Two phase boundary layers in laminar film boiling

The laminar film boiling is analyzed by means of an integral procedure. The method treats the film boiling as a two phase boundary layer problem; thereby the effect of the interfacial shear on the heat transfer rate can be investigated. The problem is attacked by simultaneously solving the vapor and liquid boundary layer equations. An extensive comparison of the predicted results with the exact solutions substantiates the validity of the present integral procedure. Even the details of the velocity and temperature profiles turn out to be in close agreement with the exact solutions.     

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.     

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.     

On anti-symmetric waves in an unbounded piezoelectric plate with axisymmetric electrodes

In this paper we consider an unbounded piezoelectric plate with electrodes at its surfaces, which are axisymmetric with respect to an axis normal to the plate. The electromechanical waves in this plate are assumed to have an electric potential which is symmetric with respect to the middle plane of the plate. The electric field outside the plate will appear to be important. This external electric field is investigated, particularly in connection with the resonances of the plate.     

Bending stresses in out-of-round uni- and multi-axial internally pressurized strength specimens

A derivation is presented for the bending stresses in internally pressurized slightly elliptical tubular or ring specimens used in the testing of uni- and multiaxial strength. The solution obtained shows that the bending stresses are directly proportional to Young's modulus of elasticity of the material being tested. A numerical example applicable to strength testing of aluminum oxide demonstrates, that a slight out-of-round-ness can result in bending stresses which are an appreciable percentage of the observed values of the fracture strength. It is concluded that strength testing by means of the internal pressurization of tubular or ring specimens requires close dimensional control in specimen manufacture.     

Über die Anwendung von Liapunovs direkter Methode auf Stabilitätsprobleme kontinuierlicher,nichtkonservativer Systeme

Übersicht Es wird gezeigt, daß das klassische Energiekriterium für die Stabilität eines kontinuierlichen zweidimensionalen Systems eine eingeschränkte Aussagekraft hat, da es die Kleinheit der interessierenden Größen meist im quadratischen Mittel sichert. Ferner ergibt es sich, daß das klassische Energiekriterium für gewisse nichtkonservative Probleme, bei denen mitgehende Lasten vorkommen, hinreichend bleibt. Für diese nichtkonservativen Probleme ist die Stabilität unter tangential mitgehenden Lasten zudem größer als sie es bei denselben Problemen unter konservativen, richtungstreuen Lasten ist.

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Summary It is shown that the classical energy criterion for the stability of a continuous two-dimensional system is of a restricted value, since mostly the smallness in the mean-square of deflection and its derivatives can be guaranteed. Furthermore it results that the classical energy criterion for stability remains sufficient for certain nonconservative problems with follower forces. Besides, such nonconservative systems, subject to tangential forces, prove to be more stable as the same systems subject to conservative, unidirectional forces.

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Herrn Professor Dr. H. Ziegler in tiefer Verehrung zum 60. Geburtstag gewidmet.

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Die Arbeit wurde vom National Research Council of Canada unter Grant No. A 7297 unterstützt.     

„Fouling“-Vorgänge bei der Verdampfung von Argon im waagerecht durchströmten Rohr

Zusammenfassung Bei Messungen des Wärmeübergangs beim Strömungssieden von Argon wurde ein zeitlicher Anstieg der Wandtemperatur beobachtet, während die Einstellparameter Wärmestromdichte, Massenstromdichte, Dampfgehalt und Systemdruck konstant blieben. Dieser Temperaturanstieg endete nach mehreren Stunden bei einer beträchtlich höheren Wandtemperatur oder mit dem Eintreten der Siedekrise. Als Ursache für dieses Phänomen wurde die Ablagerung von im Argon gelöstem CO₂ an der Heizfläche ermittelt, wobei zu beachten ist, daß die Reinheit des verwendeten Argons größer als 99,996 vol-% war. In weiteren Versuchen wurde der Einfluß der CO₂-Konzentration auf diesen Fouling-Vorgang bestimmt und ein Verfahren entwickelt, um das CO₂ aus dem Meßkreislauf zu entfernen. Der Ablagerungsmechanimus und der Einfluß der Betriebsparameter auf die Ablagerungsgeschwindigkeit können mit Modellvorstellungen zur Blasenbildung erklärt werden.

