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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

A complete model of turbulence

A set of constitutive equations suitable for a priori computation of turbulent shear flows has been developed. Since no properties of a given turbulent flow need be known in advance in order to obtain a solution, the equations comprise a complete model of turbulence. Perturbation analysis shows that the model predicts a composite five-layer structure for an incompressible turbulent boundary layer, viz, a defect layer, a law-of-the-wall layer, a viscous sublayer, a near-surface roughness layer, and a viscous superlayer at the boundary-layer edge. Analysis of the defect layer demonstrates the key improvement of the model over its predecessor, the Saffman-Wilcox two-equation model of turbulence. Examination of model-predicted sublayer structure yields model-parameter boundary conditions appropriate for surfaces with roughness and mass injection. Results of numerical computations of compressible and incompressible equilibrium boundary layers show that, for such flows, the model is as accurate as mixing-length theory. Applications to transitional boundary layers and to nonequilibrium relaxation of a boundary layer passing from a rough to a smooth surface indicate that the model's applicability extends far beyond that of mixing-length theory's.

Wilcox, D. C.↗
NASA NTRS2007

GVIPS Models and Software

Two reports discuss, respectively, (1) the generalized viscoplasticity with potential structure (GVIPS) class of mathematical models and (2) the Constitutive Material Parameter Estimator (COMPARE) computer program. GVIPS models are constructed within a thermodynamics- and potential-based theoretical framework, wherein one uses internal state variables and derives constitutive equations for both the reversible (elastic) and the irreversible (viscoplastic) behaviors of materials. Because of the underlying potential structure, GVIPS models not only capture a variety of material behaviors but also are very computationally efficient. COMPARE comprises (1) an analysis core and (2) a C++-language subprogram that implements a Windows-based graphical user interface (GUI) for controlling the core. The GUI relieves the user of the sometimes tedious task of preparing data for the analysis core, freeing the user to concentrate on the task of fitting experimental data and ultimately obtaining a set of material parameters. The analysis core consists of three modules: one for GVIPS material models, an analysis module containing a specialized finite-element solution algorithm, and an optimization module. COMPARE solves the problem of finding GVIPS material parameters in the manner of a design-optimization problem in which the parameters are the design variables.

Arnold, Steven M.↗
NASA NTRS1989

The effect of an unsteady three-dimensional wake on elastic blade-flapping eigenvalues in hover

This paper describes the formulation of a finite-state inflow model based on an acceleration potential and a helical wake geometry. The states of the model are coefficients of an inflow expansion in terms of a Fourier series (azimuthally) and of special polynomials (radially). The integrals over the wake are done in closed-form to obtain a set of ordinary differential equations for the inflow coefficients. The forcing functions for these equations are generalized forces which are integrals of the blade loading exactly as in structural dynamics. This model implicitly includes (for the hover case) Prandtl-Goldstein tip losses, dynamic inflow, and Theodorsen/Loewy lift deficiency. Thus, it is a fully three-dimensional unsteady wake model. Here, this model is coupled with elastic-blade equations in hover and eigenvalues are found. The results show that the three-dimensional wake has a large effect on the flap damping of all modes.

Peters, David A.↗
NASA NTRS1987

Aeroelastic analysis of launch vehicles in transonic flight

A numerical study of the aeroelastic stability of launch vehicles in transonic flight is performed. The unsteady aerodynanic flowfield about the vehicle is simulated using the three dimensional Reynolds-Averaged Navier-Stokes equations with the account for turbulent mixing done by a two-layer algebraic eddy viscosity model. The structural-dynamic formulation considers free-free flexural vibration of an elongated beam with variable properties and the equations are cast in modal form. The two sets of equations are integrated simultaneously in time, after a initial perturbation, to ascertain the aeroelastic stability of a given configuration. The method propossed is described in detail, and results that illustrate its application are presented.

