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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Opening remarks: Current and future activities

It is our custom to present the Propagation Program's recent accomplishments and future plans at the onset of a NAPEX meeting. A summary is presented. The data analysis phase of the U.S. Olympus Campaign is nearing its end. The final report will be published by Virginia Tech by midsummer 1993. The report will comprise measurement analysis for 12 months. It will include monthly and annual attenuation statistics and statistics on scintillation effects. A number of prediction models will be presented. The ACTS propagation experiment preparations are moving forward as expected. The mobile/personal channel characterization efforts continued during the last year. Data collected by the University of Texas in 1992 are being analyzed and will become available by September 1993. We have recently started a study to characterize LEO mobile/personal channels. Topics such as indoor reception, tree shadowing, blockage, and delay spread will be investigated. These results will become available in one to two years from now. We have also collected Ka-band mobile data using Olympus 20 GHz beacon transmissions. The University of Texas has already collected five years of low-elevation angle, 11GHz propagation data. The work on database for propagation models has progressed very well. The first release is ready, and the participants of this meeting will receive a copy of the software. We had an active year where CCIR is concerned. An input document on land mobile satellite system (LMSS) propagation models was submitted to the U.S. national committee and was approved and sent to Geneva.

Davarian, Faramaz↗
NASA NTRS1998

Reduction of Jet Penetration in a Cross-Flow by Using Tabs

A tab placed suitably on a nozzle that produces a jet in a cross-flow can reduce the penetration of the jet. This effect, achieved when the tab is placed on the windward side of the nozzle relative to the cross flow, may be of interest in film cooling applications. Wind tunnel experiments are carried out, in the momentum ratio (J) range of 10-90, to investigate the tab geometry that would maximize this effect. The preliminary results show that a 'delta tab' having a base width approximately fifty percent of the nozzle diameter may be considered optimum. With a given tab size, the effect is more pronounced at higher J. Reduction in jet penetration by as much as 40% is observed. Comparable reduction in jet penetration is also obtained when a triangular shaped tab is placed flush with the tunnel wall or with its apex tilted down into the jet nozzle (the 'delta tab' being the configuration in which the apex is tilted up). However, the delta tab involves the least flow blockage and pressure loss. Relative to the baseline case, the lateral spreading of the jet is found to be more with the delta tab but less with other orientations of the tab.

Zaman, K. B. M. Q.↗
NASA NTRS2004

Acoustic, Flow Related, and Performance Related Experimental Results for Generation 1.5 High Speed Civil Transport (HSCT) 2-Dimensional Exhaust Nozzles

The principle objectives of the current program were to experimentally investigate the repeatability of acoustic and aerodynamic characteristics of 2D-CD mixer-ejector nozzles and the effects on the acoustic and aerodynamic characteristics of 2D mixer-ejectors due to (1) the configurational variations, which include mixers with aligned CD chutes, aligned convergent chutes, and staggered CD chutes and aerodynamic cycle variables, (2) treatment variations by using different treatment materials, treating the ejector with varying area, location, and treatment thickness for a mixer-ejector configuration, and (3) secondary inlet shape (i.e., a more realistic inlet) and the blockage across the inlet (a possible fin-like structure needed for installation purpose) by modifying one of the inlet of a mixer-ejector configuration. The objectives also included the measurement dynamic pressures internal to the ejector for a few selected configuration to examine the internal noise characteristics.

Salikuddin, M.↗
NASA NTRS2017

Extinction by a Homogeneous Spherical Particle in an Absorbing Medium

We use a recent computer implementation of the first principles theory of electromagnetic scattering to compute far-field extinction by a spherical particle embedded in an absorbing unbounded host. Our results show that the suppressing effect of increasing absorption inside the host medium on the ripple structure of the extinction efficiency factor as a function of the size parameter is similar to the well-known effect of increasing absorption inside a particle embedded in a nonabsorbing host. However, the accompanying effects on the interference structure of the extinction efficiency curves are diametrically opposite. As a result, sufficiently large absorption inside the host medium can cause negative particulate extinction. We offer a simple physical explanation of the phenomenon of negative extinction consistent with the interpretation of the interference structure as being the result of interference of the field transmitted by the particle and the diffracted field due to an incomplete wave front resulting from the blockage of the incident plane wave by the particle's geometrical projection.

