SEARCH · Engineering Papers

Results for “Transition Model”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22

All sources NASA NTRS DOE OSTI

NASA NTRS1986

The prominence-corona interface and its relationship to the chromosphere-corona transition

The classical model of the chromosphere-corona transition does not account for the observed behavior of the differential emission measure for T approx. less than 100,000 K. Several models have been proposed to resolve this discrepancy in physically different ways. Because the observed differential emission measure at the prominence-corona interface is on average nearly the same as in the chromosphere-corona transition, prominences offer a fresh testing ground for models tailored to the chromosphere-corona transition. The researcher considered three such models and concluded that none extends in a natural way to the environment of prominences. The researcher advanced a simple idea involving thermal conduction both along and across the magnetic field from the corona into cool threads.

Rabin, Douglas↗

DOE OSTI2021

Antiferromagnetic character of the quantum phase transition in the Hubbard model on the honeycomb lattice

In this work we provide a unified, comprehensive treatment of all operators that contribute to the antiferromagnetic, ferromagnetic, and charge-density-wave structure factors and order parameters of the hexagonal Hubbard Model. We use the Hybrid Monte Carlo algorithm to perform a systematic, carefully controlled analysis in the temporal Trotter error and of the thermodynamic limit. We expect our findings to improve the consistency of Monte Carlo determinations of critical exponents. We perform a data collapse analysis and determine the critical exponent β = 0.898 (37) for the semimetal-Mott insulator transition in the hexagonal Hubbard Model. Our methods are applicable to a wide range of lattice theories of strongly correlated electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1971

Recombination luminescence in irradiated silicon-effects of uniaxial stress and temperature variations.

Demonstration that luminescence in irradiated silicon consists of a spectral group between 0.80 and 1.0 eV which seems to be independent of impurities, while a lower energy group between 0.60 and 0.80 eV is seen only in pulled crystals. The small halfwidth and temperature dependence of the sharp zero-phonon lines observed in these spectra indicate that the luminescence arises from a bound-to-bound transition. A model is proposed for the transition mechanism. Stress data taken on the 0.79-eV zero-phonon line in pulled crystals can be fit by either a tetragonal 100 (in brackets) defect symmetry or by conduction-band splitting effects. It is suggested that the 0.79-eV zero-phonon line and the 0.60- to 0.80-eV spectral group arise from the EPR G-15 center. Stress data on a zero-phonon line at 0.97 eV associated with the 0.80- to 1.0-eV spectral group can be explained by a trigonal 111 (in brackets) defect. The divacancy is tentatively suggested as responsible for this luminescence spectra.

Jones, C. E.↗

NASA NTRSDate not supplied

Model Validation and its Crucial Roles in Connecting Space Weather Research, Operations, and End-User Needs

Model validation is indispensable to transitioning a model from research to operations. In this presentation, we focus on the performance of models for specific operational purposes/user needs, which include careful selection of the corresponding specific Essential Space Environment Quantity (ESEQ) and a proper set of metrics. Initial results of our community-wide validation efforts under the COSPAR ISWAT Initiative with relevance to surface charging and internal charging will be shown. We will highlight processes involved in performing such tasks, such as setting up unified guidelines, data preparation, assessment using different metrics, etc. We will also present setbacks and challenges associated with such efforts so further progress can be made.

Yihua Zheng↗

NASA NTRS2017

Measured Boundary Layer Transition and Rotor Hover Performance at Model Scale

An experiment involving a Mach-scaled, 11:08 f t: diameter rotor was performed in hover during the summer of 2016 at NASA Langley Research Center. The experiment investigated the hover performance as a function of the laminar to turbulent transition state of the boundary layer, including both natural and fixed transition cases. The boundary layer transition locations were measured on both the upper and lower aerodynamic surfaces simultaneously. The measurements were enabled by recent advances in infrared sensor sensitivity and stability. The infrared thermography measurement technique was enhanced by a paintable blade surface heater, as well as a new high-sensitivity long wave infrared camera. The measured transition locations showed extensive amounts, x=c>0:90, of laminar flow on the lower surface at moderate to high thrust (CT=s > 0:068) for the full blade radius. The upper surface showed large amounts, x=c > 0:50, of laminar flow at the blade tip for low thrust (CT=s < 0:045). The objective of this paper is to provide an experimental data set for comparisons to newly developed and implemented rotor boundary layer transition models in CFD and rotor design tools. The data is expected to be used as part of the AIAA Rotorcraft SimulationWorking Group

