Publications

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Flight testing provides an opportunity to characterize a system under realistic, combined environments. Unfortunately, the prospect of characterizing flight environments is often accompanied by restrictive instrumentation budgets, thereby limiting the information collected during flight testing. Instrumentation selection is often a result of bargaining to characterize environments at key locations/sub-systems, but may be inadequate to characterize the overall environments or performance of a system. This work seeks to provide an improved method for flight environment characterization through a hybrid experimental-analytical method, modal response extraction, and model expansion. Topics of discussion will include hardware design, assessment of hardware under flight environments, instrumentation planning, and data acquisition challenges. Ground testing and model updating to provide accurate models for expansion will also be presented.

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Component mode synthesis (CMS) is a widely employed model reduction technique used to reduce the computational cost associated with the dynamic analysis of complex engineering structures. To generate CMS models, specifically the formulation of Craig and Bampton, both normal fixed-interface modes and constraint modes of the component’s structure are calculated. These modes are used in conjunction with the component level mass and stiffness matrices to generate reduced mass and stiffness matrices used in the final analyses. For some component models, the most computationally expensive part of this procedure is calculating the component normal modes information. Several different approaches are utilized to investigate the sensitivity of system level responses to variations in several aspects of the CMS models. One approach evaluates changes due to modifications of the reduced mass and stiffness matrices assuming that the mode shapes do not change. The second approach assumes that the mode shapes change but the reduced mass and stiffness matrices do not change. An example is presented to show the influence of these two approaches.

Six degree of freedom (6-DOF) subsystem/component testing is becoming a desirable method, for field test data and the stress environment can be better replicated with this technology. Unfortunately, it is a rare occasion where a field test can be sufficiently instrumented such that the subsystem/component 6-DOF inputs can be directly derived. However, a recent field test of a Sandia National Laboratory system was instrumented sufficiently such that the input could be directly derived for a particular subsystem. This input is compared to methods for deriving 6-DOF test inputs from field data with limited instrumentation. There are four methods in this study used for deriving 6-DOF input with limited instrumentation. In addition to input comparisons, response measurements during the flight are compared to the predicted response of each input derivation method. All these methods with limited instrumentation suffer from the need to inverse the transmissibility function.

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This work presents a modal test on a cylindrical bolted structure that initially appeared to be a routine model calibration experiment. However, while reviewing the test data the structure appeared to have two pairs of ovaling modes with identical shapes. Assuming this to be the result of an uninstrumented component of the test article, extensive efforts were conducted to identify this feature. When all options were exhausted, the interaction between the structure and the air contained within was investigated. Contrary to the typical assumption that the fluid-structure interactions are negligible for such a thick walled cylinder, analysis showed that for this test article the acoustic modes of the internal air significantly impacted the structural response. In this case, the acoustic and the structural modes coincided in frequency, causing the first ovaling modes to split into two pairs at different frequencies. Experimental and analytical results are presented that describe this structural-acoustic mode coupling phenomenon.

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This paper investigates the effectiveness of modeling thin composite structures with hex shell elements for structural dynamics simulation. The current finite element modeling method for an existing three-layer composite aerospace structure uses solid 8-noded hex elements. It is relatively expensive in terms of the number of degrees of freedom and element count. A finer mesh typically results in a more accurate solution, however, the computation time increases. Modal analysis was used to test if a single layer of hex shell elements for each material could replace multiple layers of solid hex elements, enabling computational savings. Element aspect ratio was varied on a solid hex model of a frustum part to optimize the technique. The hex shell modeling technique was then applied to the existing three-layer composite structure. The analysis results, when compared to validation data obtained from tests performed on the actual hardware, exhibit very satisfactory agreement. A single layer of hex shell elements are capable of providing solutions that are equivalent to multiple layers of hex elements. A considerable savings in element count and solution equations result. A broader understanding of modeling options for future, more efficient methods of modeling composite shell structures is also obtained. ©2010 Society for Experimental Mechanics Inc.

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This article is the second of two that consider the treatment of fluid-solid interaction problems where the solid experiences wave loading and large bulk Lagrangian displacements. In part-I, we presented the formulation for the edge-based unstructured-grid Euler solver in the context of a discontinuous- Galerkin framework with the extensions used to treat internal fluid-solid interfaces. A super-sampled L² projection was used to construct level-set data from the Lagrangian interface, and a narrow-band approach was used to identify and construct appropriate ghost data and boundary conditions at the fluid-solid interface. A series of benchmark problems were used to verify the treatment of the fluid-solid interface conditions with a static interface position. In this paper, we consider the treatment of dynamic interfaces and the associated large bulk Lagrangian displacements of the solid.We present the coupled dynamic fluid-solid system, and develop an explicit, monolithic treatment of the fully-coupled system. The conditions associated with moving interfaces and their implementation are discussed. A comparison of moving vs. fixed reference frames is used to verify the dynamic interface treatment. Lastly, a series of two and and three-dimensional projectile and shock-body interaction calculations are presented. Ultimately, the use of the Lagrangian interface position and a super-sampled projection for fast level-set construction, the narrow-band extraction of ghost data, and monolithic explicit solution algorithm has proved to be a computationally efficient means for treating shock induced fluid-solid interaction problems.

Here, this paper is the first of two that consider the treatment of fluid-solid interaction problems under shock wave loading, where the solid experiences large bulk Lagrangian displacements. This work addresses the issues associated with using a level-set as a generalized interface for fluid-solid coupling where unstructured overlapping grids are used for the fluid and solid domains. In part-I of this work, we outline the formulation used for the edge-based unstructured-grid Euler solver in the context of the discontinuous-Galerkin method. The identification of the fluid-solid interface on the unstructured fluid mesh uses a super-sampled L² projection technique, that in conjunction with a Lagrangian interface position, permits fast identification of the interface and the concomitant imposition of boundary conditions. The use of a narrow-band approach for the identification of the wetted interface is presented with the details of the construction of interface conditions. A series of computations are presented to demonstrate the validity of the current approach on problems with static interfaces. In part-II, we present the coupled dynamic fluid-solid system, and present an explicit monolithic algorithm for the treatment of the fully-coupled system. The interface conditions associated with moving interfaces is considered, and a comparison of moving vs. static reference frames is used to evaluate the dynamic interface treatment. Finally, a series of two and and three-dimensional projectile and shock-body calculations are presented.

Here, this paper considers the issues central to fast level-set construction for the general treatement of moving interfaces in coupled fluid-solid interaction problems where the Lagrangian solid experiences large bulk motion. The central idea is based on a super-sampled L² projection, that in conjunction with a Lagrangian interface position, permits rapid identification of the solid interface in the fluid mesh and enables the imposition of boundary conditions for the fluid. A series of convergence studies are presented in terms of numerical quadrature and mesh refinement to illustrate the effectiveness of the super-sampled projection on unstructured grids. The extraction of the interface location based on distance functions is compared to the super-sampled projection method. Finally, it is shown that the extraction of an interface location based on a zero level-set converges as O(h) when compared to the exact interface location - suggesting that the availability of a Lagrangian interface description is always preferred.

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