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The evaluation of the predictive accuracy of dynamic models for future large space structures is addressed. Mass and stiffness uncertainties derived from a comparison of analytical and experimental modes are used to evaluate the uncertainty of response predictions based on the analytical model.
Luminous X-ray gas coronae in the dark matter halos of massive spiral galaxies are a fundamental prediction of structure formation models, yet only a few such coronae have been detected so far. In this paper, we study the hot X-ray coronae beyond the optical disks of two 'normal' massive spirals, NGC 1961 and NGC 6753. Based on XMM-Newton X-ray observations, hot gaseous emission is detected to {approx}60 kpc-well beyond their optical radii. The hot gas has a best-fit temperature of kT {approx} 0.6 keV and an abundance of {approx}0.1 Solar, and exhibits a fairly uniform distribution, suggesting that the quasi-static gas resides in hydrostatic equilibrium in the potential well of the galaxies. The bolometric luminosity of the gas in the (0.05-0.15)r{sub 200} region (r{sub 200} is the virial radius) is {approx}6 Multiplication-Sign 10{sup 40} erg s{sup -1} for both galaxies. The baryon mass fractions of NGC 1961 and NGC 6753 are f{sub b,NGC1961} {approx} 0.11 and f{sub b,NGC6753} {approx} 0.09, which values fall short of the cosmic baryon fraction. The hot coronae around NGC 1961 and NGC 6753 offer an excellent basis to probe structure formation simulations. To this end, the observations are confronted with the moving mesh code AREPO and the smoothed particle hydrodynamics code GADGET. Although neither model gives a perfect description, the observed luminosities, gas masses, and abundances favor the AREPO code. Moreover, the shape and the normalization of the observed density profiles are better reproduced by AREPO within {approx}0.5r{sub 200}. However, neither model incorporates efficient feedback from supermassive black holes or supernovae, which could alter the simulated properties of the X-ray coronae. With the further advance of numerical models, the present observations will be essential in constraining the feedback effects in structure formation simulations.
Zintl phases are compounds that have shown promise for thermoelectric applications. The title solid–solution Zintl compounds were prepared from the elements as single crystals using a tin flux for compositions x = 0, 1, 2, and 3. Eu 11 Cd 6 Sb 12–x As x (x < 3) crystallize isostructurally in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr 11 Cd 6 Sb 12 structure type (Pearson symbol mC58). Efforts to make the As compositions for x exceeding ~3 resulted in structures other than the Sr 11 Cd 6 Sb 12 structure type. Single-crystal X-ray diffraction indicates that As does not randomly substitute for Sb in the structure but is site specific for each composition. The amount of As determined by structural refinement was verified by electron microprobe analysis. Electronic structures and energies calculated for various model structures of Eu 11 Cd 6 Sb 10 As 2 (x = 2) indicated that the preferred As substitution pattern involves a mixture of three of the six pnicogen sites in the asymmetric unit. In addition, As substitution at the Pn4 site opens an energy gap at the Fermi level, whereas substitution at the other five pnicogen sites remains semimetallic with a pseudo gap. Thermoelectric properties of these compounds were measured on hot-pressed, fully densified pellets. Samples show exceptionally low lattice thermal conductivities from room temperature to 775 K: 0.78–0.49 W/mK for x = 0; 0.72–0.53 W/mK for x = 1; and 0.70–0.56 W/mK for x = 2. Eu 11 Cd 6 Sb 12 shows a high p-type Seebeck coefficient (from +118 to 153 μ V/K) but also high electrical resistivity (6.8 to 12.8 mΩ·cm). The value of zT reaches 0.23 at 774 K. The properties of Eu 11 Cd 6 Sb 12–x As x are interpreted in discussion with the As site substitution.
A NASTRAN analysis of the external tank (ET) substructure of the 1/8-scale space shuttle structural dynamics model is described. The NASTRAN hydroelastic procedures were used to form a model of the liquid oxygen portion of the ET. Large computer storage requirements and running times were required unless these procedures were modified. Several possibilities were demonstrated including the substitution of the real for the complex eigenvalue routine and the use of the OMIT capability to reduce the number of fluid coordinates. A NASTRAN model of the complete ET was formed and reduced to 252 degrees of freedom. A review of the eigenvector extracted, using the unsymmetrical inverse power method, indicated that the structural OMIT's resulted in unsatisfactory modal deflections. Modifications to the NASTRAN hydroelastic capability appeared necessary to generate a successful ET model. An approach is described which, by assuming incompressibility, reduces the fluid representation to a symmetric mass matrix which can be added to the structural mass. The problem is solved using faster and more efficient eigenvalue routines. Using this approach the ET NASTRAN model was analyzed for three separate weight conditions.
