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Semirigid flutter theory is used. Calculations are made on airfoils with fundamental bending frequencies up to 2000 radian per second. Centrifugal force can under certain conditions reduce the critical flutter speed.
The ability to fabricate design critical and man-rated aerospace structures using materials near the limits of their capabilities requires a comprehensive and dependable assurance program. The quality assurance program must rely heavily on nondestructive testing methods for thorough inspection to assess properties and quality of hardware items. A survey of nondestructive testing methods is presented to provide space program managers, supervisors and engineers who are unfamiliar with this technical area with appropriate insight into the commonly accepted nondestructive testing methods available, their interrelationships, used, advantages and limitations. Primary emphasis is placed on the most common methods: liquid penetrant, magnetic particle, radiography, ultrasonics and eddy current. A number of the newer test techniques including thermal, acoustic emission, holography, microwaves, eddy-sonic and exo-electron emission, which are beginning to be used in applications of interest to NASA, are also discussed briefly.
The PPN formalism which encompasses the post-Newtonian limit of nearly every metric theory of gravity is used to analyze stellar stability. This analysis enables one to infer, for any given gravitation theory, the extent to which post-Newtonian effects induce instabilities in white dwarfs, in neutron stars, and in supermassive stars. It also reveals the extent to which our current empirical knowledge of post-Newtonian gravity (based on solar-system experiments) actually guarantees that relativistic instabilities exist. In particular, it shows that: (1) for conservative theories of gravity, current solar-system experiments guarantee that the critical adiabatic index, for the stability of stars against radial pulsations exceeds the Newtonian value of 4/3 and (2) for nonconservative theories, current experiments do not permit any firm conclusion about the sign of the critical adiabatic index, and (3) in the PPN approximation to every metric theory, the standard Schwarzschild criterion for convection is valid.
Reaction-rate theory and experiment are given a critical review from the engineers' point of view. Rates of heavy-particle, collision-induced reaction in gas phase are formulated in terms of the cross sections and activation energies for reaction. The effect of cross section function shape and of excited state contributions to reaction both cause the slope of Arrhenius plots to differ from the true activation energy, except at low temperature. The master equations for chemically reacting gases are introduced, and dissociation and ionization reactions are shown to proceed primarily from excited states about kT from the dissociation or ionization limit. Collision-induced vibration, vibration-rotation, and pure rotation transitions are treated, including three-dimensional effects and conservation of energy, which have usually been ignored. The quantum theory of transitions at potential surface crossing is derived, and results are found to be in fair agreement with experiment in spite of some questionable approximations involved.
Bifurcation theory is used to analyze the nonlinear dynamic stability characteristics of an aircraft subject to single-degree-of-freedom. The requisite moment of the aerodynamic forces in the equations of motion is shown to be representable in a form equivalent to the response to finite amplitude oscillations. It is shown how this information can be deduced from the case of infinitesimal-amplitude oscillations. The bifurcation theory analysis reveals that when the bifurcation parameter is increased beyond a critical value at which the aerodynamic damping vanishes, new solutions representing finite amplitude periodic motions bifurcate from the previously stable steady motion. The sign of a simple criterion, cast in terms of aerodynamic properties, determines whether the bifurcating solutions are stable or unstable. For the pitching motion of flat-plate airfoils flying at supersonic/hypersonic speed and for oscillation of flaps at transonic speed, the bifurcation is subcritical, implying either the exchanges of stability between steady and periodic motion are accompanied by hysteresis phenomena, or that potentially large aperiodic departures from steady motion may develop.
The modern theory of second order phase transitions is very successful in calculating the critical exponents as an asymptotic expansion in powers of epsilon = 4 - D, the deviation of D = 3, the spatial dimension of the actual physical system from that of the abstract four-dimensional reference model. This remarkable mathematical 'tour de force' leaves unanswered, however, many fundamental questions concerning the exact nature of how the fluctuations interact. I discuss here some experiments which would help to further our understanding of the equilibrium critical properties. Especially promising would be a measurement of the temperature dependence of the turbidity very close to the critical point. This has the promise of determining the small and elusive but fundamentally important anomalous dimension exponent eta. I also review various ways of measuring the critical transport coefficients and point out some cases where ground based experiments may usefully supplement flight experiments.
