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At least 739 records · Page 41

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NASA NTRS1981

Propagating substorm injection fronts

It is argued that a series of two-satellite observations leads to a clarification of substorm plasma injection, in which boundary motion plays a major role. Emphasis is put on a type of event characterized by abrupt, dispersionless changes in electron intensity and a coincident perturbation that consists of both a field magnitude increase and a small rotation toward more dipolar orientation. Comparing plasma observations at two points, it is found that in active, preinjection conditions the two most important features of the plasma sheet are: (1) the low-energy convection boundary for near-zero energy particles, determined by the magnitude of the large-scale convection electric field; and (2) the precipitation-flow boundary layer between the hot plasma sheet and the atmospherically contaminated inner plasma sheet.

Moore, T. E.↗

NASA NTRS1981

Laminar free convection boundary layer heat transfer over non-isothermal surface

Accurate heat transfer results are provided for the case of nonisothermal objects. A steady, laminar, free convection boundary layer flow over two-dimensional or rotationally symmetrical bodies of nonuniform surface temperature situated in an ambient fluid of undisturbed temperature is considered analytically. The surface heat flux is given in terms of the Nusselt number and wall derivatives of universal functions for Prandtl numbers of 0.72 and 100 are provided. The method is shown to be valid up to a temperature/radius ratio of 130 deg.

Lin, F. N.↗

NASA NTRS1982

Turbulent boundary layer drag reduction using riblets

An experimental study of low-speed turbulent boundary layer flow over longitudinally grooved surfaces (i.e., riblets) is discussed. Results obtained with a highly accurate drag balance indicate that v-groove riblet surfaces can produce consistent net drag reductions as large as 8 percent provided the height and spacing of the grooves in terms of law of the wall variables are less than 25 wall units. Momentum balances confirmed these direct drag measurements. Conditionally sampled data indicate that the burst frequency for riblets is approximately the same as that for a flat plate but turbulence intensity is reduced. Attempts to optimize the net drag reduction by varying riblet cross-sectional geometry and alignment are also discussed.

Walsh, M. J.↗

NASA NTRS1982

Two dimensional stagnation point flow of a dusty gas near an oscillating plate

Necessary improvements to a paper on the flow of a dusty gas by Datta and Mishra (1980) are presented. Particular attention is given to the importance of particle phase compressibility and the hyperbolic nature of the particle momentum conservation equation which prohibits downstream (wall) boundary conditions for the solid phase. Fundamental differences between particulate and ordinary flow boundary layers are discussed, and the correct conservation equations are written.

Fernandez De La Mora, J.↗

NASA NTRS1983

Space Shuttle heating analysis with variation in angle of attack and surface condition

The effects of variations in the angle of attack and surface catalycity condition on the Space Shuttle surface heating along the windward centerline are evaluated. Further, the time history of the nonequilibrium-to-equilibrium surface heating ratio and the boundary edge quantities are analyzed. Results show that a + or - 5% change in the angle of attack does not appreciably influence either the nonequilibrium heating or the nonequilibrium-to-equilibrium heat transfer ratio at higher (75 km) or lower (48 km) altitudes. The variation in the recombination rate parameter is found to affect the surface heating most at an altitude of about 75 km for the STS-2 flight. A maximum reduction of 49% in heating due to nonequilibrium chemistry is obtained at about a 75 km altitude in the nose region of the Orbiter for the STS-2 flight data. In addition, the nonequilibrium effects at the boundary layer edge are found to become less significant for altitudes less than 65 km even though the boundary layer flow may still be in nonquilibrium.

Gupta, R. N.↗

NASA NTRS1983

The influence of coastal shape on winter mesoscale air-sea interaction

In cold air outbreaks, the combination of coastal shape and sea surface isotherms has a profound effect in the establishment of mesoscale atmospheric circulation, due to differential heating resulting from both overwater path length and underlying sea surface temperature (SST) variations. Where coastal effects are dominant, a mesoscale front forms downstream of the point which marks the major bend in the coastline's orientation. The strength of the induced mesoscale circulation depends on the original contrast between the land air temperature and the SST. It is noted that where the coastline and the isotherm pattern are approximately normal to the mean boundary layer flow, and thermal contrast is sufficiently large, the cloud streets formed downstream will be convective in nature, and oriented with the axis of roll vortices along the wind direction.

Atlas, D.↗

NASA NTRS1982

Convex curvature effects on the heated turbulent boundary layer

A convexly curved and isothermally heated wall with a 45-cm radius of curvature is subjected to turbulent boundary layer flow measurements in order to determine wall heat transfer rates and mean velocity and temperature profiles. Significant curvature effects are noted, with Stanton number and skin friction coefficient reductions of 35-40 percent by comparison with flat plate values for the same momentum or enthalpy thickness Reynolds numbers. Profiles of mean velocity and temperature show a more rapid growth of the wake regions, and a shortening of the log-linear region, as a result of curvature. Turbulent Prandtl numbers deduced from the mean temperature profiles under the assumption of a wall thermal law were found to be increased by 40-50 percent by this strong convex curvature.

