The structure of the laminar sublayer.
Shear stress transmission to solid boundary of turbulent shear flow by thin laminar sublayer governed by pressure fluctuations
SHEARING STRESS↗
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Vertical extent of zonal winds on Venus
A phenomenological study of the Venus winds and atmosphere structure suggests that the region of fast retrograde winds is confined between about 45 and 60 km altitude on the night side and is somewhat broader on the day side. Other aspects of the discussed kinematical model are negligible surface winds, speeds increasing with altitude to approximately 45 km, a wind shear between 60 and 65 km, and slow atmospheric motions above this altitude. Large scale Kelvin-Helmholtz waves associated with the shear at the upper boundary of the highspeed layer may be visible as ultraviolet cloud features, and the possibility of differential heating between light and dark UV features being the cause of the winds is considered.
Crisp, D.↗
Bi-directional, buried-wire skin-friction gage
A compact, nonobtrusive, bi-directional, skin-friction gage was developed to measure the mean shear stress beneath a three-dimensional boundary layer. The gage works by measuring the heat flux from two orthogonal wires embedded in the surface. Such a gage was constructed and its characteristics were determined for different angles of yaw in a calibration experiment in subsonic flow with a Preston tube used as a standard. Sample gages were then used in a fully three-dimensional turbulent boundary layer on a circular cone at high relative incidence, where there were regimes of favorable and adverse pressure gradients and three-dimensional separation. Both the direction and magnitude of skin friction were then obtained on the cone surface.
Higuchi, H.↗
Variation of horizontal winds with height /surface to 150 meters/
The data used in the reported analysis consist of 79 five-second intervals from the Automatic Data Acquisition System (ADAS) at the tower facility. Measurements were recorded on July 3, 1973 from 1931 through 2152 GMT during various wind conditions. Peak wind speed was determined for each 5-second interval and level and classified as low, moderate, high, or gale-force. Attention is given to peak wind speeds, wind speed differences determined from peak and associated speeds, and wind speed differences determined from the entire data record. Considering frequency and intensity of peak wind speeds, levels from 30 meters and up are significant ones. Considering frequency and magnitude of wind shears, these analyses indicate that the 60-30 layer is the significant one.
Alexander, M.↗
Turbulence measurements in the boundary layer of a low-speed wind tunnel using laser velocimetry
Laser velocimeter measurements in an incompressible, turbulent boundary layer along the wall of a low-speed wind tunnel are presented. The laser data are compared with existing hot-wire anemometer measurements of a flat plate, incompressible, turbulent, boundary layer with zero pressure gradient. An argument is presented to explain why previous laser velocimeter measurements in zero pressure gradient, turbulent boundary layers have shown an unexpected decrease in turbulent shear stresses near the wall.
Schairer, E. T.↗
Potential for improved silicon ribbon growth through thermal environment control
The Ribbon-to-Ribbon (RTR) process for growth of silicon ribbon is described. This process involves the fabrication of a microcrystalline ribbon of silicon and subsequent grain size enhancement through a laser recrystallization process. The microribbon is obtained from a Thermal Expansion Shear Separation (TESS) process which allows a CVD layer of silicon to be separated from a temporary molybdenum substrate. Efforts to achieve increased solar cell efficiencies and higher area production rates have been problematical. Furnaces, which are necessary for thermal stress control, have been shown to contribute contamination to substrates resulting in degraded efficiencies. Recent results with a new furnace design indicate efficiencies in excess of 9% will be routine. Limitations to area throughput arise due to fundamental linear velocity limitations and width limitations necessary to prevent the occurrence of thermal buckling. Calculations are reported which show the influence of thermal profile on buckling tendencies, and a proposed electron beam technique is considered which promises high throughput with minimal buckling.
Gurtler, R. W.↗
Forecasting of Clear Air Turbulence using a Diagnostic Richardson Number Tendency formulation
The results of several case studies of Clear Air Turbulence (CAT) using the Diagnostic Richardson Number Tendency (DRT) formulation are highlighted. The performance of this technique in resolving regions of documented CAT encounters is encouraging. Its operational adaptability seems particularly attractive in that the input data can be supplied by the currently operational Rawinsonde system. A CAT index is calculated deterministically, sensing synoptic-scale changes in static stability and vertical wind shear conducive for supporting meso-scale CAT layers. This index reveals volumes of the troposphere which act as 'source regions' of Kelvin-Helmholtz instabilities. The results suggest that these regions are particularly efficient with respect to the synoptic/meso-scale energetical coupling necessary for supporting significantly turbulent layers. The output highlights specific regions of the atmosphere which can be interpreted operationally in terms of CAT-encounter probabilities.
