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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 1,261 records · Page 70
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Experimental and numerical study of a turbulent boundary layer with pressure gradients

Results are presented of an experimental and numerical study of a turbulent boundary layer with pressure gradients conducted using the recent 'fringe method' with its numerical advantages and good inflow quality. After an inflow transient good agreement is observed; the differences, of up to 13 percent, are discussed. Moderate deviations from the law of the wall are found in the velocity profiles of the simulation. They are fully correlated with the pressure gradient, are in fair quantitative agreement with the experimental results of Nagano et al. (1992), and are roughly the opposite of uncorrected mixing-length-model predictions. Large deviations from the wall scaling are observed for other quantities, notably for the turbulence dissipation rate. The a(1) structure parameter drops mildly in the upper layer with adverse pressure gradient.

Spalart, Philippe R.↗
NASA NTRS1995

Estimating Vibrational Powers Of Parts In Fluid Machinery

In new method of estimating vibrational power associated with component of fluid-machinery system, physics of flow through (or in vicinity of) component regarded as governing vibrations. Devised to generate scaling estimates for design of new parts of rocket engines (e.g., pumps, combustors, nozzles) but applicable to terrestrial pumps, turbines, and other machinery in which turbulent flows and vibrations caused by such flows are significant. Validity of method depends on assumption that fluid flows quasi-steadily and that flow gives rise to uncorrelated acoustic powers in different parts of pump.

Harvey, S. A.↗
NASA NTRS2017

Development and Testing of a Multi-Transducer System for Measuring Height of Condensed Water in Steam Pipes with Steady-State and Turbulent Flow Conditions

One of the critical requirements for ensuring the safety of a steam pipe is to monitor the condensed water level under operation. For this objective, the authors initially developed methods and obtained preliminary test results based on the use of ultrasonic pulse-echo transducers and enhanced signal-processing tool; however, the methodology needed further development in order to obtain measurements in turbulent dynamic flow conditions. To improve the reliability of measurements taken in turbulent flow conditions, an experimental system was developed using multiple transducers driven by a multiplexer, and a data acquisition module capable of operating in any flow conditions. The system consists of a simulation testbed, which allows testing the performance over a range of flow rates and water levels and to observe flow conditions and patterns, as well as measure actual water level, flow velocities, wave conditions, etc. In this paper, we present the development details that include description of the testbed for simulating the flow of condensed water, the multiple transducers arrangement, the signal processing method, and the test results of both steady state and turbulent flow.

Lih, Shyh-Shiuh↗
DOE OSTI2024

On detrending stream velocity time series for robust tidal flow turbulence characterization

We investigated the impact of detrending techniques on turbulence quantities from tidal stream flow data, focusing on the autocorrelation function, $ρ_{uu}$, and velocity spectrum, $Φ(f)$. Standard detrending methods, including high-pass frequency-based and polynomial-based techniques, are examined, alongside a proposed alternative method, the empirical mode decomposition (EMD). Our results highlight that intervals of flow acceleration and deceleration, typical in tidal and riverine flows, significantly affect the estimation of turbulence quantities using high-pass frequency filtering and polynomial detrending of varying orders. These methods can strongly influence $ρ_{uu}$ and $Φ(f)$, thereby affecting the accurate estimation of derived quantities. We examine two variations of detrending data using EMD; the first removes only the EMD residue, and the second removes both the residue and the largest scale intrinsic mode function (IMF). By comparing the detrended spectra with the modeled von Kármán spectra, we demonstrate that the second variation (i.e., removing the residue and the largest scale IMF) successfully removed the large-scale trend of the data while retaining the energy of other scales.

Detrend↗
NASA NTRS1979

High speed interference heating loads and pressure distributions resulting from elevon deflections

Effects of elevon-induced three-dimensional shock-wave turbulent boundary-layer interactions on hypersonic aircraft surfaces are analyzed. Detailed surface pressure and heating rate distributions obtained on wing-elevon-fuselage models representative of aft portions of hypersonic aircraft are compared with analytical and experimental results from other sources. Examples are presented that may be used to evaluate the adequacy of current theoretical methods for estimating the effects of three-dimensional shock-wave turbulent boundary-layer interactions on hypersonic aircraft surfaces.

