Residual flexibility test method for verification of constrained structural models

Application of the free-suspension residual flexibility modal test method to the International Space Station Pathfinder structure is described. The Pathfinder, a large structure of the general size and weight of Space Station module elements, was also tested in a large fixed-base fixture to simulate Shuttle Orbiter payload constraints. After correlation of the Pathfinder finite element model to residual flexibility test data, the model was coupled to a fixture model, and constrained modes and frequencies were compared to fixed-base test. modes. The residual flexibility model compared very favorably to results of the fixed-base test. This is the first known direct comparison of free-suspension residual flexibility and fixed-base test results for a large structure. The model correlation approach used by the author for residual flexibility data is presented. Frequency response functions (FRF) for the regions of the structure that interface with the environment (a test fixture or another structure) are shown to be the primary tools for model correlation that distinguish or characterize the residual flexibility approach. A number of critical issues related to use of the structure interface FRF for correlating the model are then identified and discussed, including (1) the requirement of prominent stiffness lines, (2) overcoming problems with measurement noise which makes the antiresonances or minima in the functions difficult to identify, and (3) the use of interface stiffness and lumped mass perturbations to bring the analytical responses into agreement with test data. It is shown that good comparison of analytical-to-experimental FRF is the key to obtaining good agreement of the residual flexibility values.

Development of a large modal survey test fixture for measuring constrained-boundary modes and frequencies of space station modules and other shuttle payloads is described. Problems with fixture-to-test article interface mechanisms and the adverse effects on modal data are discussed, as well as redesign of the mechanisms. In view of the difficulties experienced in fixture development and fixed-base testing, alternative freesuspension techniques are discussed for deriving constrained vibration mode shapes for space shuttle payloads using unconstrained or free-boundary dynamic properties. These methods are the residual flexibility method, which requires measurement of a set of freeboundary modes plus the residual flexibility of the payload-to-orbiter interfaces, and the mass-additive approach, which involves the measurement of unconstrained modes of a payload with large masses attached to the interfaces.

A technique is presented for estimating the residual flexibility between nonexcited structural degrees of freedom from experimental structural vibration data. Using this method, one can include the residual flexibility estimated from modal measurements in the computation of measured flexibility for experiments with incomplete reciprocity, i.e., when the response and excitation measurement sensors are not fully collocated. The method can also be used to estimate the unknown entries in the residual flexibility matrix for experimental component mode synthesis when excitations are not provided at all interface degrees of freedom. A general solution is presented that contains an unknown positive semidefinite contribution. The general solution satisfies modal orthogonality in the limit that all of the structural degrees of freedom are instrumented and when the positive semidefinite contribution lies in a nullspace defined by the stiffness matrix and the modal flexibility. With a limited number of measurements, modal orthogonality is shown to be satisfied to the extent that the measured modes are preserved by static condensation. A rank-deficient solution is presented that allows the residual to be used in the computation of the flexibility matrix without further modeling assumptions. Numerical and experimental results that demonstrate the application of the method to both flexibility matrix convergence and experimental component mode synthesis are presented.

BASEBAND METHODS OF COMPONENT MODE SYNTHESIS FOR NON-PROPORTIONALLY DAMPED SYSTEMS

Forecasting Multivariate Time Series with Linear Restrictions using Constrained Structural State-space Models

dSysMap: exploring the edgetic role of disease mutations.

RNA-binding proteins (RBPs) are crucial regulators of gene expression, often composed of defined domains interspersed with flexible, intrinsically disordered regions. Determining the structure of ribonucleoprotein (RNP) complexes involving such RBPs necessitates integrative structural modeling due to their lack of a single stable state. In this study, we integrate magnetic resonance, mass spectrometry, and small-angle scattering data to determine the solution structure of the polypyrimidine-tract binding protein 1 (PTBP1/hnRNP I) bound to an RNA fragment from the internal ribosome entry site (IRES) of the encephalomyocarditis virus (EMCV). This binding, essential for enhancing the translation of viral RNA, leads to a complex structure that demonstrates RNA and protein compaction, while maintaining pronounced conformational flexibility. Acting as an RNA chaperone, PTBP1 orchestrates the IRES RNA into a few distinct conformations, exposing the RNA stems outward. This conformational diversity is likely common among RNP structures and functionally important. Our approach enables atomic-level characterization of heterogeneous RNP structures.

Large structures are being considered for future missions in space. They will make efficient use of material and hence are likely to be quite flexible. Flexible structures are distributed parameter systems with infinite-dimensional discrete spectra. They are often approximated by using a finite number of modes. When this is done, using finite element methods, many more elements must be used than the number of modes required, to ensure acceptable accuracy in the required modes. In this paper, we derive an infinite dimensional model for a planar flexible structure, an equilateral triangular truss. Exact poles (natural frequencies) and zeros are determined for transfer functions from control torques to local attitude angles, and simple control logic is given that stabilizes the attitude and all the vibration modes.