Scattering↗
NASA NTRS1999

Spreading Characteristics of Compressible Jets from Nozzles of Various Geometries

The spreading characteristics of jets from several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number range of 0.3-2.0. The effect of 'tabs' for a rectangular and a round nozzle is also included in the comparison. Compared to a round jet, the jets from the asymmetric nozzles spread only slightly more at subsonic conditions whereas at supersonic conditions, when 'screech' occurs, they spread much more. The dynamics of the azimuthal vortical structures of the jet, organized and intensified under the screeching condition, are thought to be responsible for the observed effect at supersonic conditions. Curiously, the jet from a 'lobed' nozzle spreads much less at supersonic condition compared to all other cases; this is due to the absence of screech with this nozzle. Screech stages inducing flapping, rather than varicose or helical, flow oscillation cause a more pronounced jet spreading. At subsonic conditions, only a slight increase in jet spreading with the asymmetric nozzles contrasts previous observations by others. The present results show that the spreading of most asymmetric jets is not much different from that of a round jet. This inference is further supported by data from the rectangular orifices. In fact, jets from the orifices with small aspect ratio (AR) exhibit virtually no increase in the spreading. A noticeable increase commences only when AR is larger than about 10. Thus, 'shear layer perimeter stretching', achieved with a larger AR for a given cross-sectional area of the orifice, by itself, proves to be a relatively inefficient mechanism for increasing jet spreading. In contrast, the presence of streamwise vortices or 'natural excitation' can cause a significant increase - effects that might explain the observations in the previous investigations. Thus far, the biggest increase in jet spreading is observed with the tabs. This is true in the subsonic regime, as well as in the supersonic regime, in spite of the fact that screech is eliminated by the tabs. The characteristic spreading of the tabbed jets is explained by the induced motion of the tab-generated streamwise vortex pairs. The tabs, however, incur thrust loss; the flow blockage and loss in thrust coefficient, vis-a-vis the spreading increase, are evaluated for various configurations.

Zaman, K. B. M. Q.↗
NASA NTRS1991

A study of residual interference effects in adaptive wall testing of a 7-inch chord CAST-10-2/DOA 2 airfoil model

The residual interferences present in the testing of a CAST10-2/DOA2 airfoil model in the Langley 0.3 m Transonic Cryogenic Tunnel (0.3 m TCT) is examined. The 7.1 inch chord airfoil model was tested in the 0.3 m TCT adaptive wall test section with a nominal 13 inch square cross section. The test data obtained on the same model during different tunnel entries showed different levels of residual interference. The present study shows that for valid comparison of the test data from adaptive wall tunnels, it is necessary to account for residual interferences, in particular the blockage correction to the Mach number.

Murthy, A. V.↗
NASA NTRS1995

The NASA Langley 16-Foot Transonic Tunnel: Historical Overview, Facility Description, Calibration, Flow Characteristics, and Test Capabilities

The Langley 16-Foot Transonic Tunnel is a closed-circuit single-return atmospheric wind tunnel that has a slotted octagonal test section with continuous air exchange. The wind tunnel speed can be varied continuously over a Mach number range from 0.1 to 1.3. Test-section plenum suction is used for speeds above a Mach number of 1.05. Over a period of some 40 years, the wind tunnel has undergone many modifications. During the modifications completed in 1990, a new model support system that increased blockage, new fan blades, a catcher screen for the first set of turning vanes, and process controllers for tunnel speed, model attitude, and jet flow for powered models were installed. This report presents a complete description of the Langley 16-Foot Transonic Tunnel and auxiliary equipment, the calibration procedures, and the results of the 1977 and the 1990 wind tunnel calibration with test section air removal. Comparisons with previous calibrations showed that the modifications made to the wind tunnel had little or no effect on the aerodynamic characteristics of the tunnel. Information required for planning experimental investigations and the use of test hardware and model support systems is also provided.