Johnson, C. B.↗
NASA NTRS1985

Laser Doppler velocimetry primer

Advanced research in experimental fluid dynamics required a familiarity with sophisticated measurement techniques. In some cases, the development and application of new techniques is required for difficult measurements. Optical methods and in particular, the laser Doppler velocimeter (LDV) are now recognized as the most reliable means for performing measurements in complex turbulent flows. And such, the experimental fluid dynamicist should be familiar with the principles of operation of the method and the details associated with its application. Thus, the goals of this primer are to efficiently transmit the basic concepts of the LDV method to potential users and to provide references that describe the specific areas in greater detail.

Bachalo, William D.↗
NASA NTRS1969

Remote atmospheric probing by ground to ground line of sight optical methods

The optical effects arising from refractive-index variations in the clear air are qualitatively described, and the possibilities are discussed of using those effects for remotely sensing the physical properties of the atmosphere. The effects include scintillations, path length fluctuations, spreading of a laser beam, deflection of the beam, and depolarization. The physical properties that may be measured include the average temperature along the path, the vertical temperature gradient, and the distribution along the path of the strength of turbulence and the transverse wind velocity. Line-of-sight laser beam methods are clearly effective in measuring the average properties, but less effective in measuring distributions along the path. Fundamental limitations to the resolution are pointed out and experiments are recommended to investigate the practicality of the methods.

Lawrence, R. S.↗
NASA NTRS1971

The Determination of the Geometries of Multiple-Element Airfoils Optimized for Maximum Lift Coefficient

Optimum airfoils in the sense of maximum lift coefficient are obtained by a newly developed method. The maximum lift coefficient is achieved by requiring that the turbulent skin friction be zero in the pressure rise region on the upper surface. Under this constraint, the pressure distribution is optimized. The optimum pressure distribution consists of a uniform stagnation pressure on the lower surface, a uniform minimum pressure on the upper surface immediately downstream of the front stagnation point followed by a Stratford zero skin friction pressure rise. When multiple-element airfoils are under consideration, this optimum pressure distribution appears on every element. The parameters used to specify the pressure distribution on each element are the Reynolds number and the normalized trailing edge velocity. The newly developed method of design computes the velocity distribution on a given airfoil and modifies the airfoil contour in a systematic manner until the desired velocity distribution is achieved. There are no limitations on how many elements the airfoil to be designed can have.

Chen, A. W.↗
NASA NTRS1995

Magnetohydrodynamic turbulence in the solar wind

The fluctuations in magnetic field and plasma velocity in solar wind, which possess many features of fully developed magnetohydrodynamic (MHD) turbulence, are discussed. Direct spacecraft observations from 0.3 to over 20 AU, remote sensing radio scintillation observations, numerical simulations, and various models provide complementary methods that show that the fluctuations in the wind parameters undergo significant dynamical evolution independent of whatever turbulence might exist in the solar photosphere and corona. The Cluster mission, with high time resolution particle and field measurements and its variable separation strategies, should be able to provide data for answering many questions on MHD turbulence.

Goldstein, Melvyn L.↗
DOE OSTI2022

Adaptive Mesh Refinement Large Eddy Simulation of the Supercritical Carbon Dioxide Round Turbulent Jet

Supercritical carbon dioxide (sCO2) is of interest to a range of engineering problems, including carbon capture, utilization, and storage (CCUS) as well as advanced cycles for power generation. Non-ideal variations in physical properties of sCO2 impact the physics of these systems. In this study, we simulate turbulent sCO2 jets to gain a better understanding of these physics.We use a second order finite volume method with adaptive mesh refinement as implemented in the first-principles simulation code PeleC to perform a Large Eddy Simulation (LES) of three turbulent jets of sCO2. Additionally, we use the Soave-Redlich-Kwong equation of state to close the system and examine the impact of a cubic equation of state on the turbulent flow physics. We look at velocity and Reynolds stress profiles at different downstream locations for three cases in which the temperature of the jet andthat of the ambient fluid differ in order to capture the effects of widely varying thermal properties in the pseudocritical region. These results are then contrasted with established theory for ideal gas jets.

adaptive mesh refinement↗
NASA NTRS1996

Analytical Theory of the Destruction Terms in Dissipation Rate Transport Equations

