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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Intensive probing of clear air convective fields by radar and instrumented drone aircraft.

Clear air convective fields were probed in three summer experiments (1969, 1970, and 1971) on an S-band monopulse tracking radar at Wallops Island, Virginia, and a drone aircraft with a takeoff weight of 5.2 kg, wingspan of 2.5 m, and cruising glide speed of 10.3 m/sec. The drone was flown 23.2 km north of the radar and carried temperature, pressure/altitude, humidity, and vertical and airspeed velocity sensors. Extensive time-space convective field data were obtained by taking a large number of RHI and PPI pictures at short intervals of time. The rapidly changing overall convective field data obtained from the radar could be related to the meteorological information telemetered from the drone at a reasonably low cost by this combined technique.

Rowland, J. R.↗
NASA NTRS1973

Measurements and a model for convective velocities in the turbulent boundary layer

A physical model is presented which describes convective velocities within a flat plate turbulent boundary layer. A production zone concept is used as a basis for the physical model. The production zone concept employs the idea that packets of turbulent fluid are generated near the viscous sublayer. These packets are found to be discernible from the mean motion and may move either outward from the production zone or inward depending on their circulation relative to the fluid surrounding the packet. The packets are predicted to travel with a convective velocity different from the local mean velocity throughout most of the boundary layer. The model also predicts that the convective velocities will be functions of wave number outside the production zone.

Cliff, W. C.↗
NASA NTRS1972

Viscous stratification of the earth and convection

The shallow model of the earth's mantle is discussed along with a variety of geophysical arguments for its correctness and against the existence of deep convection. The main agrument is summarized in the proposal that the astheno sphere is less viscous (by a factor of 10 to 100) than has generally been assumed. In this shallow model, the return flow is essentially through the asthenosphere. The dynamical agent is the steep temperature gradient in the upper mantle. Speculations as to the historical variation of this gradient are advanced. The effects on the model of a nonuniform earth aggregation are considered and shown to favor shallow convection as well as a top convective layer (lithosphere plus asthenosphere) whose depth increases slowly over the earth's life, leading to a tectonic activity that increases gradually with time.

Elsasser, W. M.↗
NASA NTRS1974

Thermal instability and energy relations in the convective envelopes of slowly rotating stars

The nonstationary convection that arises due to thermal instability in the envelopes of lower Main Sequence stars (and the sun in particular) is investigated. The convection that arises in the envelopes as a result of thermal instability is of nonstationary turbulent nature. It is shown that construction of a consistent model of the zones of turbulent convection in the stars requires the use of the complete heat flux equation with consideration of the terms that appear because of turbulence. Expressions for these additional terms in terms of averaged characteristics of the motion and eddy viscosity coefficients are derived on the basis of the Prandtl-Wasiutinsky theory.

Yavorskaya, I. M.↗
NASA NTRS1975

Evaluation of T-111 forced-convection loop tested with lithium at 1370 C

A T-111 alloy (Ta-8% W-2% Hf) forced-convection loop containing molten lithium was operated 3000 hr at a maximum temperature of 1370 C. Flow velocities up to 6.3 m/sec were used, and the results of this forced-convection loop are very similar to those observed in lower velocity thermal-convection loops of T-111 containing lithium. Weight changes were determined at 93 positions around the loop. The maximum dissolution rate occurred at the maximum wall temperature of the loop and was less than 1.3 microns/year. Mass transfer of hafnium, nitrogen, and, to a lesser extent, carbon occurred from the hotter to cooler regions. Exposed surfaces in the highest temperature region were found to be depleted in hafnium to a depth of 60 microns with no detectable change in tungsten content. There was some loss in room-temperature tensile strength for specimens exposed to lithium at 1370 C, attributable to depletion of hafnium and nitrogen and to attendant grain growth.

Devan, J. H.↗
NASA NTRS1975

Modal equations for cellular convection

We expand the fluctuating flow variables of Boussinesq convection in the planform functions of linear theory. Our proposal is to consider a drastic truncation of this expansion as a possible useful approximation scheme for studying cellular convection. With just one term included, we obtain a fairly simple set of equations which reproduces some of the qualitative properties of cellular convection and whose steady-state form has already been derived by Roberts (1966). This set of 'modal equations' is analyzed at slightly supercritical and at very high Rayleigh numbers. In the latter regime the Nusselt number varies with Rayleigh number just as in the mean-field approximation with one horizontal scale when the boundaries are rigid. However, the Nusselt number now depends also on the Prandtl number in a way that seems compatible with experiment. The chief difficulty with the approach is the absence of a deductive scheme for deciding which planforms should be retained in the truncated expansion.