Azevedo, Joao Luiz F.↗
NASA NTRS1992

Modal simulation of gearbox vibration with experimental correlation

A newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center. Experimental results from the test rig were used to verify the analytical model. In this global dynamic model, the number of degrees of freedom of the system are reduced by transforming the system equations of motion into modal coordinates. The vibration of the individual gear-shaft system are coupled through the gear mesh forces. A three-dimensional, axial-lateral coupled, bearing model was used to couple the casing structural vibration to the gear-rotor dynamics. The coupled system of modal equations is solved to predict the resulting vibration at several locations on the test rig. Experimental vibration data was compared to the predictions of the global dynamic model. There is excellent agreement between the vibration results from analysis and experiment.

Choy, Fred K.↗
NASA NTRS1992

Modal simulation of gearbox vibration with experimental correlation

A newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center. Experimental results from the test rig were used to verify the analytical model. In this global dynamic model, the number of degrees of freedom of the system are reduced by transforming the system equations of motion into modal coordinates. The vibration of the individual gear-shaft system are coupled through the gear mesh forces. A three-dimensional, axial-lateral coupled, bearing model was used to couple the casing structural vibration to the gear-rotor dynamics. The coupled system of modal equations is solved to predict the resulting vibration at several locations on the test rig. Experimental vibration data was compared to the predicitions of the global dynamic model. There is excellent agreement between the vibration results from analysis and experiment.

Choy, Fred K.↗
NASA NTRS2012

Electron Beam Induced Luminescence of SiO2 Optical Coatings

Optical coatings of disordered thin film SiO,/SiO, samples undergoing electron beam bombardment exhibited a cathodoluminescence effect. Measurements were made of the absolute illuminance and emission spectra as functions of incident electron energy (0.2 keV to 30 keV). flux (approx. 0.1 nA/sq cm to > 100 nA/sq cm) and power (<50 nW/sq cm to > 3mW/sq cm) for coatings (approx.60 nm to approx.200 nm thick) on reflective metal substrates over a range of sample temperatures (approx.40 K to approx.400 K) and emission wavelengths (approx.260 nm to approx.5000 nm). Illuminance reached a plateau saturation intensity I(sub sat) approx. 10(exp -4)W/sq cm. scaling as [I(sub mc)/(I + I(sub mc)/I(sub sat)] as incident power I(sub mc) approached I(sub sat). Well below I(sub sat), illuminance scaled linearly with incident power for lower-energy non-penetrating radiation that deposited all of its energy in the dielectric film. At higher incident energies (> 1-5 keV) electrons penetrate the dielectric, depositing only a fraction of their energy in the dielectric film, where deposited energy scales as the ratio of film thickness to penetration range. Since electron range exhibits a peak below approx. 1 keV, the illuminance decreased with increasing energy for penetrating radiation. The temperature dependence of the illuminance. in general, increased with decreasing temperature. Changes in apparent color with temperature were also observed. Four bands in the UV/VIS/NIR range from 300 nm to 1000 nm were observed in spectral measurements; no significant luminescence was detected in a range from approx. 1000 nm to approx. 5000 nm. These different bands either increased or decreased in intensity and shifted slightly in energy with decreasing temperature. Our measurements are explained in terms of simple disordered band theory models based on atomic-scale stochastic transport of electrons in highly disordered insulating materials. The theoretical models provide a fundamental basis for understanding the dependence of cathodoluminescence on irradiation time, incident flux and energy. and sample thickness and temperature. The observed luminescence occurs when an incident high energy electron excites a valence band electron into the conduction band. Following the excitation. a complex series of transitions take place between the extended conduction states and the long-lived localized trapped states (or chromophores) associated with structural or compositional defects in the dielectric materials. Thc transport equations and models of thc spatial and energy distribution of the trapped states below the mobility edge, their occupancy, and transitions provide microscopic scale insights into this macroscopic phenomenon. Our observations and theoretical models are also compared with similar previous room temperature studies of similar SiO2 films.

