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At least 991 records · Page 55

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NASA NTRS1974

Calculation of turbulent shear stress in supersonic boundary layer flows

An analysis of turbulent boundary layer flow characteristics and the computational procedure used are discussed. The integrated mass and momentum flux profiles and differentials of the integral quantities are used in the computations so that local evaluation of the streamwise velocity gradient is not necessary. The computed results are compared with measured shear stress data obtained by using hot wire anemometer and laser velocimeter techniques. The flow measurements were made upstream and downstream of an adiabatic unseparated interaction of an oblique shock wave with the turbulent boundary layer on the flat wall of a two dimensional wind tunnel. A comparison of the numerical analysis and actual measurements is made and the effects of small differences in mean flow profiles on the computed shear stress distributions are discussed.

Sun, C. C.↗

NASA NTRS1974

Calculation of compressible turbulent free shear layers

Rudy and Bushnell's (1972) mixing length theory for the prediction of mean velocity fields in turbulent shear layers is extended to include supersonic free turbulent mixing. The normal momentum equation is coupled with the conventional equations of motion and is solved iteratively. Results show that transverse static pressure variation has very little effect on the mean flow variables in compressible free turbulent shear layers.

Oh, Y. H.↗

NASA NTRS1974

A technique for generating shear waves in cylindrical shells under radial impact

Experimental techniques are developed to study and measure the shear-wave velocity in an aluminum cylindrical shell subjected to a radial impact. The radial impact is obtained by exploding an electrical detonator inserted in plastic plugs mounted on the end of the shell. Strain gages, mounted on the outside surface of the shell at various axial locations, are used to obtain oscilloscope traces from which the shear-wave velocity can be calculated.

Blum, A.↗

NASA NTRS1974

Shear buckling of square perforated plates

The behavior of thin square perforated plates under the action of uniform shear deformation is studied experimentally and analytically using finite element analysis. Elastic Shear buckling strength is established as a function of the diameter of a round, centrally located hole in the plate. Post buckling behavior and the behavior of perforated plates with various ring stiffeners are also studied experimentally.

Grosskurth, J. F., Jr.↗

NASA NTRS1975

Effect of adsorbed chlorine and oxygen on shear strength of iron and copper junctions

Static friction experiments were performed in ultrahigh vacuum at room temperature on copper, iron, and steel contacts selectively contaminated with oxygen and chlorine in submonolayer amounts. The concentration of the adsorbates was determined with Auger electron spectroscopy and was measured relative to the saturation concentration of oxygen on iron (concentration 1.0). The coefficient of static friction decreased with increasing adsorbate concentration. It was independent of the metal and the adsorbate. The results compared satisfactorily with an extension of the junction growth theory to heterogeneous interfaces. The reduction in interfacial shear strength was measured by the ratio sub a/sub m where sub a is the shear strength of the interface with an adsorbate concentration of 1.0, and sub m is the strength of the clean metal interface. This ratio was 0.835 + or - 0.012 for all the systems tested.

Wheeler, D. R.↗

NASA NTRS1973

Analysis of response to wind-shears using the optimal control model of the human operator

The effects of wind-shears on the approach performance of a STOL aircraft are analyzed using the optimal-control model of the human operator. This analysis involves a time-varying situation that is more complex than is traditionally treated by human operator modelling techniques. The extensions to the time-varying case are discussed and results are presented that illustrate the effects of wind shears on category II window-performance and the effects on performance of pilot time-delay and of variations in pilot gain during approach.

Baron, S.↗

NASA NTRS1975

The effect of thin turbulent shear layers on the optical quality of imaging systems

A modified C141 transport was outfitted with a 91.5-cm reflector telescope designed to view objects radiating outside the visible window in the infrared range from 1 micron to 1000 microns. The telescope is situated in a cavity which is operated open port. Spoilers were designed which reduce turbulence-induced excitation of the cavity. The aircraft was designed to operate at altitudes up to 15 km to significantly reduce the effect of the H2O and CO2. Furthermore, the optically degrading influence of the large-scale atmospheric turbulence on land-based telescopes is replaced by the effect of the turbulent shear layer resulting from the spoiler upstream of the cavity. A mathematical model was established to describe the effect of turbulent shear layers on imaging systems and to examine the parameters of interest relevant to potential wind-tunnel experimentation.