Gough, D. O.↗
NASA NTRS1975

Skylab M518 multipurpose furnace convection analysis

An analysis was performed of the convection which existed on ground tests and during skylab processing of two experiments: vapor growth of IV-VI compounds growth of spherical crystals. A parallel analysis was also performed on Skylab experiment indium antimonide crystals because indium antimonide (InSb) was used and a free surface existed in the tellurium-doped Skylab III sample. In addition, brief analyses were also performed of the microsegregation in germanium experiment because the Skylab crystals indicated turbulent convection effects. Simple dimensional analysis calculations and a more accurate, but complex, convection computer model, were used in the analysis.

Bourgeois, S. V.↗
NASA NTRS1976

A simplified analytical solution for thermal response of a one-dimensional, steady state transpiration cooling system in radiative and convective environment

A simplified analytical method for calculation of thermal response within a transpiration-cooled porous heat shield material in an intense radiative-convective heating environment is presented. The essential assumptions of the radiative and convective transfer processes in the heat shield matrix are the two-temperature approximation and the specified radiative-convective heatings of the front surface. Sample calculations for porous silica with CO2 injection are presented for some typical parameters of mass injection rate, porosity, and material thickness. The effect of these parameters on the cooling system is discussed.

Kubota, H.↗
NASA NTRS1976

Clear air convective behavior revealed by radar chaff

A simple experiment is described in which chaff was injected into the updraft region of individual clear-air convective cells in order to clarify characteristics of such convective elements. The chaff was dispensed automatically by a small radio-controlled drone aircraft and also by hand from the ground. It is found that clear-air convective cells are roughly spherical in form, that rapid mixing occurs within a cell, and that little mixing occurs between adjacent cells. Some suggestions are offered for future work in the tracing of air motions arising from the ground.

Rowland, J. R.↗
NASA NTRS1976

The evolution of the clear air convective layer revealed by surface-based remote sensors

Results are reported for simultaneous observations of the growth and decay of the clear-air convective mixing layer near a coastline, which were made with an FM-CW radar, a high-power narrow-beam S-band radar, and an acoustic echo sounder. The main purpose of this study was to determine the relationship between the rise rate of the convective depth and the lapse rate of temperature, particularly in the morning hours. The results indicate that the three remote sensors can provide excellent mutually supporting data on the convective depth. It is found that this depth is well behaved during the day and that its rise rate varies roughly linearly with the inverse square root of the temperature lapse rate during the morning. The data suggest that some models concerning the rise rate require modification, since these models imply that the surface heat flux would have to be unreasonably large to produce the observed relationship.

Noonkester, V. R.↗
NASA NTRS1976

Stellar convection theory. I - The anelastic modal equations

Methods are developed for dealing with the various dynamical problems that arise because of convective zones in stars. A system of equations for stellar convection is derived from the full equations of compressible fluid dynamics with the aid of two major approximations. The first of these is the anelastic approximation, which involves both the filtering out of acoustic waves and a suitable linearization of the fluctuating thermodynamic variables. The second one approximates the horizontal structure of convection by expanding the motion in a set of horizontal cellular platforms and severely truncating the expansion. The resulting system of partial differential equations, referred to as the anelastic modal equations, is outlined along with suggested boundary conditions and techniques for solving the equations. Ways of assessing the overall validity of the present treatment are discussed.

Latour, J.↗
NASA NTRS1976

Convection sensitivity and thermal analyses for indium and indium-lead mixing experiment (74-18)

Sounding rocket Experiment 74-18 was designed to demonstrate the effects of the Black Brandt rocket acceleration levels (during the low-g coast phase of its flight) on the motion of a liquid metal system to assist in preflight design. Some post flight analyses were also conducted. Preflight studies consisted of heat transfer analysis and convection sensitivity and convection modeling analyses which aided in the: (1) final selection of fluid materials (indium-lead melts rather than paraffins); (2) design and timing of heater and quench system; and (3) preflight predictions of the degree of lead penetration into the pure indium segment of the fluid. Postflight studies involved: (1) updating the convection sensitivity calculations by utilizing actual flight gravity levels; and (2) modeling the mixing in the flight samples.