Dennison, J. R.↗
NASA NTRS1983

Buckling of imperfect periodic lattice structures

A simplified buckling analysis is presented for a family of periodic lattice structures such as those proposed for large space structures. A transcendental 6 x 6 matrix of eigenvalues is shown to be sufficient for modeling buckling behavior because member stiffnesses are based on an exact solution of the beam-column equation. Exact stiffnesses are derived for a curved member, thus allowing modeling of imperfect lattice structures. Comparisons of predictions of the lattice model with those available from shell and beam theory underscore the inaccuracies introduced by treating the lattice structure as a continuum. Sample calculations are provided for an isogrid cylinder and a three element double-laced truss.

Anderson, M. S.↗
NASA NTRS1994

Large-scale processes in the solar nebula

Theoretical models of the structure of a minimum mass solar nebula should be able to provide the physical context to help evaluate the efficacy of any mechanism proposed for the formation of chondrules or Ca, Al-rich inclusions (CAI's). These models generally attempt to use the equations of radiative hydrodynamics to calculate the large-scale structure of the solar nebula throughout the planet-forming region. In addition, it has been suggested that chondrules and CAI's (=Ch&CAI's) may have been formed as a direct result of large-scale nebula processing such as passage of material through high-temperature regions associated with the global structure of the nebula. In this report we assess the status of global models of solar nebula structure and of various related mechanisms that have been suggested for Ch and CAI formation.

Boss, A. P.↗
NASA NTRS2001

The Numerical Analysis of a Turbulent Compressible Jet

A numerical method to simulate high Reynolds number jet flows was formulated and applied to gain a better understanding of the flow physics. Large-eddy simulation was chosen as the most promising approach to model the turbulent structures due to its compromise between accuracy and computational expense. The filtered Navier-Stokes equations were developed including a total energy form of the energy equation. Subgrid scale models for the momentum and energy equations were adapted from compressible forms of Smagorinsky's original model. The effect of using disparate temporal and spatial accuracy in a numerical scheme was discovered through one-dimensional model problems and a new uniformly fourth-order accurate numerical method was developed. Results from two- and three-dimensional validation exercises show that the code accurately reproduces both viscous and inviscid flows. Numerous axisymmetric jet simulations were performed to investigate the effect of grid resolution, numerical scheme, exit boundary conditions and subgrid scale modeling on the solution and the results were used to guide the three-dimensional calculations. Three-dimensional calculations of a Mach 1.4 jet showed that this LES simulation accurately captures the physics of the turbulent flow. The agreement with experimental data was relatively good and is much better than results in the current literature. Turbulent intensities indicate that the turbulent structures at this level of modeling are not isotropic and this information could lend itself to the development of improved subgrid scale models for LES and turbulence models for RANS simulations. A two point correlation technique was used to quantify the turbulent structures. Two point space correlations were used to obtain a measure of the integral length scale, which proved to be approximately 1/2 D(sub j). Two point space-time correlations were used to obtain the convection velocity for the turbulent structures. This velocity ranged from 0.57 to 0.71 U(sub j).

DeBonis, James R.↗
NASA NTRS1993

Structural similitude and design of scaled down laminated models

The excellent mechanical properties of laminated composite structures make them prime candidates for wide variety of applications in aerospace, mechanical and other branches of engineering. The enormous design flexibility of advanced composites is obtained at the cost of large number of design parameters. Due to complexity of the systems and lack of complete design based informations, designers tend to be conservative in their design. Furthermore, any new design is extensively evaluated experimentally until it achieves the necessary reliability, performance and safety. However, the experimental evaluation of composite structures are costly and time consuming. Consequently, it is extremely useful if a full-scale structure can be replaced by a similar scaled-down model which is much easier to work with. Furthermore, a dramatic reduction in cost and time can be achieved, if available experimental data of a specific structure can be used to predict the behavior of a group of similar systems. This study investigates problems associated with the design of scaled models. Such study is important since it provides the necessary scaling laws, and the factors which affect the accuracy of the scale models. Similitude theory is employed to develop the necessary similarity conditions (scaling laws). Scaling laws provide relationship between a full-scale structure and its scale model, and can be used to extrapolate the experimental data of a small, inexpensive, and testable model into design information for a large prototype. Due to large number of design parameters, the identification of the principal scaling laws by conventional method (dimensional analysis) is tedious. Similitude theory based on governing equations of the structural system is more direct and simpler in execution. The difficulty of making completely similar scale models often leads to accept certain type of distortion from exact duplication of the prototype (partial similarity). Both complete and partial similarity are discussed. The procedure consists of systematically observing the effect of each parameter and corresponding scaling laws. Then acceptable intervals and limitations for these parameters and scaling laws are discussed. In each case, a set of valid scaling factors and corresponding response scaling laws that accurately predict the response of prototypes from experimental models is introduced. The examples used include rectangular laminated plates under destabilizing loads, applied individually, vibrational characteristics of same plates, as well as cylindrical bending of beam-plates.

