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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Direct numerical simulations of a temporally evolving mixing layer subject to forcing

The vortical evolution of mixing layers subject to various types of forcing is numerically simulated using pseudospectral methods. The effect of harmonic forcing and random noise in the initial conditions is examined with some results compared to experimental data. Spanwise forcing is found to enhance streamwise vorticity in a nonlinear process leading to a slow, secondary growth of the shear layer. The effect of forcing on a chemical reaction is favorably compared with experimental data at low Reynolds numbers. Combining harmonic and subharmonic forcing is shown to both augment and later destroy streamwise vorticity.

Claus, Russell W.↗
NASA NTRS1989

Direct numerical simulations of a temporally evolving mixing layer subject to forcing

The vortical evolution of mixing layers subject to various types of forcing is numerically simulated using pseudospectral methods. The effect of harmonic forcing and random noise in the initial conditions is examined with some results compared to experimental data. Spanwise forcing is found to enhance streamwise vorticity in a nonlinear process leading to a slow, secondary growth of the shear layer. The effect of forcing on a chemical reaction is favorably compared with experimental data at low Reynolds numbers. Combining harmonic and subharmonic forcing is shown to both augment and later destroy streamwise vorticity.

Claus, R. W.↗
NASA NTRS1993

Direct numerical simulation of turbulent flow over a backward-facing step

The objectives of this study are as follows: (1) to conduct a direct numerical simulation of turbulent backward facing step flow using inflow and outflow conditions; and (2) to provide data in the form of Reynolds stress budgets for Reynolds averaged modeling. The report presents the basic statistical data and comparisons with the concurrent experiments of Jovic and Driver and budgets of turbulent kinetic energy.

Le, Hung↗
NASA NTRS1992

Numerical simulations of turbulent premixed H2/O2/N2 flames with complex chemistry

Premixed stoichiometric H2/O2/N2 flames propagating in two-dimensional turbulence were studied using direct numerical simulation (simulations in which all fluid and thermochemical scales are fully resolved) including realistic chemical kinetics and molecular transport. Results are compared with earlier zero-chemistry (flame sheet) and one-step chemistry simulations. Consistent with the simpler models, the turbulent flame with realistic chemistry aligns preferentially with extensive strain rates in the tangent plane and flame curvature probability density functions are close to symmetric with near-zero means. By contrast to simple-chemistry results with non-unity Lewis numbers (ratio of thermal to species diffusivity), local flame structure does not correlate with curvature but rather with tangential strain rate. Turbulent straining results in substantial thinning of the flame relative to the steady unstrained laminar case. Heat release and H2O2 contours remain thin and connected ('flamelet-like') while species including H-atom and OH are more diffuse. Peak OH concentration occurs well behind the peak heat-release zone. The feasibility of incorporating realistic chemistry into full turbulence simulations to address issues such as pollutant formation in hydrocarbon-air flames is suggested.

Baum, M.↗
NASA NTRS1994

Direct numerical simulations of on-demand vortex generators: Mathematical formulation

The objective of the present research is the development and application of efficient adaptive numerical algorithms for the study, via direct numerical simulations, of active vortex generators. We are using innovative computational schemes to investigate flows past complex configurations undergoing arbitrary motions. Some of the questions we try to answer are: Can and how may we control the dynamics of the wake? What is the importance of body shape and motion in the active control of the flow? What is the effect of three-dimensionality in laboratory experiments? We are interested not only in coupling our results to ongoing, related experimental work, but furthermore to develop an extensive database relating the above mechanisms to the vortical wake structures with the long-range objective of developing feedback control mechanisms. This technology is very important to aircraft, ship, automotive, and other industries that require predictive capability for fluid mechanical problems. The results would have an impact in high angle of attack aerodynamics and help design ways to improve the efficiency of ships and submarines (maneuverability, vortex induced vibration, and noise).

Koumoutsakos, Petros↗
NASA NTRS1995

Interactive visualization of numerical simulation results: A tool for mission planning and data analysis

We report on the development of an interactive system for visualizing and analyzing numerical simulation results. This system is based on visualization modules which use the Application Visualization System (AVS) and the NCAR graphics packages. Examples from recent simulations are presented to illustrate how these modules can be used for displaying and manipulating simulation results to facilitate their comparison with phenomenological model results and observations.

