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At least 127 records · Page 7

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NASA NTRS1988

A Three-Dimensional, Time-Dependent Numerical Modeling of Super-Sonic, Super-Alfvenic MHD Flow

A three-dimensional, time-dependent, MHD disturbance propagating through a steady inhomogeneous MHD flow is analyzed by a numerical model based on the Lax-Wendroff finite difference method. The initial steady MHD flow is assumed to be supersonic and super-Alfvenic. The background steady MHD flow is obtained by dropping the time-dependent terms in the unsteady MHD equations. A transient MHD disturbance corresponding to a MHD shock is generated at the lower radial boundary surface. The resulting development and propagation of 3-D MHD disturbances interacting with the background steady MHD flow are numerically simulated in this first application to the field of interplanetary physics. Detailed descriptions of the mathematical formulation and numerical methodology are presented. Example computations are performed to demonstrate the validity of the formulation and the capabilities of the model.

Han, S. M.↗

NASA NTRS1980

A numerical model of the zonal mean circulation of the middle atmosphere

The paper presents a simulation of the zonally averaged circulation in the middle atmosphere using a numerical model based on the primitive equations in log pressure coordinates. The circulation is driven radiatively by heating due to solar ultraviolet absorption by ozone and infrared cooling due to carbon dioxide and ozone; Rayleigh friction with a a short time constant above 70 km is included to simulate the strong mechanical dissipation which is hypothesized to exist in the vicinity of the mesopause due to turbulence associated with gravity waves and tides near the mesopause.

Holton, J. R.↗

DOE OSTI2025

Impact of mixed grain size sediment input on braided river Morphodynamics: Insights from experimental and numerical modeling

The rate of sediment supply has significant impacts on river morphology, making it crucial to understand the geomorphic changes and grain size distribution dynamics in rivers. However, the effects of varying grain size sediment input on morphological changes in braided channels remain poorly understood. This study is the first to investigate the bar development and sediment sorting processes in braided channels with non-uniform sediment inputs using both numerical and experimental approaches. We applied a two-dimensional numerical model, Nays2DH to confirm and generalize experimental results. The model reproduced key experiment results, including 1) stream elevation changes, and 2) grain size distribution. Using this validated model, we explored the morphological changes and sorting process in a braided river with sediment inputs. The numerical experiments demonstrate that sediment input controls the elevation of the stream bed and the grain size distribution. Notably, both the elevation and grain-size distribution become relatively stable in downstream of the channel. Additionally, the simulation results suggest that an increased sediment supply leads to greater channel complexity, with bed surface armoring decreasing.

Nays2DH↗

NASA NTRS2014

How to Develop and Interpret a Credibility Assessment of Numerical Models for Human Research: NASA-STD-7009 Demystified

In the wake of the Columbia accident, the NASA-STD-7009 [1] credibility assessment was developed as a unifying platform to describe model credibility and the uncertainties in its modeling predictions. This standard is now being adapted by NASAs Human Research Program to cover a wide range of numerical models for human research. When used properly, the standard can improve the process of code development by encouraging the use of best practices. It can also give management more insight in making informed decisions through a better understanding of the models capabilities and limitations.To a newcomer, the abstractions presented in NASA-STD-7009 and the sheer volume of information that must be absorbed can be overwhelming. This talk is aimed at describing the credibility assessment, which is the heart of the standard, in plain terms. It will outline how to develop a credibility assessment under the standard. It will also show how to quickly interpret the graphs and tables that result from the assessment and how to drill down from the top-level view to the foundation of the assessment. Finally, it will highlight some of the resources that are available for further study.

numerical analysis↗

NASA NTRS1991

Numerical modeling of solidification and convection in a viscous pure binary eutectic system

The solidification and convection of the pure binary eutectic silicate system diopside-anorthite (Di-An) is numerically modeled. A mass-weighted enthalpy of fusion is used to account for the second solid phase (An) which crystallizes at the solidus temperature. Variable under-relaxation is used to aid convergence of the momentum equations in the implementation of the SIMPLER algorithm used to solve the two-dimensional continuum conservation equations. Numerical experiments of the solidification of Di80 melt show that a large temperature drop occurs across the solid and mush regions which decreases convective vigor in the liquid. Interesting compositional segregation patterns are produced during the solidification of Di80.