Capone, Francis J.↗
NASA NTRS1985

Scaling laws for testing of high lift airfoils under heavy rainfall

The results of studies regarding the effect of rainfall about aircraft are briefly reviewed. It is found that performance penalties on airfoils have been identified in subscale tests. For this reason, it is of great importance that scaling laws be dveloped to aid in the extrapolation of these data to fullscale. The present investigation represents an attempt to develop scaling laws for testing subscale airfoils under heavy rain conditions. Attention is given to rain statistics, airfoil operation in heavy rain, scaling laws, thermodynamics of condensation and/or evaporation, rainfall and airfoil scaling, aspects of splash back, film thickness, rivulets, and flap slot blockage. It is concluded that the extrapolation of airfoil performance data taken at subscale under simulated heavy rain conditions to fullscale must be undertaken with caution.

Bilanin, A. J.↗
NASA NTRS1985

Technology for pressure-instrumented thin airfoil models, phase 1

A network of channels was chemically milled into one surface of a pair of matched plates having bond planes which were neither planar or profiled to match the contour of the trailing edge of a supercritical airfoil for testing in cryogenic wind tunnels. Vacuum brazing bonded the plates together to create a network of pressure passages without blockages or cross leaks. The greatest success was achieved with the smaller samples and planar bonding surfaces. In larger samples, problems were encountered due to warpage created by the relief of residual stresses. Successful bonds were formed by brazing A286, Nitronic 40 and 300 series stainless steels at 1065 C using AMS 4777B brazing alloy, but excessive grain growth occurred in samples of 200 grade 18 nickel maraging steels. Good bonds were obtained with maraging steel using a 47 percent Nickel-47 percent Palladium-6 percent Silicon alloy and brazing at 927 C. Electro-Discharge-Machining was an effective method of cutting profiled bond planes and airfoil contours. Orifices of good definition were obtained when the EDM wire cut passed through predrilled holes. Possible configurations for joints between small segments and the larger main wing were also studied.

Wigley, D. A.↗
NASA NTRS1973

Investigations of three dimensional flow characteristics in a three bladed rocket pump inducer

Because of the long and narrow passages between the blades of a rocket pump inducer, major effects from turbulence and viscosity make the flow through such a device truly three-dimensional and therefore difficult to measure or predict. To investigate the flow characteristics, conventional and three-dimensional hot-wire probe measurements were conducted in a model inducer 3 ft in diameter. Blade-to-blade variation of the three components of velocity at the exit and the corresponding turbulence intensities were obtained by means of hotwire probes aligned in the three coordinate directions. Analysis of the three-dimensional inviscid flow field led to qualitative prediction of the flow characteristics. Extensive measurements and theoretical analyses of the flow field showed that blade blockage, viscosity, and blade boundary layer interactions are the dominant influences on flow in this type of turbomachinery.

Poncet, A.↗
NASA NTRS2020

Effect of Atmospheric Wind and Upstream Buildings on the Flow Quality in the Test Section of the 80-by 120-Foot Wind Tunnel

The effect of upwind buildings on flow quality in the test section of the 80- by-120-Foot Wind Tunnel housed at the National Full-Scale Aerodynamics Complex at NASA Ames Research Center was modeled at reduced scale. A 1/50th-scale model of the 80x120 leg was placed on the turntable in the test section of the full-scale NFAC 80x120 wind tunnel. Ambient wind was simulated by means of a fixed linear array of 6 blowers mounted upstream of the NFAC turntable. NFAC turntable rotation was used to change ambient (atmospheric) wind direction. Models of existing upwind buildings and the planned Google Campus buildings (Googleplex) were constructed at 1/50th-scale. In addition, a variety of upwind blockage conditions were studied, using rectangular bodies, to simulate either a 2D or 3D blockage directly ahead of the inlet. Computer-controlled surveys of test section flow quality were measured using a Cobra probe to provide all three orthogonal velocity components. Surprisingly, measurements indicate that test section flow quality will be unaffected or slightly improved by the upwind presence of the Google Campus. A possible explanation is discussed.