Overmeyer, Austin D.↗

NASA NTRS1988

Transition mixing study empirical model report

The empirical model developed in the NASA Dilution Jet Mixing Program has been extended to include the curvature effects of transition liners. This extension is based on the results of a 3-D numerical model generated under this contract. The empirical model results agree well with the numerical model results for all tests cases evaluated. The empirical model shows faster mixing rates compared to the numerical model. Both models show drift of jets toward the inner wall of a turning duct. The structure of the jets from the inner wall does not exhibit the familiar kidney-shaped structures observed for the outer wall jets or for jets injected in rectangular ducts.

Srinivasan, R.↗

NASA NTRS2007

Turbulence Modeling for the Simulation of Transition in Wall Shear Flows

Our research involves study of the behavior of k-epsilon turbulence models for simulation of bypass-level transition over flat surfaces and turbine blades. One facet of the research has been to assess the performance of a multitude of k-epsilon models in what we call "natural transition", i.e. no modifications to the k-e models. The study has been to ascertain what features in the dynamics of the model affect the start and end of the transition. Some of the findings are in keeping with those reported by others (e.g. ERCOFTAC). A second facet of the research has been to develop and benchmark a new multi-time scale k-epsilon model (MTS) for use in simulating bypass-level transition. This model has certain features of the published MTS models by Hanjalic, Launder, and Schiestel, and by Kim and his coworkers. The major new feature of our MTS model is that it can be used to compute wall shear flows as a low-turbulence Reynolds number type of model, i.e. there is no required partition with patching a one-equation k model in the near-wall region to a two-equation k-epsilon model in the outer part of the flow. Our MTS model has been studied extensively to understand its dynamics in predicting the onset of transition and the end-stage of the transition. Results to date indicate that it far superior to the standard unmodified k-epsilon models. The effects of protracted pressure gradients on the model behavior are currently being investigated.

Crawford, Michael E.↗

NASA NTRS1996

A Model for NASA-KSC's Privatization Transition

This paper describes a model for government agencies that are considering privatization of all or part of their functions. Privatization encompasses the transitioning of government functions from government run to contractor run. The model developed in this paper is used to analyze the National Aeronautics and Space Administration's (NASA) decision to privatize space shuttle operations at the Kennedy Space Center (KSC). Several specific recommendations are given to KSC as they attempt to operationalize this privatization decision at the Center and to transition to a new relationship with their contractors.

Lavelle, Jerome P.↗

NASA NTRS1999

Unified Model Deformation and Flow Transition Measurements

The number of optical techniques that may potentially be used during a given wind tunnel test is continually growing. These include parameter sensitive paints that are sensitive to temperature or pressure, several different types of off-body and on-body flow visualization techniques, optical angle-of-attack (AoA), optical measurement of model deformation, optical techniques for determining density or velocity, and spectroscopic techniques for determining various flow field parameters. Often in the past the various optical techniques were developed independently of each other, with little or no consideration for other techniques that might also be used during a given test. Recently two optical techniques have been increasingly requested for production measurements in NASA wind tunnels. These are the video photogrammetric (or videogrammetric) technique for measuring model deformation known as the video model deformation (VMD) technique, and the parameter sensitive paints for making global pressure and temperature measurements. Considerations for, and initial attempts at, simultaneous measurements with the pressure sensitive paint (PSP) and the videogrammetric techniques have been implemented. Temperature sensitive paint (TSP) has been found to be useful for boundary-layer transition detection since turbulent boundary layers convect heat at higher rates than laminar boundary layers of comparable thickness. Transition is marked by a characteristic surface temperature change wherever there is a difference between model and flow temperatures. Recently, additional capabilities have been implemented in the target-tracking videogrammetric measurement system. These capabilities have permitted practical simultaneous measurements using parameter sensitive paint and video model deformation measurements that led to the first successful unified test with TSP for transition detection in a large production wind tunnel.

