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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Wind tunnel measurements of three-dimensional wakes of buildings

Measurements relevant to the effect of buildings on the low level atmospheric boundary layer are presented. A wind tunnel experiment was undertaken to determine the nature of the flow downstream from a gap between two transversely aligned, equal sized models of rectangular cross section. These building models were immersed in an equilibrium turbulent boundary layer which was developed on a smooth floor in a zero longitudinal pressure gradient. Measurements with an inclined (45 degree) hot-wire were made at key positions downstream of models arranged with a large, small, and no gap between them. Hot-wire theory is presented which enables computation of the three mean velocity components, U, V and W, as well as Reynolds stresses. These measurements permit understanding of the character of the wake downstream of laterally spaced buildings. Surface streamline patterns obtained by the oil film method were used to delineate the separation region to the rear of the buildings for a variety of spacings.

Logan, E., Jr.↗
NASA NTRS1982

Some observations of separated flow on finite wings

Wind tunnel test results for aspects of flow over airfoils exhibiting single and multiple trailing edge stall 'mushroom' cells are reported. Rectangular wings with aspect ratios of 4.0 and 9.0 were tested at Reynolds numbers of 480,000 and 257,000, respectively. Surface flow patterns were visualized by means of a fluorescent oil flow technique, separated flow was observed with a tuft wand and a water probe, spanwise flow was studied with hot-wire anemometry, smoke flow and an Ar laser illuminated the centerplane flow, and photographs were made of the oil flow patterns. Swirl patterns on partially and fully stalled wings suggested vortex flow attachments in those regions, and a saddle point on the fully stalled AR=4.0 wing indicated a secondary vortex flow at the forward region of the separation bubble. The separation wake decayed downstream, while the tip vortex interacted with the separation bubble on the fully stalled wing. Three mushroom cells were observed on the AR=9.0 wing.

Winkelmann, A. E.↗
NASA NTRS1983

Subsonic high-angle-of-attack aerodynamic characteristics of a cone and cylinder with triangular cross sections and a cone with a square cross section

Experiments were conducted in the 12-Foot Pressure Wind Tunnel at Ames Research Center on three models with noncircular cross sections: a cone having a square cross section with rounded corners and a cone and cylinder with triangular cross sections and rounded vertices. The cones were tested with both sharp and blunt noses. Surface pressures and force and moment measurements were obtained over an angle of attack range from 30 deg to 90 deg and selected oil-flow experiments were conducted to visualize surface flow patterns. Unit Reynolds numbers ranged from 0.8x1,000,000/m to 13.0x1,000,000/m at a Mach number of 0.25, except for a few low-Reynolds-number runs at a Mach number of 0.17. Pressure data, as well as force data and oil-flow photographs, reveal that the three dimensional flow structure at angles of attack up to 75 deg is very complex and is highly dependent on nose bluntness and Reynolds number. For angles of attack from 75 deg to 90 deg the sectional aerodynamic characteristics are similar to those of a two dimensional cylinder with the same cross section.

Clarkson, M. H.↗
NASA NTRS1984

Satellite observations of the 1982-1983 El Nino along the U.S. Pacific coast

Satellite infrared temperature images illustrate several effects of the 1982-1983 El Nino: warm sea-surface temperatures with the greatest anomalies near the coast, weakened coastal upwelling, and changes in surface circulation patterns. Phytoplankton pigment images from the Coastal Zone Color Scanner indicate reduced productivity during El Nino, apparently related to the weakened coastal upwelling. The satellite images provide direct evidence of mesosale changes associated with the oceanwide El Nino event.

Fiedler, P. C.↗
NASA NTRS1984

Numerical and experimental determination of secondary separation on delta wings in subsonic flow

A quasi-three-dimensional boundary layer method for conical inviscid flows is presented. The model was developed to characterize the flow fields over slender delta wings, particularly the shed vortex sheet. A steady, incompressible, laminar boundary layer is assumed and a solution is obtained with the Smith (1966) finite difference code. Predictions are compared with oil flow patterns and surface pressure measurements for 74 and 80 deg delta wings at 0-20 deg angles of attack. The model is capable of accurately predicting the secondary separation lines. Inviscid velocities derived from panel methods lead, however, to inaccurate pressure distributions.