Oldenburg, Curtis M.↗

NASA NTRS1993

Numerical modeling of ooid size and the problem of Neoproterozoic giant ooids

Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numerical models are used to evaluate the effects of several processes influencing ooid size. The first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth. Several Neoproterozoic oolites contain unusually large ooids, some reaching diameters of up to 16 mm. While lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher growth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during Neoproterozoic time.

NASA Discipline Exobiology↗

NASA NTRS2002

Validated Numerical Models for the Convective Extinction of Fuel Droplets (CEFD)

The NASA Nebraska Space Grant (NSGC) & EPSCoR programs have continued their effort to support outstanding research endeavors by funding the Numerical Simulation of the Combustion of Fuel Droplets study at the University of Nebraska at Lincoln (UNL). This team of researchers has developed a transient numerical model to study the combustion of suspended and moving droplets. The engines that propel missiles, jets, and many other devices are dependent upon combustion. Therefore, data concerning the combustion of fuel droplets is of immediate relevance to aviation and aeronautical personnel, especially those involved in flight operations. The experiments being conducted by Dr. Gogos and Dr. Nayagam s research teams, allow investigators to gather data for comparison with theoretical predictions of burning rates, flame structures, and extinction conditions. The consequent improved fundamental understanding of droplet combustion may contribute to the clean and safe utilization of fossil fuels (Williams, Dryer, Haggard & Nayagam, 1997, f 2). The present state of knowledge on convective extinction of fuel droplets derives from experiments conducted under normal gravity conditions. However, any data obtained with suspended droplets under normal gravity are grossly affected by gravity. The need to obtain experimental data under microgravity conditions is therefore well justified and addresses one of the goals of NASA's Human Exploration and Development of Space (HEDS) microgravity combustion experiment.

Gogos, George↗

NASA NTRS1992

A general numerical model for wave rotor analysis

Wave rotors represent one of the promising technologies for achieving very high core temperatures and pressures in future gas turbine engines. Their operation depends upon unsteady gas dynamics and as such, their analysis is quite difficult. This report describes a numerical model which has been developed to perform such an analysis. Following a brief introduction, a summary of the wave rotor concept is given. The governing equations are then presented, along with a summary of the assumptions used to obtain them. Next, the numerical integration technique is described. This is an explicit finite volume technique based on the method of Roe. The discussion then focuses on the implementation of appropriate boundary conditions. Following this, some results are presented which first compare the numerical approximation to the governing differential equations and then compare the overall model to an actual wave rotor experiment. Finally, some concluding remarks are presented concerning the limitations of the simplifying assumptions and areas where the model may be improved.

Paxson, Daniel W.↗

NASA NTRS1982

A preliminary study of the application of HCMM satellite data to define initial and boundary conditions for numerical models: A case study in St. Louis, Missouri

Infrared and visible HCMM data were used to examine the potential application of these data to define initial and boundary conditions for mesoscale numerical models. Various boundary layer models were used to calculate the distribution of the surface heat flux, specific humidity depression (the difference between the specific humidity in the air at approxmately the 10 m level and the specific humidity at the ground), and the eddy vicosity in a 72 km by 72 km area centered about St. Louis, Missouri. Various aspects of the implications of the results on the meteorology of St. Louis are discussed. Overall, the results indicated that a reasonable estimate of the surface heat flux, urban albedo, ground temperature, and specific humidity depression can be obtained using HCMM satellite data. Values of the ground-specific humidity can be obtained if the distribution of the air-specific humidity is available. More research is required in estimating the absolute magnitude of the specific humidity depression because calculations may be sensitive to model parameters.