Steinmetz, W. J.↗

NASA NTRS1975

Numerical simulation of turbulence in the presence of shear

The numerical calculations are presented of the large eddy structure of turbulent flows, by use of the averaged Navier-Stokes equations, where averages are taken over spatial regions small compared to the size of the computational grid. The subgrid components of motion are modeled by a local eddy-viscosity model. A new finite-difference scheme is proposed to represent the nonlinear average advective term which has fourth-order accuracy. This scheme exhibits several advantages over existing schemes with regard to the following: (1) the scheme is compact as it extends only one point away in each direction from the point to which it is applied; (2) it gives better resolution for high wave-number waves in the solution of Poisson equation, and (3) it reduces programming complexity and computation time. Examples worked out in detail are the decay of isotropic turbulence, homogeneous turbulent shear flow, and homogeneous turbulent shear flow with system rotation.

Shaanan, S.↗

NASA NTRS1975

Experimental study of a free turbulent shear flow at Mach 19 with electron-beam and conventional probes

An experimental study of the initial development region of a hypersonic turbulent free mixing layer was made. Data were obtained at three stations downstream of a M = 19 nozzle over a Reynolds range of 1.3 million to 3.3 million per meter and at a total temperature of about 1670 K. In general, good agreement was obtained between electron-beam and conventional probe measurements of local mean flow parameters. Measurements of fluctuating density indicated that peak root-mean-square (rms) levels are higher in the turbulent free mixing layer than in boundary layers for Mach numbers less than 9. The intensity of rms density fluctuations in the free stream is similar in magnitude to pressure fluctuations in high Mach number flows. Spectrum analyses of the measured fluctuating density through the shear layer indicate significant fluctuation energy at the lower frequencies (0.2 to 5 kHZ) which correspond to large-scale disturbances in the high-velocity region of the shear layer.

Harvey, W. P.↗

NASA NTRS1974

High shear stress behavior of some representative lubricants

Shear stress independent behavior observed for representative, synthetic, nonblended lubricants is found to be of the same magnitude as the shear stress in sliding elastohydrodynamic contacts. It is shown that dissipation heating is the only mechanism of importance in the generation of deviations from constant viscosity as measured with capillary tube viscometric methods. Newtonian end corrections for capillary tubes are found to be constant for nonblended liquid lubricants.

Jakobsen, J.↗

NASA NTRS1974

Inclusion of transverse shear deformation in finite element displacement formulations

A stiffness matrix is derived for a beam element with transverse shear deformation. It is shown that straightforward energy minimization yields the correct stiffness matrix in displacement formulations when transverse shear effects are considered. Since the TIM4 beam element does not represent the geometric boundary conditions for a cantilever beam the rotation of the normal must be retained as a grid point degree of freedom.

Narayanaswami, R.↗

NASA NTRS1975

The measurement of shear stress and total heat flux in a nonadiabatic turbulent hypersonic boundary layer

Turbulent shear stress and direct turbulent total heat-flux measurements have been made across a nonadiabatic, zero pressure gradient, hypersonic boundary layer by using specially designed hot-wire probes free of strain-gauging and wire oscillation. Heat-flux measurements were in reasonably good agreement with values obtained by integrating the energy equation using measured profiles of velocity and temperature. The shear-stress values deduced from the measurements, by assuming zero correlation of velocity and pressure fluctuations, were lower than the values obtained by integrating the momentum equation. Statistical properties of the cross-correlations are similar to corresponding incompressible measurements at approximately the same momentum-thickness Reynolds number.

Mikulla, V.↗

NASA NTRS1975

On the passage of radiation through inhomogeneous, moving media. XII - Polarized waves in a plane, sheared medium

Using the complete set of Maxwell equations, the ray paths and Stokes parameters of a polarized wave are followed which passes through a slab of differentially shearing material of variable refractive index. The polarization state of the signal leaving the top of the slab is related to the polarization state of the initial wave. In general, the two are not the same. These calculations were made to illustrate the point that the polarization properties of observed pulsar signals may be very different from those of the emitted signals due to propagation effects in the differentially shearing magnetospheres that most pulsars are believed to possess. While it is recognized that the precise amount of polarization variations is model dependent, the calculations show that even for simple situations a noticeable polarization variation occurs. Accordingly, the calculations reported here are an educative device: they show that it is, perhaps, unwise to assume that the observed polarization properties of pulsar signals are the same as the emitted signals.

