Publications

A study has been performed to quantify the uncertain performance of an environment sensing safety system which senses the unique payload trajectory environment created during parachute deployment. Models for the underlying sources of uncertainty, specifically the parachute inflation uncertainty and the uncertainty in performance of the mechanical switch sensors, have been identified and validated. DAKOTA, a Sandia-developed software package for uncertainty quantification, has been used to perform a probabalistic study of system performance, accounting for sources of uncertainty in the parachute inflation process and in the environment sensing process. This study has been performed across the payload release envelope with margins and uncertainties quantified for a critical sytem performance requirement. © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Abstract not provided.

An experiment to measure surface pressure data on a series of three stainless steel simulated parachute ribbons was conducted. During the first phase of the test, unsteady pressure measurements were made on the windward and leeward sides of the ribbons to determine the statistical properties of the surface pressures. Particle Image Velocimetry (PIV) measurements were simultaneously made to establish the velocity field in the wake of the ribbons and its correlation with the pressure measurements. In the second phase of the test, steady-state pressure measurements were made to establish the pressure distributions. In the third phase, the stainless steel ribbons were replaced with nylon ribbons and PIV measurements were made in the wake. A detailed error analysis indicates that the accuracy of the pressure measurements was very good. However, an anomaly in the flow field caused the wake behind the stainless steel ribbons to establish itself in a stable manner on one side of the model. This same stability was not present for the nylon ribbon model although an average of the wake velocity data indicated an apparent 2{degree} upwash in the wind tunnel flow field. Since flow angularity upstream of the model was not measured, the use of the data for code validation is not recommended without a second experiment to establish that upstream boundary condition.

A numerical flow model is developed to simulate two-dimensional fluid flow past immersed, elastically supported tube arrays. This work is motivated by the objective of predicting forces and motion associated with both deep-water drilling and production risers in the oil industry. This work has other engineering applications including simulation of flow past tubular heat exchangers or submarine-towed sensor arrays and the flow about parachute ribbons. In the present work, a vortex method is used for solving the unsteady flow field. This method demonstrates inherent advantages over more conventional grid-based computational fluid dynamics. The vortex method is non-iterative, does not require artificial viscosity for stability, displays minimal numerical diffusion, can easily treat moving boundaries, and allows a greatly reduced computational domain since vorticity occupies only a small fraction of the fluid volume. A gridless approach is used in the flow sufficiently distant from surfaces. A Lagrangian remap scheme is used near surfaces to calculate diffusion and convection of vorticity. A fast multipole technique is utilized for efficient calculation of velocity from the vorticity field. The ability of the method to correctly predict lift and drag forces on simple stationary geometries over a broad range of Reynolds numbers is presented.

Abstract not provided.

In this paper the authors present the current status of an unsteady 3D parachute simulation code which is being developed at Sandia National Laboratories under the Department of Energy's Accelerated Strategic Computing Initiative (ASCI). The Vortex Inflation PARachute code (VIPAR) which embodies this effort will eventually be able to perform complete numerical simulations of ribbon parachute deployment, inflation, and steady descent. At the present time they have a working serial version of the uncoupled fluids code which can simulate unsteady 3D incompressible flows around bluff bodies made up of triangular membrane elements. A parallel version of the code has just been completed which will allow one to compute flows over complex geometries utilizing several thousand processors on one of the new DOE teraFLOP computers.

The prediction of potential flow about zero thickness membranes by the boundary element method constitutes an integral component of the Lagrangian vortex-boundary element simulation of flow about parachutes. To this end, the vortex loop (or the panel) method has been used, for some time now, in the aerospace industry with relative success [1, 2]. Vortex loops (with constant circulation) are equivalent to boundary elements with piecewise constant variation of the potential jump. In this case, extending the analysis in [3], the near field potential velocity evaluations can be shown to be {Omicron}(1). The accurate evaluation of the potential velocity field very near the parachute surface is particularly critical to the overall accuracy and stability of the vortex-boundary element simulations. As we will demonstrate in Section 3, the boundary integral singularities, which arise due to the application of low order boundary elements, may lead to severely spiked potential velocities at vortex element centers that are near the boundary. The spikes in turn cause the erratic motion of the vortex elements, and the eventual loss of smoothness of the vorticity field and possible numerical blow up. In light of the arguments above, the application of boundary elements with (at least) a linear variation of the potential jump--or, equivalently, piecewise constant vortex sheets--would appear to be more appropriate for vortex-boundary element simulations. For this case, two strategies are possible for obtaining the potential flow field. The first option is to solve the integral equations for the (unknown) strengths of the surface vortex sheets. As we will discuss in Section 2.1, the challenge in this case is to devise a consistent system of equations that imposes the solenoidality of the locally 2-D vortex sheets. The second approach is to solve for the unknown potential jump distribution. In this case, for commonly used C{sup o} shape functions, the boundary integral is singular at the collocation points. Unfortunately, the development of elements with C{sup 1} continuity for the potential jumps is quite complicated in 3-D. To this end, the application of Galerkin ''smoothing'' to the boundary integral equations removes the singularity at the collocation points; thus allowing the use of C{sup o} elements and potential jump distributions [4]. Successful implementations of the Galerkin Boundary Element Method to 2-D conduction [4] and elastostatic [5] problems have been reported in the literature. Thus far, the singularity removal algorithms have been based on a posterior and mathematically complex reasoning, which have required Taylor series expansion and limit processes. The application of these strategies to 3-D is expected to be significantly more complicated. In this report, we develop the formulation for a ''Regularized'' Galerkin Boundary Element Method (RGBEM). The regularization procedure involves simple manipulations using vector calculus to reduce the singularity of the hypersingular boundary integral equation by two orders for C{sup o} elements. For the case of linear potential jump distributions over plane triangles the regularized integral is simplified considerably to a double surface integral of the Green function. This is the case implemented and tested in this report. Using the example problem of flow normal to a square flat plate, the linear RGBEM predictions are demonstrated here to be more accurate, to converge faster, and to be significantly less spiked than the solutions obtained by the vortex loop method.