Vukovich, F. M.↗

NASA NTRS1992

Preconditioning for stratospheric sudden warmings - Sensitivity studies with a numerical model

Results are presented of numerical experiments performed to determine what constitutes preconditioning for stratospheric sudden warmings. A number of 20-d runs were performed with a 3D primitive equation model. Runs varied in that they had different (zonally symmetric) initial fields and different lower boundary planetary wave fields, both based on NMC data for particular days. Initial and boundary data were used from the two minor and the sole major sudden warmings of January-February 1979 and some minor and the sole major sudden warmings of January-February 1989. To isolate the properties of the initial conditions from preconditioned cases and those with no warming, hybrid initial fields were created by combining data from two different days, one preconditioned and the other not. Runs with the hybrid initial conditions indicate that it is the lower stratospheric winds that are important in characterizing a preconditioned flow. It is suggested that it would be possible for a preconditioned state to exist for at least a short time without a warming occurring.

Smith, Anne K.↗

NASA NTRS1994

Transient Numerical Modeling of the Combustion of Bi-Component Liquid Droplets: Methanol/Water Mixture

This study shows that liquid mixtures of methanol and water are attractive candidates for microgravity droplet combustion experiments and associated numerical modeling. The gas phase chemistry for these droplet mixtures is conceptually simple, well understood and substantially validated. In addition, the thermodynamic and transport properties of the liquid mixture have also been well characterized. Furthermore, the results obtained in this study predict that the extinction of these droplets may be observable in ground-based drop to tower experiments. Such experiments will be conducted shortly followed by space-based experiments utilizing the NASA FSDC and DCE experiments.

Marchese, A. J.↗

NASA NTRS2009

Using Numerical Modeling to Simulate Space Capsule Ground Landings

Experimental work is being conducted at the National Aeronautics and Space Administration s (NASA) Langley Research Center (LaRC) to investigate ground landing capabilities of the Orion crew exploration vehicle (CEV). The Orion capsule is NASA s replacement for the Space Shuttle. The Orion capsule will service the International Space Station and be used for future space missions to the Moon and to Mars. To evaluate the feasibility of Orion ground landings, a series of capsule impact tests are being performed at the NASA Langley Landing and Impact Research Facility (LandIR). The experimental results derived at LandIR provide means to validate and calibrate nonlinear dynamic finite element models, which are also being developed during this study. Because of the high cost and time involvement intrinsic to full-scale testing, numerical simulations are favored over experimental work. Subsequent to a numerical model validated by actual test responses, impact simulations will be conducted to study multiple impact scenarios not practical to test. Twenty-one swing tests using the LandIR gantry were conducted during the June 07 through October 07 time period to evaluate the Orion s impact response. Results for two capsule initial pitch angles, 0deg and -15deg , along with their computer simulations using LS-DYNA are presented in this article. A soil-vehicle friction coefficient of 0.45 was determined by comparing the test stopping distance with computer simulations. In addition, soil modeling accuracy is presented by comparing vertical penetrometer impact tests with computer simulations for the soil model used during the swing tests.

Heymsfield, Ernie↗

NASA NTRS2021

Analytical and Numerical Modeling of Sensor Port Acoustics

The purpose of this Technical Publication (TP) is to provide background, relevant theory, and examples for acoustic response analysis of a sensor port. A new approach is devised theoretically and computationally that captures the true acoustic response of a sensor port. This TP summarizes the acoustics back-ground, the port response theoretical development, and provides comparisons of a port acoustic response using an analytical model and computational acoustics. The effects of nonlinear acoustics and acoustic propagation in liquids is also examined. Additionally, this TP describes the design of a specialized filter using the predicted sensor port response that can be applied to data for correction.

sensor port↗

NASA NTRS2015

Numerical Modeling of the Transient Chilldown Process of a Cryogenic Propellant Transfer Line

Before cryogenic fuel depots can be fully realized, efficient methods with which to chill down the spacecraft transfer line and receiver tank are required. This paper presents numerical modeling of the chilldown of a liquid hydrogen tank-to-tank propellant transfer line using the Generalized Fluid System Simulation Program (GFSSP). To compare with data from recently concluded turbulent LH2 chill down experiments, seven different cases were run across a range of inlet liquid temperatures and mass flow rates. Both trickle and pulse chill down methods were simulated. The GFSSP model qualitatively matches external skin mounted temperature readings, but large differences are shown between measured and predicted internal stream temperatures. Discrepancies are attributed to the simplified model correlation used to compute two-phase flow boiling heat transfer. Flow visualization from testing shows that the initial bottoming out of skin mounted sensors corresponds to annular flow, but that considerable time is required for the stream sensor to achieve steady state as the system moves through annular, churn, and bubbly flow. The GFSSP model does adequately well in tracking trends in the data but further work is needed to refine the two-phase flow modeling to better match observed test data.

