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At least 613 records · Page 34

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NASA NTRS1983

Galvanic coupling between D6AC steel, 6061-T6 aluminum, Inconel 718 and graphite-epoxy composite material: Corrosion occurrence and prevention

The effects of galvanic coupling between D6AC steel, 6061-T6 aluminum, Inconel 718, and graphite-epoxy composite material (G/E) in 3.5% NaCl were studied. Measurements of corrosion potentials, galvanic currents and corrosion rates of the bare metals using weight-loss methods served to establish the need for corrosion protection in cases where D6AC steel and 6061-T6 aluminum are galvanically coupled to G/E in salt water while Inconel 718 was shown to be compatible with G/E. Six tests were made to study corrosion protective methods for eliminating galvanic corrosion in the cases of D6AC steel and 6061-T6 aluminum coupled to G/E. These results indicate that, when the G/E is completely coated with paint or a paint/polyurethane resin combination, satisfactory protection of the D6AC steel is achieved with either a coat of zinc-rich primer or a primer/topcoat combination. Likewise, satisfactory corrosion protection of the aluminum is achieved by coating it with an epoxy coating system.

Danford, M. D.↗

NASA NTRS1983

Acoustic responses of coupled fluid-structure system by acoustic-structural analogy

The use of an analogy between structural mechanics and acoustics makes it possible to solve fluid-structural interaction (FSI) problems using an existing structural analysis computer program. This method was implemented in MSC/NASTRAN program and the FSI analysis was performed using two dimensional coupled fluid beam model to assess and evaluate the adequacy of this approach. The coupled modal analysis of 3-D model is also briefly discussed. The normal mode, modal frequency response and transient response analysis of 2-D coupled fluid beam system is presented. The significant reduction of the acoustic pressure response at the fluid structure interface is observed as a result of fluid structure interaction.

Shin, Y. S.↗

NASA NTRS1984

Basic experimental study of the coupling between flow instabilities and incident sound

Whether a solid trailing edge is required to produce efficient coupling between sound and instability waves in a shear layer was investigated. The differences found in the literature on the theoretical notions about receptivity, and a need to resolve them by way of well-planned experiments are discussed. Instability waves in the shear layer of a subsonic jet, excited by a point sound source located external to the jet, were first visualized using an ensemble averaging technique. Various means were adopted to shield the sound reaching the nozzle lip. It was found that the low frequency sound couples more efficiently at distances downstream of the nozzle. To substantiate the findings further, a supersonic screeching jet was tested such that it passed through a small opening in a baffle placed parallel to the exit plane. The measured feedback or screech frequencies and also the excited flow disturbances changed drastically on traversing the baffle axially thus providing a strong indication that a trailing edge is not necessary for efficient coupling between sound and flow.

Ahuja, K. K.↗

NASA NTRS1983

Proposed solution methodology for the dynamically coupled nonlinear geared rotor mechanics equations

The equations which describe the three-dimensional motion of an unbalanced rigid disk in a shaft system are nonlinear and contain dynamic-coupling terms. Traditionally, investigators have used an order analysis to justify ignoring the nonlinear terms in the equations of motion, producing a set of linear equations. This paper will show that, when gears are included in such a rotor system, the nonlinear dynamic-coupling terms are potentially as large as the linear terms. Because of this, one must attempt to solve the nonlinear rotor mechanics equations. A solution methodology is investigated to obtain approximate steady-state solutions to these equations. As an example of the use of the technique, a simpler set of equations is solved and the results compared to numerical simulations. These equations represent the forced, steady-state response of a spring-supported pendulum. These equations were chosen because they contain the type of nonlinear terms found in the dynamically-coupled nonlinear rotor equations. The numerical simulations indicate this method is reasonably accurate even when the nonlinearities are large.

