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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 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

An optimization-based approach to parameter learning for fractional type nonlocal models

Nonlocal operators of fractional type are a popular modeling choice for applications that do not adhere to classical diffusive behavior; however, one major challenge in nonlocal simulations is the selection of model parameters. In this study we propose an optimization-based approach to parameter identification for fractional models with an optional truncation radius. We formulate the inference problem as an optimal control problem where the objective is to minimize the discrepancy between observed data and an approximate solution of the model, and the control variables are the fractional order and the truncation length. For the numerical solution of the minimization problem we propose a gradient-based approach, where we enhance the numerical performance by an approximation of the bilinear form of the state equation and its derivative with respect to the fractional order. Several numerical tests in one and two dimensions illustrate the theoretical results and show the robustness and applicability of our method.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Adsorption of Tracer Gases in Geological Media: Experimental Benchmarking

The interaction of xenon with containment barriers can play a crucial role in the detection of underground nuclear explosions. Independent field experiments and laboratory-derived models have observed depressed surface level xenon concentrations and subsequently predicted geological capture rates. This work sought to experimentally verify those predictions by analyzing xenon transport though a two-bulb diffusion apparatus adapted for the study of geologic media. Novel to this work is the timescale over which these experiments were carried out, allowing the system to reach equilibrium, rather than relying on numerical models to treat the equilibrium concentration of each gas as a fit coefficient.

Byers, Maggie↗
DOE OSTI2020

Baroclinic Residual Circulation and Mass Transport Due to Internal Tides

Baroclinic (BC) tidal residual circulation due to internal tides is investigated around islands over a shallow ridge using a numerical ocean model. Internal tides enhance vertical mixing over shallow slopes, leading to horizontal density gradients that drive BC residual circulation along the main thermocline. For a strongly stratified summer case, the vertical diffusivity estimated by the Mellor and Yamada turbulence closure model exceeds 1 × 10 -2 m 2 s -1 , and the velocity of BC residual circulations reaches 0.2 m s -1 . The magnitude of BC residual circulation is larger than that of barotropic residual tidal circulation, implying that BC residual circulation due to internal tides plays an important role in coastal ocean circulation. Furthermore, BC residual flow accounts for an equal percentage (5%) of the total tidal kinetic energy as the barotropic residual flow under summer stratification conditions. Results from a coupled sediment resuspension and transport model show the growth of intermediate nepheloid layers formed by strong bottom shear stress and BC residual flow. The residual component contributes largely (23% of the total in summer) to the total suspended sediment flux. Seasonal variability is explored, with weaker winter stratification leading to reduced mixing, and thus weaker BC residual circulation and sediment flux. Lastly, the magnitude of the BC residual circulation is also shown to be proportional to the square of the tidal amplitude.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Predictive Large-Eddy Simulation of Supercritical-Pressure Reactive Flows in the Cold Ignition Regime

This report describes a one-year study performed under DOE sponsorship, continuing the investigation of high-pressure turbulent reactive flows. The interest is in the effect of the chemical species distribution in high-pressure turbulent flows in the presence of strong temperature gradients as would occur during reactions in realistic flows. The prime example of such flows are boundary layers in which the wall is at a lower temperature than that of the fluid, as would be the case in Diesel engines. Previous DOE BES work in the program further highlighted the importance of the boundary layer as a configuration for fundamental studies: soot formation in boundary layers is still a problem poorly understood and depends on the availability of particular chemical species at that location, and when boundary layer Large Eddy Simulations results were compared with measurements, the agreement was unfavorable, showing that this important ‘unit’ problem is not well understood. To understand this unit problem, modeling and Direct Numerical Simulations of this unit problem were conducted for the simplest possible multispecies system, that is a binary-species system. The results discovered a new phenomenon, that is, Soret effect induced uphill diffusion. The far-reaching implication is that through an imposed wall/fluid temperature difference it is possible to control the distribution of the species in the boundary layer. Because the results have been documented in one paper published in the refereed literature, and also in conference papers, this final report is in the form of an Executive Summary succinctly describing the results and putting them in perspective with respect to existing information. The refereed and conference papers published are individually listed as Appendices and attached to this report. One manuscript is still in progress and is thus not listed.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1985