Simon, T. W.↗

NASA NTRS1983

Transonic empirical configuration design process

This lecture describes some of the experimental research pertaining to transonic configuration development conducted by the Transonic Aerodynamics Branch of the NASA Langley Research Center. Discussions are presented of the following: use of florescent oil films for the study of surface boundary layer flows; the severe effect of wind tunnel wall interference on the measured configuration drag rise near the speed of sound as determined by a comparison between wind tunnel and free air results; the development of a near sonic transport configuration incorporating a supercritical wing and an indented fuselage, designed on the basis of the area rule with a modification to account for the presence of local supersonic flow above the wing; a device for improving the transonic pitch up of swept wings with very little added drag at the cruise condition; a means for reducing the large transonic aerodynamic interference between the wing, fuselage, nacelle and pylon for a for a fuselage mounted nacelle having the inlet above the wing; and methods for reducing the transonic interference between flows over a winglet and the wing.

Whitcomb, R. T.↗

NASA NTRS1983

Doppler lidar signal and turbulence study

Wind fields were measured with the ground-based NASA/MSFC lidar are compared with the in situ NASA RB-57 aircraft measurements. The mean wind fields, the turbulence intensities, and the turbulence spectra determined from measurements by both systems are in very good agreement. Turbulence intensities and spectra were calculated from the fluctuations with time in the radial wind speed component. The second moment or Doppler frequency spectral width of the lidar measurements was also compared with turbulence intensities measured by the aircraft. These second moments could only be resolved at the very low altitudes (in three range bins). Turbulence intensities estimated from the spectral width data were an order of magnitude higher than those measured by the aircraft. An interesting boundary layer evolved during the progress of the experiment. The breakup of a stable boundary layer resulted in winds blowing in one direction above 600 m msl and in the opposite direction below that level. Both the aircraft and the lidar systems clearly identified this unusual boundary layer flow and showed the identical trends.

Frost, W.↗

NASA NTRS1984

Embedded shear layer computations for increased drag reduction

One of the most promising methods of minimizing drag is the reduction of skin friction by injection of low momentum fluid into the near-wall region of turbulent boundary layer flows. This method could be made more effective by limiting the spread rate of the resulting mixing region. In order to achieve a better understanding of how this goal might be achieved, numerical investigations of the relevant fluid dynamic processes governing these regions have been conducted. A compact finite-difference algorithm has been applied to the complete form of the governing conservation equations for a two-dimensional laminar mixing layer. The ability of this computational approach to model successfully the formation and interaction of the large scale vortical structures which dominate such flow fields is verified in the present study. Parameters which affect the spread rate of the mixing region are also identified. In addition, the relative importance of viscous and momentum transport effects in the vortex interactions is determined.

Gatski, T. B.↗

NASA NTRS1983

A new explicit method for the numerical solution of parabolic differential equations

A new method is derived for solving parabolic partial differential equations arising in transient heat conduction or in boundary-layer flows. The method is based on a combination of the modified differential quadrature (MDQ) method with the rational Runge-Kutta time-integration scheme. It is fully explicit, requires no matrix inversion, and is stable for any time-step for the heat equations. Burgers equation and the one- and two-dimensional heat equations are solved to demonstrate the accuracy and efficiency of the proposed algorithm. The present method is found to be very accurate and efficient when results are compared with analytic solutions.

Satofuka, N.↗

NASA NTRS1984

Spacecraft contamination: Sources and prevention; Thermophysics Conference, 18th, Montreal, Canada, June 1-3, 1983, Selected Papers

Results of current research in the field of spacecraft contamination are presented. An overview of the various sources of contamination is given, showing the many and diverse mechanisms associated with the degradation of thermal and optical performance of spacecraft surfaces and optics. Some of the causes of contamination are discussed, including spallation of foam insulation on cryogenic fuel tanks, dispersion of particle contamination about a spacecraft, particle contamination of optical sensors, and the influence of the location of venting systems. Contamination prevention is addressed. Infrared transmittance and infrared optical properties of contaminants are considered along with the effects and properties of mixtures of exhaust species, of plume boundary layer flow, and of the contamination associated with bipropellant attitude control thrusters proposed for the Galileo spacecraft. Also discussed are: degradation of 'clean' thermal control surfaces over four years in orbit, in-orbit measurement of solar absorptance, and contamination from postfire efflux of a solid-propellant rocket motor.