Simitses, G. J.↗
NASA NTRS1980

On the performance of explicit and implicit algorithms for transient thermal analysis

The status of an effort to increase the efficiency of calculating transient temperature fields in complex aerospace vehicle structures is described. The advantages and disadvantages of explicit and implicit algorithms are discussed. A promising set of implicit algorithms, known as the GEAR package is described. Four test problems, used for evaluating and comparing various algorithms, have been selected and finite element models of the configurations are discribed. These problems include a space shuttle frame component, an insulated cylinder, a metallic panel for a thermal protection system and a model of the space shuttle orbiter wing. Calculations were carried out using the SPAR finite element program, the MITAS lumped parameter program and a special purpose finite element program incorporating the GEAR algorithms. Results generally indicate a preference for implicit over explicit algorithms for solution of transient structural heat transfer problems when the governing equations are stiff. Careful attention to modeling detail such as avoiding thin or short high-conducting elements can sometimes reduce the stiffness to the extent that explicit methods become advantageous.

Adelman, H. M.↗
NASA NTRS2003

Collaborative Research: Atlantic Ocean Tropical/Subtropical Processes from Seasonal to Decadal Time Scales: Model/Data, Model/Model Comparison and Model/Data Synthesis Through Assimilation

The effort of this first year of research has been focused on the assimilation of TOPEX/Poseidon altimetric data into a primitive equation model of the Atlantic tropical/subtropical circulation. A reduced-rank, stationary Kalman filter has been constructed to assimilate the altimetric sea surface height anomaly (SHA) into the model. The goal is to assess how the inter-hemispheric transports between the Atlantic subtropics and tropics are affected by the assimilation and how the subsurface thermocline structure , and its variability ,is dynamically constrained by the SHA. The model is a reduced-gravity primitive equation GCM of the upper Atlantic Ocean between 30 S and 30 N. The assimilation scheme is an approximation to the extended Kalman filter in which the error covariances of the state estimates are calculated only in a reduced- dimension subspace. The subspace is defined by the leading empirical orthogonal functions calculated from an unconstrained model calculation. Both an identical twin experiment using simulated SHA observations and assimilation of the real TOPEX data were performed. Results from the twin experiments demonstrate the ability of the method to constrain the ocean circulation and the subsurface temperature structure. The impact on the subsurface temperature structure of TOPEX assimilation was assessed using data from expandable bathythermographs. This showed a substantial improvement in the estimated temperature variability only within 13 degrees in latitude around the equator. The impact of TOPEX SHA assimilation on zonally integrated meridional transport across different latitudes was also estimated. Again within 13 degrees from the equator both the mean amplitude and interannual variability of the surface and subsurface transports were significantly enhanced, while the transports were insensitive to the assimilation in the subtropics.

Malannotte-Rizzoli, Paola↗
NASA NTRS2012

Comparison of MSL RCS Jet Computations With Flow Visualization and Velocimetry

Numerical predictions of the Mars Science Laboratory (MSL) reaction control system (RCS) jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide (NO) planar laser-induced fluorescence (PLIF) data. The steady Reynolds Averaged Navier Stokes (RANS) equations using the Baldwin-Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental PLIF data used for comparison consists of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the PLIF signal equation, computational flow images (CFI) were produced and directly compared to the qualitative PLIF data. Post processing of the experimental and simulation data enabled the jet trajectory to be extracted for a more quantitative comparison. The two-dimensional velocity fields were reconstructed through interpolation of a series of single-component velocity profiles. Each distribution of single-component velocity was obtained using molecular tagging velocimetry (MTV). After validating the numerical model, the numerical solution was further examined to gain insight into hypersonic jet-in-crossflow interaction. Future NO-PLIF experiments are proposed based on this analysis.

