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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Measured and calculated optical property profiles in the mixed layer and free troposphere

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground-based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self-consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction-to-backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.

Rosen, James M.↗
NASA NTRS1985

Unsteady turbulent boundary-layer experiments with rapidly changing free-stream conditions

Selected data sets produced by the Stanford Unsteady Turbulent Boundary Layer Program are compared to analytic solutions of the Stokes equations for different kinds of imposed, organized unsteadiness. Good agreement is found, qualifying the facility as one that faithfully produces flows of different prescribed unsteady conditions. Data sets from the facility are therefore considered especially suitable for comparison with predictions of turbulence models for unsteady flows. New data describing the development of an unsteady separation bubble are presented.

Brereton, G. J.↗
NASA NTRS2005

Alloy NASA-HR-1

NASA-HR-1 is a high-strength Fe-Ni-base superalloy that resists high-pressure hydrogen environment embrittlement (HEE), oxidation, and corrosion. Originally derived from JBK-75, NASA-HR-1 has exceptional HEE resistance that can be attributed to its gamma-matrix and eta-free (Ni3Ti) grain boundaries. The chemistry was formulated using a design approach capable of accounting for the simultaneous effects of several alloy additions. This approach included: (1) Systematically modifying gamma-matrix compositions based on JBK-75; (2) Increasing gamma (Ni3(Al,Ti)) volume fraction and adding gamma-matrix strengthening elements to obtain higher strength; and (3) Obtaining precipitate-free grain boundaries. The most outstanding attribute of NASA-HR-1 is its ability to resist HEE while showing much improved strength. NASA-HR-1 has approximately 25% higher yield strength than JXK-75 and exhibits tensile elongation of more than 20% with no ductility loss in a hydrogen environment at 5 ksi, an achievement unparalleled by any other commercially available alloy. Its Cr and Ni contents provide exceptional resistance to environments that promote oxidation and corrosion. Microstructural stability was maintained by improved solid solubility of the gamma-matrix, along with the addition of alloying elements to retard eta (Ni3Ti) precipitation. NASA-HR-1 represents a new system that greatly extends the compositional ranges of existing HEE-resistant Fe-Ni-base superalloys.

Chen, Po-Shou↗
NASA NTRS1983

The structure of a separating turbulent boundary layer. IV - Effects of periodic free-stream unsteadiness

Measurements were obtained of the sinusoidal unsteadiness of the free stream velocity during the separation of the turbulent boundary layer. Data were gathered by single wire and cross-wire, anemometry upstream of flow detachment, by laser Doppler velocimetry to detect the movement of the flow in small increments, and by a laser anemometer in the detached zone to measure turbulence and velocities. The study was restricted to a sinusoidal instability frequency of 0.61 and a ratio of oscillation amplitude to mean velocity of 0.3. Large amplitude and phase variations were found after the detachment, with unsteady effects producing hysteresis in the relationships between flow parameters. The detached shear layer decreased in thickness with increasing free-stream velocity and increases in the Reynolds shear stress. Deceleration of the free stream velocity caused thickening in the shear layer and upstream movement of the flow reversal location. The results are useful for studies of compressor blade and helicopter rotors in transition.

Simpson, R. L.↗
NASA NTRS2007

Experiments in Transitional Boundary Layers With Emphasis on High Free-Stream Disturbance Level, Surface Concave Curvature and Strong Favorable Streamwise Pressure Gradient Effects

Experiments on boundary layer transition with flat, concave and convex walls and various levels of free-stream disturbance and with zero and strong streamwise acceleration have been conducted. Measurements of both fluid mechanics and heat transfer processes were taken. Examples are profiles of mean velocity and temperature; Reynolds normal and shear stresses; turbulent streamwise and cross-stream heat fluxed; turbulent Prandtl number; and streamwise variations of wall skin friction and heat transfer coefficient values. Free-stream turbulence levels were varied over the range from about 0.3 percent to about 8 percent. The effects of curvature on the onset of transition under low disturbance conditions are clear; concave curvature leads to an earlier and more rapid transition and the opposite is true for convex curvature This was previously known but little documentation of the transport processes in the flow was available

Simon, T. W.↗
NASA NTRS1991

Free-stream turbulence and concave curvature effects on heated, transitional boundary layers

