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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

A method for three-dimensional modeling of wind-shear environments for flight simulator applications

A computational method for modeling severe wind shears of the type that have been documented during severe convective atmospheric conditions is offered for use in research and training flight simulation. The procedure was developed with the objectives of operational flexibility and minimum computer load. From one to five, simple down burst wind models can be configured and located to produce the wind field desired for specific simulated flight scenarios. A definition of related turbulence parameters is offered as an additional product of the computations. The use of the method to model several documented examples of severe wind shear is demonstrated.

Bray, R. S.↗
NASA NTRS1984

Simulated flight through JAWS wind shear - In-depth analysis results

The Joint Airport Weather Studies (JAWS) field experiment was carried out in 1982 near Denver. An analysis is presented of aircraft performance in the three-dimensional wind fields. The fourth dimension, time, is not considered. The analysis seeks to prepare computer models of microburst wind shear from the JAWS data sets for input to flight simulators and for research and development of aircraft control systems and operational procedures. A description is given of the data set and the method of interpolating velocities and velocity gradients for input to the six-degrees-of-freedom equations governing the motion of the aircraft. The results of the aircraft performance analysis are then presented, and the interpretation classifies the regions of shear as severe, moderate, or weak. Paths through the severe microburst of August 5, 1982, are then recommended for training and operational applications. Selected subregions of the flow field defined in terms of planar sections through the wind field are presented for application to simulators with limited computer storage capacity, that is, for computers incapable of storing the entire array of variables needed if the complete wind field is programmed.

Frost, W.↗
NASA NTRS1983

Correlation of compressive and shear stress with spalling of plasma-sprayed ceramic materials

Ceramics on metal substrates for potential use as high temperature seals or other applications are exposed to forces originating from differences in thermal expansion between the ceramic and the metal substrate. This report develops a relationship between the difference in expansion of the ceramic and the substrate, defines conditions under which shear between the ceramic and the substrate occurs, and those under which bending forces are produced in the ceramic. The off-axis effect of compression forces resulting from high temperature plastic flow of the ceramic producing buckling of the ceramic is developed. Shear is associated with the edge or boundary stresses on the component while bending is associated with the distortion of an interior region. Both modes are significant in predicting life of the ceramic. Previously announced in STAR as N83-27016

Mullen, R. L.↗
NASA NTRS1984

Postbuckling of long orthotropic plates loaded in combined transverse compression and longitudinal compression or shear

The postbuckling behavior of simply supported, rectangular, isotropic and orthotropic composite plates under combined loading is determined analytically. Results are presented for a long plate loaded beyond its buckling load in either a combination of longitudinal and transverse compression or a combination of transverse compression and inplane shear. Results show that orthotropic plates may behave differently than isotropic plates for these loading conditions. Results also show that the postbuckling behavior of plates in shear with constrained edge displacements is markedly different than their behavior with stress free edges.

Stein, M.↗
NASA NTRS1984

The thermal instability in a sheared magnetic field - Filament condensation with anisotropic heat conduction

The condensation-mode growth rate of the thermal instability in an empirically motivated sheared field is shown to depend upon the existence of perpendicular thermal conduction. This typically very small effect (perpendicular conductivity/parallel conductivity less than about 10 to the -10th for the solar corona) increases the spatial-derivative order of the compressible temperature-perturbation equation, and thereby eliminates the singularities which appear when perpendicular conductivity = 0. The resulting growth rate is less than 1.5 times the controlling constant-density radiation rate, and has a clear maximum at a cross-field length of order 100 times and a width of about 0.1 the magnetic shear scale for solar conditions. The profiles of the observable temperature and density perturbations are independent of the thermal conductivity, and thus agree with those found previously. An analytic solution to the short-wavelength incompressible case is also given.

Van Hoven, G.↗
NASA NTRS1984

The coupling between scales in shear flows

The relationship between large and small-scale motions in turbulent shear flows is studied using short-time correlation measurements. Boundary layers, plane and axisymmetric mixing layers, plane wakes, and the far fields of plane and circular jets are examined. The coupling is found to be significant in all flows. Phase reversal across the shear region is found to occur in the boundary layers and the mixing layers only. This is interpreted to mean that in these layers the streamwise extent of the large structure over which the small-scale activity reaches a peak moves from one transverse end of the large structure over which its ensemble-averaged u-fluctuation is positive to another where it is negative. In the far field of jets and wakes, the small-scale peak activity remains confined to that half of the streamwise extent of the large structure where its ensemble-averaged u-fluctuation is positive.

