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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Application of an upwind Navier-Stokes code to two-dimensional transonic airfoil flow

An upwind-biased implicit approximate factorization Navier-Stokes algorithm is applied to a variety of steady transonic airfoil cases, using the NACA 0012, RAE 2822, and Jones supercritical airfoils. The thin-layer form of the compressible Navier-Stokes equations is used. Both the CYBER 205 and CRAY 2 supercomputers are utilized, with average computational speeds of about 18 and 16 microsec/gridpoint/iteration, respectively. Lift curves, drag polars, and variations in drag coefficient with Mach number are determined for the NACA 0012 and Jones supercritical airfoils. Also, several cases are computed for comparison with experiment. The effect of grid density and grid extent on a typical turbulent airfoil solution is shown. An algebraic eddy-viscosity turbulence model is used for all of the computations.

Rumsey, Christopher L.↗
NASA NTRS1986

Atmospheric boundary layer modification in the marginal ice zone

A case study of the Andreas et al. (1984) data on atmospheric boundary layer modification in the marginal ice zone is made. The model is a two-dimensional, multilevel, linear model with turbulence, lateral and vertical advection, and radiation. Good agreement between observed and modeled temperature cross sections is obtained. In contrast to the hypothesis of Andreas et al., the air flow is found to be stable to secondary circulations. Adiabatic lifting and, at long fetches, cloud top longwave cooling, not an air-to-surface heat flux, dominate the cooling of the boundary layer. The accumulation with fetch over the ice of changes in the surface wind field is shown to have a large effect on estimates of the surface wind stress. It is speculated that the Andreas et al. estimates of the drag coefficient over the compact sea ice are too high.

Bennett, Theodore J., Jr.↗
NASA NTRS1986

Stability of the sunlit cometary ionopause

The stability of the sheared MHD flow at the sunlit cometary ionopause at several heliocentric distances is examined, taking into account the effects of ion-neutral drag, sources, curvature, and compressibility. If the ionosphere is regarded as a magnetic field-free cavity, whether it is stable or unstable to the Kelvin-Helmholtz (K-H) mode depends on the value of the velocity shear assumed. Also, for any assumed value of the shear the instability is greater for smaller wavelengths (about 100 km) than for larger (at least 1000 km). It is also shown that even if the ionopause is initially unstable, a modest penetration of the interplanetary magnetic field into the ionosphere could lead to its restabilization. Which of the three instabilities, K-H, flute, or drag, dominates depends on the relative magnitude of the shear, the rather uncertain ion-neutral drag coefficient, and the density ratio across the ionopause. The apparent marginal instability of the ionopause of comet Giacobini-Zinner and the stability of the ionopause of comet Halley are explained essentially in terms of the different solar wind conditions encountered by the two comets.

Ershkovich, A. I.↗
NASA NTRS1986

Effect of an impact-generated gas cloud on the acceleration of solid ejecta

A hypothesis that vaporization of a large amount of volatiles such as H2O and CO2 in the Martian surface by the impact of a large object could have accelerated solid ejecta to earth is examined. A hydrodynamic model is used to approximate a hemispherical gas cloud expanding into an atmosphere and entraining solid ejecta. Account is taken of the target material, the impactor materials, mass vaporized, impact velocity, drag coefficient, and crater sizes. A Martian crater larger than 30 km diam is found to be a necessary remnant of any impact that could have produced the shergottites, nakhlites and Chassigny meteorites which have been found on earth and possess similarities to analyzed Martian rocks.

Vickery, Ann↗
NASA NTRS1987

Drag reduction on a bluff body at yaw angles to 30 degrees

The base separation alleviation and drag reduction effectiveness of transverse rectangular grooves and longitudinal v-grooves in the afterbody shoulder region of a bluff body is investigated for body yaw angles of 0-30 deg. The grooves are found to be beneficial in reducing both freestream and axial drag coefficients at yaw angles of up to 25 deg.

Howard, Floyd G.↗
NASA NTRS1986

Time-dependent mixed convection heat transfer from a sphere in a micro-gravity environment

A fundamental problem of interest for crystal growth in micro-gravity applications involves the mixed convection heat transfer from a sphere in a uniform flow of fluid at a differing temperature. Under the combined influence of the imposed free stream as well as an induced buoyancy force due to thermal expansion of the fluid, the heat transfer from the sphere will be different from that of either the pure forced convection flow or the pure free convection flow. For the present study, the method of matched asymptotic expansions is applied to the laminar flow problem of an impulsively heated, impulsively started sphere in an originally quiescent fluid. Time series expansions are developed for the dependent variables by acknowledging the existence of two distinct regions: one, an inner region, near the sphere, in which viscous effects are significant; and two, an outer region in which the fluid may be treated as inviscid. The time series expansions are developed in terms of the Reynolds number and Richardson number (Buoyancy Parameter), and the relevant heat transfer and drag coefficients are calculated and plotted.

