Synthesis and Structural Investigation of Brightly Colored Organoammonium Violurates

Nine new organoammonium violurates [R¹R²R³NH][C₄H₂N₃O₄] [R¹ = R² = H, R³ = c‐C₃H₅ ( 2 ), R³ = tBu ( 3 ), R³ = adamantyl ( 4 ), R³ = C₆H₂Me₂‐4,5‐NH₂‐2 ( 5 ); R¹ = H, R² = R³ = Et ( 6 ), iPr ( 7 ); R¹ = H, R²/R³ = (–CH₂–)₄ ( 8 ); R¹ = R² = R³ = Et ( 9 ); R¹ = R² = Me, R³ = (CH₂)₂NMe₂ ( 10 )] were prepared by treatment of violuric acid ( 1 ) with a variety of primary, secondary, and tertiary amines. With the exception of orange 5 , all these violurate salts form bright blue or blue‐purple crystalline solids. The acidic triethylammonium violurate [NHEt₃]H[C₄H₂N₃O₄]₂ · H₂O ( 9a ) was isolated in the form of red‐violet, plate‐like crystals by the reaction of violuric acid hydrate with triethylamine in a molar ratio of 2:1 in ethanol. All compounds were fully characterized by their IR and NMR (¹H, ¹³C) data as well as elemental analyses. X‐ray crystal structures determinations of 2 , 7 , and 9a revealed supramolecular self‐assembly through networks of N–H ··· N and N–H ··· O hydrogen bonds in the crystalline state.     

Lyapunov, Bohl and Sacker-Sell Spectral Intervals for Differential-Algebraic Equations

Lyapunov and exponential dichotomy spectral theory is extended from ordinary differential equations (ODEs) to nonautonomous differential-algebraic equations (DAEs). By using orthogonal changes of variables, the original DAE system is transformed into appropriate condensed forms, for which concepts such as Lyapunov exponents, Bohl exponents, exponential dichotomy and spectral intervals of various kinds can be analyzed via the resulting underlying ODE. Some essential differences between the spectral theory for ODEs and that for DAEs are pointed out. It is also discussed how numerical methods for computing the spectral intervals associated with Lyapunov and Sacker-Sell (exponential dichotomy) can be extended from those methods proposed for ODEs. Some numerical examples are presented to illustrate the theoretical results.      

Hybrid URANS/LES Turbulence Simulation of Vortex Shedding Behind a Triangular Flameholder

In this study a detached eddy simulation (DES) model, which belongs to the group of hybrid URANS/LES turbulence models, is used for the simulation of vortex shedding behind a triangular obstacle. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS shear-stress transport (SST) model is used. In the other regions with higher grid resolution a LES model, which uses a transport equation for the turbulent subgrid energy, is applied. The DES model is first investigated for two standard test cases, namely decaying homogeneous isotropic turbulence and the backward facing step, respectively. For the decaying homogeneous isotropic turbulence test case the evolution of the energy spectra in wavenumber space for different times are studied for both the DES and a Smagorinsky type LES model. Different grid resolutions are analyzed with a special emphasis on the modeling constant connecting the filter length scale to the grid size. The results are compared to experimental data. The backward facing step test case is used to study the model behavior for a case with a transition region between a RANS modeling approach close to the wall and LES based modeling in the intense shear flow region. The final application is the simulation of the vortex shedding behind a triangular obstacle. First, the influence of the inlet condition formulation is studied in detail as they can have a significant influence especially for LES based models. Detailed comparisons between simulation and experiment for the flow structure past the obstacle and statistical quantities such as the shedding frequency are shown. Finally the additional temporal and spatial information provided by the DES model is used to show the predicted anisotropy of turbulence.     

Development and Analysis of Wall Models for Internal Combustion Engine Simulations Using High-speed Micro-PIV Measurements

The performance, efficiency and emissions of internal combustion (IC) engines are affected by the thermo-viscous boundary layer region and heat transfer. Computational models for the prediction of engine performance typically rely on equilibrium wall-function models to overcome the need for resolving the viscous boundary layer structure. The wall shear stress and heat flux are obtained as boundary conditions for the outer flow calculation. However, these equilibrium wall-function models are typically derived by considering canonical flow configurations, introducing substantial modeling assumptions that are not necessarily justified for in-cylinder flows. The objective of this work is to assess the validity of several model approximations that are commonly introduced in the development of wall-function models for IC-engine applications. This examination is performed by considering crank-angle resolved high-resolution micro-particle image velocimetry (µ-PIV) measurements in a spark-ignition direct-injection single cylinder engine. Using these measurements, the performance of an algebraic equilibrium wall-function model commonly used in RANS and LES IC-engine simulations is evaluated. By identifying shortcomings of this model, a non-equilibrium differential wall model is developed and two different closures are considered for the determination of the turbulent viscosity. It is shown that both wall models provide adequate predictions if applied inside the viscous sublayer. However, the equilibrium wall-function model consistently underpredicts the shear stress if applied in the log-layer. In contrast, the non-equilibrium wall model provides improved predictions of the near-wall region and shear stress irrespective of the wall distance and the piston location. By utilizing the experimental data, significant adverse pressure gradients due to the large vortical motion inside the cylinder (induced by tumble, swirl and turbulence) are observed and included in the non-equilibrium wall model to further improve the model performance. These investigations are complemented by developing a consistent wall heat transfer model, and simulation results are compared against the equilibrium wall-function model and Woschni’s empirical correlation.     