Given that next-generation infrastructures will contain large numbers of grid-connected inverters and these interfaces will be satisfying a growing fraction of system load, it is imperative to analyze the impacts of power electronics on such systems. However, since each inverter model has a relatively large number of dynamic states, it would be impractical to execute complex system models where the full dynamics of each inverter are retained. To address this challenge, we derive a reduced-order structure-preserving model for parallel-connected grid-tied three-phase inverters. Here, each inverter in the system is assumed to have a full-bridge topology, LCL filter at the point of common coupling, and the control architecture for each inverter includes a current controller, a power controller, and a phase-locked loop for grid synchronization. We outline a structure-preserving reduced-order inverter model for the setting where the parallel inverters are each designed such that the filter components and controller gains scale linearly with the power rating. By structure preserving, we mean that the reduced-order three-phase inverter model is also composed of an LCL filter, a power controller, current controller, and PLL. That is, we show that the system of parallel inverters can be modeled exactly as one aggregated inverter unit and this equivalent model has the same number of dynamical states as an individual inverter in the paralleled system. Numerical simulations validate the reduced-order models.
Next-generation power networks will contain large numbers of grid-connected inverters satisfying a significant fraction of system load. Since each inverter model has a relatively large number of dynamic states, it is impractical to analyze complex system models where the full dynamics of each inverter are retained. To address this challenge, we derive a reduced-order structure-preserving model for parallel-connected grid-tied three-phase inverters. Here, each inverter in the system is assumed to have a full-bridge topology, LCL filter at the point of common coupling, and the control architecture for each inverter includes a current controller, a power controller, and a phase-locked loop for grid synchronization. We outline a structure-preserving reduced-order inverter model with lumped parameters for the setting where the parallel inverters are each designed such that the filter components and controller gains scale linearly with the power rating. By structure preserving, we mean that the reduced-order three-phase inverter model is also composed of an LCL filter, a power controller, current controller, and PLL. We show that the system of parallel inverters can be modeled exactly as one aggregated inverter unit and this equivalent model has the same number of dynamical states as any individual inverter in the system. Numerical simulations validate the reduced-order model.
Our work proposes a model-reduction methodology that preserves Lagrangian structure and achieves computational efficiency in the presence of high-order nonlinearities and arbitrary parameter dependence. As such, the resulting reduced-order model retains key properties such as energy conservation and symplectic time-evolution maps. We focus on parameterized simple mechanical systems subjected to Rayleigh damping and external forces, and consider an application to nonlinear structural dynamics. To preserve structure, the method first approximates the system's “Lagrangian ingredients''---the Riemannian metric, the potential-energy function, the dissipation function, and the external force---and subsequently derives reduced-order equations of motion by applying the (forced) Euler--Lagrange equation with these quantities. Moreover, from the algebraic perspective, key contributions include two efficient techniques for approximating parameterized reduced matrices while preserving symmetry and positive definiteness: matrix gappy proper orthogonal decomposition and reduced-basis sparsification. Our results for a parameterized truss-structure problem demonstrate the practical importance of preserving Lagrangian structure and illustrate the proposed method's merits: it reduces computation time while maintaining high accuracy and stability, in contrast to existing nonlinear model-reduction techniques that do not preserve structure.
A simple model for the production and propagation of magnetized loops in the Galactic center is developed. It is found that a magnetized model can reproduce the filamentary, asymmetric structures on all scales from 100 pc to subparsec scales in the Galactic center. The radio and molecular gas morphology on all scales inside the Galactic center lobe may be explained in terms of the expansion of strongly magnetized loops and their interaction with the 2-5 pc torus and 30-50 pc portion of the center lobe. The magnetic loops have a toroidal field which explains the braided appearance of the radio continuum emission on the 2 pc scale. The barlike feature on the 2 pc radio continuum maps can be viewed as a piece of a magnetic loop with field strength about 0.01 G which is colliding and reconnecting with the ionized inner edge of the 2 pc molecular torus. The loop-torus interaction feeds energetic particles and shear Alfven waves into the torus, and deposits mass and energy.