open-circuit wind tunnel↗
NASA NTRS1959

An Investigation of the Performance of Various Reaction Control Devices

An investigation of a small-scale reaction control devices in still air with both subsonic and supersonic internal flows has shown that lateral forces approaching 70 percent of the resultant force of the undeflected jet can be obtained. These results were obtained with a tilted extension at a deflection of 40 deg. The tests of tilted extensions indicated an optimum length-to-diameter ratio of approximately 0.75 to 1.00, dependent upon the deflection angle. For the two geometric types of spoiler tabs tested, blockage-area ratio appears to be the only variable affecting the lateral force developed. Usable values of lateral force were developed by the full-eyelid type of device with reasonably small losses in the thrust and weight flow. Somewhat larger values of lateral force were developed by injecting a secondary flow normal to the primary jet, but for conditions of these tests the losses in thrust and weight flow were large. Relatively good agreement with other investigations was obtained for several of the devices. The agreement of the present results with those of an investigation made with larger-scale equipment indicates that Reynolds number may not be critical for these tests. In as much as the effects of external flow could influence the performance and other factors affecting the choice of a reaction control for a specific use, it would appear desirable to make further tests of the devices described in this report in the presence of external flow.

Hunter, Paul A.↗
NASA NTRS1983

Spectral radiance errors in remote sensing ground studies due to nearby objects

Attention is given to the error that can occur in all radiometric measurements owing to the presence of nearby objects. When a researcher positions himself on the side of a target point opposite the sun, his body gives rise to two erroneous effects. First, it blocks a portion of the incoming diffuse sky radiance to the target point, and second, it reflects incoming diffuse and direct solar irradiance and ground exitance onto the target point. It is noted that the same phenomenon occurs for any nearby object, whether it be a field truck, a building structure, or a row of trees. This error deriving from nearby objects is often not recognized by researchers or is considered insignificant with no knowledge of its magnitude. The approach taken here is to mathematically model the radiant transfers that take place between the global irradiance, panel, or scene and the object and to report the magnitude of this error for various solar zenith angles, wavelengths, size and distances of objects (steradian blockage), and spectral reflectances of the scene and object. The scene, object, and panel are assumed to be Lambertian, and the object is always located on the side of the target point opposite the sun.

Kimes, D. S.↗
NASA NTRS2003

Enhanced Mixing in a Rectangular Duct

An experimental investigation of the mixing of non-reacting opposed rows of jets injected normal to a confined rectangular crossflow has been conducted. Planar Mie-scattering was used to measure the time-average concentration distribution of the jet fluid in planes perpendicular to the duct axis. Particular emphasis was placed on the study of closely spaced orifice configurations applicable to the mixing zone of an RQL combustor. Baseline studies were performed of mixing under "ideal" conditions, i.e., plenum fed jets injecting into a crossflow uniform in velocity and turbulence intensity. In addition, more practical ("non-ideal") issues encountered during hardware design were also studied. As in other studies, mixing effectiveness, determined using a spatial unmixedness parameter based on the variance of mean jet concentration distributions, was found to be optimum when the spacing-to-duct-height ratio was inversely proportional to the square root of the jet-to-mainstream momentum-flux ratio. This relationship is suitable for design under ideal flow conditions. Inlet flow boundary conditions of the jet and approach flow (mainstream) were found to strongly influence mixing performance, but no attempt was made to determine optimum performance under non-ideal conditions. The tests performed do offer some guidance as to expected mixing behavior for several common variables likely to be imposed by hardware constraints. Additionally, in this study it was found that for rows of orifices with opposite centerlines inline, mixing was similar for blockages up to 89 percent (previous crossflow mixing studies concerned with dilution zone configurations, blockages were typically less than 50 percent). Lower levels of unmixedness were obtained as a function of downstream location when axial injection length was minimized. Mixing may be enhanced if orifice centerlines of opposed rows are staggered, but blockage must be =50 percent in this configuration. Round hole and "square" orifice shapes had similar performance. Other variations in orifice shape did not substantially augment overall mixing performance. Furthermore an isothermal mixing data set was generated and used by CFDRC as input to a NO(x) inference code.

