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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

LOADS: a computer program for determining the shear, bending moment and axial loads for fuselage type structures

LOADS determines rigid body vehicle shears, bending moments and axial loads on a space vehicle due to aerodynamic loads and propellant inertial loads. An example hand calculation is presented and was used to check LOADS. A brief description of the program and the equations used are presented. LOADS is operational on the Univac 1110, occupies 10505 core and typically takes less than one(1) second of CAU time to execute.

Nolte, W. E.↗
NASA NTRS1978

Excitation of instability waves in a two-dimensional shear layer by sound

The excitation of instability waves in a plane compressible shear layer by sound waves is studied. The problem is formulated mathematically as an inhomogeneous boundary-value problem. A general solution for abitrary incident sound wave is found by first constructing the Green's function of the problem. Numerical values of the coupling constants between incident sound waves and excited instability waves for a range of flow Mach number are calculated. The effect of the angle of incidence in the case of a beam of acoustic waves is analyzed. It is found that for moderate subsonic Mach numbers a narrow beam aiming at an angle between 50 to 80 deg to the flow direction is most effective in exciting instability waves.

Tam, C. K. W.↗
NASA NTRS1978

The prediction of turbulence and wind shear associated with thunderstorm gust fronts

The parametric dependence of storm simulation is explored on the basis of the previously reported (1977) second-order closure model of turbulence designed to simulate a thunderstorm gust front. The model simulation results are intended for extrapolation to the conditions considered most hazardous to aircraft operation. It is shown that the presistence and strength of the gust front are enhanced significantly by increasing the spread of the downdraft shaft and the velocity at the top of the domain. Turbulence develops in the regions of high shear and is most intense near the corner and within the gust front itself. While the center shaft is certainly an area to be avoided, the gust front region is probably more critical because it may extend over a large region far from the center of the downdraft, in a highly transient state.

Teske, M. E.↗
NASA NTRS1979

Refraction of sound by a shear layer - Experimental assessment

An experimental study was conducted to determine the refraction angle and amplitude changes associated with sound transmission through a circular, open jet shear layer. Both on-axis and off-axis acoustic source locations were used. Source frequency varied from 1 kHz to 10 kHz while freestream Mach number varied from 0.1 to 0.4. The experimental results were compared with an existing refraction theory which was extended to account for off-axis source positions. A simple experiment was also conducted to assess the importance of turbulence scattering between 1 kHz and 25 kHz.

Schlinker, R. H.↗
NASA NTRS1979

Stress intensity factors in bonded half planes containing inclined cracks and subjected to antiplane shear loading

The antiplane shear problem for two bonded dissimilar half planes containing a semi-infinite crack or two arbitrarily located collinear cracks is considered. For the semi-infinite crack the problem is solved for a concentrated wedge load and the stress intensity factor and the angular distribution of stresses are calculated. For finite cracks the problem is reduced to a pair of integral equations. Numerical results are obtained for cracks fully imbedded in a homogeneous medium, one crack tip touching the interface, and a crack crossing the interface for various crack angles.

Bassani, J. L.↗
NASA NTRS1979

A theory of stationary long waves. I - A simple theory of blocking. II - Resonant Rossby waves in the presence of realistic vertical shears

A theory of stationary long waves in the atmosphere is developed, with attention given to the blocking phenomenon caused by the resonant amplification of large-scale planetary waves in a uniform zonal flow and to resonant Rossby waves in an atmosphere with winds varying with height. A simple model is presented to illustrate the temporal behavior of Rossby waves forced by topography and differential heating of the land and the sea, using a beta-plane approximation and assuming a spatially uniform zonal wind which may vary in time, and quasi-geostrophic disturbances. Results are then extended to the case of resonant Rossby waves in the presence of realistic vertical shears. Numerical experiments in which the wind profiles are varied in a number of physically possible manners reveal the most favorable wind configurations for resonance.

Tung, K. K.↗
NASA NTRS1981

Biaxial method for in-plane shear testing

Method for obtaining uniform shear deformation yields more accurate values for material mechanical properties than uniaxial picture frame techniques. Forces applied are one-half usual magnitude, reducing transmitted force and related pin deformations. Biaxial method installs square sandwich specimen in stiff frame with pinned corners. Frictional effects are negligible, and stiffening of honeycomb core is corrected for in results.

Bush, H. G.↗
NASA NTRS1980

Turbulent structures in wall-bounded shear flows observed via three-dimensional numerical simulators

Three recent simulations of tubulent shear flow bounded by a wall using the Illiac computer are reported. These are: (1) vibrating-ribbon experiments; (2) study of the evolution of a spot-like disturbance in a laminar boundary layer; and (3) investigation of turbulent channel flow. A number of persistent flow structures were observed, including streamwise and vertical vorticity distributions near the wall, low-speed and high-speed streaks, and local regions of intense vertical velocity. The role of these structures in, for example, the growth or maintenance of turbulence is discussed. The problem of representing the large range of turbulent scales in a computer simulation is also discussed.

Leonard, A.↗
NASA NTRS1980

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 1: Pressure distribution. Part 2: Wall shear stress. Part 3: Simplified formulas for the prediction of surface pressures and skin friction

An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.