Summary of process research analysis efforts

A summary of solar-cell process research analysis efforts was presented. Process design and cell design are interactive efforts where technology from integrated circuit processes and other processes are blended. The primary factors that control cell efficiency are: (1) the bulk parameters of the available sheet material, (2) the retention and enhancement of these bulk parameters, and (3) the cell design and the cost to produce versus the finished cells performance. The process sequences need to be tailored to be compatible with the sheet form, the cell shape form, and the processing equipment. New process options that require further evaluation and utilization are lasers, robotics, thermal pulse techniques, and new materials. There are numerous process control techniques that can be adapted and used that will improve product uniformity and reduced costs. Two factors that can lead to longer life modules are the use of solar cell diffusion barriers and improved encapsulation.

Burger, D. R.↗
NASA NTRS1987

Numerical solution of the radiation transport equation in disk geometry

An efficient numerical method for solving the problem of radiation transport in a dusty medium with two dimensional (2-D) disk geometry is described. It is a generalization of the one-dimensional quasi-diffusion method in which the transport equation is cast in diffusion form and then solved as a boundary value problem. The method should be applicable to a variety of astronomical sources, the dynamics of which are angular-momentum dominated and hence not accurately treated by spherical geometry, e.g., protoplanetary nebulae, circumstellar disks, interstellar molecular clouds, accretion disks, and disk galaxies. The computational procedure and practical considerations for implementing the method are described in detail. To illustrate the effects of 2-D radiation transport, some model results (dust temperature distributions and IR flux spectra) for externally heated, interstellar dust clouds with spherically symmetric and disk geometry are compared.

Spagna, George F., Jr.↗
NASA NTRS1995

Application of the space-time conservation element and solution element method to two-dimensional advection-diffusion problems

The existing 2-D alpha-mu scheme and alpha-epsilon scheme based on the method of space-time conservation element and solution element, which were constructed for solving the linear 2-D unsteady advection-diffusion equation and unsteady advection equation, respectively, are tested. Also, the alpha-epsilon scheme is modified to become the V-E scheme for solving the nonlinear 2-D inviscid Burgers equation. Numerical solutions of six test problems are presented in comparison with their exact solutions or numerical solutions obtained by traditional finite-difference or finite-element methods. It is demonstrated that the 2-D alpha-mu, alpha-epsilon, and nu-epsilon schemes can be used to obtain numerical results which are more accurate than those based on some of the traditional methods but without using any artificial tuning in the computation. Similar to the previous 1-D test problems, the high accuracy and simplicity features of the space-time conservation element and solution element method have been revealed again in the present 2-D test results.

Wang, Xiao-Yen↗
NASA NTRS2023

Numerical Investigation of Film Coefficient Approximation for Chemically Reacting Boundary-Layer Flows

Aerothermal analysis of spacecraft planetary entry is heavily dependent on heritage engineering models. The film coefficient heat transfer model examined in this paper estimates the convective heating to the vehicle for a laminar, dissociated, chemically reacting boundary layer for an Earth atmosphere. This model requires information about the vehicle and flowfield for a given trajectory point and estimates a proportional relationship between enthalpy potential and convective heat flux. In practice it is the aerothermal engineer who must decide which assumptions are appropriate for his/her application. This work looks at numerous CFD simulations for an arbitrary, axisymmetric flight vehicle to analyze the relative importance of both the mass and energy constraints imposed at the wall boundary, as well as the effect of various diffusion models. Within the subset of tested energy boundary conditions, it is found that the most desirable energy boundary condition is the radiative equilibrium boundary condition, which permits conservative estimates of convective heat flux, but also generates flowfield-dependent spatial thermal distributions along the surface. Other key findings are presented in an effort to make the film coefficient engineering model readily available to design engineers across industry.