Hommel, Mark J.↗
NASA NTRS1987

Spanwise structures in transitional flow around circular cylinders

An experimental investigation of the flow around a smooth circular cylinder at the beginning of the critical transition, where the drag coefficient starts to decrease, was carried out. The measurements disclosed the presence of spanwise structures occurring symmetrically on both sides of the cylinder, associated with large amplitude, low frequency flow components. Mean pressure maps and spanwise cross correlations of pressures and velocities are presented.

Kim, H. J.↗
NASA NTRS1987

Lobed nozzle afterbody research

The effect of three-dimensional, longitudinal contouring of jet engine nozzle afterbodies is investigated experimentally in a water tunnel facility. Both laminar and turbulent Reynolds numbers are tested for three different afterbody configurations. Results indicate that forced mixer lobe technology can be applied to nozzle afterbodies and decrease drag coefficient by as much as 70 percent. Improved exhaust mixing is also possible through such contouring. Exhaust gas attenuation benefits of 125 percent have been recorded.

Russell, William D.↗
NASA NTRS1987

Effect of a round airfoil nose on leading-edge suction

Kulfan (1979) assumed that the angle of attack for initial vortex separation on a slender wing with rounded leading edges could be obtained by equating the leading-edge suction (LES) and nose drag coefficients. In the present study, this assumption is examined and is shown to predict reasonably well the initial angle of attack at which laminar separation occurs near the airfoil nose. However, the assumption is shown to be slightly less accurate for thick or cambered airfoils. Attainable LES estimated by Kulfan's method seemed to agree well with that obtained from an airfoil aerodynamics code and experimental data on a NACA 64A009 airfoil at M = 0.4 and Re = 0.86 x 10 to the 6th.

Lan, C. Edward↗
NASA NTRS1987

High Reynolds number transonic tests on a NACA 0012 airfoil in the Langley 0.3-meter transonic cryogenic tunnel

Tests were conducted in the two-dimensional test section of the Langley 0.3-m Transonic Cryogenic Tunnel on a NACA 0012 airfoil to obtain aerodynamic data as a part of the Advanced Technology Airfoil Test (ATAT) program. The test program covered a Mach number range of 0.30 to 0.82 and a Reynolds number range of 3.0 to 45.0 x 10 to the 6th. The stagnation pressure was varied between 1.2 and 6.0 atmospheres and the stagnation temperature was varied between 300 K and 90 K to obtain these test conditions. Plots of the spanwise variation of drag coefficient as a function of normal force coefficient and the variation of the basic aerodynamic characteristics with angle of attack are shown. The data are presented uncorrected for wall interference effects and without analysis.

Ladson, Charles L.↗
NASA NTRS1988

Experimental investigation of hypersonic aerodynamics

An extensive series of ballistic range tests were conducted at the Ames Research Center to determine precisely the aerodynamic characteristics of the Galileo entry probe vehicle. Figures and tables are presented which summarize the results of these ballistic range tests. Drag data were obtained for both a nonablated and a hypothesized ablated Galileo configuration at Mach numbers from about 0.7 to 14 and at Reynolds numbers from 1000 to 4 million. The tests were conducted in air and the experimental results were compared with available Pioneer Venus data since these two configurations are similar in geometry. The nonablated Galileo configuration was also tested with two different center-of-gravity positions to obtain values of pitching-moment-curve slope which could be used in determining values of lift and center-of-pressure location for this configuration. The results indicate that the drag characteristics of the Galileo probe are qualitatively similar to that of Pioneer Venus, however, the drag of the nonablated Galileo is about 3 percent lower at the higher Mach numbers and as much as 5 percent greater at transonic Mach numbers of about 1.0 to 1.5. Also, the drag of the hypothesized ablated configuration is about 3 percent lower than that of the nonablated configuration at the higher Mach numbers but about the same at the lower Mach numbers. Additional tests are required at Reynolds numbers of 1000, 500, and 250 to determine if the dramatic rise in drag coefficient measured for Pioneer Venus at these low Reynolds numbers also occurs for Galileo, as might be expected.

Intrieri, Peter F.↗
NASA NTRS1988

A system for remote measurements of the wind stress over the ocean

The DISSTRESS system for remote measurements of the surface wind stress over the ocean from ships and buoys is described. It is fully digital, utilizing the inertial dissipation technique. Parallel processing allows anemometer data to be filtered in natural frequency space; that is, the filter cutoffs shift linearly with the mean wind speed of the data to be filtered. The construction of the digital Butterworth bandpass filters is presented in detail. The performance of the system is evaluated by analyzing the results from 28 days of operation during the Frontal Air-Sea Interaction Experiment. The mean wind speed is checked, the anemometer response function is established, and drag coefficients are compared to previous studies. The capability of the system is demonstrated by continuous time series of the friction velocity computed every 20 min. The conclusion is that the surface wind stress can be measured more reliably and accurately (20 percent) with this system than from anemometer wind speeds and a bulk formula.