Modeled dissipation rate transport equations are often derived by invoking various hypotheses to close correlations in the corresponding exact equations. D. C. Leslie suggested that these models might be derived instead from Kraichnan's wavenumber space integrals for inertial range transport power. This suggestion is applied to the destruction terms in the dissipation rate equations for incompressible turbulence, buoyant turbulence, rotating incompressible turbulence, and rotating buoyant turbulence. Model constants like C(epsilon 2) are expressed as integrals; convergence of these integrals implies the absence of Reynolds number dependence in the corresponding destruction term. The dependence of C(epsilon 2) on rotation rate emerges naturally; sensitization of the modeled dissipation rate equation to rotation is not required. A buoyancy related effect which is absent in the exact transport equation for temperature variance dissipation, but which sometimes improves computational predictions, also arises naturally. Both the presence of this effect and the appropriate time scale in the modeled transport equation depend on whether Bolgiano or Kolmogorov inertial range scaling applies. A simple application of these methods leads to a preliminary, dissipation rate equation for rotating buoyant turbulence.

Rubinstein, Robert↗
NASA NTRS2021

Coupling of the FUN3D Unstructured Flow Solver and the LASTRAC Stability Code to Model Transition

We develop an iterative automated method to predict transition locations in boundary-layer flows by using the FUN3D solver to perform flow simulations and the LASTRAC code for linear stability computations. The coupling of FUN3D and LASTRAC allows for a robust physics-based approach to model boundary-layer transition by analyzing the growth of different instability waves and then using that information to iteratively update the resulting transition location. There is no user involvement during the iterative computations. We apply this automated method to subsonic flow over a flat plate with a sharp leading edge. The final solution has regions of laminar and turbulent flow with a transition onset location that agrees with experiments and stability-based correlations. This iterative automated method is also applied to an NLF(1)-0416 airfoil for conditions with and without a separation bubble. Along with predicting transition locations, we compare the streamwise distributions of surface-pressure and skin-friction coefficients to a transport-equation-based model. We consider a 6:1 prolate spheroid at three angles of attack, namely, five, ten, and fifteen degrees, where mixed-mode transition occurs due to both Tollmien-Schlichting and crossflow instabilities. The skin-friction contours and transition fronts at every angle of attack from our iterative automated method show good agreement with past experimental and computational results in the literature for the 6:1 prolate spheroid.

Transition↗
NASA NTRS1989

The winds of O-stars. I - An analysis of the UV line profiles with the SEI method

The theoretical UV P Cygni profiles of the UV resonance lines for C IV, N V, Si IV, C III, and N IV of 26 O-type stars and one B star were calculated using the SEI method described by Lamers et al. (1987), taking into account the effects of turbulence in the wind, limb darkening, photospheric lines, and interstellar Ly alpha. The results were compared with profiles observed with the IUE satellite. The profile fits were found to be very accurate, showing significant improvement over previously obtained fits with the Sobolev (1958) method. The SEI method and the line fitting analysis are described and the results are presented in terms of the column densities and distributions of absorbing atoms, and the velocity laws.

Groenewegen, M. A. T.↗
NASA NTRS1996

Application of an Upwind High Resolution Finite-Differencing Scheme and Multigrid Method in Steady-State Incompressible Flow Simulations

The analysis and design of a submarine propulsor requires the ability to predict the characteristics of both laminar and turbulent flows to a higher degree of accuracy. This report presents results of certain benchmark computations based on an upwind, high-resolution, finite-differencing Navier-Stokes solver. The purpose of the computations is to evaluate the ability, the accuracy and the performance of the solver in the simulation of detailed features of viscous flows. Features of interest include flow separation and reattachment, surface pressure and skin friction distributions. Those features are particularly relevant to the propulsor analysis. Test cases with a wide range of Reynolds numbers are selected; therefore, the effects of the convective and the diffusive terms of the solver can be evaluated separately. Test cases include flows over bluff bodies, such as circular cylinders and spheres, at various low Reynolds numbers, flows over a flat plate with and without turbulence effects, and turbulent flows over axisymmetric bodies with and without propulsor effects. Finally, to enhance the iterative solution procedure, a full approximation scheme V-cycle multigrid method is implemented. Preliminary results indicate that the method significantly reduces the computational effort.