An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-stream turbulence levels was performed. Results show that for transition of a flat-plate, the two forms of boundary layer behavior, identified as laminar-like and turbulent-like, cannot be thought of as separate Blasius and fully-turbulent profiles, respectively. Thus, simple transition models in which the desired quantity is assumed to be an average, weighted on intermittency, of the theoretical laminar and fully turbulent values is not expected to be successful. Deviation of the flow identified as laminar-like from theoretical laminar behavior is shown to be due to recovery after the passage of a turbulent spot, while deviation of the flow identified as turbulent-like from the full-turbulent values is thought to be due to incomplete establishment of the fully-turbulent power spectral distribution. Turbulent Prandtl numbers for the transitional flow, computed from measured shear stress, turbulent heat flux and mean velocity and temperature profiles, were less than unity. For the curved-wall case with low free-stream turbulence intensity, the existence of Gortler vortices on the concave wall within both laminar and turbulent flows was established using liquid crystal visualization and spanwise velocity and temperature traverses. Transition was found to occur via a vortex breakdown mode. The vortex wavelength was quite irregular in both the laminar and turbulent flows, but the vortices were stable in time and space. The upwash was found to be more unstable, with higher levels of u' and u'v', and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were found to be near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No evidence of streamwise vortices was seen in the high turbulence intensity case.

Kim, J.↗
NASA NTRS1959

Details of Exact Low Prandtl Number Boundary-Layer Solutions for Forced and For Free Convection

A detailed report is given of exact (numerical) solutions of the laminar-boundary-layer equations for the Prandtl number range appropriate to liquid metals (0.003 to 0.03). Consideration is given to the following situations: (1) forced convection over a flat plate for the conditions of uniform wall temperature and uniform wall heat flux, and (2) free convection over an isothermal vertical plate. Tabulations of the new solutions are given in detail. Results are presented for the heat-transfer and shear-stress characteristics; temperature and velocity distributions are also shown. The heat-transfer results are correlated in terms of dimensionless parameters that vary only slightly over the entire liquid-metal range. Previous analytical and experimental work on low Prandtl number boundary layers is surveyed and compared with the new exact solutions.

Sparrow, E. M.↗
NASA NTRS1997

Effect of Free Stream Turbulence and Other Vortical Disturbances on a Laminar Boundary Layer

This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulence Reynolds number or the downstream distance (or both) is small enough so that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e., the Navier Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes, and the corresponding individual Fourier component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. The calculated boundary-layer disturbances are in good quantitative agreement with the experimentally observed Klebanoff modes when strong low-frequency anisotropic effects are included in the free-stream turbulence spectrum. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.

Goldstein, Marvin E.↗
NASA NTRS1991

Free-stream turbulence and concave curvature effects on heated, transitional boundary layers, volume 1

An experimental investigation of the transition process on flat-plate and concave curved-wall boundary layers for various free-streem turbulence levels was performed. Where possible, sampling according to the intermittency function was made. Such sampling allowed segregation of the signal into two types of behavior: laminar-like and turbulent-like. The results from the investigation are discussed. Documentation is presented in two volumes. Volume one contains the text of the report including figures and supporting appendices. Volume two contains data reduction program listings and tabulated data.

Kim, J.↗
NASA NTRS1989

Boundary-layer receptivity to long-wave free-stream disturbances

The present treatment of the early stages of boundary layer-transition phenomena, where the unsteady motion is of small amplitude and can be accordingly treated as a small perturbation of an appropriate mean flow, elaborates the Heinrich et al. (1988) discussion of the role played by this 'receptivity' stage: in which the unsteady flow exhibits the same harmonic time-dependence as the externally-imposed forcing. Freestream disturbance wavelengths are noted to often be much longer than the Tollmien-Schlichting wavelength. Attention is given to the variety of wavelength-reduction mechanisms able to couple the long-wavelength, freestream disturbances to the comparatively short Tollmien-Schlichting waves.

Goldstein, M. E.↗
NASA NTRS2022

Dilution of Boundary Layer Cloud Condensation Nucleus Concentrations by Free Tropospheric Entrainment During Marine Cold Air Outbreaks

Recent aircraft measurements over the northwest Atlantic enable an investigation of how entrainment from the free troposphere (FT) impacts cloud condensation nucleus (CCN) concentrations in the marine boundary layer (MBL) during cold-air outbreaks (CAOs), motivated by the role of CCN in mediating transitions from closed to open-cell regimes. Observations compiled over eight flights indicate predominantly far lesser CCN concentrations in the FT than in the MBL. For one flight, a fetch-dependent MBL-mean CCN budget is compiled from estimates of sea-surface fluxes, entrainment of FT air, and hydrometeor collision-coalescence, based on in-situ and remote-sensing measurements. Results indicate a dominant role of FT entrainment in reducing MBL CCN concentrations, consistent with satellite-observed trends in droplet number concentration upwind of CAO cloud-regime transitions over the northwest Atlantic. Relatively scant CCN may widely be associated with FT dry intrusions, and should accelerate cloud-regime transitions where underlying MBL air is CCN-rich, thereby reducing regional albedo.