Mathematical theory predicts that small changes in container shape or in contact angle can give rise to large shifts of liquid in a microgravity environment. This phenomenon was investigated in the Interface Configuration Experiment on board the USML-2 Space Shuttle flight. The experiment's "double proboscis" containers were designed to strike a balance between conflicting requirements of sizable volume of liquid shift (for ease of observation) and abruptness of the shift (for accurate determination of critical contact angle). The experimental results support the classical concept of macroscopic contact angle and demonstrate the role of hysteresis in impeding orientation toward equilibrium.
Mathematical theory predicts that small changes in container shape or in contact angle can give rise to large shifts of liquid in a microgravity environment. This phenomenon was investigated in the Interface Configuration Experiment on board the USMT,2 Space Shuttle flight. The experiment's "double proboscis" containers were designed to strike a balance between conflicting requirements of sizable volume of liquid shift (for ease of observation) and abruptness of the shift (for accurate determination of critical contact angle). The experimental results support the classical concept of macroscopic contact angle and demonstrate the role of hysteresis in impeding orientation toward equilibrium.
The concepts of 'frozen-in magnetic field lines' and 'field-line reconnection', which are frequently used in discussions of the theory of the magnetosphere, have been criticized by Alfven and Falthammar (1971), by Heikkila (1973), and by Alfven (1975). In the present paper, it is demonstrated that both concepts are unnecessary and often misleading. The frozen-in concept is shown to belong to the pseudo-plasma formalism which is useful only in special cases.
The results of an experimental investigation on the fracture behavior of unidirectional and multidirectional laminated graphite/epoxy composites is reported. Critical tensile fracture stresses for constant head rate uniaxial specimens containing single-edge notches, double-edge notches, and centrally located circular holes are presented. Results are reported for loads and notches at various angles to the fiber direction. Evidence of notch sensitivity and stable crack growth is presented. Self-similar crack growth is shown to occur for only a limited set of circumstances. Critical stresses are compared to the theories of Waddoups et al and Whitney et al. These comparisons are presented using isotropic and orthotropic stress concentration factors together with a finite width correction factor. The results tend to show that use of only isotropic correction factors allow good correlation between theory and experiment.
A filamentary composite spherical pressure vessel is modeled as a pseudoisotropic (or transversely isotropic) composite shell, with the effects of the liner and fill tubes omitted. Equations of elasticity, macromechanical and micromechanical formulations, and laminate properties are derived for the application of an internally pressured spherical composite vessel. Viscoelastic properties for the composite matrix are used to characterize time-dependent behavior. Using the maximum strain theory of failure, burst pressure and critical strain equations are formulated, solved in the Laplace domain with an associated elastic solution, and inverted back into the time domain using the method of collocation. Viscoelastic properties of HBFR-55 resin are experimentally determined and a Kevlar/HBFR-55 system is evaluated with a FORTRAN program. The computed reduction in burst pressure with respect to time indicates that the analysis employed may be used to predict the time-dependent response of a filamentary composite spherical pressure vessel.
An analytic solution is presented for a stratified fluid of arbitrary constant Richardson number. By computer aided analysis the perturbation fields, including that of the Richardson number can be calculated. The results of the linear analytic model were compared with nonlinear simulations, leading to the following conclusions: (1) the perturbations in the Richardson number field, when small, are produced primarily by the perturbations of the shear; (2) perturbations of in the Richardson number field, even when small, are not symmetric, the increase being significantly larger than the decrease (the linear analytic solution and the nonlinear simulations both confirm this result); (3) as the perturbations grow, this asymmetry increases, but more so in the nonlinear simulations than in the linear analysis; (4) for large perturbations of the shear flow, the static stability, as represented by N2, is the dominating mechanism, becoming zero or negative, and producing convective overturning; and (5) the convectional measure of linearity in lee wave theory, NH/U, is no longer the critical parameter (it is suggested that (H/u sub 0) (du sub 0/dz) takes on this role in a shearing flow).
The principle of minimum energy was applied to epitaxial interfaces to show the interrelationship beteen misfit, overgrowth thickness and misfit dislocation spacing. The low energy dislocation configurations were presented for selected interfacial geometries. A review of the interfacial energy calculations was made and a critical assessment of the agreement between theory and experiment was presented. Modes of misfit accommodation were presented with emphasis on the distinction between kinetic effects and equilibrium conditions. Two-dimensional and three-dimensional overgrowths were treated together with interdiffusion-modified interfaces, and several models of interfacial structure were treated including the classical and the current models. The paper is concluded by indicating areas of needed investigation into interfacial structure.