Bourgeois, S. V.↗
NASA NTRS1977

Boiling incipience and convective boiling of neon and nitrogen

Forced convection and subcooled boiling heat transfer data for liquid nitrogen and liquid neon were obtained in support of a design study for a 30 tesla cryomagnet cooled by forced convection of liquid neon. The cryogen data obtained over a range of system pressures, fluid flow rates, and applied heat fluxes were used to develop correlations for predicting boiling incipience and convective boiling heat transfer coefficients in uniformly heated flow channels. The accuracy of the correlating equations was then evaluated. A technique was also developed to calculate the position of boiling incipience in a uniformly heated flow channel. Comparisons made with the experimental data showed a prediction accuracy of + or - 15 percent.

Papell, S. S.↗
NASA NTRS1976

Umbral boundaries, convection, and the depth of sunspots

The boundary between the umbra and penumbra of a sunspot is consistently observed to be very sharp, on the order of 500 km. Approximating the sunspot as a static region in a homogeneous medium with a radiative surface, temperature distributions resulting from a variety of convective motions exterior to the sunspot are calculated. The calculations suggest that, for the exterior convection to produce the observed boundary, the maximum depth of the region of inhibited convection below a sunspot umbra is on the order of 1,000 km.

Isenberg, P. A.↗
NASA NTRS1978

Studies of convection in a solidifying system with surface tension at reduced gravity

The low gravity environment of Earth's orbit is being seriously considered for experimentation on the production of materials in space. Most of such materials processes inevitably involve either the solidification of melt or the melting of solids. Inherent in most fluid mechanisms with temperature gradients is convective motion. A study is presented for the onset of convection in a solidifying system in an environment which is similar to that encountered in space processing. Since the study is for a low gravity condition, the only driving mechanism considered is that due to the variation of surface tension force at the free surface of the melt layer. Two simple solidification models were considered, one in which the solidification process enters in the perturbation system and another in which the melt is solidifying at a constant rate. The results show that the solidification process will bring about convection in the melt earlier than otherwise.

Antar, B. N.↗
NASA NTRS1977

Numerical model of the convecting F2 ionosphere at high latitudes

In previous work Knudsen (1974) presented a model for the convection field of the high latitude F layer and evaluated the time-dependent behavior of a tube of F layer plasma carried around the polar regions by the field. The present paper describes the initial results of a more detailed numerical study of the behavior of the F layer tubes, where it is assumed that the tubes are subjected to time-dependent ionization rates from both solar photons and precipitating energetic electrons. The numerical results are presented in the form of a map view of N-m F2 contours, electron concentration in vertical section over the magnetic pole from noon to midnight, and several vertical profiles of electron concentration for both convecting and nonconvecting flux tubes. The proposed convection field produced a tongue of F layer plasma extending from the dayside of the cleft over the polar cap with concentrations consistent with those observed by Isis 2.

Knudsen, W. C.↗
NASA NTRS1977

Boiling incipience and convective boiling of neon and nitrogen

Forced convection and subcooled boiling heat transfer data for liquid nitrogen and liquid neon were obtained in support of a design study for a 30 tesla cryomagnet cooled by forced convection of liquid neon. This design precludes nucleate boiling in the flow channels as they are too small to handle vapor flow. Consequently, it was necessary to determine boiling incipience under the operating conditions of the magnet system. The cryogen data obtained over a range of system pressures, fluid flow rates, and applied heat fluxes were used to develop correlations for predicting boiling incipience and convective boiling heat transfer coefficients in uniformly heated flow channels. The accuracy of the correlating equations was then evaluated. A technique was also developed to calculate the position of boiling incipience in a uniformly heated flow channel. Comparisons made with the experimental data showed a prediction accuracy of plus or minus 15 percent

Papell, S. S.↗
NASA NTRS1977

Observation of the development of individual clear air convective cells

A series of radar observations has been used to monitor the development of clear air convective cells. It is suggested that an airfield may be a source of such cells. The cells first appear at a distance of about 11 km, and are observed to be produced every four minutes. The emergence of separate cells supports the bubble theory of convection. After reaching maximum height, a typical decrease of 100-200 m occurs. Various methods used to estimate convective cell energy yield values of 10 to the 12th, 4 x 10 to the 11th, and 10 to the 11th J.

Arnold, A.↗