Johansen, Craig T.↗
NASA NTRS2013

Comparison of Mars Science Laboratory Reaction Control System Jet Computations With Flow Visualization and Velocimetry

Numerical predictions of the Mars Science Laboratory reaction control system jets interacting with a Mach 10 hypersonic flow are compared to experimental nitric oxide planar laser-induced fluorescence data. The steady Reynolds Averaged Navier Stokes equations using the Baldwin-Barth one-equation turbulence model were solved using the OVERFLOW code. The experimental fluorescence data used for comparison consists of qualitative two-dimensional visualization images, qualitative reconstructed three-dimensional flow structures, and quantitative two-dimensional distributions of streamwise velocity. Through modeling of the fluorescence signal equation, computational flow images were produced and directly compared to the qualitative fluorescence data.

Bathel, Brett F.↗
NASA NTRS1975

Structure of turbulent jets and wakes

The structure of two-dimensional turbulent jets and wakes is studied using two eddy viscosity models. The turbulent energy equation is used together with the mean momentum equations, and the system is closed by introducing eddy coefficients. The fine structure turbulence is obtained by applying proper boundary conditions at the mean turbulent interface position. The results, after a second averaging process, are compared with available experimental data.

Wang, F. C.↗
NASA NTRS1995

Coronal roots of solar wind streams: 3-D MHD modeling

Weak (discontinuous) solutions of the 3-D MHD equations look like a promising tool to model the transonic solar wind with structural elements: current sheets, coronal plumes etc. Using the observational information about various coronal emissions one can include these structural elements into the 3-D MHD solar wind model by embedding the discontinuities of given type. Such 3-D MHD structured solar wind is calculated self-consistently: variants are examined via numerical experiments. In particular, the behavior of coronal plumes in the transonic solar wind flow, is modeled. The input information for numerical modeling (for example, the magnetic field map at the very base of the solar corona) can be adjusted so that fast stream arises over the center of the coronal hole, over the coronal hole boundaries and, even, over the region with closed magnetic topology. 3-D MHD equations have the analytical solution which can serve as a model of supersonic trans-alfvenic solar wind in the (5-20) solar radii heliocentric distance interval. The transverse, nonradial total (gas + magnetic field) pressure balance in the flow is the corner-stone of this solution. The solution describes the filamentation (ray-like structure of the solar corona) and streaming (formation of high-speed streams with velocities up to 800 km/sec) as a consequence of the magnetic field spatial inhomogeneous structure and trans-alfvenic character of the flow. The magnetic field works in the model as a 'controller' for the solar wind streaming and filamentation.

Pisanko, Yu. V.↗
NASA NTRS1973

Turbulent kinetic energy equation and free mixing

Calculation of free shear flows was carried out to investigate the usefulness of several concepts which were previously successfully applied to wall flows. The method belongs to the class of differential approaches. The turbulence is taken into account by the introduction of one additional partial differential equation, the transport equation for the turbulent shear stress. The structure of turbulence is modeled after Bradshaw et al. This model was used successfully in boundary layers and its applicability to other flows is demonstrated. The work reported differs substantially from that of an earlier attempt to use this approach for calculation of free flows. The most important difference is that the region around the center line is treated by invoking the interaction hypothesis (concerning the structure of turbulence in the regions separated by the velocity extrema). The compressibility effects on shear layer spreading at low and moderate Mach numbers were investigated. In the absence of detailed experiments in free flows, the evidence from boundary layers that at low Mach numbers the structure of turbulence is unaffected by the compressibility was relied on. The present model was tested over a range of self-preserving and developing flows including pressure gradients using identical empirical input. The dependence of the structure of turbulence on the spreading rate of the shear layer was established.

Morel, T.↗