Integrated-plane photoelastic method—Application of photoelastic isodynes

It has been shown by Pindera and Mazurkiewicz that a new type of scattered-light modulation in the plane of a two-dimensional photoelastic object can be obtained when the stationary integrated photoelastic method developed by Pindera and Straka is applied in a scanning mode and when the transfer function of the photoelastic system satisfies certain conditions. The new type of light modulation, called field of isodynes by the authors, carries information on stress components normal to the direction of propagation of primary beam, and on corresponding total-force component. The points where this stress component is equal to zero can be easily determined. The classical scattered-light modulation along a chosen line represents a cross section of a corresponding isodynes field. It is shown that these features of the method of isodynes make it possible to easily determine the distribution and values of normal stress components at any arbitrary rectilinear cross section, and to check immediately the accuracy of measurements. The experimental determination of contact stresses and contact regions using the method of isodynes is especially simple and elegant.     

Potentiostat for use in electro-chemical experiments

Summary In many electro-chemical experiments it is desirable that one of the current-carrying electrodes of an electrolytic bath has a constant potential. This may be realized by manual control of the bath current. Owing to the sudden and unpredictable potential fluctuations of the electrode concerned, this is very difficult and even impossible in some special cases, e.g. when passivating an Fe-electrode in aqueous H₂SO₄. In addition there may be a considerable time-lag between the potential and the current fluctuations. The different types of potentiostats described so far seem to have too large a time-lag or/and too low a sensitivity for this purpose. The authors describe a potentiostat with proportional and integrating properties resulting in a stabilization of the potential within 1/2 mV in favourable sections of the potential current characteristic. Even with an electrode with a very steep potential current characteristic (e.g. the above mentioned Fe-electrode) the ripple never exceeds a few mV. The bath current, the maximum value of which is 30 mA, is the differential current of two electronic tubes. This type of circuit offers various advantages.     

Measurements on a molecular beam apparatus for mass spectrometric study of reactions in flames

Summary The formation of a molecular beam in sampling 1 torr to 1 atm. gases (Ar or H₂) is studied for three diameters of the sampling orifice. To modulate the beam, use is made of a mechanical chopper placed very near the sampling orifice. This enables us to measure the scattering of the beam by collisions between beam molecules. From our measurements it follows that these collisions are very important for >0.020 mm at an initial pressure of 1 atm. We discuss the consequences of our results for mass spectrometric studies of flames.     

Approximate analytical solutions for solute transport in two-layer porous media

Mathematical models for transport in layered media are important for investigating how restricting layers affect rates of solute migration in soil profiles; they may also improve the analysis of solute displacement experiments. This study reports an (approximate) analytical solution for solute transport during steady-state flow in a two-layer medium requiring continuity of solute fluxes and resident concentrations at the interface. The solutions were derived with Laplace transformations making use of the binomial theorem. Results based on this solution were found to be in relatively good agreement with those obtained using numerical inversion of the Laplace transform. An expression for the flux-averaged concentration in the second layer was also obtained. Zero- and first-order approximations for the solute distribution in the second layer were derived for a thin first layer representing a water film or crust on top of the medium. These thin-layer approximations did not perform as well as the binomial solution, except for the first-order approximation when the Peclet number,P, of the first layer, was low (P<5). Results of this study indicate that the ordering of two layers will affect the predicted breakthrough curves at the outlet of the medium. The two-layer solution was used to illustrate the effects of dispersion in the inlet or outlet reservoirs using previously published data on apparatus-induced dispersion.The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

An investigation of interior ballistics ignition phase

An axisymmetric viscous two-phase model is presented which describes the transient combustion of granular propellants during the ignition phase of a ballistic charge. Details of the model are presented along with computational results for a low-pressure ballistic simulator. Predicted pressure time evolutions are compared with experimental data of a real test-firing in which an unexpected pressure excursion occurred. Gun propellant breakup effects, due to bed compaction, are taken into consideration to explain the discrepancies between the numerical and experimental results. Finally, a discussion is presented of the mechanisms by which the behavior of pressure waves can be strongly influenced and thus controlled by the manner in which the propelling charge is ignited.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Modelluntersuchungen des Wärmeaustausches und Strömungswiderstandes an den Heizflächen mit Diagonalflossen

This paper presents the results of an experimental study on the heat transfer and pressure drop in diagonally finned tube banks. The investigations have been carried out using the napthalene analogy. The performance of a heat exchanger with fins arranged a certain angle to the flow direction with the plain tube heat exchanger have been compared.     