Mitchell, L. D.↗

NASA NTRS1983

Nonlinear effects of elastic coupling in unsymmetric laminates

Classical lamination theory predicts the room-temperature shape of all unsymmetrically laminated, elevated-temperature cure composites to be a saddle shape. Experimental observation indicates, however, that in many cases the room-temperature shape is cylindrical. In addition, a second cylindrical shape can often be obtained from the first by a simple snap-through action. It is the elastic couplings between inplane and out-of-plane deformations which are inherent in unsymmetric laminates that are responsible for the room-temperature shape. However, the couplings are so strong that geometrically nonlinear effects are produced. These are not accounted for in the classical theory. This paper reviews a theory developed to explain the effects of the coupling on laminate shape. The theory is based on a minimization of the laminate's total potential energy. The theory accounts for geometric nonlinearities. Because the problem is nonlinear, approximate solutions are sought by using a Rayleigh-Ritz procedure. Because of the observed snap-through of some laminates, stability of the predicted shapes is examined. Numerical results and some limited experimental data are presented for several laminates.

Hyer, M. W.↗

NASA NTRS1984

Coupling coefficient of gain-guided lasers

An analytical model is presented for the coupling coefficient for two gain-guided coupled waveguides, e.g., semiconductor laser arrays. A common parabolic gain distribution is assumed for the lasers, and the effective dielectric constant distribution is approximated in terms of the bulk refraction index, wavelength, power filling factor, and the antiguiding factor. The fundamental mode is then formulated and used in an integral for the coupling coefficient. The dependence of the coefficient of various waveguide parameters is described.

Katz, J.↗

NASA NTRS1985

A coupled ice-ocean model of ice breakup and banding in the marginal ice zone

A coupled ice-ocean numerical model for the marginal ice zone is considered. The model consists of a nonlinear sea ice model and a two-layer (reduced gravity) ocean model. The dependence of the upwelling response on wind stress direction is discussed. The results confirm earlier analytical work. It is shown that there exist directions for which there is no upwelling, while other directions give maximum upwelling in terms of the volume of uplifted water. The ice and ocean is coupled directly through the stress at the ice-ocean interface. An interesting consequence of the coupling is found in cases when the ice edge is almost stationary. In these cases the ice tends to break up a few tenths of kilometers inside of the ice edge.

Smedstad, O. M.↗

NASA NTRS1985

A simulation of high latitude F-layer instabilities in the presence of magnetosphere-ionosphere coupling

A simulation of inertial high-latitude ionospheric interchange instabilities, including magnetospheric coupling effects is presented. It is shown that the primary magnetosphere-ionosphere coupling effect is to incorporate the inertia of the magnetospheric plasma in the analysis. The following conclusions are drawn from the simulation: (1) magnetospheric coupling effects reduce the growth rate of the interchange instability, (2) striations produced by the inertial interchange instability develop in a different manner than in the noninertial regime, and (3) striations produced in the inertial regime are more isotropic and spread out, resulting in irregularities oriented perpendicular to those produced in the noninertial case.

Mitchell, H. G., Jr.↗

NASA NTRS1984

Dilute plasma coupling currents to a high voltage solar array in weak magnetic fields

The plasma coupling current to an approximately 2000 sq cm array was measured for externally biased positive and negative voltages on the array to 1000 V in applied magnetic field strengths from 0 to 0.93 G. The plasma density varied from 2,000 to 1.3 million electrons/cu cm. It was found that the magnetic field primarily increased the plasma coupling current for negative biases. For positive bias, the current could increase or decrease depending on the voltage, field strength, and plasma density. It was also found that the plasma coupling current was not very sensitive to how the plane of the array was oriented relative to the magnetic field.

Grier, N. T.↗

NASA NTRS1985

The effect of limiting aerodynamic and structural coupling in models of mistuned bladed disk vibration

A model has been developed for studying the effect of mistuning on bladed disk vibration which has the unique feature that the extent of aerodynamic and structural interaction which it simulates can be readily varied from full coupling of all blades on the disk to coupling of each blade with only its nearest neighbors. Simulations utilizing the resulting algorithm show that limited coupling models may be used to predict the statistical distribution of blade amplitudes that characterizes the mistuning effect, which in turn determines stage durability. This approach is used to study the effect of changing various system parameters on amplitude scatter. Gas density, the number of blades on the disk, disk stiffness, and the engine order of the excitation are considered. The results are used to draw some conclusions about how to improve laboratory tests and component design.