Boundary Layer Cloud Condensation Nucleus Concentr↗
NASA NTRS1983

The structure of a separating turbulent boundary layer. V Frequency effects on periodic unsteady free-stream flows

The results of experimental trials to obtain measurements of the effects of frequency on the sinusoidal variations in the free stream velocity over a separating turbulent boundary layer are reported. An oscillation frequency of 0.596 Hz was examined, in conjunction with a free stream velocity of 10.18 m/sec. Hot-wire anemometers measured the velocity upstream of the separation and the phase-averaged skin friction, while the velocities in the detached flow zone and the downstream velocity were assayed with a laser anemometer. Large amplitude and phase variations developed at the beginning of separation, producing hysteresis in the relationships between flow parameters. Increases in the free stream velocity decreased the detached shear layer thickness, while in the deceleration segment of the cycle the separated shear layer grew thicker. The results are in a frequency range that are significant for axial compressor blades and helicopter blades.

Simpson, R. L.↗
NASA NTRS1977

Creep degradation in oxide-dispersion-strengthened alloys

Oxide dispersion strengthened Ni-base alloys in wrought bar form are studied for creep degradation effects similar to those found in thin gage sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and three types of advanced ODS-NiCrAl alloys. Tensile test specimens were exposed to creep at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, the appearance of dispersoid-free bands, grain boundary cavitation, and internal oxidation in the microstructure were interpreted as creep degradation effects. This work showed that many ODS alloys are subject to creep damage. Degradation of tensile properties occurred after very small amounts of creep strain, ductility being the most sensitive property. All the ODS alloys which were creep damaged possessed a large grain size. Creep damage appears to have been due to diffusional creep which produced dispersoid-free bands around boundaries acting as vacancy sources. Low angle and possibly twin boundaries acted as vacancy sources.

Whittenberger, J. D.↗
NASA NTRS1977

Effect of prior creep at 1365 K on the room temperature tensile properties of several oxide dispersion strengthened alloys

An experimental study was conducted to determine whether oxide dispersion-strengthened (ODS) Ni-base alloys in wrought bar form are subject to creep degradation effects similar to those found in thin-gage sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and advanced ODS-NiCrAl types; the alloys included microstructures ranging from an essentially perfect single crystal to a structure consisting of very small elongated grains. Tensile test specimens were exposed to creep at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, appearance of dispersoid free bands, grain boundary cavitation, and/or internal oxidation are interpreted as creep degradation effects. The amount of degradation depends on creep strain, and degradation appears to be due to diffusional creep which produces dispersoid free bands around grain boundaries acting as vacancy sources.

Whittenberger, J. D.↗
NASA NTRS1984

Effect of combined pressure and temperature distortion orientation on high-bypass-ratio turbofan engine stability

Total-temperature, static-pressure and total-pressure distributions were measured in the inlet duct upstream of the engine inlet and within the fan and compressor of a YTF34 turbofan engine. Free-stream and boundary layer yaw angle variations were measured between a rotable screen assembly and the engine inlet. Total pressure distortions were generated using three 180 deg extent screens and total temperature distortions were generated using a rotatable hydrogen burner. Reynolds number index upstream of the rotatable screen assembly was maintained at 0.5 (based on the undistorted sectors at station 1, the inlet flow measuring station). The engine mechanical fan speed at sea level condition was rated at 7005 rpm. The engine was tested at a corrected fan speed of 90 percent of rated condition. Yaw angle increased between the rotatable screen assembly and the engine inlet. The largest variation in free-stream and boundary layer yaw angle occurs when the combined distortions are 180 deg out-of-phase. Static-pressure distortion increased exponentially as flow approached the engine. Total-pressure distortions were attenuated between the engine inlet and the compressor exit. Total-temperature distortion persisted through the compressor for all four combined distortions investigated.

Soeder, R. H.↗
NASA NTRS1988

Measurement of sound emitted by flying projectiles with aeroacoustic sources

Training projectiles with axisymmetric ring cavities that produce intense tones in an airstream were shot in a straight-line trajectory. A ground-based microphone was used to obtain the angular distribution of sound intensity produced from the flying projectile. Data reduction required calculation of Doppler and attenuation factors. Also, the directional sensitivity of the ground-mounted microphone was measured and used in the data reduction. A rapid angular variation of sound intensity produced from the projectile was found that can be used to plot an intensity contour map on the ground. A full-scale field test confirmed the validity of the aeroacoustic concept of producing a relatively intense whistle from the projectile, and the usefulness of short-range flight tests that yield acoustic data free of uncertainties associated with diffraction, reflection, and refraction at jet boundaries in free-jet tests.

Cho, Y. I.↗