Aeroelastic analysis of parachute deceleration systems with empirical aerodynamics

A technique for the aeroelastic solution of parachute decelerators is presented in this work. The methodology uses empirical aerodynamics, based on a filling-time inflation model and Ludtke's area law, coupled to two explicit structural solution approaches. A mass-spring-damper technique allows solving the deployment of the system (when the grid is highly distorted) efficiently, and a finite element model is used for the accurate calculation of the structural loads and stresses during parachute opening and steady flight. The coupling strategy is staggered and the models share the same mesh. The methodology is intended for practical calculations of deceleration systems, and provides useful performance and structural data minimizing model complexity and computational cost. The suitability of the proposed technique is assessed by comparisons with reference test drop data.Peer ReviewedPostprint (author's final draft

Efficient aeroelastic analysis of wind loads on inflatable hangars

Wind loads play a crucial role in inflatable structures. Unfortunately, design loads from safety regulations grossly overestimate the real aerodynamic forces. Thus, a more accurate estimation of wind loads is desirable. Conventional CFD approaches (e.g. LES) struggle with the complexities of the flow field (intricate geometry and massive flow separation) and require a very high computational effort. We present a cost-efficient tool for the aeroelastic analysis of inflatable hangars. It uses a staggered solution scheme with an explicit finite-element structural solver and potential flow aerodynamics. To account for large areas of separated flow typical of blunt shapes, a semi-empirical correction is applied to the inviscid solution. The streamlines of the potential solution are computed and, for each one, the separation point is predicted with Stratford’s criterion. Finally, an empirical correction is applied to the inviscid pressure field. We present validation benchmarks as well as a real life application example. Over the majority of the flow field, the pressure field agrees well with high-fidelity computations, yielding similar global loads for structural sizing. This is achieved with a small fraction of the computational effort required by conventional CFD approaches.Postprint (published version

Straightforward one-pot stereoselective synthesis of substituted tetrahydrofurans from 1,3-butadienes and aldehydes.

International audienceA new and efficient multicomponent reaction involving a double-allylation sequence is reported. In situ generated bimetallic reagents are prepared from disilane and added onto a range of aliphatic aldehydes to afford a direct access to trisubstituted tetrahydrofurans in good to excellent diastereoselectivity (up to 70%)

A numerical investigation of wind tunnel model deformations caused by the twin-sting support system

This work presents a wing deformation analysis of a twin-sting-mounted commercial aircraft model. Twin-sting arrangements minimize flow disturbances around the model fuselage and tail; on the other hand, they cause important changes in the flow field around the wing and also increase aerodynamic interference at the wing and aeroplastic effects on the wing. In some cases, these effects can alter the normal downwash developed behind the wing, modifying the flow pattern at the tail. Consequently, when tail aerodynamics is a major concern, this kind of support interference should be carefully evaluated. The methodology developed in this work employs an unstructured FEM-based flow solver for computing aerodynamic loads. These loads are then transferred to a finite element structural model in order to assess the geometrical deformation of the wing caused by the torsional moment exerted by the supporting mechanism. The analysis described involves there different twin-sting support configurations taking into account angle of attack variations and Mach numbers spanning from subsonic to high transonic ranges.Preprin

Explicit dynamic analysis of thin membrane structures

CIMNE Publicaciones nº 351An explicit dynamic structural solver developed at CIMNE for the analysis of parachutes is presented. The canopy fabric has a negligible out-of-plane stiffness, therefore its numerical study presents important challenges. Both the large changes in geometry and the statically indeterminate character of the system are problematic from the numerical point of view. This report covers the reasons behind the particular choice of solution scheme as well as a detailed description of the underlying algorithm. Both the theoretical foundations of the method and details of implementation aiming at improving computational efficiency are given. Benchmark cases to assess the accuracy of the solution as well as examples of practical application showing the performance of the code are finally presented.Preprin

