Zur analytischen Behandlung des nichtlinearen aeroelastischen Galloping-Problems

Übersicht Es wird ein graphisch-numerisches Verfahren zur Lösung der nichtlinearen Galloping-Differential- gleichung dargelegt. Die Einzelheiten des Lösungsverfahrens werden beschrieben und an Hand eines typischen Beispiels näher erläutert.

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Summary A graphical-numerical method for the solution of the non-linear Galloping differential equation is presented. The details of the analytical procedure are discussed and explained by means of a typical example.

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Diese Arbeit entstand im Rahmen eines einjährigen Stipendiums der Alexander von Humboldt-Stiftung. Der Stipendiat Dr.-Ing. V. Manea ist Mitarbeiter im Institut de Mecanica Fluidelor si Constructii Aerospatiale, Bukarest.     

Akkommodationskoeffizienten von Edelgasen an Pt im Temperaturbereich von 80 K bis 450 K

Zusammenfassung Die Akkommodationskoeffizienten von He, Ne und Ar wurden von 80 K bis 450 K, die von Kr und Xe von 250 K bis 450 K an gasbedeckten und teilweise gereinigten Pt-Oberflächen gemessen. Zur Anwendung kam die stationäre Hitzdrahtmethode bei sehr niedrigen Drucken.Der Fehler in den Bestimmungen an der gasbedeckten Oberfläche wird auf 2–3% geschätzt. Die Messungen an der teilweise gereinigten Oberfläche wurden extrapoliert aus dem zeitlichen Anstieg des Akkommodationskoeffizienten nach einer Aufheizung des Hitzdrahtes auf 900 K. Diese Ergebnisse sind mit einem wesentlich höheren Fehler behaftet.

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Accommodation coefficients of noble gases on Pt-surfaces from 80 K up to 450 K

Accommodation coefficients of He, Ne and Ar have been measured from 80 K up to 450 K, those of Kr and Xe from 250 K up to 450 K on gas covered and partially cleaned Pt-surfaces by the stationary hot wire method at very low pressures.The error of determinations on gas covered surfaces is estimated to be about 2 or 3%. The measurements on partially cleaned surfaces were extrapolated from the increase of the accommodation coefficient with time after a heating of the Pt-wire up to 900 K. In this case the error will be considerably higher.

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Bezeichnungen A Apparaturkonstante - a_i Konstante - C_v Molwärme bei konstantem Volumen - F Oberfläche des Meßdrahtes aus Pt - f_i Druckexpansionsfaktor - g (T) Korrekturfunktion, die die thermomolekulare Druck-differenz in verschieden temperierten Teilen des Versuchsraumes berücksichtigt - I Heizstrom im Pt-Draht - l Länge des Pt-Drahtes - M Molmasse - p Druck - q_D vom Pt-Draht durch Drahtwärmeleitung abgeführte Leistung - q_E dem Pt-Draht zugeführte elektrische Leistung - q_G vom Pt-Draht durch Gaswärmeleitung abgeführte Leistung - q_s vom Pt-Draht durch Strahlung abgeführte Leistung - R allgemeine Gaskonstante - R_i Widerstand des Pt-Drahtes - r Radius des Pt-Drahtes - T Gastemperatur - t₀ Raumtemperatur - T Differenz der Temperatur des Meßdrahtes und der Gas-temperatur - t Zeit - t_A charakteristische Adsorptionszeit - V_i Volumina - z Koordinate in Längsrichtung des Pt-Drahtes - Akkommodationskoeffizient (AK) - ₀ AK der teilweise gereinigten Pt-Oberfläche - _G AK der gasbedeckten Pt-Oberfläche - mittlere freie Weglänge - _Pt Wärmeleitfähigkeit des Pt     

A photomechanics material for elastoplastic stress analysis

The object of this investigation was to develop a technique or method for elastoplastic stress analysis using the optical effects of transparent materials. Of paramount importance was the selection and characterization of a suitable model material. In particular, it was desirable that the material be able to undergo large plastic strains while, at the same time, exhibiting a suitable level of optical response. A mixture of flexible and rigid polyester resins was found suitable, i.e., the mixture exhibited large strains and good optical response. It was found that unload birefringence (fringe order immediately upon removal of load) could be used to determine strain for a uniaxial-stress field. In particular, it provided a means for evaluating stress- and strain-concentration factors. Comparisons with other methods showed that the proposed method was reliable and gave results that are similar to those by other means. The usefulness of the material and method for two- and three-dimensional problems awaits further study.     

Evaluation of the energy efficiency of heat addition upstream of the body in a supersonic flow

Criteria for evaluating the energy efficiency of heat addition upstream of the body in a supersonic gas flow are obtained. Based on the functional objectives of flying vehicles and the thermodynamic model of the process, estimates are obtained for missile and aircrafttype vehicles. The minimum Mach numbers at which heat addition upstream of the body is reasonable are evaluated. The increase in the flight range in the cruising regime for an aircrafttype vehicle and on the active trajectory for a missiletype vehicle is evaluated. Estimates for fuel economy in launching an aerospace plane into an Earth orbit are given. It is shown that a significant part of the fuel should be spent on producing energy for gas heating in order to obtain a noticeable effect. The minimum necessary efficiency" of conversion of the fuel energy into the gasheating energy is evaluated.     