Keller, J. L.↗
Influence of layer thickness on the strength of angle-ply laminates
Experimental results are presented showing that the strength and toughness of finite width angle-ply laminates can be increased significantly by using an alternating layer stacking sequence as opposed to a clustered configuration. The ultimate tensile stress of an alternating plus/minus theta laminate of the form (+ or - theta) sub 2 sub s can be much as 1.5 times that of a clustered configuration of the form theta sub 2/-theta sub 2 sub s. Further, the toughness of the alternating layer configuration can be as much as 2.7 times that of the clustered configuration. These differences are explained analytically through consideration of the influence of layer thickness on the magnitude of the interlaminar shear stress and by examination of failed specimens. It is shown that the two laminate configurations exhibit distinctly different failure modes for some fiber angles. Both laminate configurations exhibit catastrophic failure with the damage limited essentially to a small region defined by the length of a single crack across the width of the specimen, parallel to the fiber direction. Results are presented for T300/5208 graphite-epoxy for fiber orientations of 10, 30, and 45 deg.
Herakovich, C. T.↗
Effects of coating rectangular microscopic electrophoresis chamber with methylcellulose
One of the biggest problems in obtaining high accuracy in microscopic electrophoresis is the parabolic flow of liquid in the chamber due to electroosmotic backflow during application of the electric field. In chambers with glass walls the source of polarization leading to electroosmosis is the negative charge of the silicare and other ions that form the wall structure. It was found by Hjerten, who used a rotating 3.0 mm capillary tube for free zone electrophoresis, that precisely neutralizing this charge was extremely difficult, but if a neutral polymer matrix (formaldehyde fixed methylcellulose) was formed over the glass (quartz) wall the double layer was displaced and the viscosity at the shear plane increased so that electroosmotic flow could be eliminated. Experiments were designed to determine the reliability with which methylcellulose coating of the Zeiss Cytopherometer chamber reduced electroosmotic backflow and the effect of coating on the accuracy of cell electrophoretic mobility (EPN) determinations. Fixed rat erythrocytes (RBC) were used as test particles.
Plank, L. D.↗
Effects of thermochemistry, nonequilibrium, and surface catalysis on the design of hypersonic vehicles
An account is given of the function of physical aspects of a gas on the characteristics of the flow and of the heating associated with hypersonic flight. At the high temperatures encountered, the thermal and chemical characteristics of the air in a hypersonic vehicle's shock layer are altered in ways which depend on the atomic and molecular structure of N and O and their ions; similar effects exist in scramjet propulsion systems. These properties in turn influence the character of shock waves and expansions, and hence the pressure, temperature, and velocity distributions. Transport properties affecting the boundary-layer structure will also affect heat flux and shear stress.
Scott, Carl D.↗
Effect of interleaves on fracture of laminated composites. I - Analysis. II - Solution and results
The effect of interleaves placed between the fiber-reinforced plies of multilayered composite laminates is investigated analytically, with the composite approximated by linearly elastic isotropic media consisting of half-planes separated from a finite-width layer by thin interleaves of low extensional and shear moduli. Integral equations are derived which describe the effect of interleaves on damage growth. Numerical solutions are then developed which are based on Gaussian integration techniques and Hadamard's concept of differentiation of Cauchy integrals. Critical stresses and displacements are calculated to investigate the influence of relative elastic properties and geometry of the interleafed composite on interface debonding, extent and suppression and delamination, and stress relieving in the undamaged plies.
Kaw, A. K.↗
Venus magmatic and tectonic evolution
Two years beyond the initial mapping by the Magellan spacecraft, hypotheses for the magmatic and tectonic evolution of Venus have become refined and focused. We present our view of these processes, attempting to synthesize aspects of a model for the tectonic and magmatic behavior of the planet. The ideas presented should be taken collectively as an hypothesis subject to further testing. The quintessence of our model is that shear and buoyancy forces in the upper boundary layer of mantle convection give rise to a spatially and temporally complex pattern of strain in a one-plate Venusian lithosphere and modulate the timing and occurrence of magmatism on a global basis.
Phillips, R. J.↗
Some Basic Aspects of Magnetohydrodynamic Boundary-Layer Flows
An appraisal is made of existing solutions of magnetohydrodynamic boundary-layer equations for stagnation flow and flat-plate flow, and some new solutions are given. Since an exact solution of the equations of magnetohydrodynamics requires complicated simultaneous treatment of the equations of fluid flow and of electromagnetism, certain simplifying assumptions are generally introduced. The full implications of these assumptions have not been brought out properly in several recent papers. It is shown in the present report that for the particular law of deformation which the magnetic lines are assumed to follow in these papers a magnet situated inside the missile nose would not be able to take up any drag forces; to do so it would have to be placed in the flow away from the nose. It is also shown that for the assumption that potential flow is maintained outside the boundary layer, the deformation of the magnetic lines is restricted to small values. The literature contains serious disagreements with regard to reductions in heat-transfer rates due to magnetic action at the nose of a missile, and these disagreements are shown to be mainly due to different interpretations of reentry conditions rather than more complicated effects. In the present paper the magnetohydrodynamic boundary-layer equation is also expressed in a simple form that is especially convenient for physical interpretation. This is done by adapting methods to magnetic forces which in the past have been used for forces due to gravitational or centrifugal action. The simplified approach is used to develop some new solutions of boundary-layer flow and to reinterpret certain solutions existing in the literature. An asymptotic boundary-layer solution representing a fixed velocity profile and shear is found. Special emphasis is put on estimating skin friction and heat-transfer rates.