Film coefficient↗
DOE OSTI2017

Multidimensional Cyclic Voltammetry Simulations of Pseudocapacitive Electrodes with a Conducting Nanorod Scaffold

This paper aims to understand the effect of nanoarchitecture on the performance of pseudocapacitive electrodes consisting of conducting scaffold coated with pseudocapacitive material. To do so, two-dimensional numerical simulations of ordered conducting nanorods coated with a thin film of pseudocapacitive material were performed. The simulations reproduced three-electrode cyclic voltammetry measurements based on a continuum model derived from first principles. Two empirical approaches commonly used experimentally to characterize the contributions of surface-controlled and diffusion-controlled charge storage mechanisms to the total current density with respect to scan rate were theoretically validated for the first time. Moreover, the areal capacitive capacitance, attributed to EDL formation, remained constant and independent of electrode dimensions, at low scan rates. However, at high scan rates, it decreased with decreasing conducting nanorod radius and increasing pseudocapacitive layer thickness due to resistive losses. By contrast, the gravimetric faradaic capacitance, due to reversible faradaic reactions, decreased continuously with increasing scan rate and pseudocapacitive layer thickness but was independent of conducting nanorod radius. Note that the total gravimetric capacitance predicted numerically featured values comparable to experimental measurements. Finally, an optimum pseudocapacitive layer thickness that maximizes total areal capacitance was identified as a function of scan rate and confirmed by scaling analysis.

42 ENGINEERING↗
DOE OSTI2017

An improved random walk algorithm for the implicit Monte Carlo method

In this work, we introduce a modified Implicit Monte Carlo (IMC) Random Walk (RW) algorithm, which increases simulation efficiency for multigroup radiative transfer problems with strongly frequency-dependent opacities. To date, the RW method has only been implemented in “fully-gray” form; that is, the multigroup IMC opacities are group-collapsed over the full frequency domain of the problem to obtain a gray diffusion problem for RW. This formulation works well for problems with large spatial cells and/or opacities that are weakly dependent on frequency; however, the efficiency of the RW method degrades when the spatial cells are thin or the opacities are a strong function of frequency. To address this inefficiency, we introduce a RW frequency group cutoff in each spatial cell, which divides the frequency domain into optically thick and optically thin components. In the modified algorithm, opacities for the RW diffusion problem are obtained by group-collapsing IMC opacities below the frequency group cutoff. Particles with frequencies above the cutoff are transported via standard IMC, while particles below the cutoff are eligible for RW. This greatly increases the total number of RW steps taken per IMC time-step, which in turn improves the efficiency of the simulation. We refer to this new method as Partially-Gray Random Walk (PGRW). We present numerical results for several multigroup radiative transfer problems, which show that the PGRW method is significantly more efficient than standard RW for several problems of interest. In general, PGRW decreases runtimes by a factor of ∼2–4 compared to standard RW, and a factor of ∼3–6 compared to standard IMC. While PGRW is slower than frequency-dependent Discrete Diffusion Monte Carlo (DDMC), it is also easier to adapt to unstructured meshes and can be used in spatial cells where DDMC is not applicable. This suggests that it may be optimal to employ both DDMC and PGRW in a single simulation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

An optimization-based approach to parameter learning for fractional type nonlocal models

Nonlocal operators of fractional type are a popular modeling choice for applications that do not adhere to classical diffusive behavior; however, one major challenge in nonlocal simulations is the selection of model parameters. In this work we propose an optimization-based approach to parameter identification for fractional models with an optional truncation radius. We formulate the inference problem as an optimal control problem where the objective is to minimize the discrepancy between observed data and an approximate solution of the model, and the control variables are the fractional order and the truncation length. For the numerical solution of the minimization problem we propose a gradient-based approach, where we enhance the numerical performance by an approximation of the bilinear form of the state equation and its derivative with respect to the fractional order. Several numerical tests in one and two dimensions illustrate the theoretical results and show the robustness and applicability of our method.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS2015