Liscinsky, D. S.↗
NASA NTRS2000

Acoustics and Trust of Separate-Flow Exhaust Nozzles With Mixing Devices for High-Bypass-Ratio Engines

The NASA Glenn Research Center recently completed an experimental study to reduce the jet noise from modern turbofan engines. The study concentrated on exhaust nozzle designs for high-bypass-ratio engines. These designs modified the core and fan nozzles individually and simultaneously. Several designs provided an ideal jet noise reduction of over 2.5 EPNdB for the effective perceived noise level (EPNL) metric. Noise data, after correcting for takeoff thrust losses, indicated over a 2.0-EPNdB reduction for nine designs. Individually modifying the fan nozzle did not provide attractive EPNL reductions. Designs in which only the core nozzle was modified provided greater EPNL reductions. Designs in which core and fan nozzles were modified simultaneously provided the greatest EPNL reduction. The best nozzle design had a 2.7-EPNdB reduction (corrected for takeoff thrust loss) with a 0.06-point cruise thrust loss. This design simultaneously employed chevrons on the core and fan nozzles. In comparison with chevrons, tabs appeared to be an inefficient method for reducing jet noise. Data trends indicate that the sum of the thrust losses from individually modifying core and fan nozzles did not generally equal the thrust loss from modifying them simultaneously. Flow blockage from tabs did not scale directly with cruise thrust loss and the interaction between fan flow and the core nozzle seemed to strongly affect noise and cruise performance. Finally, the nozzle configuration candidates for full-scale engine demonstrations are identified.

Naseem H Saiyed↗
NASA NTRS1994

Design of a variable area diffuser for a 15-inch Mach 6 open-jet tunnel

The Langley 15-inch Mach 6 High Temperature Tunnel was recently converted from a Mach 10 Hypersonic Flow Apparatus. This conversion was effected to improve the capability of testing in Mach 6 air at relatively high reservoir temperatures not previously possible at Langley. Elevated temperatures allow the matching of the Mach numbers, Reynolds numbers, and ratio of wall-to-adiabatic-wall temperatures (TW/Taw) between this and the Langley 20-inch Mach 6 CF4 Tunnel. This ratio is also matched for Langley's 31-inch Mach 10 Tunnel and is an important parameter useful in the simulation of slender bodies such as National Aerospace Plane (NASP) configurations currently being studied. Having established the nozzle's operating characteristics, the decision was made to install another test section to provide model injection capability. This test section is an open-jet type, with an injection system capable of injecting a model from retracted position to nozzle centerline between 0.5 and 2 seconds. Preliminary calibrations with the new test section resulted in Tunnel blockage. This blockage phenomenon was eliminated when the conical center body in the diffuser was replaced. The issue then, is to provide a new and more efficient variable area diffuser configuration with the capability to withstand testing of larger models without sending the Tunnel into an unstart condition. Use of the 1-dimensional steady flow equation with due regard to friction and heat transfer was employed to estimate the required area ratios (exit area / throat area) in a variable area diffuser. Correlations between diffuser exit Mach number and area ratios, relative to the stagnation pressure ratios and diffuser inlet Mach number were derived. From these correlations, one can set upper and lower operating pressures and temperatures for a given diffuser throat area. In addition, they will provide appropriate input conditions for the full 3-dimensional computational fluid dynamics (CFD) code for further simulation studies.

