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The time-dependent structure of the Jovian ionosphere is examined. Diurnal variation of appreciable magnitude is revealed in the lower ionosphere. The upper ionosphere remains more or less intact at nighttime, as in the case of the earth's ionosphere. There is considerable difference in the height-integrated electrical conductivities on the day and night sides.
An application of the water vapor diffusion technique is examined whereby the permeated water vapor is vented to space vacuum to alleviate on-board waste storage and provide supplemental cooling. The work reported herein deals primarily with the vapor diffusion-heat rejection (VD-HR) as it applies to the Space Shuttle. A stack configuration was selected, designed and fabricated. An asymmetric cellulose acetate membrane, used in reverse osmosis application was selected and a special spacer was designed to enhance mixing and promote mass transfer. A skid-mount unit was assembled from components used in the bench unit although no attempt was made to render it flight-suitable. The operating conditions of the VD-HR were examined and defined and a 60-day continuous test was carried out. The membranes performed very well throughout the test; no membrane rupture and no unusual flux decay was observed. In addition, a tentative design for a flight-suitable VD-HR unit was made.
Gentle precisely controlled process is used to draw polybenzimidazole (PBI) fibers to denier as low as 0.17 per fiber. Yarns of lightweight fibers could be useful in applications where lightweight textiles must withstand high temperatures, corrosion, or radiation.
A study to determine the feasibility of preparing ultralow denier polybenzimidazole (PBI) yarns was undertaken. Conditions that presently yield multifilament yarns with bundle deniers ranging from 75 to 15,000 were used as a baseline. From this starting point, process parameters were identified that give five filament yarns with yarn densiers as low as 0.80. Physical properties from such ultralow denier yarns were at levels that would permit subsequent fabrication into fabrics.
The feasibility of using a 45 deg-honeycomb for improving the flow quality in wind tunnels was investigated. The results of the experiments indicate that the turbulence levels in a wind tunnel using the 45 deg-honeycomb would be comparable to those obtainable using a conventional honeycomb, i.e., a honeycomb which is positioned normal to the mean flow in the settling chamber. However, this is true only when a 45 deg-screen is mounted immediately downstream of the honeycomb, and when some distance is provided between the trailing edges of the corner turning vanes and the upstream side of the 45 deg-honeycomb. This distance is required for adequate decay of the turning-vane wakes, thereby providing a reasonably uniform mean flow entering the honeycomb. Results demonstrate that this distance should be at least twice the spacing between the turning vanes. Even though the resulting turbulence intensity is the same downstream of both the 45 deg-honeycomb and the conventional honeycomb, there would have to be a significant improvement in the test-section turbulence intensity using the 45 deg-honeycomb to justify the additional expense in fabricating and installing this type of honeycomb.
Digitally acquired and processed results from an experimental investigation of grid generated turbulence of various scales through and downstream of nine matched cubic contour contractions ranging in area ratio from 2 to 36, and in length to inlet diameter ratio from 0.25 to 1.50 are reported. An additional contraction with a fifth order contour was also utilized for studying the shape effect. Thirteen homogeneous and nearly isotropic test flow conditions with a range of turbulence intensities, length scales and Reynolds numbers were generated and used to examine the sensitivity of the contractions to upstream turbulence. The extent to which the turbulence is altered by the contraction depends on the incoming turbulence scales, the total strain experienced by the fluid, as well as the contraction ratio and the strain rate. Varying the turbulence integral scale influences the transverse turbulence components more than the streamwise component. In general, the larger the turbulence scale, the lesser the reduction in the turbulence intensity of the transverse components. Best agreement with rapid distortion theory was obtained for large scale turbulence, where viscous decay over the contraction length was negligible, or when a first order correction for viscous decay was applied to the results.
A 45 deg-honeycomb flow manipulator, mounted parallel to the corner turning vanes, was investigated for improving the flow quality in wind tunnels with little or no settling chamber length. This manipulator permits increased turbulence decay distance in comparison to a conventional honeycomb arrangement. The resulting turbulence levels in a wind tunnel using the 45 deg-honeycomb are comparable to those obtained using a conventional honeycomb, but only when a 45 deg-screen is mounted immediately downstream of the honeycomb and when some separation distance between the turning vanes and the 45 deg-honeycomb is provided for adequate decay of the turning vane wakes (at least twice the spacing between the turning vanes).
Experiments were carried out on the mixing of streams with and without swirl and with and without density differences in a coannular geometry. Three variations of flow visualization techniques were used: direct excitation, collisional excitation, and quenching of the tracer. The axial variation of the flow for the case with swirl in the inner annulus is investigated in some detail. From the flow visualization pictures, it is concluded that the initial Helmholtz instability of the tangential shear layer gives rise to small scale peripheral strucutres, then it is possible that the inner swirling stream drives an instability which turns these small scale peripheral structures into ones of larger scale, thereby rearranging the initial vorticity distribution of the flow field. The pictures also indicate the occurrence of interchange of fluid between the annuli. It is also concluded that the interaction of density field with the resulting radial pressure gradient may have influenced the evolution of the flow structures.