Large, William G.↗
NASA NTRS1988

High altitude atmospheric modeling

Five empirical models were compared with 13 data sets, including both atmospheric drag-based data and mass spectrometer data. The most recently published model, MSIS-86, was found to be the best model overall with an accuracy around 15 percent. The excellent overall agreement of the mass spectrometer-based MSIS models with the drag data, including both the older data from orbital decay and the newer accelerometer data, suggests that the absolute calibration of the (ensemble of) mass spectrometers and the assumed drag coefficient in the atomic oxygen regime are consistent to 5 percent. This study illustrates a number of reasons for the current accuracy limit such as calibration accuracy and unmodeled trends. Nevertheless, the largest variations in total density in the thermosphere are accounted for, to a very high degree, by existing models. The greatest potential for improvements is in areas where we still have insufficient data (like the lower thermosphere or exosphere), where there are disagreements in technique (such as the exosphere) which can be resolved, or wherever generally more accurate measurements become available.

Hedin, Alan E.↗
NASA NTRS1988

Viscous/inviscid interaction procedure for high-amplitude oscillating airfoils

A coupling procedure between an inviscid potential theoretical panel method and an unsteady interactive boundary layer calculation procedure has been developed for calculating unsteady airloads on oscillating airfoils in the high-incidence regime. A coordinate transformation into a time-dependent surface coordinate has been incorporated into the boundary layer code to avoid a quasi-steady treatment of the viscous flow equations in the leading edge region. The upper and lower sides of the airfoil are calculated with the complete set of unsteady equations. The interactive region of the boundary layer calculation can be straightforwardly extended over the complete upper surface of the airfoil while accounting for interaction areas in the region of strong adverse pressure gradients with incipient separation. Comparisons of sample calculations with experimental data show reasonably good agreement even for unsteady drag coefficients.

Geissler, W.↗
NASA NTRS1989

The effect of a row of Helmholtz resonators on the turbulence of a boundary layer

Laser velocimetry and hot-film data are presented from an experimental investigation in which a tightly-packed row of Helmholtz resonators was used to introduce a momentum oscillation into a turbulent boundary layer. This device has been proposed as a basis for flow-turbulence control. The resonators are found to suck in high-momentum fluid and expel low or zero-momentum fluid into the flow. The interaction of the resonators produces a local increase in the boundary layer momentum thickness which corresponds to a drag coefficient of the order of 0.01.

Flynn, Kevin P.↗
NASA NTRS1989

On the global nature of the solar wind interaction with Comet Halley

Data obtained by two instruments of the RPA-Copernic experiment aboard Giotto during its encounter with Comet Halley are used to determine the positions of several sharp boundaries delineating transitions from one flow state to another. Production rates of the neutrals are obtained, along with ion-neutral drag coefficients. It is suggested that the cometopause observed between the shock and the ionopause coincides with the expected position of a previously proposed collisionopause.

Mendis, D. A.↗
NASA NTRS1989

Wind stress over the Arabian Sea from ship reports and Seasat scatterometer data

Seasat scatterometer data over the Arabian Sea are used to build wind-stress fields during July and August 1978. They are first compared with 3-day wind analyses from ship data along the Somali coast. Seasat scatterometer specifications of 2-m/s and 20-deg accuracy are fulfilled in almost all cases. The exceptions are for winds stronger than 14 m/s, which are underestimated by the scatterometer by 15 percent. Wind stress is derived from these wind data using a bulk formula with a drag coefficient depending on the wind intensity. A successive-correction objective analysis is used to build the wind-stress field over the Arabian Sea with 2 x 2-deg and 6-day resolution. The final wind-stress fields are not significantly dependent on the objective analysis because of the dense coverage of the scatterometer. The combination of scatterometer and coastal ship data gives the best coverage to resolve monsoon wind structures even close to the coast. The final wind stress fields show wind features consistent with other monthly mean wind stress field. However, a high variability is observed on the 6-day time scale.

Perigaud, C.↗
NASA NTRS1990

Effects of nonplanar outboard wing forms on a wing

It is possible for a constant span to obtain better aerodynamic performance from a wing with a nonplanar outboard wing form than from a wing with a planar outboard form, despite the added drag from the increased wetted area. Furthermore, the semispan rolling-moment characteristics indicate the lower wing-root bending moment for some nonplanar configurations. These conclusions are based on an experimental and computational investigation of the aerodynamic characteristics of planar and nonplanar outboard wing forms. Seven different configurations - planar rectangular, nonplanar rising arc, nonplanar drooping arc, planar sheared, sheared with dihedral, sheared with anhedral, and planar elliptical - were investigated for two different spans. Flow-visualization photographs indicate that there are three vortex systems associated with the sheared forms. The lower induced drag coefficients of nonplanar wings are believed to accrue from the movement of vorticity away from the center-of-span line, resulting, in some instances, in induced efficiencies higher than that of a planar elliptical wing. Flow surveys indicate that the effective span, as determined by the location of the tip vortex, might not be a sufficient yardstick of the induced performance of a nonplanar wing.

Naik, D. A.↗