Numerical tools for the analysis of parachutes

The design and evaluation of parachute-payload systems is a technology field in which numerical analysis tools can make very important contributions. This work describes a new development from CIMNE in this area, a coupled fluid-structural solver for unsteady simulations of ram-air type parachutes. For an efficient solution of the aerodynamic problem, an unsteady panel method has been chosen exploiting the fact that large areas of separated flow are not expected under nominal flight conditions of ram-air parachutes. A dynamic explicit finite element solver is used for the structure. This approach yields a robust solution even when highly non-linear effects due to large displacements and material response are present. The numerical results show considerable accuracy and robustness. An added benefit of the proposed aerodynamic and structural techniques is that they can be easily vectored and thus suitable for use in parallel architectures. The main features of the computational tools are described and several numerical examples are provided to illustrate the performance and capabilities of the technique

A 3D low-order panel method for unsteady aerodynamic problems

An unsteady low-order panel method for three-dimensional subsonic analyses is presented. The method, which is based on well-established techniques in computational aerodynamics, is intended to achieve a cost-effective solution of unsteady flows around arbitrary aerodynamic configurations. This work has two main objectives. First, to relax geometry discretization requirements and, second, to simplify the treatment of problems in which the analysis configuration moves along specified flight paths and/or changes its geometry during the simulation. Following this aim, a time-marching solution procedure is adopted in conjunction with a free-wake model which avoids iterative solutions for wake shape and position. The suitability of the present approach for solving typical aerodynamic problems is illustrated by means of several numerical examples.Preprin

Fonction de transfert pour l'estimation de la composition chimique des eaux fossiles à partir des diatomées. Calibration sur des lacs salés du Lipez, SW de l'Altiplano bolivien

L'étude des assemblages de diatomées et de la composition chimique des eaux a été effectuée dans 13 lacs salés peu profonds localisés dans le Sud de l'Altiplano Bolivien. Les points et les dates de prélèvement sont les mêmes pour les deux types d'étude. Les relations entre les assemblages de diatomées et les variables chimiques mesurées sont effectuées dans le but d'estimer ces variables dans le passé à partir des diatomées fossiles conservées dans les sédiments. La méthode des moyennes pondérées (WA et WA-PLS régression) a permis d'estimer les optima et les tolérances de 61 espèces à la salinité et aux différents éléments chimiques dont le bore et le lithium.Diatom assemblages and water chemistry were studied in 13 shallow salt lakes in the southern part of the Bolivian Altiplano. At each locality bottom sediment and water samples were collected simultaneously. Relationships between the composition of the diatom assemblages and variations in water chemistry were collated in order to permit the estimation of ancient water chemistries based on changes in the make up of fossil diatom associations in older sediments. Weighted Averages treated by Partial Least Squares regression (WA and WA-PLS methods) allowed an estimation of optima and the relative tolerances of 61 species to variations in salinity and to the relative quantities of the 15 chemical elements studied, among them boron and lithium

A-posteriori error estimation for the finite point method with applications to compressible flow

An a-posteriori error estimate with application to inviscid compressible flow problems is presented. The estimate is a surrogate measure of the discretization error, obtained from an approximation to the truncation terms of the governing equations. This approximation is calculated from the discrete nodal differential residuals using a reconstructed solution field on a modified stencil of points. Both the error estimation methodology and the flow solution scheme are implemented using the Finite Point Method, a meshless technique enabling higher-order approximations and reconstruction procedures on general unstructured discretizations. The performance of the proposed error indicator is studied and applications to adaptive grid refinement are presented.Peer ReviewedPostprint (author's final draft

Shape optimization in aeronautical applications using neural networks

An optimization methodology based on neural networks was developed for use in 2D optimal shape design problems. Neural networks were used as a parameterization scheme to represent the shape function, and an edge-based high-resolution scheme for the solution of the compressible Euler equations was used to model the flow around the shape. The global system incorporates neural networks and the Euler fluid solver into the C++ Flood optimization framework containing a library of optimization algorithms. The optimization scheme was applied to a minimal drag problem in an unconstrained optimization case and a constrained case in hypersonic flow using evolutionary training algorithms. The results indicate that the minimum drag problem is solved to a high degree of accuracy but at high computational cost. For more complex shapes, parallel computing methods are required to reduce computational time.Preprin