The capacity is derived under both peak and average signal power constraints and without a signal bandwidth constraint. A random telegraph wave type signalling scheme of Kabanov is known to achieve capacity provided enough signalling bandwidth is available. In the absence of received background noise, an optimally coded PPM system is shown to achieve capacity with greatly reduced bandwidth as compared to Kabanov signals.
Tropospheric OH is believed to play a major role in atmospheric photochemistry, and, via its chemical interactions OH ultimately may affect the climate, stratospheric ozone levels, and the pH of rain. A valuable tool for predicting the global OH abundance as a function of latitude and altitude is the two-dimensional diagnostic model for tropospheric OH. This model can also be applied to budget studies of species which interact with OH, such as CH4, CO, and O3. In addition to laboratory-obtained kinetic data, important variables include the rate constants for several reactions, the rate of heterogeneous removal of soluble species, and the global abundance and spatial variability in H2O, O3, CO, and NOx. The uncertainties in these parameters imply over a factor of 3 variability in the calculated global OH concentration as well as significant uncertainties in the latitudinal and vertical OH distribution.
Calculation of eletric field vectors within and near an isolated thundercloud (which has a given volume charge distribution) is envisaged. The maximum field strength within the thundercloud is calculated. The effects of screening layers, both above and below the thundercloud may be investigated, as well time-dependent potential problem. The study can lead to a better understanding of the charge distributions in a thundercloud, which in turn may shed some light on the actual mechanism of charging of a thundercloud.
A Markov diffusion model was used to determine an approximate probability density for the random gain. This approximate density preserves the correct second-order statistics and appears to be in reasonably good agreement with experimental data. The receiver operating curve for a pulse counter detector of PMT cathode emission events was analyzed using this density. The error performance of a simple binary direct detection optical communication system was also derived.
A thermal balance test (controlled flux intensity) on a simple black dummy spacecraft using IR lamps was performed and evaluated, the latter being aimed specifically at thermal mathematical model (TMM) verification. For reference purposes the model was also subjected to a solar simulation test (SST). The results show that the temperature distributions measured during IR testing for two different model attitudes under steady state conditions are reproducible with a TMM. The TMM test data correlation is not as accurate for IRT as for SST. Using the standard deviation of the temperature difference distribution (analysis minus test) the SST data correlation is better by a factor of 1.8 to 2.5. The lower figure applies to the measured and the higher to the computer-generated IR flux intensity distribution. Techniques of lamp power control are presented. A continuing work program is described which is aimed at quantifying the differences between solar simulation and infrared techniques for a model representing the thermal radiating surfaces of a large communications spacecraft.
The capacity of a free space optical channel using a direct detection receiver is derived under both peak and average signal power constraints and without a signal bandwidth constraint. The addition of instantaneous noiseless feedback from the receiver to the transmitter does not increase the channel capacity. In the absence of received background noise, an optimally coded PPM system is shown to achieve capacity in the limit as signal bandwidth approaches infinity. In the case of large peak to average signal power ratios, an interleaved coding scheme with PPM modulation is shown to have a computational cutoff rate far greater than ordinary coding schemes.
Simultaneous measurements of the streamwise and radial velocity components at two points, one on and one off the centerline with variable radial separation, were digitally recorded and processed at several stations along a four to one contraction with controlled upstream turbulence conditions. Various statistical quantities are presented including spectra and coherence functions. The integral L sub ux, L sub um, L sub vx, L sub vm were also estimated and their variation along the contraction is examined.
Vorticity from all surfaces and isolated objects in the vicinity of the fan intake, including the outside surfaces of the fan housing, were identified as the major sources for disturbances leading to blade passing frequency noise. The previously proposed mechanism based on atmospheric turbulence is refuted. Flow visualization and hot wire techniques were used in three different facilities to document the evolution of various types of disturbances, including the details of the mean flow and turbulence characteristics. The results suggest that special attention must be devoted to the design of the inlet and that geometric modeling may not lead to adequate simulation of the in flight characteristics. While honeycomb type flow manipulators appear to be effective in reducing some of the disturbances, higher pressure drop devices that generate adequate turbulence, for mixing of isolated nonuniformities, may be necessary to suppress the remaining disturbances. The results are also applicable to the design of inlets of open return wind tunnels and similar flow facilities.
The low velocity impact response of graphite-epoxy laminates is investigated theoretically and experimentally. A nine-node isoparametric finite element in conjunction with an empirical contact law was used for the theoretical investigation. Flat laminates subjected to pendulum impact were used for the experimental investigation. Theoretical results are in good agreement with strain gage experimental data. The collective results of the investigation indicate that the theoretical procedure describes the impact response of the laminate up to about 150 in/sec. impact velocity.
Harmonic wave and wave front propagations in a graphite/epoxy laminate are investigated using a plate theory that includes transverse shear deformation. Transient waves produced by impact of a steel ball are studied experimentally and by using finite elements. The statically measured law of contact between the steel ball and the laminate is used in the finite element program to compute the dynamic contact force. It is found that use of this contact law in conjunction with the finite element modeling of the laminate yields excellent agreement with the experimental results.