The analytical models and results of a structural dynamics investigation of the reference initial operation and evolutionary configurations of the nine foot bay space station are presented. This investigation was carried out between April and August 1984 as part of a team effort to define a reference configuration for the first U.S. manned space station. The results presented herein serve as a guide, a point of departure and a standard for future NASA and contractor studies leading to the design of the Space Station. The reference initial operation configuration of the nine foot bay station was found to be very flexible, with its lowest mode between 0.096 and 0.138 Hertz depending on station attachments. However, for the transient load cases which were then available, internal member loads had positive margins of safety and preliminary results indicate that laboratory experiments which require quiescent conditions can be satisfied down to the order of 0.0001 g's.
Lignin, a major biomass component, can be an excellent source for different monomers in the polymer industry. However, the complex and heterogeneous structure of lignin poses a significant challenge for designing energy-efficient processes for depolymerization. As many proposed depolymerization processes are solvothermal, it is essential to understand the structure and dynamics of lignin in solution, in particular aqueous solution. Here, we utilize molecular dynamics simulations to understand the effect of water models on the structure and dynamics of different model lignin oligomers (softwood and hardwood) as a function of temperature. We have examined three different water models: TIP3P, TIP4P/Ew, and flexible SPC/Fw. We find that the diffusion constant of lignin oligomers in an aqueous solution differs significantly depending on the water model used. The diffusion constant of lignin in the TIP3P water model is almost twice as large as that in SPC/Fw and TIP4P/Ew. The softwood and hardwood oligomers adopt an extended structure in TIP3P water compared to SPC/Fw and TIP4P/Ew. Given the different levels of sensitivity of transport and structural properties of aqueous lignin on water models, it is important to take these into account when discussing results from a specific water model.
We examine basic issues involved in synthesizing resonance-line profiles from 1-D, dynamical models of highly structured hot-star winds. Although these models exhibit extensive variations in density as well as velocity, the density scale length is still typically much greater than the Sobolev length. The line transfer is thus treated using a Sobolev approach, as generalized by Rybicki & Hummer (1978) to take proper account of the multiple Sobolev resonances arising from the nonmonotonic velocity field. The resulting reduced-lambda-matrix equation describing nonlocal coupling of the source function is solved by iteration, and line profiles are then derived from formal solution integration using this source function. Two more approximate methods that instead use either a stationary or a structured, local source function yield qualitatively similar line-profiles, but are found to violate photon conservation by 10% or more. The full results suggest that such models may indeed be able to reproduce naturally some of the qualitative properties long noted in observed UV line profiles, such as discrete absorption components in unsaturated lines, or the blue-edge variability in saturated lines. However, these particular models do not yet produce the black absorption troughs commonly observed in saturated lines, and it seems that this and other important discrepancies (e.g., in acceleration time scale of absorption components) may require development of more complete models that include rotation and other 2-D and/or 3-D effects.
A monograph which assesses the state of the art of space vehicle design and development is presented. The monograph presents criteria and recommended practices for determining the structural data and a mathematical structural model of the vehicle needed for accurate prediction of structure and control-system interaction; for design to minimize undesirable interactions between the structure and the control system; and for determining techniques to achieve the maximum desirable interactions and associated structural design benefits. All space vehicles are treated, including launch vehicles, spacecraft, and entry vehicles. Important structural characteristics which affect the structural model used for structural and control-system interaction analysis are given.
Errors and previously implicit assumptions treated explicitly. Paper discusses errors in mathematical models of physical systems and problem of identifying model from system input and output. Problem approached by explicitly taking account of erroneous choices of model structure. Paper concerned specifically with linear or weakly nonlinear models.