Burner, Alpheus W.↗

DOE OSTI2020

Metal-insulator transition in a random Hubbard model

In this work, we examine the metal-insulator transition in a half-filled Hubbard model of electrons with random and all-to-all hopping and exchange, and an on-site nonrandom repulsion, the Hubbard $\textit{U}$. We argue that recent numerical results of Cha et al. can be understood in terms of a deconfined critical point between a disordered Fermi liquid and an insulating spin glass. We find a deconfined critical point in a previously proposed large $\textit{M}$ theory, which generalizes the SU(2) spin symmetry to SU($\textit{M}$) and obtains exponents for the electron and spin correlators which agree with those of Cha et al. This critical point is expected to have the maximal quantum Lyapunov exponent. We also present a renormalization group analysis, and argue for the presence of an additional metallic spin glass phase at half-filling and small $\textit{U}$.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1990

Energy balance in the solar transition region. I - Hydrostatic thermal models with ambipolar diffusion

The energy balance in the lower transition region is analyzed by constructing theoretical models which satisfy the energy balance constraint. The energy balance is achieved by balancing the radiative losses and the energy flowing downward from the corona. This energy flow is mainly in two forms: conductive heat flow and hydrogen ionization energy flow due to ambipolar diffusion. Hydrostatic equilibrium is assumed, and, in a first calculation, local mechanical heating and Joule heating are ignored. In a second model, some mechanical heating compatible with chromospheric energy-balance calculations is introduced. The models are computed for a partial non-LTE approach in which radiation departs strongly from LTE but particles depart from Maxwellian distributions only to first order. The results, which apply to cases where the magnetic field is either absent, or uniform and vertical, are compared with the observed Lyman lines and continuum from the average quiet sun. The approximate agreement suggests that this type of model can roughly explain the observed intensities in a physically meaningful way, assuming only a few free parameters specified as chromospheric boundary conditions.

Fontenla, J. M.↗

NASA NTRS1984

Long-term dynamical stability and transition in a spectral model

The relationship between blocking and multiple equilibria was investigated in a two layer model of the Northern Hemisphere January heating and boundary conditions are imposed. The truncation varies over global wavenumbers 4-8 and the coefficients of horizontal diffusion of heat and potential vorticity are varied about realistic values. It is shown that while severely truncated models can exhibit multiple quasi equilibria under realistic domain, forcing, and boundary conditions, higher resolution is required to allow realistic transitions to be mediated by strong baroclinic wave activity. It is indicated that for extended range forecasting blocking transitions are not likely to be forecast well unless the associated synoptic events can be forecasted statistically.

Wobus, R. L.↗

DOE OSTI2021

Modeling and simulation of transitional Rayleigh–Taylor flow with partially averaged Navier–Stokes equations

In this work, the partially averaged Navier–Stokes (PANS) equations are used to predict the variable-density Rayleigh–Taylor (RT) flow at Atwood number 0.5 and maximum Reynolds number 500. This is a prototypical problem of material mixing, featuring laminar, transitional, and turbulent flow, instabilities and coherent structures, density fluctuations, and production of turbulence kinetic energy by both shear and buoyancy mechanisms. These features pose numerous challenges to modeling and simulation, making the RT flow ideal to develop the validation space of the recently proposed PANS Besnard–Harlow–Rauenzahn-linear eddy viscosity model closure. The numerical simulations are conducted at different levels of physical resolution and test three approaches to set the parameters $f_\phi$ defining the range of physically resolved scales. The computations demonstrate the efficiency (accuracy vs cost) of the PANS model predicting the spatiotemporal development of the RT flow. Results comparable to large-eddy simulations and direct numerical simulations are obtained at significantly lower physical resolution without the limitations of the Reynolds-averaged Navier–Stokes equations in these transitional flows. The data also illustrate the importance of appropriate selection of the physical resolution and the resolved fraction of each dependent quantity $\phi$ of the turbulent closure, $f_\phi$. These two aspects determine the ability of the model to resolve the flow phenomena not amenable to modeling by the closure and, as such, the computations’ fidelity.