NAVIER STOKES EQUATIONS↗
NASA NTRS1987

Stability characteristics of a supersonic boundary layer and their relation to the position of the laminar-turbulent transition point

By comparing the calculated results with experimental data, it is demonstrated that the position of the laminar-boundary transition point of a boundary layer can be estimated by using the e-exp-n method. The effect of the Mach number, pressure gradient, and heat transfer on the laminar-turbulent transition is discussed. It is found that under conditions of strong cooling, the effect of the pressure gradient on the position of the transition point is less pronounced than in the absence of heat transfer.

Lysenko, V. I.↗
DOE OSTI2021

Constructing a new predictive scaling formula for ITER's divertor heat-load width informed by a simulation-anchored machine learning

Understanding and predicting divertor heat-load width λq is a critically important problem for an easier and more robust operation of ITER with high fusion gain. Previous predictive simulation data for λ q using the extreme-scale edge gyrokinetic code XGC1 [S. Ku et al., Phys. Plasmas 25, 056107 (2018)] in the electrostatic limit under attached divertor plasma conditions in three major US tokamaks [C. S. Chang et al., Nucl. Fusion 57, 116023 (2017)] reproduced the Eich and Goldston attached-divertor formula results [formula #14 in T. Eich et al., Nucl. Fusion 53, 093031 (2013) and R. J. Goldston, Nucl. Fusion 52, 013009 (2012)] and furthermore predicted over six times wider λ q than the maximal Eich and Goldston formula predictions on a full-power (Q = 10) scenario ITER plasma. After adding data from further predictive simulations on a highest current JET and highest-current Alcator C-Mod, a machine learning program is used to identify a new scaling formula for λ q as a simple modification to the Eich formula #14, which reproduces the Eich scaling formula for the present tokamaks and which embraces the wide λ q XGC for the full-current Q = 10 ITER plasma. Additionally, the new formula is then successfully tested on three more ITER plasmas: two corresponding to long burning scenarios with Q = 5 and one at low plasma current to be explored in the initial phases of ITER operation. The new physics that gives rise to the wider λ q XGC is identified to be the weakly collisional, trapped-electron-mode turbulence across the magnetic separatrix, which is known to be an efficient transporter of the electron heat and mass. Electromagnetic turbulence and high-collisionality effects on the new formula are the next study topics for XGC1.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Predicting large-scale pool fire dynamics using an unsteady flamelet- and large-eddy simulation-based model suite

A low-Mach, unstructured, large-eddy-simulation-based, unsteady flamelet approach with a generalized heat loss combustion methodology (including soot generation and consumption mechanisms) is deployed to support a large-scale, quiescent, 5-m JP-8 pool fire validation study. The quiescent pool fire validation study deploys solution sensitivity procedures, i.e., the effect of mesh and time step refinement on capturing key fire dynamics such as fingering and puffing, as mesh resolutions approach O(1) cm. A novel design-order, discrete-ordinate-method discretization methodology is established by use of an analytical thermal/participating media radiation solution on both low-order hexahedral and tetrahedral mesh topologies in addition to quadratic hexahedral elements. Here, the coupling between heat losses and the flamelet thermochemical state is achieved by augmenting the unsteady flamelet equation set with a heat loss source term. Soot and radiation source terms are determined using flamelet approaches for the full range of heat losses experienced in fire applications including radiative extinction. The proposed modeling and simulation paradigm are validated using pool surface radiative heat flux, maximum centerline temperature location, and puffing frequency data, all of which are predicted within 10% accuracy. Simulations demonstrate that under-resolved meshes predict an overly conservative radiative heat flux magnitude with improved comparisons as compared to a previously deployed hybrid Reynolds-averaged Navier–Stokes/eddy dissipation concept-based methodology.

42 ENGINEERING↗
DOE OSTI2023

Evolution and hot electron generation of laser–plasma instabilities in direct-drive inertial confinement fusion

A series of 2D in-plane plane wave particle-in-cell simulations find distinctive paths of laser-plasma instability evolution in OMEGA-scale implosions, depending on the initial electron temperature. At low temperatures, two-plasmon decay (TPD) dominates in both initial growth and the steady state. At high temperatures, the initial dominant modes switch to stimulated Raman scattering, but TPD still dominates a steady state characterized by cavitation and Langmuir turbulence. A hot electron scaling is also obtained from the simulations that, when combined with laser/plasma conditions from hydro simulations, can predict hot electron generation in implosions that do not employ smoothing-by-spectral-dispersion (SSD). It also shows that under the same laser/plasma conditions, SSD can reduce hot electron generation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