field. In addition, the generalized-time integration scheme for fluids is adapted to be used with the fractional step technique.Postprint (published version

Modeling of High Pressure Confined Inflatable Structures

Safety of transportation tunnels is a top priority among transportation agencies and public administrators and a very important aspect in the daily operation of a tunnel system. However, it is always a challenge to create and integrate protection systems in existing tunnels to prevent or at least mitigate the occurrence of hazardous events such as spread of smoke or noxious fumes, flooding, among others. Typically there two ways for preventing or mitigating the occurrence of hazardous events: one is the implementation of permanent solutions and, the second one, is the use of temporary solutions. Permanent solutions usually have relatively high sealing efficiency due to their solid and rigid sealing mechanisms such as bulkheads and floodgates. However, they can be extremely expensive and sometimes difficult to build or install due to physical, economical or operational constraints. On the other hand, temporary solutions, which can be relatively low cost and easy to install, offer a temporary countermeasure while permanent repairs are implemented. The development of flexible structures, such as inflatable plugs for temporary solutions is becoming a viable alternative for protection of transportation tunnels and other similar critical civil infrastructure.;The Resilient Tunnel System (RTS) is a passive tunnel protection system developed at West Virginia University (WVU). This system is intended to prevent or minimize the damage induced by hazardous events by creating a compartment to contain the threat. The Resilient Tunnel System implements inflatable structures at specific locations of the tunnel to seal up the tunnel and create a compartment to isolate the compromised region. WVU has conducted several validation tests on full scale inflatable structures designed to mitigate flooding in an actual rail transportation tunnel and in specially built testing facilities. However, testing at full scale either in an actual tunnel or in specially built testing facilities, is a very complex and resource demanding task. It can take several iterations to achieve desired results which cannot be accurately predicted in advance. Therefore, the development of numerical models using Finite Element Analysis becomes imperative in order to: first, reproduce experimental work done at WVU using different prototypes at different scales; and then use the calibrated models as predicting tool that can anticipate the outcome of experiments and eventually reduce its number due to the intrinsic complexity and cost.;This dissertation aims to present the results of the development of Finite Element Models of confined inflatable structures designed to withstand flooding pressures. Models of different prototypes were created and analyzed in order to reproduce experimental results. Numerical results show that the adjusted models can reproduce experimental results, ranging from deployment, full pressurization and induced failure, with a great degree of accuracy providing a reliable predicting tool for evaluation of alternative configurations and parametric studies

Through thickness air permeability and thermal conductivity analysis for textile materials

Woven fabrics have found enormous application in our daily life and in industry because of their flexibility, strength and permeability. The aim of this work was to create a general model for through thickness air permeability and thermal conductivity for different types of textile fabrics because of their applications in industries and everyday life. An analytical model to predict through thickness air permeability was developed. The objective was to create a model which will take into consideration the two primary mechanisms of air flow in fabrics: through the gaps between yarns and through the yarns. Through thickness air permeability was measured according to British Standard BS EN ISO 9237: 1995. Several fabrics were tested including plain weave, twill weave and satin weave fabrics. The analytical model is a combination Kulichenko and Van Langenhove's analytical model which predicts the permeability through gaps between yams with Gebart's model to predict permeability within yams. Analytical predictions were compared to the experimental data. Computational modelling of through thickness air permeability using Computational Fluid Dynamics CFD software is presented in this thesis. The Polymer Composites Research Group in the University of Nottingham has created a textile schema, named TexGen. The prerequisites of this software were to be able to model various types of textile structures. A CFD model using CFX 11.0 was developed to be able to predict fabric permeability. In addition, an analytical model was developed for fabrics deformed by shear, compaction and tension. Experimental work for through thickness air permeability of sheared fabric was used to verify predicted results. An analytical model for thermal conductivity of fabrics was developed including the influence of moisture content on thermal conductivity. Two existing approaches for single-layer fabrics are described and compared: rule of mixtures and thermal resistance approach. A me6iod for thermal conductivity prediction for multiple layer fabrics is presented. The results are compared to the experimental data and analysed. Some predicted results were in excellent and good agreement with experimental data whereas other predicted results were in poor agreement with experimental data as they were dramatically affected by the assumptions made in the analytical model

Immersogeometric cardiovascular fluid–structure interaction analysis with divergence-conforming B-splines

This paper uses a divergence-conforming B-spline fluid discretization to address the long-standing issue of poor mass conservation in immersed methods for computational fluid–structure interaction (FSI) that represent the influence of the structure as a forcing term in the fluid subproblem. We focus, in particular, on the immersogeometric method developed in our earlier work, analyze its convergence for linear model problems, then apply it to FSI analysis of heart valves, using divergence-conforming B-splines to discretize the fluid subproblem. Poor mass conservation can manifest as effective leakage of fluid through thin solid barriers. This leakage disrupts the qualitative behavior of FSI systems such as heart valves, which exist specifically to block flow. Divergence-conforming discretizations can enforce mass conservation exactly, avoiding this problem. To demonstrate the practical utility of immersogeometric FSI analysis with divergence-conforming B-splines, we use the methods described in this paper to construct and evaluate a computational model of an in vitro experiment that pumps water through an artificial valve

Aeronautical engineering: A continuing bibliography with indexes (supplement 269)

This bibliography lists 539 reports, articles, and other documents introduced into the NASA scientific and technical information system in August, 1991. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

MODELLING OF WHIPLASH TRAUMA; PARAMETRIC STUDY OF REAR-END COLLISION AND DEVELOPMENT OF HEAD-RESTRAINT SYSTEM

Whiplash is a common neck injury people usually suffer from after a rear car accident. Over the past decade, both engineers and physicians were trying to analyze the biomechanics of the injury to develop an effective prevention system design. Car Manufacturers and researchers developed various types of head-restraints, including re-active and pro-active systems, to protect the neck against whiplash. A few works have been done on developing a robust tracking head-restraint system to adjust its position automatically relative to the occupant’s head position. The current study illustrates the effect of head-restraint position and material properties on whiplash injury using finite element modelling. Accordingly, a tracking head-restraint system was developed to maintain the optimum head-restraint position while driving to effectively protect the neck against whiplash.Qatar National Research Fund (a member of Qatar Foundation) through the National Priorities Research Program NPRP # 6-292- 2-127

Aeronautical engineering: A continuing bibliography with indexes (supplement 304)

This bibliography lists 453 reports, articles, and other documents introduced into the NASA scientific and technical information system in May 1994. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Aeronautical engineering: A cumulative index to a continuing bibliography (supplement 274)

This publication is a cumulative index to the abstracts contained in supplements 262 through 273 of Aeronautical Engineering: A Continuing Bibliography. The bibliographic series is compiled through the cooperative efforts of the American Institute of Aeronautics and Astronautics (AIAA) and the National Aeronautics and Space Administration (NASA). Seven indexes are included: subject, personal author, corporate source, foreign technology, contract number, report number, and accession number