Mechanisms of initial oxidation of silicon for the formation of silicon dioxide have been investigated. The oxidation of silicon in an atomic oxigen plasma environment is found to exhibit two distinct and linear oxide growth curves for each of the plasma powers used in ashing (25, 50, and 100 watts). Data obtained indicate that the exponent to the pressure in the oxide growth rate formula changes from 1.4 + or - 0.1 to 0.7 + or - 0.1 as one crosses the critical thickness. These data contradict the theory predicting that this exponent should be 1 for both regimes. The activation energy for oxidation in the zone reaction regime is found to be 0.17 eV, in contrast to the published value of 1-2 eV for thermally grown oxides.
A detailed analysis of the radio spectrum across the lobes of Cygnus A is presented in order to critically test the synchroton spectral aging theory. The results are in good agreement with the jet model for powerful radio galaxies, involving particle acceleration at the hot spots and outflow into the radio lobes, with subsequent energy loss due to synchrotron radiation. The hot spot spectra are well represented by a spectral aging model involving continuous injection of relativistic particles. Both hot spots have spectral break frequencies around 10 GHz. An injection index of 0.5 is found for both hot spots, consistent with diffusive shock acceleration at a strong nonrelativistic shock in a Newtonian fluid. The LF hot spot emission spectrum falls below the injected power law. This effect is isolated to the hot spots, and is best explained by a low-energy cutoff in the particle distribution.
A method used by Tsien to derive similarity rules for hypersonic flows is utilized to derive Von Karman's similarity rules for transonic flows. At the lower limit of the transonic region of flow the theory yields a formula for the critical stream Mach numbers of a given family of symmetrical profiles. It is further shown that this formula can also be obtained by means of the Prandtl-Glauert small-perturbation method. Investigation of the behavior of the similarity parameter in the region where the thickness coefficient approaches zero and the critical stream Mach number approaches unity shows that it possesses a limiting value characteristic of the prescribed family of shapes.
Simulations of planar, premixed turbulent flames with heat release were used to study turbulent transport. Reynolds stress and Reynolds flux budgets were obtained and used to guide the investigation of important physical effects. Essentially all pressure terms in the transport equations were found to be significant. In the Reynolds flux equations, these terms are the major source of counter-gradient transport. Viscous and molecular terms were also found to be significant, with both dilatational and solenoidal terms contributing to the Reynolds stress dissipation. The BML theory of premixed turbulent combustion was critically examined in detail. The BML bimodal pdf was found to agree well with the DNS data. All BML decompositions, through the third moments, show very good agreement with the DNS results. Several BML models for conditional terms were checked using the DNS data and were found to require more extensive development.
Analysis of 746 new V-band observations of the RR Lyrae star AH Cam obtained during 1989 - 1992 clearly show that its light curve cannot be described by a single period. In fact, at first glance, the Fourier spectrum of the photometry resembles that of a double-mode pulsator, with peaks at a fundamental period of 0.3686 d and an apparent secondary period of 0.2628 d. Nevertheless, the dual-mode solution is a poor fit to the data. Rather, we believe that AH Cam is a single-mode RR Lyrae star undergoing the Blazhko effect: periodic modulation of the amplitude and shape of its light curve. What was originally taken to be the period of the second mode is instead the 1-cycle/d alias of a modulation sidelobe in the Fourier spectrum. The data are well described by a modulation period of just under 11 d, which is the shortest Blazhko period reported to date in the literature and confirms the earlier suggestion by Goranskii. A low-resolution spectrum of AH Cam indicates that it is relatively metal rich, with delta-S less than or = 2. Its high metallicity and short modulation period may provide a critical test of at least one theory for the Blazhko effect. Moskalik's internal resonance model makes specific predictions of the growth rate of the fundamental model vs fundamental period. AH Cam falls outside the regime of other known Blazhko variables and resonance model predictions, but these are appropriate for metal-poor RR Lyrae stars. If the theory matches the behavior of AH Cam for a metal-rich stellar model, this would bolster the resonance hypothesis.