Bandyopadhyay, P. R.↗
NASA NTRS1984

A model of the excitation of orderly structures in a shear layer

The artificial excitation of shear layers is investigated theoretically and experimentally. The present paper describes quantitatively the coupling between exciting sound field and shear layer fluctuations. The mathematical model is restricted to low Strouhal numbers at which large scale structures are occurring. The theory does not contain any empirical constants and it is confirmed by the experiments in the expected validity range.

Bechert, D. W.↗
NASA NTRS1985

Nonlinear analysis of laminated shells including transverse shear strains

Numerical results obtained using a doubly curved, shear deformable shell element are presented for geometrically nonlinear analysis of laminated composite shells. The element is based on an extension of Sanders' shell theory and accounts for the von Karman strains and transverse shear strains. The sample numerical results presented here for the geometrically nonlinear analysis of laminated composite shells should serve as references for future investigations.

Reddy, J. N.↗
NASA NTRS1985

Nonlinear theory for laminated and thick plates and shells including the effects of transverse shearing

Nonlinear strain displacement relations for three-dimensional elasticity are determined in orthogonal curvilinear coordinates. To develop a two-dimensional theory, the displacements are expressed by trigonometric series representation through-the-thickness. The nonlinear strain-displacement relations are expanded into series which contain all first and second degree terms. In the series for the displacements only the first few terms are retained. Insertion of the expansions into the three-dimensional virtual work expression leads to nonlinear equations of equilibrium for laminated and thick plates and shells that include the effects of transverse shearing. Equations of equilibrium and buckling equations are derived for flat plates and cylindrical shells. The shell equations reduce to conventional transverse shearing shell equations when the effects of the trigonometric terms are omitted and to classical shell equations when the trigonometric terms are omitted and the shell is assumed to be thin.

Stein, M.↗
NASA NTRS1985

Postbuckling of flat unstiffened graphite-epoxy plates loaded in shear

An experimental and analytical study was conducted of the postbuckling response and failure characteristics of 8-, 16-, and 24-ply unstiffened graphite/epoxy plates under shear loading, including orthotropic, quasi-orthotropic, entirely +, -45 deg ply, and (0/90)s-class plates. Postbuckling stiffness is noted to be influenced by the stacking sequence and by the inplane boundary condition imposed on the test section by a picture frame test fixture. The ratio of failure load to buckling load was higher for specimens that had higher width-to-thickness ratios. It is noted that high displacement gradients may induce transverse shear stresses, and that delamination was the failure mechanism for many of the 8-ply specimens. An adhesive failure between the composite specimen and metal edge reinforcements initiated failure of the 16- and 24-ply specimens.

Rouse, M.↗
NASA NTRS1985

A higher-order shear deformation theory of laminated elastic shells

A higher-order shear deformation theory of elastic shells is developed for shells laminated of orthotropic layers. The theory is a modification of the Sanders' theory and accounts for parabolic distribution of the transverse shear strains through thickness of the shell and tangential stress-free boundary conditions on the boundary surfaces of the shell. The Navier-type exact solutions for bending and natural vibration are presented for cylindrical and spherical shells under simply supported boundary conditions.

Reddy, J. N.↗
NASA NTRS1985

Analysis of low-altitude wind speed and direction shears

Horizontal wind profile measurements recorded at the NASA 150-Meter Ground Winds Tower Facility at the Kennedy Space Center, Fla., are analyzed to evaluate wind shears known to be hazardous to the ascent and descent of conventional aircraft and the Space Shuttle. Twenty 5-s intervals of high (between 10 and 18 m/s) and gale force (18 to 33 m/s) surface winds provided instantaneous recordings every 0.1 s per speed, direction, and tower level from 3-150 m. Mathematical (maximum, mean, standard deviation) and graphical (percentage frequency distribution) descriptions of absolute, positive, and negative speed and direction shears with altitude (six vertical layers) and along flight path (one horizontal distance) are presented as functions of the intensity categories and significant values.