Many industrial machines nowadays are sold based on analysis performed on mathematical models of the rotors, bearings, substructures, and other components. The validity of the analysts therefore depends on the accuracy of the models themselves. When the rotor is available, modal testing may be used to validate the model of the rotor by comparing the calculated and measured free-free natural frequencies and mode shapes. This work presents additional tools for the verification of analytical models against experimental data. These tools use models of the rotor constructed from the measured data and the analytical model. A comparison of the first six calculated and measured free-free natural frequencies and mode shapes for a multi-mass flexible rotor is presented. The natural frequencies compare within 1.8%. The calculated and measured mode shapes were used to construct independent reduced order models of the rotor. These models were used to perform forced response and stability analyses. Forced response functions are presented comparing the forced response characteristics obtained from the two models. This provides a comparison between the measured and calculated forced response functions for the same number of modes. For the stability analysis, identical bearing models were added to both reduced order models. The eigenvalues were calculated using both models for a range of bearing stiffness and damping coefficients and were plotted for comparison.

Model reduction is a technique commonly used to reduce the computation time of structural dynamic models. The results of the reduced model can then be expanded to full space using the transformation matrix developed in the reduction process. This work focuses on expanding the mode shapes of system models that are comprised of reduced component models. Typically, this expansion requires the full space system model to be computed in order to obtain the system model transformation matrix needed to perform the expansion. Computing the full space system model to determine the expansion matrix does not save computation time and therefore defeats the purpose of using model reduction. This paper proposes using the expansion matrices of the individual components to expand the assembled system model modes. In this work, System Equivalent Reduction Expansion Process (SEREP) is used for reduction and expansion. The accuracy of the expanded system model is shown to be dependent on the modes retained in the reduced component models. Recent work on Variability Improvement of Key Inaccurate Node Groups (VIKING) has shown that over specifying the number of modes used in the reduction/expansion process that span the space of the system model modes significantly improves the results. The VIKING technique is the basis for the expansion process developed in this work. Multiple analytical cases are presented to show how the selection of component modes affects the expansion results. The analytical cases demonstrate that accurate system model expansion results can be obtained when a sufficient set of component modes that span the space of the system model modes are used.

The current trend toward construction of high rise building with the use of lightweight, high strength material and advance construction methods have led to making comparatively light and flexible structures, which have relatively low natural damping. Results make structures more prone to environmental excitation such as earthquake loads. An asymmetry in a building which may arise due to unsymmetrical distribution of stiffness, mass, damping and strength characteristics. The responses of asymmetric structure under earthquake are characterized by translation and rotational degree of freedom. This paper includes an experimental investigation of multi-storied asymmetric building subjected to harmonic loading with and without MTMD. Experimentation of multi-storied structural model is done on shake table and observation was recorded. The displacements and time history of frame model were recorded and compare with the frames having multiple tuned mass dampers, to study the effectiveness of MTMD for leading toward response

This paper assesses the accuracy of a number of structural-model order-reduction techniques as applied to strain-actuated (smart) flight-vehicle structures modeled with finite elements (FE). Model order reduction is an essential facet when working in a design-oriented optimization environment. Therefore, to eventually build a design-oriented multidisciplinary optimization (MDO) capability for large-scale strain-actuated structures (e.g., aircraft, spacecraft), insight into the affects of model order reduction on the accuracy of design constraints and sensitivities must first be gained. In this paper reduced-order models are solved using mode displacement (MD) and mode acceleration (MA) methods. Furthermore, the effect of the chosen reduction bases on model accuracy is also investigated. Specifically, the total number of basis modes used is varied. Fictitious masses (FMs) may be added to areas of the structure where local effects predominate, and mass-normalized static solution vectors (Ritz vectors) of important actuation modes may be appended to the bases. By examining the convergence of design constraints and their sensitivities for a variety of model order-reduction approaches (solution methods and reduction bases), valuable insight is gained into the technical challenges that must be addressed when creating a robust MDO capability for strain-actuated structures.

A physical domain-based substructuring as a framework for dynamic modeling and reanalysis of systems

Nanodiscs™ consist of small phospholipid bilayer discs surrounded and stabilized by amphiphilic protein belts. Nanodiscs and their confinement and stabilization of nanometer sized pieces of phospholipid bilayer are highly interesting from a membrane physics point of view. We demonstrate how the detailed structure of Di-Lauroyl-Phosphatidyl Choline (DLPC) nanodiscs may be determined by simultaneous fitting of a structural model to small-angle scattering data from the nanodiscs as investigated in three different contrast situations, respectively two SANS contrasts and one SAXS contrast. The article gives a detailed account of the underlying structural model for the nanodiscs and describe how additional chemical and biophysical information can be incorporated in the model in terms of molecular constraints. We discuss and quantify the contribution from the different elements of the structural model and provide very strong experimental support for the nanodiscs as having an elliptical cross-section and with poly-histidine tags protruding out from the rim of the protein belt. The analysis also provides unprecedented information about the structural conformation of the phospholipids when these are localized in the nanodiscs. The model paves the first part of the way in order to reach our long term goal of using the nanodiscs as a platform for small-angle scattering based structural investigations of membrane proteins in solution.

This paper presents a gradient-based aerodynamic shape optimization framework that utilizes a Graphics Processing Unit (GPU) for solving both flow and adjoint equations. It is built based on a GPU-accelerated flow solver that has been developed previously. Hence, the focus of this work is on how to solve the adjoint equations on the GPU and subsequently compute the gradients. The adjoint equations are right-preconditioned by a block Incomplete Lower Upper (ILU) preconditioner and solved by a restarted Generalized Minimum Residual (GMRES) method. The exact residual Jacobian matrix in the adjoint equations is computed using finite difference and a distance-2 graph coloring algorithm. With the adjoint-based gradients, the steepest descent method with momentum is employed for constrained aerodynamic shape optimization of a wing-body configuration at a transonic flow condition.