A numerical investigation of transient detonation in granulated material

A two-phase model based upon principles of continuum mixture theory is numerically solved to predict the evolution of detonation in a granulated reactive material. Shock to detonation transition (SDT) is considered whereby combustion is initiated due to compression of the material by a moving piston. In particular, this study demonstrates the existence of a SDT event which gives rise to a steady two-phase Chapman-Jouguet (CJ) detonation structure consisting of a single lead shock in the gas and an unshocked solid; this structure has previously been independently predicted by a steady-state theory. The unsteady model equations, which constitute a non-strictly hyperbolic system, are numerically solved using a modern high-resolution method. The numerical method is based on Godunov's method, and utilizes an approximate solution for the two-phase Riemann problem. Comparisons are made between numerical predictions and known theoretical results for 1) an inert two-phase shock tube problem, 2) an inert compaction wave structure, and 3) a reactive two-phase detonation structure; in all cases, good agreement exists. Received 4 August 1995 / Accepted 17 February 1996     

Schnelle Berechnung der mittleren Temperaturdifferenz in luftgekühlten Kreuzstromwärmeaustauschern

Zusammenfassung Es wird eine bekannte, explizite Näherungsgleichung mit zehn neuen Sätzen von jeweils sechzehn empirischen Konstanten vorgestellt zur schnellen Berechnung der mittleren Temperaturdifferenz von nunmehr insgesamt achtzehn Kreuzstromführungen. Die mittlere Temperaturdifferenz wird aus der dimensionslosen Abkühlung des Prozeßstromes und der Zahl der Übergangseinheiten auf der Luftseite berechnet.

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Fast calculation of mean temperature difference in air-cooled cross-flow heat exchangers

A known approximate explicit equation together with ten new sets of sixteen empirical coefficients is presented for the fast calculation of the mean temperature difference of by now all together eighteen crossflow arrangements. The mean temperature difference is calculated from the effectiveness of the process stream and the number of transfer units on the air side.

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Bezeichnungen b_{i, K} Konstanten in Gl. (6) - c_p spezifische Wärmekapazität der Luft bei Konstanten Druck - Wärmekapazitätsstrom der Luft - F Austauschfläche, Fläche - f mittlerer absoluter Fehler von r - i, k Exponenten und Summenzähler in Gl. (6) - K Wärmedurchgangskoeffizient - m, n Maximalwerte von i bzw. k - p, q dimensionslose Temperaturänderungen beider Fluide, definiert durch Gln. (2) und (3) - r dimensionslose mittlere Temperaturdifferenz, definiert durch Gl. (1) - s Zahl der Übergangseinheiten (NTU) auf der Luftseite, definiert durch Gl. (4) - T Temperatur des Prozeßstroms - w Anströmgeschwindigkeit der Luft - z Zahl der Punkte - endliche Differenz - Dichte der Luft Indices I eine Rohrreihe - 1, 2 Eintritt, Austritt - a Anströmquerschnitt - Ge Gegenstrom - Gl Gleichstrom - M Mittelwert - max Maximalwert (nicht Mittelwert) Herrn Prof. em. Dr.-Ing. Dr.-Ing. E. h. H. Hausen zur Vollendung seines 85. Lebensjahres     

A theory of finite elastic consolidation

Presented in this paper is a general theory describing the consolidation of a porous elastic soil. The formulation allows for the occurrence of finite geometry changes and finite elastic strains during the consolidation process. The governing equations have been cast in a rate form and the laws which determine deformation and pore fluid flow, i.e. Hooke's law and Darcy's law, are presented in a frame indifferent manner. A numerical technique is described that provides an approximate solution to the governing equations. The theory and the solution technique are illustrated by several examples of practical interest.