Basu, P.↗

NASA NTRS1986

Linear dynamic coupling in geared rotor systems

The effects of high frequency oscillations caused by the gear mesh, on components of a geared system that can be modeled as rigid discs are analyzed using linear dynamic coupling terms. The coupled, nonlinear equations of motion for a disc attached to a rotating shaft are presented. The results of a trial problem analysis show that the inclusion of the linear dynamic coupling terms can produce significant changes in the predicted response of geared rotor systems, and that the produced sideband responses are greater than the unbalanced response. The method is useful in designing gear drives for heavy-lift helicopters, industrial speed reducers, naval propulsion systems, and heavy off-road equipment.

David, J. W.↗

NASA NTRS1985

Phase shifting two coupled circadian pacemakers - Implications for jet lag

Two Van der Pol oscillators with reciprocal linear velocity coupling are utilized to model the response of the human circadian timing system to abrupt displacements of the environmental time cues (zeitgebers). The core temperature rhythm and sleep-wake cycle simulated by the model are examined. The relationship between the masking of circadian rhythms by environmental variables and behavioral and physiological events and the rates of resynchronization is studied. The effects of zeitgeber phase shifts and zeitgeber strength on the resynchronization rates are analyzed. The influence of intrinsic pacemakers periods and coupling strength on resynchronization are investigated. The simulated data reveal that: resynchronization after a time zone shift depends on the magnitude of the shift; the time of day of the shift has little influence on resynchronization; the strength of zeitgebers affects the rate and direction of the resynchronization; the intrinsic pacemaker periods have a significant effect on resynchronization; and increasing the coupling between the oscillators results in an increase in the rate of resynchronization. The model data are compared to transmeridian flight studies data and similar resynchronization patterns are observed.

Gander, P. H.↗

NASA NTRS1986

Observations of ionospheric magnetospheric coupling - DE and Chatanika coincidences

Observations from several experiments on board the Dynamics Explorer 1 and 2 (DE 1 and 2) spacecraft and ground-based radar measurements from the Chatanika radar are combined in order to examine the details of ionospheric/magnetospheric coupling in the local evening sector. DE 1 and DE 2 were in coplanar polar orbits that provided measurements almost simultaneously in time and magnetically coincident with the Chatanika radar from L = 3 to L = 17. The coupling processes are inferred from the density, temperature, composition, and angular distributions of the low-energy plasma observed from the E region of the ionosphere to magnetospheric altitudes of 2.5 earth radii. Plasma characteristics of the plasmasphere, main trough, auroral zone, and polar cap can be studied in this data set. The observations imply that as L increases, the dominant coupling mechanism between the ionosphere and magnetosphere in the measured energy range changes from equilibrium diffusion to perpendicular acceleration and finally to parallel acceleration.

Green, J. L.↗

NASA NTRS1986

A numerical model of magnetosphere-ionosphere coupling Preliminary results

A three-dimensional simulation model was developed to study magnetosphere-ionosphere coupling in the auroral region. One-fluid MHD equations are adopted to model the magnetosphere, and current density continuity equations were solved consistently to model the ionosphere. In the preliminary simulation runs described here, the electrodynamics of region 1 field-aligned currents were modeled. Initially, the electric field is taken to be in the magnetospheric equatorial plane. Alfven waves then propagate down to the ionosphere, accompanying a field-aligned current and exciting the electrostatic potential in the ionosphere by electrodynamic coupling. The results of the preliminary runs directly correspond with the fundamental characteristics of global magnetosphere-ionosphere coupling. These characteristics include an ionospheric electrostatic potential which varies in its development in time, depending upon the ratio of ionospheric resistance to magnetospheric impedance, the flowing of field-aligned currents into the ionosphere on the dawnside and out on the duskside, and the distribution along the geomagnetic field of the amplitude of the field-aligned current density in proportion to the field intensity.