Modulated Taylor-Couette Flow: Onset of Spiral Modes

The linear stability of the flow between concentric cylinders, with the inner cylinder rotating at a constant angular velocity and the outer cylinder with an angular velocity varying harmonically about a zero mean, is addressed. The bifurcations of the base state are analyzed using Floquet theory, paying particular attention to non-axisymmetric bifurcations which are dominant in significant regions of parameter space. In these regions the spiral modes of the unforced system become parametrically excited and dominant. This is typical behavior of parametrically forced extended systems, where some modes are stabilized, but others are simultaneously excited. The flow structure of the bifurcated states are examined in detail, paying particular attention to the dynamic implications of their symmetries, and in particular how and when subsequent period doublings are inhibited. Received 5 November 2001 and accepted 29 March 2002 Published online: 2 October 2002 RID="*" ID="*" This work was supported by NSF Grants INT-9732637 and CTS-9908599 (U.S.A.) and MCYT Grants PB97-0685 and BFM2001-2350 (Spain). Communicated by H.J.S. Fernando     

A new micromechanical formulation of martensite kinetics driven by temperature and/or stress

Summary Martensitic transformation behavior of alloys is studied under the arbitrary action of a thermal and/or a triaxial mechanical load-stress state by solving a transformation kinetic equation presented recently by the same authors. Numerical and analytical solutions reveal that the transformation behavior is almost path-independent. Lines of constant volume fraction of martensite are nearly parallel in the stress-temperature plane. Some new analytical formulae for martensitic transformation kinetics are presented.

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Eine neue mikromechanische Formulierung der Martensit-Kinetik unter Temperatur- und/order Spannungsbelastung

Übersicht Es wird das Verhalten der martensitischen Umwandlung von Legierungen unter beliebiger thermischer und/oder dreiachsiger mechanischer Spannungsbelastung untersucht, indem eine neue Gleichung der Umwandlungskinetik gelöst wird. Diese Gleichung wurde vor kurzem von denselben Autoren vorgestellt. Die numerischen und die analytischen Lösungen zeigen, daß das Transformationsverhalten nahezu pfadunabhängig ist. Die Linien gleicher Volumsfraktion von Martensit sind nahezu parallel im Spannungs-Temperatur-Diagramm. Es werden auch einige neue analytische Formeln für die Umwandlungskinetik von Martensit präsentiert.

     

Effect of an inhibitor on high-speed turbulent flames and the transition to detonation

The influence of an inhibitor (CF₃Br or Halon 1301) on the propagation of high-speed turbulent flames, quasi-detonations and the transition to detonation has been investigated for methane-air, propane-air and acetylene-air mixtures. The experiments are carried out in a 13 m tube (15 cm diameter) filled with regularly spaced orifice plates (blockage ratio of 0.39) to ensure rapid flame acceleration. In all cases, the addition of the inhibitor reduces the turbulent flame velocity and extinguishes the flame with sufficient inhibitor concentration (2.7% and 7.5% for methane-air and propane-air, respectively). For acetylene-air mixtures, the quasi-detonation speed is progressively reduced with increasing inhibitor concentration and eventually causes the failure of the quasi-detonation and transition back to a fast turbulent flame. The inhibitor also narrows the propagation limits in all cases. To elucidate the inhibition mechanism, detailed modelling of both the turbulent flame structure as well as the chemical kinetics are required.     

Determination of the complex shear modulus of structural adhesives using a doubly clamped sandwich beam

A method is described for calculating the complex shear modulus of structural adhesives from dynamic test data on a doubly clamped sandwich beam in which the adhesive is contained as a thin layer. The accuracy of the technique is validated by finite-element analysis. Values of modulus obtained using the doubly clamped beam technique are found to be in excellent agreement with values obtained from dynamic tests on bulk specimens of an epoxy adhesive.     

Non-linear creep damage under one-dimensional variable tensile stress

A non-linear damage relation, containing the axial strain history and a time integral over the stress history, is proposed for the case of one-dimensional time dependent tensile stress. Non-linear steady and transient creep terms are included in the axial strain relation, and elastic and creep Poisson's ratios are introduced into the lateral strain relation. Using these relations, complete damage solutions are obtained for the constant stress rate, step stress, relaxation and constant load tests. Observations are made concerning the associated rupture times.     

Corona-induced electrohydrodynamic instabilities in low conducting liquids

The rose-window electrohydrodynamic (EHD) instability has been observed when a perpendicular field with an additional unipolar ion injection is applied onto a low conducting liquid surface. This instability has a characteristic pattern with cells five to 10 times greater than those observed in volume instabilities caused by unipolar injection. We have used corona discharge from a metallic point to perform some measurements of the rose-window instability in low conducting liquids. The results are compared to the linear theoretical criterion for an ohmic liquid. They confirmed that the minimum voltage for this instability is much lower than that for the interfacial instability in high conducting liquids. This was predicted theoretically in the dependence of the critical voltage as a function of the non-dimensional conductivity. It is shown that in a non-ohmic liquid the rose window appears as a secondary instability after the volume instability.