Navier Stokes equations↗

NASA NTRS1990

A study of turbulence models for prediction of transitional boundary layers

Calculations of two-dimensional transitional boundary layer flows in zero and favorable pressure gradients are presented. The major focus is on the evaluation of current turbulence models to predict quantities such as skin-friction and heat transfer coefficients. Three turbulence models using the mixing length concept along with a one-equation model are considered. These models are tested by comparison with the experiments of Blair and Werle, who investigated flows over a heated flat plate for various levels of free-stream turbulence.

Abid, Ridha↗

DOE OSTI2022

Clean Energy Transition for Transportation Systems: Modeling Implications

Transportation is currently the least-diversified energy demand sector, with over 90% of energy use coming from petroleum. As a result, transportation recently became the largest source of GHG emissions in the U.S. and mobility needs for passenger and freight are growing rapidly. However, after over a century of petroleum dominance, new disruptive technologies and business models offer a pathway to decarbonize the sector. Transportation is at a turning point. On the horizon lies a future where affordable and abundant renewable electricity can be used to power cost-competitive battery electric vehicles (EVs) and produce energy-dense low-carbon fuels enabling to fully decarbonize transportation systems across all modes. Exploring the clean energy transition for the multitude of different transportation systems requires new analytical modeling and approaches. This talk reviewed current work at the National Renewable Energy Laboratory (NREL) to develop and use innovative tools and analytics approaches to inform the transformation to a sustainable mobility future and the integration of transportation systems with the broader energy sector.

ADVANCED PROPULSION SYSTEMS↗

NASA NTRS1985

Transition in Turbines

The concept of a large disturbance bypass mechanism for the initiation of transition is reviewed and studied. This mechanism, or some manifestation thereof, is suspected to be at work in the boundary layers present in a turbine flow passage. Discussion is presented on four relevant subtopics: (1) the effect of upstream disturbances and wakes on transition; (2) transition prediction models, code development, and verification; (3) transition and turbulence measurement techniques; and (4) the hydrodynamic condition of low Reynolds number boundary layers.

Source record↗

NASA NTRS1979

A compressible solution of the Navier-Stokes equations for turbulent flow about an airfoil

A compressible time dependent solution of the Navier-Stokes equations including a transition turbulence model is obtained for the isolated airfoil flow field problem. The equations are solved by a consistently split linearized block implicit scheme. A nonorthogonal body-fitted coordinate system is used which has maximum resolution near the airfoil surface and in the region of the airfoil leading edge. The transition turbulence model is based upon the turbulence kinetic energy equation and predicts regions of laminar, transitional, and turbulent flow. Mean flow field and turbulence field results are presented for an NACA 0012 airfoil at zero and nonzero incidence angles of Reynolds number up to one million and low subsonic Mach numbers.

Shamroth, S. J.↗

DOE OSTI2025

Characterizing Near-Surface Moisture Increase during the Clear-Sky Afternoon-to-Evening Transition Using a Single-Column Model

The afternoon-to-evening transition is the period when the atmospheric boundary layer transitions from convective to stable conditions. One noticeable feature during this transition period is the rapid increase in water vapor concentration near the surface. However, the mechanism behind the increase in water vapor remains poorly understood. This study investigated the processes contributing to the water vapor increase and the impacts of the land cover and horizontal advection on the water vapor increase using a single-column model for the clear-sky condition. Numerical experiments were conducted on three cases at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site. Evapotranspiration was found to be the main moisture source to the water vapor increase, and that change of the near-surface vertical temperature gradient from negative to positive is the trigger for this increase during the afternoon-to-evening transition. This is because the near-surface turbulence divergence term decreases due to the change in the buoyancy profile caused by vertical temperature gradient change. The impact of horizontal advection on water vapor varies, and it can either lead to an increase or a decrease in water vapor, depending on the spatial horizontal water vapor gradient. This study also found that land cover can influence the timing of the water vapor increase since different land covers may have different Bowen ratios. Water vapor increases earlier under conditions with smaller Bowen ratios compared to those with larger values.

Atmosphere-land interaction↗