Hess, Robert V.↗
Advanced Modeling Strategies for the Analysis of Tile-Reinforced Composite Armor
A detailed investigation of the deformation mechanisms in tile-reinforced armored components was conducted to develop the most efficient modeling strategies for the structural analysis of large components of the Composite Armored Vehicle. The limitations of conventional finite elements with respect to the analysis of tile-reinforced structures were examined, and two complementary optimal modeling strategies were developed. These strategies are element layering and the use of a tile-adhesive superelement. Element layering is a technique that uses stacks of shear deformable shell elements to obtain the proper transverse shear distributions through the thickness of the laminate. The tile-adhesive superelement consists of a statically condensed substructure model designed to take advantage of periodicity in tile placement patterns to eliminate numerical redundancies in the analysis. Both approaches can be used simultaneously to create unusually efficient models that accurately predict the global response by incorporating the correct local deformation mechanisms.
Davila, Carlos G.↗
Turbulent mixing layers in the interstellar medium of galaxies
We propose that turbulent mixing layers are common in the interstellar medium (ISM). Injection of kinetic energy into the ISM by supernovae and stellar winds, in combination with density and temperature inhomogeneities, results in shear flows. Such flows will become turbulent due to the high Reynolds number (low viscosity) of the ISM plasma. These turbulent boundary layers will be particularly interesting where the shear flow occurs at boundaries of hot (approximately 10(exp 6) K) and cold or warm (10(exp 2) - 10(exp 4) K) gas. Mixing will occur in such layers producing intermediate-temperature gas at T is approximately equal to 10(exp 5.0) - 10(exp 5.5) that radiates strongly in the optical, ultraviolet, and EUV. We have modeled these layers under the assumptions of rapid mixing down to the atomic level and steady flow. By including the effects of non-equilibrium ionization and self-photoionization of the gas as it cools after mixing, we predict the intensities of numerous optical, infrared, and ultraviolet emission lines, as well as absorption column densities of C 4, N 5, Si 4, and O 6.
Slavin, J. D.↗
Laser interferometer skin-friction measurements of crossing-shock-wave/turbulent-boundary-layer interactions
Wall shear stress measurements beneath crossing-shock-wave/turbulent boundary-layer interactions have been made for three interactions of different strengths. The interactions are generated by two sharp fins at symetric angles of attack mounted on a flat plate. The shear stress measurements were made for fin angles of 7 and 11 deg at Mach 3 and 15 deg at Mach 3.85. The measurements were made using a laser interferometer skin-friction meter, a device that determines the wall shear by optically measuring the time rate of thinning of an oil film placed on the test model surface. Results of the measurements reveal high skin-friction coefficients in the vicinity of the fin/plate junction and the presence of quasi-two-dimensional flow separation on the interaction center line. Additionally, two Navier-Stokes computations, one using a Baldwin-Lomax turbulence model and one using a k-epsilon model, are compared with the experimental results for the Mach 3.85, 15-deg interaction case. Although the k-epsilon model did a reasonable job of predicting the overall trend in portions of the skin-friction distribution, neither computation fully captured the physics of the near-surface flow in this complex interaction.
Garrison, T. J.↗
Direct measurements at Mach 6 of turbulent skin-friction reduction by injection from single and multiple flush slots
The surface skin-friction reduction downstream of one to four successive flush slots injecting at an angle of 10 deg into a turbulent Mach 6 boundary layer has been determined from direct measurements of surface shear. Increasing the number of succeeding slots increases the skin-friction reduction, but this trend reverses at high injection rates. The incremental improvement in skin-friction reduction decreases with increasing number of slots. Comparison with previously reported step slot data indicates that step slots with tangential injection are more effective in reducing skin friction than the present flush slot configuration. Finite-difference predictions are in reasonable agreement with skin-friction and boundary-layer profile data.
Srokowski, A. J.↗
The effects of spherical geometry on baroclinic instability
A baroclinic stability analysis is performed for a simple family of zonal shear profiles over a sphere, using a two-layer, quasi-geostrophic model. The stability properties and the structure of the most unstable waves are qualitatively similar to those on a beta-plane. However, the spherical geometry plays a major role in locating some of the important features of the most unstable waves. In particular, the locations of the maximum wave amplitude, maximum eddy heat fluxes, and maximum convergence of the eddy angular momentum flux are all well correlated with the location of the maximum excess of the vertical shear over the minimum value necessary for local instability on a sphere. Consequently the eddy momentum flux tends to generate a mid-latitude jet even if there is no preexisting mid-latitude jet in the basic state zonal flow. These findings suggest some of the elements needed for parameterizing the meridional variations of baroclinic eddy fluxes accurately.
Moura, A. D.↗