Roux, J. A.↗

NASA NTRS1985

Flame flashback in a premixed dump combustor

A stainless steel, rectangular center-dump, premixed-prevaporized combustor was used at the NASA Lewis Research Center to study flashback in controlled conditions simulating gas turbine combustor operations under steady-state and transient conditions. The combustor allowed visual access to the combustion area, and was operated at inlet air temperatures from 600 to 850 K, premixer wall temperatures from 450 to 900 K, and average premixer velocity from 40 to 80 ft/s. A parametric study revealed a slight decrease in the flashback equivalence ratio as the inlet air temperatures increased. It also indicated that the premixer wall temperature and premixer velocity are not governing parameters of flashback. It is suggested that flashback occurs through the premixer wall boundary layer flow reversal caused by combustion instability.

Proctor, M. P.↗

NASA NTRS1985

Experimental study of forward-located jet-interaction nozzles

The effects of forward-mounted sonic jet-interaction nozzles with one, three equal, or one large and two small orifices on the transitional or turbulent boundary-layer flow over a biconic (9-deg-25-arcmin/6-deg) model at angle of attack -10 to the 15 deg and Reynolds number 8.10 x 10 to the 6th/ft are investigated experimentally using pressure taps, force measurements, oil-flow visualization, and schlieren photography in the 20-in. Mach-6 wind tunel at NASA Langley. The injection flow rates are 0.055 and 0.026 lbm/s, with stagnation pressure 500 psia, stagnation temperature 1000 R, and dynamic pressure 8.7 psia. The results are presented in diagrams, graphs, and photographs and characterized. Moment amplification factors of 1.0 or better (maximum 1.8 at angle of attack 15 deg and flow rate 0.055 lbm/s) are observed for all nozzle types, but the configuration with three unequal orifices is found to give improvements of 10 percent over the single-orifice nozzle at a given flow rate.

Cavalleri, R. J.↗

NASA NTRS1985

Boundary layer control of nocturnal convection associated with a synoptic scale system

Well defined nocturnal convection, together with distinct maximum precipitation events in Nebraska, were associated with strong diurnal wind oscillations at 850 mb in Oklahoma, Kansas and Missouri in the present study of pronounced precipitation and low level circulation variations over the Great Plains on May 5-10, 1979. A numerical prediction model, used to relate boundary layer flow modulations to synoptic scale flow for this case, is driven by a diurnal cycle. Experimental forecasts displayed diurnal wind modulations at 500 m, with rising motions of the order of 4 cm/sec at the 2200 m level, along the leading edge of the nocturnal jet.

Astling, E. G.↗

NASA NTRS1986

Aeolian processes aboard a space station: Saltation and particle trajectory analysis

The Carousel wind tunnel (CWT) proposed to study aeolian processes aboard a space station consists of two concentric rotating drums. The space between the two drums comprises the wind tunnel test section. Differential rates of rotation of the two drums would provide a wind velocity with respect to either drum surface. Preliminary results of measured velocity profiles made in a CWT prototype indicate that the wall bounded boundary layer profiles are suitable to simulate flat plate turbulent boundary layer flow. The two dimensional flat plate Cartesian coordinate equations of motion of a particle moving through the air are explained. In order to assess the suitability of CWT in the analysis of the trajectories of windblown particles, a series of calculations were conducted comparing cases for gravity with those of zero gravity. Results from the calculations demonstrate that a wind tunnel of the carousel design could be fabricted to operate in a space station environment and that experiments could be conducted which would yield significant results contributing to the understanding of the physics of particle dynamics.

White, B. R.↗

NASA NTRS1986

Wall layer models for the calculation of velocity and heat transfer in turbulent boundary layers

In the computation of turbulent boundary-layer flows and internal flows, a substantial amount of mesh points and computational effort is required to adequately resolve the intense temperature and velocity profile variations in the near wall region. In this study, analytical profile approximations are obtained for the mean velocity and temperature distribution in the wall layer; these profiles are based upon consideration of the observed coherent structure of the time-dependent wall-layer flow. The profile approximations are relatively simple analytical formulae which satisfy all the required compatibility conditions at the wall and the logarithmic behavior at the edge of the wall layer. The Reynolds analogy for heat transfer is not used in the present theory. A general method for utilizing the present wall-layer profile models in a prediction method is indicated.

Walker, J. D. A.↗

NASA NTRS1986

Wall temperature control of low-speed body drag

The use of thermal means to control drag under turbulent boundary layer conditions is examined. Numerical calculations are presented for both skin friction and (unseparated) pressure drag for turbulent boundary-layer flows over a fuselage-like body with wall heat transfer. In addition, thermal control of separation on a bluff body is investigated. It is shown that a total drag reduction of up to 20 percent can be achieved for wall heating with a wall-to-total-freestream temperature ratio of 2. For streamlined slender bodies, partial wall heating of the forebody can produce almost the same order of total drag reduction as the full body heating case. For bluff bodies, the separation delay from partial wall cooling of the afterbody is approximately the same as for the fully cooled body.

Lin, J. C.↗