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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Long nonlinear waves in stratified shear flows

The propagation of finite-amplitude internal waves in a shear flow is considered for wavelengths that are long compared to the shear-layer thickness. Both singular and regular modes are investigated, and the equation governing the amplitude evolution is derived. The theory is generalized to allow for a radiation condition when the region outside the stratified shear layer is unbounded and weakly stratified. In this case, the evolution equation contains a damping term describing energy loss by radiation which can be used to estimate the persistence of solitary waves or nonlinear wave packets in realistic environments. A continuous three-layer model is studied in detail and closed-form expressions are obtained for the phase speed and the coefficients of the nonlinear and dispersive terms in the amplitude equation as a function of Richardson number.

Maslowe, S. A.↗
NASA NTRS1981

Experimental study of combustion in a turbulent free shear layer formed at a rearward facing step

A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward facing step. The mean and rms averages of the turbulent velocity flow field are determined by LDV for both reacting (equivalence ratio 0.57) and nonreacting flows (Reynolds number 15,000-37,000 based on step height). The effect of combustion is to shift the layer toward the recirculation zone and reduce the flame spread. For reacting flow, the growth rate is unchanged except very near the step. The probability density function of the velocity is bimodial near the origin of the reacting layer and single-peaked but often skewed elsewhere. Large-scale structures dominate the reacting shear layer. Measurements of their passing frequency from LDV are consistent with high-speed Schlieren movies of the reacting layer and indicate that the coalescence rate of the eddies in the shear layer is reduced by combustion.

Pitz, R. W.↗
NASA NTRS1981

Propagation in a shearing plasma. III - Magnetic field effects and pulsar microstructure periods

Effects of a magnetic field on wave and pulse propagation in a shearing plasma are investigated. Equations for the wave fields are derived from Maxwell's equations and the Minkowski relations for an anisotropic medium, when the magnetic field is parallel to the shearing velocity. The independent propagation modes are elliptically polarized in the stationary frame and vary with the velocity of the medium. Numerical calculations are presented which show the propagation of waves and pulses through a plane-parallel shearing plasma in the case of weak and strong fields. It is found that in a weak field, propagation effects are very similar to those previously found in field-free models: the transmitted pulses acquire temporal modulations whose period is frequency dependent. In a strong field, the period of the temporal modulations is independent of frequency. Transmitted pulses can also undergo polarization changes as a result of phase differences between modes. These results are discussed in relation to their significance to propagation in a pulsar magnetosphere and, in particular, to a model for pulse microstructure.

Harding, A. K.↗
NASA NTRS1981

Flow separation in shear-layer-driven cavities

A study of the shear-layer flow over a range of open-top cavity configurations is reported. Emphasis is placed on the effect that altering the cavity's span length and aspect ratio has on the development of the shear layer. Computational results are obtained using an interactive method which adapts the compressible boundary-layer model for the flow above the cavity and incompressible Navier-Stokes equations within the enclosure. Interaction of this composite model with the outer, inviscid supersonic flow is also considered in one case. The results show that the location of the stagnation points is sensitive primarily to the variation of the span. When the span was fixed and the aspect ratio varied, the shear layer was nearly unaffected except at aspect ratios less than 0.5. Interaction with the outer flow had a smoothing effect on the shape of the dividing streamline but did not significantly affect the location of the stagnation points.

Brandeis, J.↗
NASA NTRS1981

Magnitude and frequency of wind speed shears from 3 to 150 meters

An analysis is presented of high resolution wind profile measurements recorded at the NASA 150-m ground winds tower facility, showing wind speed shear frequency and magnitude distributions for six vertical layers of the atmosphere and one vertical distance. Vertical wind shear is defined as the change of wind speed with height, and its magnitudes were derived by algebraically subtracting lower level wind speeds from those of higher levels and dividing the distance between levels. Horizontal wind shear is understood to be change of wind speed with horizontal distance, and its magnitudes were derived by algebraically subtracting the wind speed at a short tower from that at a tall one and dividing by the distance between towers.