Alexander, M. B.↗
NASA NTRS1986

Effects of external environments on the short beam shear strength of filament wound graphite/epoxy

Filament wound graphite/epoxy samples were immersed in seawater, deionized water, and toluene at room temperature and 80 deg C for 5, 15, and 43 days, and in methanol at room temperature for 15 and 43 days. The percent weight gains and short beam shear strengths were determined after environmental exposure. Samples immersed in deionized water and seawater had higher percent weight gains than those immersed in toluene at room temperature and 80 deg C. The percent weight gains for samples immersed in methanol at room temperature were comparable to those of deionized water and seawater immersed samples. A comparison of percent decreases in short beam shear strengths could not be made due to a large scatter in data. This may indicate defects in samples due to machining or variations in material properties due to processing.

Penn, B. G.↗
NASA NTRS1985

The effect of contraction on a homogeneous turbulent shear flow

In the present study, a homogeneous flow was generated and allowed to reach its asymptotic state. A two-dimensional contraction was inserted a short distance downstream. Both mean velocity and turbulence-stress profiles (normal and shear) were measured at several points along the flow direction. The pressure-strain terms were evaluated from the energy balance made along the wind-tunnel axis. On the basis of the obtained results, it is concluded that a uniformly sheared turbulent flow with approximate transverse homogeneity remains in its condition at all stages of its development through a two-dimensional contraction. Small deviations from homogeneity observed upstream of contraction become much less visible towards the end of contraction. The mean-flow development can be predicted adequately by the Bernoulli equation.

Sreenivasan, K. R.↗
NASA NTRS1985

Shear-lag analysis of notched laminates with interlaminar debonding

The fracture behavior of a debonded zone of finite width with no longitudinal damage in the unidirectional ply is predicted and the solution is then extended to include longitudinal matrix yielding and splitting in the unidirectional ply at the crack tip. The shear-lag assumption is used to describe the shear transfer between fibers. The fracture behavior of the laminate is studied as a function of initial crack length, constraint ratio, and width of the debonded zone. Results indicate that debonding can reduce the maximum fiber stress at the crack tip on the order of ten percent. This effect is maximum for a debonded width of two or three fiber spacings and is independent of the initial crack length. As the debond width grows beyond this point, the maximum stress increases. For widths of about ten fiber spacings or more, the maximum fiber stress is larger than for the fully bonded case. In the presence of longitudinal matrix damage the same general behavior is found; however, the location of the maximum fiber stress is quite complex.

Kaw, A. K.↗
NASA NTRS1986

Numerical simulation of unsteady flow in an axisymmetric shear layer

Numerical simulation of unsteady flow in an axisymmetric subsonic shear layer is accomplished by solving the time-dependent compressible Navier-Stokes equations. The objective of the effort is to investigate by numerical means the influence of various flow parameters on the shear layer behavior. The parameters investigated include the velocity ratio of two streams, total temperature, and nozzle lip thickness. The computations are performed on a CRAY-IS computer using McCormack's explicit finite difference scheme. The computed results generally show qualitative agreement with experimental data.

Scott, J. N.↗
NASA NTRS1985

Analysis of laminated composite plates using a higher-order shear deformation theory

A higher-order deformation theory is used to analyse laminated anisotropic composite plates for deflections, stresses, natural frequencies and buckling loads. The theory accounts for parabolic distribution of the transverse shear stresses, and requires no shear correction coefficients. A displacement finite element model of the theory is developed, and applications of the element to bending, vibration and stability of laminated plates are discussed. The present solutions are compared with those obtained using the classical plate theory and the three-dimensional elasticity theory.

Phan, N. D.↗
NASA NTRS1986

Ideal condensations due to perpendicular thermal conduction in a sheared magnetic field

Cool condensations generated by a radiative thermal instability in a sheared magnetic field have previously been the bases of solar filament formation models. Through the assumption of fully anisotropic heat flow, a new set of condensation modes are here obtained which become singular in the limit of vanishing perpendicular thermal conductivity. The growth rates are noted to typically be greater than those reported previously for sheared field condensations. The fastest growth is exhibited by modes possessing the fewest oscillations.

Van Hoven, G.↗