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81.
Asghar Ghorbani Gerd Steinhilber Detlev Markus Ulrich Maas 《Combustion Theory and Modelling》2013,17(2):188-222
In this paper, a new formulation of the projection approach is introduced for stand-alone probability density function (PDF) methods. The method is suitable for applications in low-Mach number transient turbulent reacting flows. The method is based on a fractional step method in which first the advection–diffusion–reaction equations are modelled and solved within a particle-based PDF method to predict an intermediate velocity field. Then the mean velocity field is projected onto a space where the continuity for the mean velocity is satisfied. In this approach, a Poisson equation is solved on the Eulerian grid to obtain the mean pressure field. Then the mean pressure is interpolated at the location of each stochastic Lagrangian particle. The formulation of the Poisson equation avoids the time derivatives of the density (due to convection) as well as second-order spatial derivatives. This in turn eliminates the major sources of instability in the presence of stochastic noise that are inherent in particle-based PDF methods. The convergence of the algorithm (in the non-turbulent case) is investigated first by the method of manufactured solutions. Then the algorithm is applied to a one-dimensional turbulent premixed flame in order to assess the accuracy and convergence of the method in the case of turbulent combustion. As a part of this work, we also apply the algorithm to a more realistic flow, namely a transient turbulent reacting jet, in order to assess the performance of the method. 相似文献
82.
83.
Chunkan Yu Paola Breda Michael Pfitzner Ulrich Maas 《Proceedings of the Combustion Institute》2021,38(2):2645-2653
For general reacting flows the numerical simulation faces two main challenges. One is the high dimensionality and stiffness of the governing conservation equations due to detailed chemistry, which can be solved by using simplified chemical kinetics. The other one is the difficulty of modeling the coupling of turbulence with thermo-chemical source term. The probability density function (PDF) method allows to calculate turbulent reacting flows by solving the thermal-chemical source term in closed form. Usually, the PDF method for turbulent processes such as mixing processes and the reduction method for chemical kinetics are developed separately. However, coupling of both processes plays an important role for the numerical accuracy. To investigate the importance of coupling between turbulence and simplified chemistry, two different coupling strategies for mixing and reduced chemistry are discussed and tested for the well-known Sandia Flames E and F, in which there is a strong interaction between turbulence and chemical kinetics. The EMST mixing model is chosen for turbulent mixing, while the Reaction-Diffusion Manifolds (REDIMs) is used as simplified chemistry. However, the proposed strategies are also valid for other mixing models and manifold based simplified chemistry. 相似文献
84.
85.
W. E. J. R. Maas C. H. M. Papavoine W. S. Veeman G. H. Werumeus Buning J. M. J. Vankan 《Journal of Polymer Science.Polymer Physics》1994,32(5):785-789
A previously published new solid-state nuclear magnetic resonance (NMR) method is applied to the interdiffusion of poly(methacrylate) (PMMA) and poly(vinylidene fluoride) (PVF2) above their Tg. Via a variation of the cross-polarization technique magnetization is transferred from protons to fluorines. When this magnetization is made to disappear at the fluorine sites, only those protons that are distant from fluorines greater than the distance over which cross-polarization functions will retain their magnetization. In this way we detect the fraction of PMMA near (ca. 20 Å) PVF2. Starting from sheets of PMMA and PVF2, which are then heated at 190°C for a variable time, and applying the above technique, we can determine the fractions of PMMA and PVF2 that have diffused within a distance of a few Å of each other. The intrinsic diffusion coefficients of PMMA and PVF2 determined from such experiments compare well with literature data. Initial attempts to fit the experimental data suggest that the concentration dependence of the diffusion coefficients cannot be neglected. © 1994 John Wiley & Sons, Inc. 相似文献
86.
87.
S. Lederer H. Winter HP. Winter F. Aumayr 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(3):505-511
The energy loss of hydrogen atoms with energies of 400
eV and 1 keV is studied in coincidence with the number of emitted
electrons during grazing scattering from atomically clean and flat
KI(001) and LiF(001) surfaces. The energy loss spectra for
specific numbers of emitted electrons are analyzed in terms of a
binary interaction model based on the formation of transient
negative ions via local capture of valence band electrons from
anion sites. Based on computer simulations we derive for this
interaction scenario probabilities for the production of surface
excitons, for electron loss to the conduction band of KI, for
emission of electrons, and for formation of negative hydrogen
ions. The pronounced differences of data obtained for the two
surfaces are attributed to the different electronic structures of
KI and LiF. 相似文献
88.
Towards wafer-scale integration of high repetition rate passively mode-locked surface-emitting semiconductor lasers 总被引:3,自引:0,他引:3
D.?Lorenser H.J.?UnoldEmail author D.J.H.C.?Maas A.?Aschwanden R.?Grange R.?Paschotta D.?Ebling E.?Gini U.?Keller 《Applied physics. B, Lasers and optics》2004,79(8):927-932
One of the most application-relevant milestones that remain to be achieved in the field of passively mode-locked surface-emitting semiconductor lasers is the integration of the semiconductor absorber into the gain structure, enabling the realization of ultra-compact high-repetition-rate laser devices suitable for wafer-scale integration. We have recently succeeded in fabricating the key element in this concept, a quantum-dot-based saturable absorber with a very low saturation fluence, which for the first time allows stable mode locking of surface-emitting semiconductor lasers with the same mode areas on gain and absorber. Experimental results at high repetition rates of up to 30 GHz are shown. PACS 42.55.Px; 42.60.Fc; 42.82.Gw 相似文献
89.
90.
Formation of Organosilicon Compounds. 79. NMR-Spectroscopical Investigation of 1,3,5-Trisilacyclohexanes and 1,3,5,7-Tetrasilaadamantanes For several groups of isomeric 1,3,5-trisilacyclohexanes and 1,3,5,7-tetrasilaadamantanes, structure assignment and conformation analysis of given by elucidation of their 1H-NMR spectra. 相似文献