Lee, M. A.↗

NASA NTRS1975

Shear-flexible finite-element models of laminated composite plates and shells

Several finite-element models are applied to the linear static, stability, and vibration analysis of laminated composite plates and shells. The study is based on linear shallow-shell theory, with the effects of shear deformation, anisotropic material behavior, and bending-extensional coupling included. Both stiffness (displacement) and mixed finite-element models are considered. Discussion is focused on the effects of shear deformation and anisotropic material behavior on the accuracy and convergence of different finite-element models. Numerical studies are presented which show the effects of increasing the order of the approximating polynomials, adding internal degrees of freedom, and using derivatives of generalized displacements as nodal parameters.

Noor, A. K.↗

NASA NTRS1976

Ten deg off-axis tensile test for intralaminar shear characterization of fiber composites

A combined theoretical and experimental investigation was conducted to assess the suitability of the 10 deg off-axis tensile test specimen for the intralaminar shear characterization of unidirectional composites. Composite mechanics, a combined-stress failure criterion, and a finite element analysis were used to determine theoretically the stress-strain variation across the specimen width and the relative stress and strain magnitudes at the 10 deg plane. Strain gages were used to measure the strain variation across the specimen width at specimen midlength and near the end tabs. Specimens from Mod-I/epoxy, T-300/epoxy, and S-glass/epoxy were used in the experimental program. It was found that the 10 deg off-axis tensile test specimen is suitable for intralaminar shear characterization and it is recommended that it should be considered as a possible standard test specimen for such a characterization.

Chamis, C. C.↗

NASA NTRS1975

Gravity shear waves atop the cirrus layer of intense convective storms

Recent visual satellite photographs of certain intense convective storms have revealed concentric wave patterns. A model for the generation and growth of these waves is proposed. The proposed initial generating mechanism is similar to the effect noticed when a pebble is dropped into a calm pond. The penetration of the tropopause by overshooting convection is analogous to the pebble's penetration of the water's surface. The model for wave growth involves instability due to the wind shear resulting from the cirrus outflow. This model is based on an equation for the waves' phase speed which is similar to the Helmholtz equation. It, however, does not assume an incompressible atmosphere, but rather assumes density is a logarithmic function of height. Finally, the model is evaluated on the two mid-latitude and three tropical cases. The data indicate that shearing instability may be a significant factor in the appearance of these waves.

Stobie, J. G.↗

NASA NTRS1976

Wind shear detection using measurement of aircraft total energy change

Encounters with wind shears are of concern and have caused major accidents, particularly during landing approaches. Changes in the longitudinal component of the wind affect the aircraft by changing its kinetic energy with respect to the air. It is shown that an instrument which will measure and display the rate of change of total energy of the aircraft with respect to the air will give a leading indication of wind shear problems. The concept is outlined and some instrumentation and display considerations are discussed.

Joppa, R. G.↗

NASA NTRS1976

Estimates of the low-level wind shear and turbulence in the vicinity of Kennedy International Airport on 24 June 1975

A study was conducted to estimate the type of wind and turbulence distributions which may have existed at the time of the crash of Eastern Airlines Flight 66 while attempting to land. A number of different wind and turbulence profiles are predicted for the site and date of the crash. The morning and mid-afternoon predictions are in reasonably good agreement with magnitude and direction as reported by the weather observer. Although precise predictions cannot be made during the passage of the thunderstorm which coincides with the time of the accident, a number of different profiles which might exist under or in the vicinity of a thunderstorm are presented. The profile that is most probable predicts the mean headwind shear over 100 m (300 feet) altitude change and the average fluctuations about the mean headwind distribution. This combination of means and fluctuations leads to a reasonable probability that the instantaneous headwind shear would equal the maximum value reported in the flight recorder data.

Lewellen, W. S.↗