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Fouling-phenomena during the evaporation of argon within a horizontal tube

During heat transfer measurements at flow boiling of argon a transient increase of the wall temperature has been observed, though the parameters such as heat flux, mass velocity, flow quality and system pressure were kept constant. This temperature drifting ended after several hours at a considerably higher wall temperature or with the boiling crisis. Although the purity of the test substance was higher than 99.996 vol-%, this phenomenon was caused by the solidification of CO₂ on the heating surface. Further investigations showed the influence of the CO₂-concentration on the drifting as well as a method to remove the CO₂ out of the test fluid. Both, fouling mechanism and influence of the parameters on the fouling, can be explained satisfactorily by means of bubble-growth mechanisms.

     

Arguments for and against Engesser's buckling formulas

Summary To account for shear in buckling pioblems, one uses either the approach of Engesser or the socalled modified approach, proposed by Haringx in connection with helical springs. Both are based on onedimensional reasoning. While most authors seem to prefer the second method, Nänni [4] has shown by means of the three-dimensional theory of elasticity that, for bars, the first one is superior. The present paper is based on a more fundamental one-dimensional approach and confirms Nänni's and hence Engesser's results. It also shows that, together with shear, shortening of the bar must be taken into account, and it improves Engesser's formulas accordingly. It finally shows why Haringx' approach is correct for springs.

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Argumente für und gegen Engessers Knickformeln

Übersicht Um bei Knickproblemen den Schub zu berücksichtigen, verwendet man entweder das Engessersche oder das sogenannte modifizierte Verfahren, das von Haringx zur Behandlung von Schraubenfedern vorgeschlagen worden ist. Während die meisten Autoren die zweite Methode vorzuziehen scheinen, hat Nänni [4] mittels der dreidimensionalen Elastizitätstheorie gezeigt, daß bei Stäben die erste überlegen ist. Die vorliegende Arbeit stützt sich auf ein fundamentaleres eindimensionales Verfahren und bestätigt Nännis und damit Engessers Ergebnisse. Sie zeigt auch, daß mit dem Schub zusammen die Verkürzung des Stabes zu berücksichtigen ist, und sie verbessert Engessers Formeln dementsprechend. Schließlich zeigt sie, weshalb das Verfahren von Haringx bei Federn korrekt ist.

     

Time resolved measurements of the flow generated by suction feeding fish

The majority of aquatic vertebrates are suction feeders: by rapidly expanding the mouth cavity they generate a fluid flow outside of their head in order to draw prey into their mouth. In addition to the biological relevance, the generated flow field is interesting fluid mechanically as it incorporates high velocities, is localized in front of the mouth, and is unsteady, typically lasting between 10 and 50 ms. Using manometry and high-speed particle image velocimetry, this is the first study to quantify pressure within and outside the mouth of a feeding fish while simultaneously measuring the velocity field outside the mouth. Measurements with a high temporal (2 ms) and spatial (<1 mm) resolution were made for several feeding events of a single largemouth bass (Micropterus salmoides). General properties of the flow were evaluated, including the transient velocity field, its relationship to pressure within the mouth and pressure at the prey. We find that throughout the feeding event a relationship exists for the magnitude of fluid speed as a function of distance from the predator mouth that is based on scaling the velocity field according to the size of the mouth opening and the magnitude of fluid speed at the mouth. The velocity field is concentrated within an area extending approximately one mouth diameter from the fish and the generated pressure field is even more local to the mouth aperture. Although peak suction pressures measured inside the mouth were slightly larger than those that were predicted using the equations of motion, we find that these equations give a very accurate prediction of the timing of peak pressure, so long as the unsteady nature of the flow is included.