Loney, Norman W.↗
NASA NTRS2001

Effectiveness of Micro-Blowing Technique in Adverse Pressure Gradients

The impact of the micro-blowing technique (MBT) on the skin friction and total drag of a strut in a turbulent, strong adverse-pressure-gradient flow is assessed experimentally over a range of subsonic Mach numbers (0.3 less than M less than 0.7) and reduced blowing fractions (0 less than or equal to 2F/C (sub f,o) less than or equal to 1.75). The MBT-treated strut is situated along the centerline of a symmetric 2-D diffuser with a static pressure rise coefficient of 0.6. In agreement with presented theory and earlier experiments in zero-pressure-gradient flows, the effusion of blowing air reduces skin friction significantly (e.g., by 60% at reduced blowing fractions near 1.75). The total drag of the treated strut with blowing is significantly lower than that of the treated strut in the limit of zero-blowing; further, the total drag is reduced below that of the baseline (solid-plate) strut, provided that the reduced blowing fractions are sufficiently high. The micro-blowing air is, however, deficient in streamwise momentum and the blowing leads to increased boundary-layer and wake thicknesses and shape factors. Diffuser performance metrics and wake surveys are used to discuss the impact of various levels of micro-blowing on the aerodynamic blockage and loss.

Welch, Gerard E.↗
NASA NTRS2020

Wind Tunnel Investigation of the Supersonic Stage Separation Aerodynamics of a Generic 0.0175-Scale Bimese Two-Stage-to-Orbit Reusable Launch Vehicle Configuration

A wind tunnel investigation was conducted of the supersonic stage separation aerodynamics of a generic two-stage-to-orbit bimese wingbody configuration in the NASA Langley Research Center Unitary Plan Wind Tunnel. Proximity and isolated model testing was conducted at Mach numbers of 2.3, 3.0, and 4.5 and a unit Reynolds number of 2.0 million per foot using 0.0175-scale models of the Langley Glide-Back Booster concept designated as the orbiter and booster in belly-to-belly and back-to-belly configurations. Longitudinal forces and moments were obtained on both models and surface static pressure measurements were obtained on the orbiter model at 328 relative proximity locations and at relative angles of attack of 0 degrees and 5 degrees. The test results supported a larger effort to develop and validate experimental and computational tools applicable to the design and simulation of stage separation and abort procedures for reusable launch vehicles composed of multiple bodies, including winged bodies. An initial proof-of-concept experiment featuring low-cost uninstrumented models was conducted to verify an emerging automated model control system and new support system hardware, and to identify potential model and support system blockage and unsteady aerodynamics/model dynamics prior to committing to higher-fidelity instrumented models. This investigation led to upgrades in the facility stage separation hardware, calibration and testing techniques and capabilities, and data analysis and documentation methodologies that have been extended to the more recent NASA Constellation and Space Launch System crew and cargo launch vehicle programs. A virtual diagnostics interface methodology was used to facilitate the design of the stage separation support hardware, to position the models in the test section, and to define the experimental test space. Advances in the facility automated model positioning system established a foundation for the development of a continuous-sweep data acquisition technique that is responsible for significant productivity improvements to the current NASA Space Launch System test program. The automated model positioning capability was leveraged to conduct a companion statistically-designed stage separation experiment requiring randomization of the relative proximity positions of the orbiter and booster models. The respective zones of influence and interference effects of the orbiter and booster were identified from three-dimensional scatter plots, contour and influence maps, and two-dimensional plotting methods. The highly-nonlinear, shock-dominated aerodynamic characteristics of the orbiter and booster in the Unitary Plan Wind Tunnel exhibited good agreement with independent test data obtained in a NASA Marshall Space Flight Center wind tunnel and with computational fluid dynamics predictions using a compressible, three-dimensional flow solver and an inviscid, unstructured Cartesian method.

Erickson, Gary E.↗