Berchem, J.↗
NASA NTRS2004

Numerical Simulation of 3-D Supersonic Viscous Flow in an Experimental MHD Channel

The 3-D supersonic viscous flow in an experimental MHD channel has been numerically simulated. The experimental MHD channel is currently in operation at NASA Ames Research Center. The channel contains a nozzle section, a center section, and an accelerator section where magnetic and electric fields can be imposed on the flow. In recent tests, velocity increases of up to 40% have been achieved in the accelerator section. The flow in the channel is numerically computed using a new 3-D parabolized Navier-Stokes (PNS) algorithm that has been developed to efficiently compute MHD flows in the low magnetic Reynolds number regime. The MHD effects are modeled by introducing source terms into the PNS equations which can then be solved in a very e5uent manner. To account for upstream (elliptic) effects, the flowfield can be computed using multiple streamwise sweeps with an iterated PNS algorithm. The new algorithm has been used to compute two test cases that match the experimental conditions. In both cases, magnetic and electric fields are applied to the flow. The computed results are in good agreement with the available experimental data.

Kato, Hiromasa↗
DOE OSTI2017

Effects of air chemistry and stiffened EOS of air in numerical simulations of bubble collapse in water

In this paper we study the effects of a stiffened gas equation of state for air at high pressure and air chemistry with dissociation in numerical simulations of bubble collapse in water. Two types of bubble collapse are studied, with one corresponding to spherically symmetric collapse simulating single-bubble sonoluminescence (SBSL), and the other a GPa-shock-induced axisymmetric bubble collapse. The numerical method is a finite-volume based solver with diffuse material interface model. Verification and validation of the solver are demonstrated by comparing to analytical solutions and experimental observations. We find that for sonoluminescence, air chemistry and equation of state for air at high pressures can have significant effects on the peak temperature and pressure attained during the evolution, and that the peak temperature is on the order of 1 eV, which is close to that observed in experiments. For shock-induced bubble collapse, the pressure inside the bubble will not be as high as that in sonoluminescence due to the absence of spherical symmetry. However, the air chemistry still has a significant effect on the temperature of the bubble. We also examine the effect of multiple bubble interaction in shock-induced bubble collapse and find several mechanisms that can significantly increase the pressure in the water. These mechanisms include frontal-distal-side-collision, shock focusing, upstream-traveling shocks, and compression of the water near the centerline by the vortex generated by bubble collapse. These have important implications for cavitation erosion.

42 ENGINEERING↗
NASA NTRS1990

Direct numerical simulation of the transitional zone

Properties of the transitional zone in channel and boundary-layer flow are determined from the results of direct numerical simulations of forced transition. Three channel flow cases produce similar behavior, particularly in the 'transition function' relevant for some algebraic eddy viscosity models of the transitional zone. Moreover, the transitional zone from the computed forced boundary-layer transition is qualitatively similar to experimental data for natural transition and to the computed channel flow transition.

Zang, Thomas A.↗
NASA NTRS1991

Numerical simulations of space processing - The reality, the myth, and the future

The present capabilities offered by the combination of advanced special and general purpose numerical codes and supercomputers are discussed. Emphasis is placed on unique demands placed by the space environment on the computation of realistic space experiments and recommendations for improving the scientific return on the space investment. It is concluded that numerical simulations can provide timely and relevant answers which can guide and improve future space experiments.

Chait, A.↗
NASA NTRS2004

Direct Numerical Simulation of Turbulent Condensation in Clouds

In this brief, we investigate the turbulent condensation of a population of droplets by means of a direct numerical simulation. To that end, a coupled Navier-Stokes/Lagrangian solver is used where each particle is tracked and its growth by water vapor condensation is monitored exactly. The main goals of the study are to find out whether turbulence broadens the droplet size distribution, as observed in in situ measurements. The second issue is to understand if and for how long a correlation between the droplet radius and the local supersaturation exists for the purpose of modeling sub-grid scale microphysics in cloud-resolving codes. This brief is organized as follows. In Section 2 the governing equations are presented, including the droplet condensation model. The implementation of the forcing procedure is described in Section 3. The simulation results are presented in Section 4 together with a sketch of a simple stochastic model for turbulent condensation. Conclusions and the main outcomes of the study are given in Section 5.

Shariff, K.↗
NASA NTRS2007

Time-Accurate Numerical Simulations of Synthetic Jet Quiescent Air

The unsteady evolution of three-dimensional synthetic jet into quiescent air is studied by time-accurate numerical simulations using a second-order accurate mixed explicit-implicit fractional step scheme on Cartesian grids. Both two-dimensional and three-dimensional calculations of synthetic jet are carried out at a Reynolds number (based on average velocity during the discharge phase of the cycle V(sub j), and jet width d) of 750 and Stokes number of 17.02. The results obtained are assessed against PIV and hotwire measurements provided for the NASA LaRC workshop on CFD validation of synthetic jets.