Simulation and Modeling the Mechanical Behavior of Textile Reinforced Composites by Combining the Binary Model and X-FEM

This paper presents a new efficient modeling strategy based on the combination of Binary Model and X–FEM. It is applied to represent the inner architecture of textile reinforced composites where the fabric is characterized by a complex geometry. Homogenization methods are used to compute the effective elastic material properties. Thereby, the discrete formulation of periodic boundary conditions is adapted. Finally, the results in terms of effective material properties reveal a good agreement with parameters obtained by experimental tests. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrogen Bonding in the Crystal Structures of New Imidazolium Triflimide Protic Ionic Liquids

Abstract  The synthesis and crystal structures of 1,3-diamino-2-methylimidazolium bis(trifluoromethylsulfonyl)imide (1), 1,3-dihydroxy-2-methylimidazolium bis(trifluoromethylsulfonyl)imide (2) and 1-(2-(diethylammonio)ethyl)-3-methylimidazolium bis(bis(trifluoromethylsulfonyl)imide) (4) are reported. The salts 1, 2 and 4 have melting points below 100 °C, the intermediate 1-(2-(diethylamino)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (3) is liquid at room temperature. Compound 1 is monoclinic, space group P2₁/n with a = 8.4979(4) ?, b = 12.2803(6) ?, c = 13.9400(7) ?, β = 93.086(4)°, and Z = 4. Compound 2 is monoclinic, space group P2₁/c with a = 7.6165(2) ?, b = 20.5323(8) ?, c = 9.7654(3) ?, β = 111.046(2)°, and Z = 4. Compound 4 is triclinic, space group with a = 8.5313(4) ?, b = 9.2157(4) ?, c = 20.5812(8) ?, α = 84.668(2)°, β = 83.738(2)°, γ = 63.096(2)°, and Z = 2. The ions in 1 build a network of N–H···O hydrogen bonds, in 2 they are linked to chains by O–H···N and bifurcated O–H···O hydrogen bonds, whereas in 4 they form pairs by N–H···O contacts. The triflimide anions adopt transoid conformations. Index Abstract  Short interionic contacts, conformational flexibility, and disorder phenomena were identified in the crystal structures of three new, low-melting, protic imidazolium triflimides.      

Attosecond photoelectron spectroscopy of electron tunneling in a dissociating hydrogen molecular ion

We demonstrate the potential of intense-field pump, attosecond probe photoelectron spectroscopy to monitor electron tunneling between the two protons during dissociative ionization of the hydrogen molecule, with attosecond temporal and Angstrom-scale spatial resolution.     

On the generalized Jacobi equation

The standard text-book Jacobi equation (equation of geodesic deviation) arises by linearizing the geodesic equation around some chosen geodesic, where the linearization is done with respect to the coordinates and the velocities. The generalized Jacobi equation, introduced by Hodgkinson in 1972 and further developed by Mashhoon and others, arises if the linearization is done only with respect to the coordinates, but not with respect to the velocities. The resulting equation has been studied by several authors in some detail for timelike geodesics in a Lorentzian manifold. Here we begin by briefly considering the generalized Jacobi equation on affine manifolds, without a metric; then we specify to lightlike geodesics in a Lorentzian manifold. We illustrate the latter case by considering particular lightlike geodesics (a) in Schwarzschild spacetime and (b) in a plane-wave spacetime.     

Stray field interaction of stacked amorphous tapes

In this study, magnetic cores made of amorphous rectangular tape layers are investigated. The quality factor Q of the tape material decreases rapidly, however, when stacking at least two tape layers. The hysteresis loop becomes non-linear, and the coercivity increases. These effects are principally independent of the frequency and occur whether tape layers are insulated or not. The Kerr-microscopy was used to monitor local hysteresis loops by varying the distance of two tape layers. The magnetization direction of each magnetic domain is influenced by the anisotropy axis, the external magnetic field and the stray field of magnetic domains of the neighboring tape layers. We found that crossed easy axes (as the extreme case for inclined axes) of congruent domains retain the remagnetization and induce a plateau of the local loop. Summarizing local loops leads to the observed increase of coercivity and non-linearity of the inductively measured loop. A high Q-factor can be preserved if the easy axes of stacked tape layers are identical within the interaction range in the order of mm.