Effects of Mesoscale Eddies in the Active Mixed Layer: Test of the Parametrisation in Eddy Resolving Simulations

In eddy resolving simulations, we test a mixed layer mesoscale parametrisation, developed recently by Canuto and Dubovikov [Ocean Model., 2011, 39, 200-207]. With no adjustable parameters, the parametrisation yields the horizontal and vertical mesoscale fluxes in terms of coarse-resolution fields and eddy kinetic energy (EKE). We compare terms of the parametrisation diagnosed from coarse-grained fields with the eddy mesoscale fluxes diagnosed directly from the high resolution model. An expression for the EKE in terms of mean fields has also been found to get a closed parametrisation in terms of the mean fields only. In 40 numerical experiments we simulated two types of flows: idealised flows driven by baroclinic instabilities only, and more realistic flows, driven by wind and surface fluxes as well as by inflow-outflow. The diagnosed quasi-instantaneous horizontal and vertical mesoscale buoyancy fluxes (averaged over 1-2 degrees and 10 days) demonstrate a strong scatter typical for turbulent flows, however, the fluxes are positively correlated with the parametrisation with higher (0.5-0.74) correlations at the experiments with larger baroclinic radius Rossby. After being averaged over 3-4 months, diffusivities diagnosed from the eddy resolving simulations are consistent with the parametrisation for a broad range of parameters. Diagnosed vertical mesoscale fluxes restratify mixed layer and are in a good agreement with the parametrisation unless vertical turbulent mixing in the upper layer becomes strong enough in comparison with mesoscale advection. In the latter case, numerical simulations demonstrate that the deviation of the fluxes from the parametrisation is controlled by dimensionless parameter estimating the ratio of vertical turbulent mixing term to mesoscale advection. An analysis using a modified omega-equation reveals that the effects of the vertical mixing of vorticity is responsible for the two-three fold amplification of vertical mesoscale flux. Possible physical mechanisms, responsible for the amplification of vertical mesoscale flux are discussed.

simulation↗
NASA NTRS1977

Numerical simulation of life cycles of advection warm fog

The formation, development and dissipation of advection warm fog is investigated. The equations employed in the model include the equation of continuity, momentum and energy for the descriptions of density, wind component and potential temperature, respectively, together with two diffusion equations for the modification of water-vapor mixing ratio and liquid-water mixing ratios. A description of the vertical turbulent transfer of heat, moisture and momentum has been taken into consideration. The turbulent exchange coefficients adopted in the model are based on empirical flux-gradient relations.

Hung, R. J.↗
NASA NTRS2023

Diffuse Auroral Intensities Produced By Whistler Mode and Electron Cyclotron Harmonic Waves

Whistler mode waves and ECH waves as observed by the THEMIS-D and THEMIS-E satellites have been analyzed. It is observed that ECH waves are very weak for pitch-angle diffusion whereas whistler mode waves are more efficient. Bounce-averaged pitch-angle diffusion rates at the edge of atmospheric loss-cone have been calculated for both waves. Further, these are used to obtain electron precipitation flux inside the loss-cone. The electron flux at the edge of the loss-cone is represented by the kappa distribution. Numerical calculation of precipitation flux is performed by varying the fitting parameters (which determine the shape of electron precipitation flux) appearing in the kappa distribution. Precipitation flux is used to obtain volume excitation rates and height-integrated volume excitation rates for the seven excitation states. Calculations have been performed using two standard atmosphere models corresponding to mean and high solar and geomagnetic activities. It has been observed from studies that the excitation rates produced by the ECH wave are several orders of magnitude smaller than the rates produced by whistler mode waves. We show that the scattering efficiency of whistler Mode is higher than that of ECH waves. The oblique whistler mode waves allow efficient scattering of electrons in diffuse aurora. Outcomes are discussed.