Chilldown↗

NASA NTRS1989

Numerical modeling of enclosure convection

A numerical study on the steady and unsteady natural convection in two-dimensional rectangular enclosures has been performed by a time-accurate ADI finite difference scheme. The study covered a range of Rayleigh numbers between 1000 and 10 to the 7th, aspect ratios between 0.2 and 10.0, and tilt angles between -90 (heating from bottom) and +90 deg (heating from top). Various Prandtl numbers have been studied, but only the results of water (Pr = 7.0) are reported here due to space limitations. The physics revealed, however, includes the convection phenomena and the Rayleigh-Benard stability, as well as the combined mechanism of these two. The onset of secondary cells is determined by using a velocity map, which is simpler and cleaner, instead of a streamline plot. The critical Ra number for the occurrence of these secondary cells is shown to be lower than can be detected by experimental studies. On the Rayleigh-Benard stability part, a second transition from stable single-cell convection to periodic multicellular convection is disclosed.

Duh, J. C.↗

NASA NTRS1981

Numerical model of long-lived Jovian vortices

The extension of the measured zonal velocity profile into the adiabatic interior of Jupiter, while eddies and large oval structures are confined to a shallow stably-stratified upper layer, are assumed in a nonlinear numerical model of long-lived Jovian vortices. In agreement of the observed flows of Jupiter, each vortex is stationary with respect to the shear flow at a critical latitude that is close to the latitude of the vortex center. The solutions obtained are strongly nonlinear, in contrast to the solitary wave solutions that are the weakly nonlinear extensions of ultralong linear waves. The merging of two stable vortices upon collision, rather than the non-interaction predicted by solitary wave theory, is in keeping with Jovian vortex observations. It is suggested that long-lived vortices maintain themselves against dissipation by absorbing smaller vortices produced by convection.

Ingersoll, A. P.↗

NASA NTRS1992

Final results of an experiment in operational forecasting of sea breeze thunderstorms using a mesoscale numerical model

Sea breeze thunderstorms during quiescent synoptic conductions account for 40 percent of Florida rainfall, and are the dominant feature of April-October weather at the Kennedy Space Center (KSC). An effort is presently made to assess the feasibility of a mesoscale numerical model in improving the point-specific thunderstorm forecasting accuracy at the KSC, in the 2-12 hour time frame. Attention is given to the Applied Regional Atmospheric Modeling System.

Lyons, Walter A.↗

DOE OSTI2026

OC6 Project Phase IV: Validation of Numerical Models for Novel Floating Offshore Wind Support Structures

The fourth phase of the Offshore Code Comparison, Collaboration, Continued, with Correlation and unCertainty (OC6) aims to assess the ability of the tools to model a novel floating wind design. The design that was selected for this project is the TetraSpar designed by Stiesdal Offshore Technologies. The platform is a unique design that features a spar like stability with a very low center of gravity, but does not require deep-water ports. This combination is achieved with a system made of two separate steel structures, a hull at the water’s surface that supports the wind turbine tower, and a keel that is suspended below the main body. This lower keel has a very high density and drives the total platform’s low center of gravity. Scaled model tests of the TetraSpar design were performed in 2018-2019 by the University of Maine. The tests were performed with 1:43 Froude scaling. The data collected in the model tests provide a detailed description of the response of the platform in various controlled and relatively well understood conditions. Comparison with these data provides a good opportunity for validation of numerical models. The OC6 Phase IV work will focus on the specific parameters used in these scaled model tests. The load cases (LC) considered in this project are as follows: LC 1.X - Equilibrium and Static Offset Tests LC 2.X - Platform Free Decays LC 3.X - Turbine Thrust (Wind Only) LC 4.X - Regular and Irregular Waves (Wave only) LC 5.X - Combined Wind and Wave Loading

17 WIND ENERGY↗