Watanabe, K.↗

NASA NTRS1986

Summary of AH-1G flight vibration data for validation of coupled rotor-fuselage analyses

Under a NASA research program designated DAMVIBS (Design Analysis Methods for VIBrationS), four U. S. helicopter industry participants (Bell Helicopter, Boeing Vertol, McDonnell Douglas Helicopter, and Sikorsky Aircraft) are to apply existing analytical methods for calculating coupled rotor-fuselage vibrations of the AH-1G helicopter for correlation with flight test data from an AH-1G Operational Load Survey (OLS) test program. Bell Helicopter, as the manufacturer of the AH-1G, was asked to provide pertinent rotor data and to collect the OLS flight vibration data needed to perform the correlations. The analytical representation of the fuselage structure is based on a NASTRAN finite element model (FEM) developed by Bell which has been extensively documented and correlated with ground vibration tests.The AH-1G FEM was provided to each of the participants for use in their coupled rotor-fuselage analyses. This report describes the AH-1G OLS flight test program and provides the flight conditions and measured vibration data to be used by each participant in their correlation effort. In addition, the mechanical, structural, inertial and aerodynamic data for the AH-1G two-bladed teetering main rotor system are presented. Furthermore, modifications to the NASTRAN FEM of the fuselage structure that are necessary to make it compatible with the OLS test article are described. The AH-1G OLS flight test data was found to be well documented and provide a sound basis for evaluating currently existing analysis methods used for calculation of coupled rotor-fuselage vibrations.

Dompka, R. V.↗

NASA NTRS1986

Design and testing of an electromagnetic coupling

Hostile environments such as the hard vacuum of space, and exposure to water or caustic fluids have fostered the development of devices which allow mechanical rotary feed throughs with positive sealing without the use of conventional dynamic seals. One such device is an electromagnetic coupling which transfers motion across a hermetic seal by means of a rotating magnetic field. Static pull-out torque and dynamic heat build-up and pull-out torque tests of a synchronous reluctance homopolar coupling are reported herein. Coupling efficiencies are estimated for a range of speeds and torques.

Anderson, William J.↗

NASA NTRS1987

Comparison of composite rotor blade models: A coupled-beam analysis and an MSC/NASTRAN finite-element model

A methodology was developed for the structural analysis of composite rotor blades. This coupled-beam analysis is relatively simple to use compared with alternative analysis techniques. The beam analysis was developed for thin-wall single-cell rotor structures and includes the effects of elastic coupling. This paper demonstrates the effectiveness of the new composite-beam analysis method through comparison of its results with those of an established baseline analysis technique. The baseline analysis is an MSC/NASTRAN finite-element model built up from anisotropic shell elements. Deformations are compared for three linear static load cases of centrifugal force at design rotor speed, applied torque, and lift for an ideal rotor in hover. A D-spar designed to twist under axial loading is the subject of the analysis. Results indicate the coupled-beam analysis is well within engineering accuracy.

Hodges, Robert V.↗

NASA NTRS1987

Aeroelastic effects in multirotor vehicles. Part 2: Methods of solution and results illustrating coupled rotor/body aeromechanical stability

This report is a sequel to the earlier report titled, Aeroelastic Effects in Multi-Rotor Vehicles with Application to Hybrid Heavy Lift System, Part 1: Formulation of Equations of Motion (NASA CR-3822). The trim and stability equations are presented for a twin rotor system with a buoyant envelope and an underslung load attached to a flexible supporting structure. These equations are specialized for the case of hovering flight. A stability analysis, for such a vehicle with 31 degrees of freedom, yields a total of 62 eigenvalues. A careful parametric study is performed to identify the various blade and vehicle modes, as well as the coupling between various modes. Finally, it is shown that the coupled rotor/vehicle stability analysis provides information on both the aeroelastic stability as well as complete vehicle dynamic stability. Also presented are the results of an analytical study aimed at predicting the aeromechanical stability of a single rotor helicopter in ground resonance. The theoretical results are found to be in good agreement with the experimental results, thereby validating the analytical model for the dynamics of the coupled rotor/support system.

Venkatesan, C.↗