Pair interaction lattice gas simulations: Flow past obstacles in two and three dimensions

Apart from the FCHC (face-centered hypercube), Nasilowski's pair interaction lattice gas (PI) is the only known lattice gas automaton for three-dimensional hydrodynamic simulations. Unfortunately, the viscosity of PI is not isotropic. In order to determine the degree anisotropy, we derive fluid dynamic equations for the regime of compressible viscid flow. From relaxation measurements of waves propagating in various directions we compute the physically relevant dissipation coefficients and compare our results with theoretical predictions. Although PI shows a high degree of anisotropy, we define the mean value of the dissipation tensor as effective shear viscosity. Using this value of v _eff ^2D =0.35, two-dimensional simulations of flow past a cylinder yield drag coefficients in quantitative agreement with wind tunnel measurements over a range of Reynolds numbers of 5–50. Three-dimensional simulations of flow past a sphere yield qualitative agreement with various references. A fit of the results to a semi-empirical curve provides an effective value of v _eff ^2D =0.21 for a range of Reynolds numbers from 0.19 to 40. In order to check for finite-size effects, we measured the mean free path and computed the Knudsen numbers. We obtained 1 lattice unit, corresponding to Kn=0.01 (2D) and Kn=0.1 (3D). We found no significant finite-size effects.     

Magnetic Resonance Imaging of Hydrodynamic Dispersion in a Saturated Porous Medium

By using nuclear magnetic resonance imaging (NMRI) we have been able to analyse dispersion at the microscopic scale during steady-state flow through water-saturated glass beads. The flow rate through the porous medium was chosen high enough in order to neglect the influence of molecular diffusion on dispersion. Velocity statistics were measured, by NMRI, within slices of increasing thickness perpendicular to the direction of flow. It took more than two bead diameters before a representative elementary volume (REV) for the mean velocity was reached. This was in a region in the middle of the column that was not influenced by the boundary conditions. There the velocity variance decreased exponentially as a function of the slice thickness, due we consider to the formation of an interconnecting streamline network. The exponential decrease in the velocity variance reflects the transition from a local pattern of stochastic–convective flow to a convective–dispersion regime at the scale of the REV. We found that the point-like preferential influx and efflux boundary condition increased velocity variances and thus enhanced longitudinal hydrodynamic dispersion. Using the transverse correlation length of longitudinal velocity variance, we derived a mean transverse dispersivity that agreed well with Saffmans (1959) model. So we have been able to provide for the first time a direct observation verification of a part of Saffmans (1959) conjectures. By NMRI we observed this value to be independent of the observation scale of the slice thickness.     

Throwing light on dark states of α-oligothiophenes of chain lengths 2 to 6: radical anion photoelectron spectroscopy and excited-state theory

In this work, we apply photodetachment photoelectron spectroscopy (PD-PES) on radical anions to access the lowest excited electronic states of neutral α-oligothiophenes nT (n = 2-6, where n denotes the number of thiophene rings) in the gas phase. Besides electron affinities, the spectra provide the energies of the T(1) and T(2) states which are otherwise difficult to investigate in neutral molecules due to spin selection rules. The assignment of the spectra is assisted by quantum chemical calculations using a combined density functional theory and multi-reference configuration interaction approach. For all α-oligothiophenes investigated in this work, the T(2) state is situated below S(1). In the gas phase, the S(1) state energies lie higher than in non-polar solution (0.2 to 0.4 eV). The geometry optimizations show that the S(0) state and especially the excited states gain planarity with increasing chain length. A non-planar structure or out-of-plane vibrational activity is needed to allow an efficient intersystem crossing (ISC) dynamics from S(1) to T(2), followed by internal conversion to T(1). Our theoretical calculations predict that in 6T a doubly excited state becomes nearly isoenergetic to S(1). This state is not observed by PD-PES, which is explained by the analysis of the calculated contributing electron configurations.     

Recent German Research on Periodical Unsteady Flow in Turbomachinery

The main source of flow unsteadiness in turbomachinery is the aerodynamical interaction of the rotor and stator blade rows. The blades and vanes, moving relatively to each other, interact because of the viscous wakes and the potential effects of the blades. In addition, the wakes and potential effects superimpose with other flow patterns, for instance the tip clearance vortices and other secondary flow phenomena. Furthermore in transonic compressors the interaction of wakes and shocks plays an important role. As a result, the real flow field is highly periodically unsteady and very complex, especially in multistage turbomachinery. Although this fact has received increasing attention within recent years, blade row interactions effects are not yet typically addressed in current design systems of turbomachinery. Actually, there is a requirement of the ability of modern design methods to predict unsteady flow features. With increasing aerodynamic loading of the blades and higher Mach numbers the consideration of rotor-stator-interactions gains in importance. It is therefore one of the challenges of the present and future design of compressors and turbines to include beneficial unsteady effects to improve the engine parameters. This requires a detailed physical understanding of the unsteady flow field and the resulting effects on the performance and flow stability. In 2000 the joint research program “Periodical Unsteady Flow in Turbomachinery” was initiated. Partners of this project are five research groups from four german universities: Technische Universität Berlin (Prof. Hourmouziadis), Universität der Bundeswehr München (Prof. Fottner, Prof. Pfitzner), Universität Karlsruhe: Institut für Thermische Strömungsmaschinen (Prof. Wittig, Dr. Dullenkopf), Institut für Hydromechanik (Prof. Rodi), Technische Universität Dresden (Prof. Vogeler), and a research group from the: German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt), (Prof. Weyer, Prof. Mönig). This 5-year program was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft) and coordinated by Professor Hourmouziadis (Technische Universität Berlin). The aim of this joint project is to contribute to an improved physical understanding of the periodical unsteady flow phenomena and to provide more reliable prediction methods of these complex flow conditions in turbomachinery. Selected aspects of flow unsteadiness in turbomachines were investigated with complementing experimental and numerical investigations. Different flow conditions of different complexity were investigated in detail. After a 3-year period of the project, first results of the research group are published in a special issue of the Journal of Flow, Turbulence and Combustion (Flow Turbulence Combust 69, 2002). After the end of the joint project, in the present paper selected results of each research group, which addresses different aspects of periodical unsteady flow in turbomachinery, are discussed. However, it is not the intention of the present paper to give a general survey on this field of research. The following topics are selected to provide insight into the work of the joint research group: (1) Experimental Investigation of Rotor-Stator-Interactions in an Axial Compressor, (2) Influence Of Periodically Unsteady Flow On The Boundary Layer Development Of A Highly Loaded Linear Turbine- And Compressor Cascade, (3) Flow Conditions on a Flat Plate under Oscillating Inlet Conditions, (4) Simultaneous Measurements of Flow and Heat Transfer in a Periodically Unsteady Flow, (5) Turbulence- and Transition Modelling for Unsteady RANS simulations, and (6) Direct Numerical Simulations of Transitional Flow in Turbine-Related Geometries.