Two ideas regarding the structure of turbulence near a clear air-water interface are used to derive a waterside gas transfer velocity kL for sparingly and slightly soluble gases. The first is that k L is proportional to the turnover velocity described by the vertical velocity structure function D ww (r), where r is separation distance between two points. The second is that the scalar exchange between the air-water interface and the waterside turbulence can be suitably described by a length scale proportional to the Batchelor scale lB=ηSc -1/2 , where Sc is the molecular Schmidt number and η is the Kolmogorov microscale defining the smallest scale of turbulent eddies impacted by fluid viscosity. Using an approximate solution to the von Kármán-Howarth equation predicting D ww (r) in the inertial and viscous regimes, prior formulations for k L are recovered including (i) k L =$\sqrt{2/15}$Sc -1/2 $v$, v K is the Kolmogorov velocity defined by the Reynolds number v K η/ν = 1 and ν is the kinematic viscosity of water; (ii) surface divergence formulations; (iii) k L ∝ SC -1/2 u * , where u * is the waterside friction velocity; (iv) k L ∝ Sc -1/2 $\sqrt{gv/u_*}$ for Keulegan numbers exceeding a threshold needed for long-wave generation, where the proportionality constant varies with wave age, g is the gravitational acceleration; and (v) k L = $\sqrt{2/15}$Sc -1/2 (vgβ o q o ) 1/4 in free convection, where qo is the surface heat flux and β o is the thermal expansion of water. The work demonstrates that the aforementioned k L formulations can be recovered from a single structure function model derived for locally homogeneous and isotropic turbulence.
The atomic structure of calcium-silicate-hydrate (C{sub 1.67}-S-H{sub x}) has been investigated by theoretical methods in order to establish a better insight into its structure. Three models for C-S-H all derived from tobermorite are proposed and a large number of structures were created within each model by making a random distribution of silica oligomers of different size within each structure. These structures were subjected to structural relaxation by geometry optimization and molecular dynamics steps. That resulted in a set of energies within each model. Despite an energy distribution between individual structures within each model, significant energy differences are observed between the three models. The C-S-H model related to the lowest energy is considered as the most probable. It turns out to be characterized by the distribution of dimeric and pentameric silicates and the absence of monomers. This model has mass density which is closest to the experimental one.
The objective of the structural deflection test was to verify the analytical models used to predict solar cell stress, load-bearing layer stress, and module deflection that result from a uniform pressure load on the module surface. The verification process consisted of measuring module deflection, stress in the solar cell, and stress in the load-bearing member as a functon of normal pressure load, pottant modulus of elasticity, pottant thickness, and cell location. Four glass superstrate modules, two plain wood substrate modules, two ribbed wood substrate modules, and a steel substrate module were tested. Consistent with analysis predictions, all unribbed specimens exhibited nonlinear load vs. deflection characteristics. With the exception of the steel module, test results and analysis predictions for panel deflectons agreed to within 10 percent. A wider variation of agreement was found for stress in the load-bearing members and cells.
Design, analysis, and testing of an experimental space station scale model is presented. The model contains hardware components with dynamic characteristics similar to those expected for other large space structures. Validation of analysis models is achieved through correlation with dynamic tests of hardware components and representative assembly configurations. A component mode synthesis analysis method is examined through comparisons with results from fully mated system models. Selection of input requirements for accurate component synthesis analysis predictions are assessed.
The crystal structure of Al{sub 9}O{sub 3}N{sub 7} was characterized by laboratory X-ray powder diffraction (CuKα{sub 1}). The title compound is trigonal with space group R3-bar m (centrosymmetric). The hexagonal unit-cell dimensions (Z=3) are a=0.30656(2) nm, c=7.2008(3) nm and V=0.58605(5) nm{sup 3}. The initial structural model was derived by the powder charge-flipping method and subsequently refined by the Rietveld method. The final structural model showed the positional disordering of two of the five types of Al sites. The maximum-entropy method-based pattern fitting method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The disordered crystal structure was successfully described by overlapping five types of domains with ordered atom arrangements. The distribution of atomic positions in one of the five types of domains can be achieved in the space group R3{sup ¯}m. The atom arrangements in the four other domains are noncentrosymmetric with the space group R3m. Two of the four types of domains are related by a pseudo-symmetry inversion, and the two remaining domains also have each other the inversion pseudo-symmetry. The very similar domain structure has been also reported for 21R-AlON (Al{sub 7}O{sub 3}N{sub 5}) in our previous study. - Graphical abstract: A bird’s eye view of electron densities up to 50% (0.074 nm{sup −3}) of the maximum on the plane parallel to (110) with the corresponding atomic arrangements of Al{sub 9}O{sub 3}N{sub 7}. Highlights: • Crystal structure of Al{sub 9}O{sub 3}N{sub 7} is determined by laboratory X-ray powder diffraction. • The atom arrangements are represented by the split-atom model. • The maximum-entropy method-based pattern fitting method is used to confirm the validity of the model. • The disordered structure is described by overlapping five types of domains with ordered atom arrangements.