Dejarnette, F. R.↗
NASA NTRS1985

Correlation and analysis of oil flow data for an air-breathing missile model

This paper will present the results of an oil flow investigation on an airbreathing missile model. This oil flow study examined the flow around the model, which can be configured with both axisymmetric and two-dimensional inlets. Flow visualization analyses were conducted for both types of geometries by examining the surface flow patterns made visible by the oil flows for Mach numbers of 2.5 and 3.95. The analysis has shown the extent of flow spillage around the inlet which has helped explain the force and moment data collected during previous testing of the model. The oil flow data has also been used to develop guidelines for modeling the location of the crossflow separation line along inlet fairings. Finally, the oil flow analysis has been used to identify unique features of the boattail flow. These boattail flow characteristics have been correlated with previous oil flow analysis of noncircular body models. This paper demonstrates the use of this type of oil flow analysis in developing missile flow field analysis and aerodynamic predictions ranging from impact angle methods through Navier-Stokes methods.

Stoy, S. L.↗
NASA NTRS1985

Conical similarity of shock/boundary-layer interactions generated by swept and unswept fins

A parametric experimental investigation has been made of the class of three-dimensional shock wave/turbulent boundary layer interactions generated by swept and unswept leading-edge fins. The fin sweepback angles were 0-65 deg at 5, 9, and 15 deg angles of attack. Two equilibrium two-dimensional turbulent boundary layers with a freestream Mach number of 2.95 and a Reynolds number of 6.3 x 10 to the 7th/m were used as incoming flow conditions. All of the resulting interactions were found to possess conical symmetry of the surface flow patterns and pressures outside of an initial inception zone. Further, these interactions were found to obey a simple conical similarity rule based on inviscid shock wave strength, irrespective of fin sweepback or angle of attack. This is one of the first demonstrations of similarity among three-dimensional interactions produced by geometrically dissimilar shock generators.

Settles, G. S.↗
NASA NTRS1985

An experimental study of three-dimensional shock wave/turbulent boundary layer interactions in a supersonic flow

Wall static pressure distributions, surface flow patterns, pitot pressures, and yaw angle profiles were measured in a skewed three-dimensional shock wave/turbulent boundary layer interaction region. The test section was axisymmetric with a constant diameter. The nominal freestream Mach number was 4. Upstream of the interaction, the boundary layer thickness was 0.31 in. (0.787 cm). The three-dimensional flow was produced by azimuthal pressure gradients which were generated by an 8-degree cone aligned with the primary flow direction, but with the cone axis displaced 0.3 in. (0.76 cm) from the channel centerline. The yaw angle was found to be a function of both the azimuthal angle and the distance from the beginning of the interaction. It was observed that yaw angle increased substantially near the wall. The maximum yaw angle for the whole flow field was obtained in the 90 degree azimuthal plane.

Chou, J. H.↗
NASA NTRS1986

Satellite detection of phytoplankton export from the mid-Atlantic Bight during the 1979 spring bloom

Analysis of Coastal Zone Color Scanner (CZCS) imagery confirms shipboard and in situ moored fluorometer observations of resuspension of near-bottom chlorophyll within surface waters (1 to 10 m) by northwesterly wind events in the mid-Atlantic Bight. As much as 8 to 16 micrograms chl/l are found during these wind events from March to May, with a seasonal increase of algal biomass until onset of stratification of the water column. Rapid sinking or downwelling apparently occurs after subsequent wind events, however, such that the predominant surface chlorophyll pattern is approx. 0.5 to 1.5 micrograms/l over the continental shelf during most of the spring bloom. Perhaps half of the chlorophyll increase observed by satellite during a wind resuspension event represents in-situ production during the 4 to 5 day interval, with the remainder attributed to accumulation of algal biomass previously produced and temporarily stored within near-bottom water. Present calculations suggest that about 10% of the primary production of the spring bloom may be exported as ungrazed phytoplankton carbon from mid-Atlantic shelf waters to those of the continental slope.