Alexander, M. B.↗
NASA NTRS1981

Analysis of interface cracks in adhesively bonded lap shear joints, part 4

Conservation laws of elasticity for nonhomogeneous materials were developed and were used to study the crack behavior in adhesively bonded lap shear joints. By using these laws and the fundamental relationships in fracture mechanics of interface cracks, the problem is reduced to a pair of linear algebraic equations, and stress intensity solutions can be determined directly by information extracted from the far field. The numerical results obtained show that: (1) in the lap-shear joint with a given adherend, the opening-mode stress intensity factor, (K sub 1) is always larger than that of the shearing-mode (K sub 2); (2) (K sub 1) is not sensitive to adherent thickness abut (K sub 2) increases rapidly with increasing thickness; and (3) (K sub 1) and (K sub 2) increase simultaneously as the interfacial crack length increases.

Wang, S. S.↗
NASA NTRS1980

An airport wind shear detection and warning system using Doppler radar

Two numerical models of aircraft performance are developed to relate atmospheric wind signals in wind-shear situations to aircraft response. Both models produce time histories of pitch angle, airspeed, height above the ground, and other parameters for simulating aircraft making precision approaches along a 3-deg glide slope to the runway. Simulations conducted using thunderstorm environment wind data indicate that the horizontal wind component is at least as important as the vertical component in determining aircraft response to wind shear. Obtaining quantitative measurements of aircraft approach or departure quality is found to depend on several factors, including the knowledge of winds along the expected flight track, clear-air detection in a quasi-horizontal path, and a near continuous estimate of approach deterioration due to shear conditions.

Mccarthy, J.↗
NASA NTRS1981

Wave-particle transport by weak electrostatic flow shear fluctuations

A description is presented of the first consistent theoretical treatment of transport due to weak electrostatic fluctuations from microinstabilities driven by a shear in plasma flow parallel to a uniform magnetic field. The model used considers electrostatic fluctuations in a Vlasov plasma with sheared bulk velocity parallel to a uniform magnetic field. The linear stability theory for the model has been studied by Gary and Schwartz (1980). In the current investigation, a calculation is performed of the wave-particle transport associated with the electrostatic flow shear instability.

Gary, S. P.↗
NASA NTRS1982

Airborne operation of an infrared low-level wind shear prediction system

Airborne testing under simulated and actual low-level wind shear conditions is underway on a NASA-Ames Learjet. An infrared CO2 band radiometer with a forward 'look distance' of 5 to 8 kilometers measures the air temperature weighted to this range ahead of the approach configured aircraft. Shear alerts occur when the difference between the forward temperature and static air temperature at the aircraft exceed a set value or when a perturbation occurs in the normally constant potential temperature. Aircraft approaches into thunderstorm downburst phenomena were simulated by approaches into cool estuarine air adjacent to much warmer air over land and by actual light wind shear conditions at Travis Air Force Base. Conditions were verified by the radiometer system with extensive on-board data acquisition.

Kuhn, P. M.↗
NASA NTRS1982

Simulation of phugoid excitation due to hazardous wind shear

Computer and manned flight simulator studies of the effect of thunderstorm wind shear on the excitation of phugoid oscillations of commercial-type aircraft are reported. Previous studies have suggested that the characteristic scale of thunderstorm wind shear drives commercial-type aircraft at the critical phugoid frequency. To further investigate this phenomenon, aircraft performance in a hypothetical sinusoidal wind shear was investigated. Sinusoidal winds at the phugoid frequency and with frequencies above and below this value were studied both analytically and in manned flight simulators. Results indicate that excessive oscillations in flight path amplitude predicted by computer analyses were not fully collaborated by the manned flight simulator studies. Some resolution of this problem is given and additional research necessary to further identify the discrepancies between the flight simulator and the computer analysis is outlined.

Frost, W.↗
NASA NTRS1982

The influence of meridional shear on planetary waves. I - Nonsingular wind profiles. II - Critical latitudes

An attempt to define a relationship between the strength and curvature of the latitudinal wind shear and the planetary waves is presented. Previous work based on the WKB method is reviewed and compared with Galerkin's method, which is found to be effective when considering low-order modes. The WKB method, however, increases in accuracy with the mode number. Both methods are applied to a linear wind shear profile and the Galerkin method is determined to be significantly superior in predicting the lowest order, and is therefore suitable for formulating the latitudinal structure of quasi-stationary Rossby waves in the middle atmosphere. In a second paper the mean zonal wind is assumed to depend on latitude only, and the effects of critical latitudes, where the Doppler shifter frequency is zero, on planetary waves are shown to include shear-induced instability in Kelvin waves. Further research to assay the extent that baroclinity alters stable winds is indicated.