Arvind K. Tripathi↗
NASA NTRS2001

Three-Dimensional Upward Flame Spreading in Partial-Gravity Buoyant Flows

Reduced-gravity environments have been used to establish low-speed, purely forced flows for both opposed- and concurrent-flow flame spread studies. Altenkirch's group obtained spacebased experimental results and developed unsteady, two-dimensional numerical simulations of opposed-flow flame spread including gas-phase radiation, primarily away from the flammability limit for thin fuels, but including observations of thick fuel quenching in quiescent environments. T'ien's group contributed some early flame spreading results for thin fuels both in opposed flow and concurrent flow regimes, with more focus on near-limit conditions. T'ien's group also developed two- and three-dimensional numerical simulations of concurrent-flow flame spread incorporating gas-phase radiative models, including predictions of a radiatively-induced quenching limit reached in very low-speed air flows. Radiative quenching has been subsequently observed in other studies of combustion in very low-speed flows including other flame spread investigations, droplet combustion and homogeneous diffusion flames, and is the subject of several contemporary studies reported in this workshop. Using NASA aircraft flying partial-gravity "parabolic" trajectories, flame spreading in purely buoyant, opposed-flow (downward burning) has been studied. These results indicated increases in flame spread rates and enhanced flammability (lower limiting atmospheric oxygen content) as gravity levels were reduced from normal Earth gravity, and were consistent with earlier data obtained by Altenkirch using a centrifuge. In this work, experimental results and a three-dimensional numerical simulation of upward flame spreading in variable partial-gravity environments were obtained including some effects of reduced pressure and variable sample width. The simulation provides physical insight for interpreting the experimental results and shows the intrinsic 3-D nature of buoyant, upward flame spreading. This study is intended to link the evolving understanding of flame spreading in purely-forced flows to the purely-buoyant flow environment, particularly in the concurrent flow regime; provide additional insight into the existence of steady flame spread in concurrent flows; and stimulate direct comparisons between opposed- and concurrent-flow flame spread. Additionally, this effort is intended to provide direct practical understanding applicable to fire protection planning for the habitable facilities in partial gravity environments of anticipated Lunar and Martian explorations.

Sacksteder, Kurt R.↗
DOE OSTI2023

Analytical closure to the spatially-filtered Euler equations for shock-dominated flows

To ensure numerical stability in the vicinity of shocks, a variety of methods have been used, including shock-capturing schemes such as weighted essentially non-oscillatory schemes, as well as the addition of artificial diffusivities to the governing equations. Centered finite difference schemes are often avoided near discontinuities due to the tendency for significant oscillations. However, such schemes have desirable conservation properties compared to many shock-capturing schemes. The objective of this work is to derive all necessary viscous/diffusion terms from first principles and then demonstrate the performance of these analytical terms within a centered differencing framework. The physical Euler equations are spatially-filtered with a Gaussian-like filter. Sub-filter scale (SFS) terms arise in the momentum and energy equations. Analytical closure is provided for each of them by leveraging the jump conditions for a shock. No SFS terms are present in the continuity or species equations. Here, this approach is tested for several problems involving shocks in one and two dimensions. Implemented within a centered difference code, the SFS terms perform well for a range of flow conditions without introducing excessive diffusion.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

Multilevel Sequential Monte Carlo Samplers for Normalizing Constants

This article considers the sequential Monte Carlo (SMC) approximation of ratios of normalizing constants associated to posterior distributions which in principle rely on continuum models. Therefore, the Monte Carlo estimation error and the discrete approximation error must be balanced. A multilevel strategy is utilized to substantially reduce the cost to obtain a given error level in the approximation as compared to standard estimators. Two estimators are considered and relative variance bounds are given. The theoretical results are numerically illustrated for two Bayesian inverse problems arising from elliptic partial differential equations (PDEs). The examples involve the inversion of observations of the solution of (i) a 1-dimensional Poisson equation to infer the diffusion coefficient, and (ii) a 2-dimensional Poisson equation to infer the external forcing.

97 MATHEMATICS AND COMPUTING↗