Rupesh, K-A. B.↗
DOE OSTI2015

Numerical simulation of photoexcited polaron states in water

We consider the dynamic polaron model of the hydrated electron state on the basis of a system of three nonlinear partial differential equations with appropriate initial and boundary conditions. A parallel numerical algorithm for the numerical solution of this system has been developed. Its effectiveness has been tested on a few multi-processor systems. A numerical simulation of the polaron states formation in water under the action of the ultraviolet range laser irradiation has been performed. The numerical results are shown to be in a reasonable agreement with experimental data and theoretical predictions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1987

Numerical Simulation Of Silicon-Ribbon Growth

Mathematical model includes nonlinear effects. In development simulates growth of silicon ribbon from melt. Takes account of entire temperature and stress history of ribbon. Numerical simulations performed with new model helps in search for temperature distribution, pulling speed, and other conditions favoring growth of wide, flat, relatively defect-free silicon ribbons for solar photovoltaic cells at economically attractive, high production rates. Also applicable to materials other than silicon.

Woda, Ben K.↗
NASA NTRS1987

Numerical simulation of shock wave diffraction by TVD schemes

An upwind total variation diminishing (TVD) scheme and a predictor-corrector symmetric TVD scheme were used to numerically simulate the blast wave diffraction on a stationary object. The objective is to help design an optimum configuration so that lateral motion is minimized and at the same time vortex shedding and flow separation are reduced during a blast wave encounter. Results are presented for a generic configuration for both a coarse grid and a fine grid to illustrate the global and local diffraction flow fields. Numerical experiments for the shock wave reflection on a wedge are also included to validate the current approach. Numerical study indicated that these TVD schemes are more stable and produced higher shock resolution than classical shock capturing methods such as the explicit MacCormack scheme.

Young, Victor Y. C.↗
DOE OSTI2018

Thermal characterization of full-scale PCM products and numerical simulations, including hysteresis, to evaluate energy impacts in an envelope application

This article presents combined measurements of fatty acid-based organic PCM products and numerical simulations to evaluate the energy benefits of adding a PCM layer to an exterior wall. The thermal storage characteristics of the PCM were measured using a heat flow meter apparatus (HFMA). The PCM characterization is based on a recent ASTM International standard test method, ASTM C1784. The PCM samples were subjected to step changes in temperature and allowed to stabilize at each temperature. By measuring the heat absorbed or released by the PCM, the temperature-dependent enthalpy functions for melting and freezing were determined.Here, the simulations were done using a previously-validated two-dimensional (2D) wall model containing a PCM layer and incorporating the HFMA-measured enthalpy functions. The wall model was modified to include the hysteresis phenomenon observed in PCMs, which is reflected in different melting and freezing temperatures of the PCM. Simulations were done with a single enthalpy curve based on the PCM melting tests, both melting and freezing enthalpy curves, and with different degrees of hysteresis between the melting and freezing curves. Significant differences were observed between the thermal performances of the modeled wall with the PCM layer under the different scenarios.

2D wall model with PCM↗
DOE OSTI2023

A direct numerical simulation study for confined non-isothermal jet impingement at moderate nozzle-to-plate distances: Capturing jet-to-ambient density effects

A direct numerical simulation (DNS) campaign is deployed for a series of confined downward oriented, non-isothermal turbulent impinging jet configurations. A baseline Reynolds number of 9960 is obtained through a precursor DNS pipe flow simulation (Re τ = 505). Three jet temperature configurations (confinement height to nozzle diameter of three) enter a cylindrical domain that share ambient and impingement plate temperatures (298.15 K). The range of jet temperatures are crafted such that the ratio of inlet to ambient density varies from unity to 0.52, showcasing the effect of density disparity on flow characteristics such as core collapse, radial mixing of momentum and energy, near-wall stagnation behavior, wall-jet profiles, and large-scale vortical structures. Surface quantities provided include mean radial heat flux and wall-shear stress profiles, and heat flux histograms at select radial stations. Results showcase increased radial normal stresses for higher temperature jets that support increased mixing, resulting in large-scale recirculation structures that are smaller, while retaining similar normalized radial wall profiles for shear stress, heat flux and pressure. Furthermore, radial plots for wall shear stress and Nusselt number showcase strong radial decay as compared to previous configurations that share similar jet and ambient temperatures. For the 373.15 K case, a Gaussian-like histogram for heat fluxes at the impingement plate transitions to a log-normal profile as radial distances increase. In contrast, the 573.15 K configuration displays a bi-modal heat flux characteristic at the impingement plate, and in similar manner to the moderate temperature counterpart, transitions to a log-normal profile at larger radial distances.

33 ADVANCED PROPULSION SYSTEMS↗