Parametric shape optimization of biaxial tensile specimen

Common cruciform specimen for biaxial tensile testing of sheet moulding compound, take damage and finally fail in uniaxially loaded areas. When using these specimen, an observation of damage initialization and failure in biaxially loaded areas is, therefore, not possible. In this paper, a parametric shape optimization is described to find a more suitable specimen shape. The parametrization of the specimen is presented. Objective functions are introduced to measure the appropriateness of specimen. A weighted summation transfers the constraint multiobjective optimization problem into a constraint scalar-valued problem. Findings of experiments suggest that a specimen shape with straight, non-tapering arms and slits along the arms is reasonable. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A new type of visco-plastic flow curve observed in certain concentrated diblock copolymer solutions

A diblock copolymer of styrene and isoprene (hydrogenated) is dissolved in C₁₁/C₁₂ n-paraffins and o-xylene (or cyclohexane) mixtures. An unusual type of flow curve is obtained in certain ranges of composition and under certain conditions of preparation. With increasing shear rates, the flow curve is first of the Bingham type, followed by a sudden shear thickening at almost discrete rate of shear and then by a second branch of almost linear increase. Time independent and reproducible behaviour can be achieved. A linear model is proposed and the possible molecular arrangement that corresponds to this model is discussed.Partly presented at the Annual Meeting of the Deutsche Rheologische Gesellschaft, May 13–15, 1985 in Berlin, FRG     

Real gas effects on the prediction of ram accelerator performance

The analysis of ram accelerator performance is based on one-dimensional modelling of the flow process that propels the projectile. The conservation equations are applied to a control volume travelling with the projectile, and quasi-steady flow is assumed. To date the solution obtained, namely the generalized thrust equation, has been based on the ideal gas assumption. At the high level of pressure that is encountered during the ram accelerator process, this assumption cannot be regarded as adequate. Thus, a more appropriate equation of state (EOS) should be used instead. Depending upon the level of pressure, several equations of state are available for dense gaseous energetic materials. The virial type of EOS can be more or less sophisticated, depending upon the extent of complexity of the intermolecular modelling, and turns out to be totally appropriate for most gaseous explosive mixtures that have been investigated at moderate initial pressures, i.e., less than 10MPa. In the present case the Boltzmann EOS was applied. It is based on very simplified molecular interactions, which makes it relatively easy to use in calculations. Moreover, the energetic EOS needs to be taken into account. This concerns all the calorimetric coefficients, as well as the thermodynamic parameters, which can no longer be expressed as only a function of temperature. The higher the pressure level, the more sophisticated these corrections become, but the main relationships that account for real gas effects are basically the same. These include the use of a general form of analytical operators applied to correct the thermodynamic functions and coefficients. The equations governing the one-dimensional model were taken as a basis for the real gas corrections and were solved analytically. The parameters which play the most crucial roles in this correction can thus be highlighted. A complete set of equations involving the real gas effects are presented in this paper. The QUARTET code was used in this investigation, especially for determining chemical equilibrium compositions. This more accurate model can better predict the projectile acceleration of the thermally choked propulsive mode. Although the present analysis is applied to the fuel-rich methane-oxygen-nitrogen mixture currently used in the ram accelerator experiments, its general formulation makes it readily applicable to any other mixture. The projectile velocity and acceleration histories determined by the Hugoniot analysis for the thermally choked ram accelerator mode, assuming the Boltzmann EOS, turn out to be in much better agreement with experimental observations up to the CJ detonation velocity than that when based on the ideal gas assumption. Received 9 August 1996 / Accepted 23 May 1997     

Pulsatile flow in a straight circular pipe with reabsorption across the wall

Summary Velocity-, stress- und pressure distribution has been investigated for the flow of a Newtonian liquid in a circular cylindrical tube with reabsorption across the wall. For constant reabsorption the nonlinear steady flow case has been treated for small cross flow Reynoldsnumbers, while for pulsatile flow the flow behavior is presented for constant and exponentially decaying reabsorption across the wall.

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Übersicht Für die pulsierende Strömung einer Newtonschen Flüssigkeit mit Absaugung längs der kreiszylindrischen Wand werden Geschwindigkeits-, Druck- und Spannungsverteilungen bestimmt. Dabei wird sowohl lineares als auch exponentiell abnehmendes Absaugevolumen behandelt. Für konstantes Absaugen längs der Wand wird die nichtlineare Lösung für kleine Querstrom-Reynoldszahlen angegeben.

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Awardee of the U. S. Special Program of the Alexander von Humboldt-Stiftung.     

Experimental Characterization of Premixed Flame Instabilities of a Model Gas Turbine Burner

In recent years, the NO _x emissions of heavy duty gas turbine burners have been significantly reduced by introducing premixed combustion. These highly premixed burners are known to be prone to combustion oscillations. In this paper, investigations of a single model gas turbine burner are reported focusing on thermo-acoustic instabilities and their interaction with the periodic fluctuations of the velocity and pressure. Phase-locked optical measurement techniques such as LDA and LIF gave insight into the mechanisms.Detailed investigations of a gas turbine combustor rig revealed that the combustor as well as the air plenum oscillate in Helmholtz modes. These instabilities could be attributed to the phase lag of the pressure oscillations between the air plenum and the combustor, which causes an acceleration and deceleration of the air flow through the burner and, therefore, alternating patterns of fuel rich and lean bubbles. When these bubbles reach the reaction zone, density fluctuations are generated which in turn lead to velocity fluctuations and, hence, keep up the pressure oscillations.With increasing the equivalence ratio strong combustion oscillations could be identified at the same frequency. Similarly as with weak oscillations, Helmholtz mode pressure fluctuations are present but the resulting velocity fluctuations in the combustor can be described as a pumping motion of the flow. By the velocity fluctuations the swirl stabilization of the flame is disturbed. At the same time, the oscillating pressure inside the combustor reaches its minimum value. Shortly after the flame expands again, the pressure increases inside the combustor. This phenomenon which is triggered by the pressure oscillations inside the air plenum seems to be the basic mechanism of the flame instability and leads to a significant increase of the pressure amplitudes.     

Model of gas-coal medium with a sorption phenomenon

Constitutive equations of a gas-coal two-phase medium with a local mass exchange due to a sorption-desorption phenomenon are presented. The equations have been deduced taking advantages of the theory of mixtures. Parameters of the constitutive equations have been calculated on the basis of results of laboratory tests published in recent papers. The governing set of equations of the theory is introduced and results of a sample calculation are presented.     

The complex dispersion relation of surface plasmon polaritons at gold/para-hexaphenylene interfaces

Two-photon photoemission electron microscopy (2P-PEEM) is used to measure the real and imaginary part of the dispersion relation of surface plasmon polaritons at different interface systems. A comparison of calculated and measured dispersion data for a gold/vacuum interface demonstrates the capability of the presented experimental approach. A systematic 2P-PEEM study on the dispersion relation of dielectric-loaded gold surfaces shows how effective the propagation of surface plasmon polaritons at a gold/para-hexaphenylene interface can be tuned by adjustment of the dielectric film thickness. Deviations of the experimental results from effective index calculations indicate the relevance of thin film peculiarities arising from the details of the growth process and corroborate the need of experimental analysis techniques for dispersion relation measurements.