Boyd, J. P.↗
NASA NTRS1982

Vector magnetic field evolution, energy storage, and associated photospheric velocity shear within a flare-productive active region

Sheared photospheric velocity fields inferred from spot motions for April 5-7, 1980, are compared with both transverse magnetic field orientation changes and with the region's flare history. Rapid spot motions and high inferred velocity shear coincide with increased field alignment along the longitudinal neutral line and with increased flare activity, while a later decrease in velocity shear precedes a more relaxed magnetic configuration and decrease in flare activity. It is estimated that magnetic reconfiguration produced by the relative velocities of the spots could cause storage of about 10 to the 32nd erg/day, while flares occurring during this time expended no more than about 10 to the 31st erg/day.

Krall, K. R.↗
NASA NTRS1983

Characterizing Shear Properties of Membranes

Method devised to obtain static shear stiffness data for membranes. Shear deflection of membrane measured under various applied static shear loads, and measurements plotted. Match attempted between experimental plot and predicted load-vs.-deflections curves until best fitting theoretical curve is found.

Simpson, B.↗
NASA NTRS1983

Material characterization of structural adhesives in the lap shear mode

A general method for characterizing structual adhesives in the bonded lap shear mode is proposed. Two approaches in the form of semiempirical and theoretical approaches are used. The semiempirical approach includes Ludwik's and Zhurkov's equations to describe respectively, the failure stresses in the constant strain rate and constant stress loading modes with the inclusion of the temperature effects. The theoretical approach is used to describe adhesive shear stress-strain behavior with the use of viscoelastic or nonlinear elastic constitutive equations. Two different model adhesives are used in the single lap shear mode with titanium adherends. These adhesives (one of which was developed at NASA Langley Research Center) are currently considered by NASA for possible aerospace applications. Use of different model adhesives helps in assessment of the generality of the method.

Sancaktar, E.↗
NASA NTRS1983

Buckling and failure characteristics of graphite-polyimide shear panels

The buckling and failure characteristics of unstiffened, blade stiffened, and hat stiffened graphite-polyimide shear panels are described. The picture frame shear test is used to obtain shear stress-strain data at room temperature and at 316 deg C. The experimental results are compared with a linear buckling analysis, and the specimen failure modes are described. The effect of the 316 deg C test temperature on panel behavior are discussed.

Shuart, M. J.↗
NASA NTRS1983

Low-level wind shear detection and warning - A systems update

The aim of the Joint Airport Weather Studies (JAWS) project, to determine quantitatively how well various low-level wind shear detection and warning systems operate in a uniform wind shear environment, is discussed. A number of low-level wind shear detection and warning systems that were used in the JAWS field project in the summer of 1982 are briefly described. These included three types of surface in situ sensors, three airborne sensors, and two Doppler radar remote sensing concepts. The tasks that are pertinent to an evaluation of the systems are stated, and some tentative impressions regarding the capabilities of the systems are presented.

Mccarthy, J.↗
NASA NTRS1983

A model of the excitation of large scale fluctuations in a shear layer

The acoustical excitation of instability waves in free shear layers is investigated theoretically and experimentally. A configuration with a thin shear layer leaving from a semi-infinite plate is considered. The excitation by sound sources in various locations, e.g., far away from the trailing edge or very near the edge lip, is analyzed. The influence of additional walls (like those in a channel) is also predicted. The theory is extended to include two streams on both sides of the shear layer having different velocities and densities. A reference quantity for the artificial excitation is suggested. The experimental data compare well with the theory. The theory does not contain any empirical constants.

Bechert, D. W.↗
NASA NTRS1982

Measurements in a pressure-driven and a shear-driven three-dimensional turbulent boundary layer

Results of mean velocity field, wall static pressure field and simultaneous, direct force measurements of the local wall shear stress magnitude and direction are reported for a pressure-driven and a shear-driven three-dimensional turbulent boundary layer. These data, particularly the direct force local wall shear data, were obtained to test the validity of several of the near-wall similarity models proposed in the literature for such flows.

Pierce, F. J.↗