Walsh, J. J.↗
NASA NTRS1984

Vortex Flap Technology: a Stability and Control Assessment

A comprehensive low-speed wind tunnel investigation was performed of leading edge vortex flaps applied to representative aircraft configurations. A determination was made of the effects of analytically- and empirically-designed vortex flaps on the static longitudinal and lateral-directional aerodynamics, stability, and control characteristics of fighter wings having leading-edge sweep angles of 45 to 76.5 degrees. The sensitivity to several configuration modifications was assessed, which included the effects of flap planform, leading- and trailing-edge flap deflection angles, wing location on the fuselage, forebody strakes, canards, and centerline and outboard vertical tails. Six-component forces and moments, wing surface static pressure distributions, and surface flow patterns were obtained using the Northrop 21- by 30-inch low-speed wind tunnel.

Carey, K. M.↗
NASA NTRS1987

Performance and aerodynamic braking of a horizontal-axis wind turbine from small-scale wind tunnel tests

Wind tunnel tests of three 20" diameter, zero twist, zero pitch wind turbine rotor models were conducted in a 7' x 10' wind tunnel to determine the performance of such rotors with NACA 23024 and NACA 64 sub 3-621 airfoil sections. Aerodynamic braking characteristics of a 38% span, 30% chord, vented aileron configuration were measured on the NACA 23024 rotor. Surface flow patterns were observed using fluorescent mini-tufts attached to the suction side of the rotor blades. Experimental results with and without ailerons are compared to predictions using airfoil section data and a momentum performance code. Results of the performance studies show that the 64 sub 3-621 rotor produces higher peak power than the 23024 rotor for a given rotor speed. Analytical studies, however, indicate that the 23024 should produce higher power. Transition strip experiments show that the 23024 rotor is much more sensitive to roughness than the 64 sub 3-621 rotor. These trends agree with analytical predictions. Results of the aileron test show that this aileron, when deflected, produces a braking torque at all tip speed ratios. In free wheeling coastdowns the rotor blade stopped, then rotated backward at a tip speed ratio of -0.6.

Cao, H. V.↗
NASA NTRS1987

Vortex-fin interaction on a fighter aircraft

Water tunnel tests have been conducted to study the flow associated with fin buffet for twin-fin fighter aircraft using 1/48th scale F/A-18 models. Flow visualization made use of colored dyes to determine vortex patterns, and surface hot-film anemometry was used to study the turbulent energy and the frequencies present in the flow. Configurations tested include the full aircraft, aircraft without fins, aircraft without leading-edge-extensions, and aircraft without wings. Test Reynolds number ranged from 4300 to 12,800, with corresponding Mach numbers less than 10 to the -6th. The flow studies show that the LEX vortices burst just forward of the fins at about 25-deg angle-of-attack. Hot film anemometer signals show that fin surface turbulence increases with angle of attack, and that dominant frequencies appear in the flow when bursting occurs. The dominant frequencies correspond to a Strouhal number of about 0.7 for all speeds tested, and for all angles of attack for which vortex bursting was present.

Wentz, William H., Jr.↗
NASA NTRS1987

Structure of supersonic turbulent flow past a sharp fin

The three-dimensional shock-wave/turbulent-boundary-layer interaction generated by a sharp fin is examined both experimentally and theoretically at Mach 3 for a fin angle 20 deg and Reynolds number 900,000. This study represents an extension of previous research for the sharp fin configuration to stronger interactions. The experimental data include surface pressure profiles, surface streamline patterns, and boundary-layer profiles of pitot pressure and yaw angle. Two separate theoretical approaches or 'models' were employed. Both models utilize the three-dimensional compressible Navier-Stokes equations in mass-averaged variables. The theoretical approach of Knight (1984 and 1985) employs the algebraic turbulent eddy-viscosity model of Baldwin and Lomax (1978), and the theoretical model of Horstman (1984) employs the two-equation turbulence model of Jones and Launder (1972) coupled with the wall function model of Viegas et al. (1985).