Adamson, T. C., Jr.↗
NASA NTRS1979

Transverse shear effect in a circumferentially cracked cylindrical shell

The objectives of the paper are to solve the problem of a circumferentially-cracked cylindrical shell by taking into account the effect of transverse shear, and to obtain the stress intensity factors for the bending moment as well as the membrane force as the external load. The formulation of the problem is given for a specially orthotropic material within the framework of a linearized shallow shell theory. The particular theory used permits the consideration of all five boundary conditions as to moment and stress resultants on the crack surface. The effect of Poisson's ratio on the stress intensity factors and the nature of the out-of-plane displacement along the edges of the crack, i.e., bulging, are also studied.

Delale, F.↗
NASA NTRS1979

The effect of nonuniform wind shear on the intensification and reflection of mountain waves

A study is made of the intensification and reflection of mountain waves when the shear of the basic wind profile is nonuniform. A two-layer atmospheric model is treated, and the wind profile in the troposphere is assumed to be parabolic. The Scorer parameter includes the wind profile curvature term, which may not be neglected if the Richardson number Ri is finite. When Ri is finite, the optimal phase difference across the troposphere for maximum surface velocity intensifications is found to be slightly greater than Pi. As Ri increases, the optimal phase difference decreases with Ri and approaches the limiting value Pi. This implies that waves approximately reverse phase between the surface and the tropopause for maximum wave intensifications in most physically realistic atmospheric situations. The concept of Eliassen and Palm concerning the additivity of the vertical wave energy fluxes is expanded (valid at least up to the parabolic wind profile), by which the upward and downward energy transporting modes are identified.

Hyun, J. M.↗
NASA NTRS1979

Effect of interfacial species on shear strength of metal-sapphire contacts

The interfacial shear strength of the metal-insulator system has been studied by means of the coefficient of static friction of copper, nickel, or gold contacts on sapphire in ultrahigh vacuum. The effect on contact strength of adsorbed oxygen, nitrogen, chlorine, and carbon monoxide on the metal surfaces is reported. It was found that exposures as low as 1 L of O2 on Ni produced observable increases in contact strength, whereas exposures of 3 L of Cl2 lead to a decrease in contact strength. These results imply that submonolayer concentrations of these species at the interface of a thin Ni film on Al2O3 should affect film adhesion similarly. The atomic mechanism by which these surface or interface phases affect interfacial strength is not yet understood.

Pepper, S. V.↗
NASA NTRS1979

Investigations of coherent structures in free turbulent shear flows

The dynamics of large-scale coherent structures in general, and studies based on flow-visualization and phase-locked hot-wire measurements of the structures in the near fields of circular jets are considered. A study involving the nature of coherent structures under conditions of stable vortex pairing induced by controlled axisymmetric acoustic excitation reveals that azimuthal coherence initially enhanced by the excitation is lost before the end of the potential core through evolution of azimuthal lobe structures and turbulent breakdown. The evolution of a spark-induced spot in an axisymmetric turbulent mixing layer is also investigated, and the coherent Reynolds stress associated with the spot is found to be much higher than the background turbulence Reynolds stress. In addition, the state of organization in a high Reynolds number unperturbed axisymmetric mixing layer is examined, and it is shown that the shear layer is infrequently organized and the motions are intensely three-dimensional, while tearing and fractional pairing occur as frequently as complete pairing.

Hussain, A. K. M. F.↗
NASA NTRS1980

Bifurcation and stability in a model of moist convection in a shearing environment

The truncated spectral system (model I) of shallow moist two-dimensional convection discussed by Shirer and Dutton (1979) is expanded to eleven coefficients (model II) in order to include a basic wind. Cloud streets, the atmospheric analog of the solutions to model II, are typically observed in an environment containing a shearing basic motion field. Analysis of the branching behavior of solutions to mode II shows that, if the basic wind direction varies with height, very complex temporal behavior is possible as the modified Rayleigh number HR is increased sufficiently. The first convective solution is periodic, corresponding to a cloud band that propagates downwind; but secondary branching to a two-dimensional torus can occur for larger values of HR. Orientation band formulas are derived whose predictions generally agree with the results of previous studies.

Shirer, H. N.↗
NASA NTRS1979

Experimental studies of combustion in a two dimensional free shear layer

The effect of combustion on the turbulent free shear layer formed at a rearward facing step has been studied. Schlieren movies confirm the importance of large scale vortices in determining entrainment and mixing behavior. The movies, long exposure schlieren photographs, and laser anemometry velocity profiles are used to observe the spreading rate of the layer and to study the vortex formation process. It is concluded that to first order, the primary effect of combustion is felt through the change in density ratio across the layer and acceleration of the flow due to volumetric expansion of the fluid in a confined duct.

Pitz, R. W.↗
NASA NTRS1980

An experimental study of the effects of swirl and density difference on mixing in shear flow

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.

Tan, C. S.↗
NASA NTRS1981

The development of convective instability, wind shear, and vertical motion in relation to convection activity and synoptic systems in AVE 4

Data from the Fourth Atmospheric Variability Experiment were used to investigate conditions/factors responsible for the development (local time rate-of-change) of convective instability, wind shear, and vertical motion in areas with varying degrees of convective activity. AVE IV sounding data were taken at 3 or 6 h intervals during a 36 h period on 24-25 April 1975 over approximately the eastern half of the United States. An error analysis was performed for each variable studied.

Davis, J. G.↗