Knight, Doyle D.↗
NASA NTRS1987

A flare event of the long-period RS Canum Venaticorum system IM Pegasi

The characteristics of a flare event detected on the long-period RS CVn system IM Pegasi are reported. The low-resolution spectrum show enhancements of up to a factor of five in some emission lines. All of the ultraviolet emission lines normally visible are enhanced significantly more than the normal 30 rotational modulation. Emission fluxes of both the quiescent and flare event are used to construct models of the density and temperature variation with height. These models reveal a downward shift of the transition region during the flare. Scaled models of the quiet and flaring solar outer atmosphere are used to estimate the filling factor of the flare event at about 30 percent of the stellar surface. The pattern of line enhancements in the flare is the same as a previous event in Lambda Andromeda observed previously.

Buzasi, Derek L.↗
NASA NTRS1988

CZCS view of an oceanic acid waste dump

Plumes from the acid waste dump in the New York Bight were visible in all nine cloud-free Coastal Zone Color Scanner images from April and May 1981. The CZCS subsurface radiance channels displayed consistent spectral characteristics, which consisted of a strong increase in the 550 nm channel and a moderate increase in the 520 nm channel relative to the surrounding coastal waters. The 443 nm channel showed no change or a slight decrease in radiance within the plumes. These anomalous radiances preclude the calculation of pigment in the dump plumes using existing algorithms. However, the high radiances of the 550 nm channel can be used to examine the flow patterns of surface water in the vicinity of the dump.

Elrod, Jane A.↗
NASA NTRS1989

Growing Gallium Arsenide On Silicon

Epitaxial layers of high quality formed on <111> crystal plane. Present work reports successful growth of 1- and 2-micrometer thick layers of n-type, 7-ohms per cm, 2-inch diameter, Si<111> substrate. Growth conducted in Riber-2300(R) MBE system. Both doped and undoped layers of GaAs grown. Chamber equipped with electron gun and camera for in-situ reflection high-energy-electron diffraction measurements. RHEED patterns of surface monitored continuously during slow growth stage.

Radhakrishnan, Gouri↗
NASA NTRS1989

A patched-grid algorithm for complex configurations directed towards the F-18 aircraft

A patched-grid algorithm for the analysis of complex configurations with an implicit, upwind-biased Navier-Stokes solver is presented. Results from both a spatial-flux and a time-flux conservation approach to patching across zonal boundaries are presented. A generalized coordinate transformation with a biquadratic geometric element is used at the zonal interface in order to treat highly stretched viscous grids and arbitrarily-shaped zonal boundaries. Applications are made to the F-18 forebody-strake configuration at subsonic, high-alpha conditions. Computed surface flow patterns compare well with ground-based and flight-test results; the large effect of Reynolds number on the forebody flowfield is shown.

Thomas, James L.↗
NASA NTRS1989

Concentric crater fill on Mars - An aeolian alternative to ice-rich mass wasting

Concentric crater fill, a distinctive martian landform represented by a concentric pattern of surface undulations confined within a crater rim, has been interpreted as an example of ice-enhanced regolith creep at midlatitudes (e.g., Squyres and Carr, 1986). Theoretical constraints on the stability and mobility of ground ice limit the applicability of an ice-rich soil in effectively mobilizing downslope movement at latitudes poleward of + or - 30 deg, where concentric crater fill is observed. High-resolution images of concentric crater fill material in the Utopia Planitia region (45 deg N, 271 deg W) show it to be an eroded, multiple-layer deposit. Layering should not be preserved if the crater fill material moved by slow deformation throughout its thickness, as envisioned in the ice-enhanced creep model. Multiple layers are also exposed in the plains material surrounding the craters, indicating a recurrent depositional process that was at least regional in extent. Mantling layers are observed in high-resolution images of many other locations around Mars, suggesting that deposition occurred on a global scale and was not limited to the Utopia Planitia region. It is suggested that an aeolian interpretation for the origin and modification of concentric crater fill material is most consistent with morphologic and theoretical constraints.

Zimbelman, J. R.↗