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1.
Matthias Degroote 《The European physical journal. Special topics》2013,218(1):1-70
The Faddeev Random Phase Approximation (FRPA) is a Green’s function method which couples collective degrees of freedom to the single particle motion by resumming an infinite number of Feynman diagrams. The Faddeev technique is applied to describe the two-particle-one-hole (2p1h) and two-hole-one-particle (2h1p) Green’s function in terms of non-interacting propagators and kernels for the particle-particle (pp) and particle-hole (ph) interactions. This results in an equal treatment of the intermediary pp and ph channels. In FRPA both the pp and ph phonons are calculated on the random phase approximation (RPA) level. In this work the equations that lead to the FRPA eigenvalue problem are derived. The method is then applied to atoms, small molecules and the Hubbard model, for which the ground state energy and the ionization energies are calculated. Special attention is directed to the RPA instability in the dissociation limit of diatomic molecules and in the Hubbard model. Several solutions are proposed to overcome this problem. 相似文献
2.
Laurent De Moerloose Liesbeth Taelman Patrick Segers Jan Vierendeels Joris Degroote 《国际流体数值方法杂志》2019,89(6):181-195
Fluid-structure interaction (FSI) simulations are used extensively to calculate the vibration of structures subjected to an internal or external flow. In the case of partitioned FSI simulations, separate flow and structure solvers are used, which requires some kind of coupling between both. The time step in both solvers is typically taken the same, but this unnecessarily leads to long calculation times when the time step is small due to stability reasons in one of the two solvers. Subcycling, the procedure where the time step of one solver is chosen smaller than the time step used in the other solver, may reduce the computational cost of the FSI simulation. The subcycling procedure can be either explicit or implicit, the latter implying the use of coupling iterations in each time step. Contrary to explicit subcycling, no stability analyses of implicit subcycling schemes are found in the literature. In this paper, the temporal stability of the implicit subcycling procedure is investigated. The one-dimensional flow in an elastic cylindrical tube is studied analytically. The results of this analysis are subsequently compared to a partitioned two-dimensional axisymmetric FSI calculation with implicit coupling between the flow and structure solvers. 相似文献
3.
E. Degroote 《Journal of Thermal Analysis and Calorimetry》2007,87(1):149-151
The processes involved
in flame spreading over liquid fuels are subject of this work. A heat and
momentum transfer analysis has been undertaken for fuel temperatures below
the flash-point that confirms (within this range of temperatures studied in
this work) that flame spreading is assisted by a convection pattern ahead
of the flame. This assistance mechanism, which is not observed for solid fuels,
is the origin (for lower temperatures) of a pulsating behaviour of the flame.
A first experimental determination of the characteristic horizontal length
of this assistance zone will be given. The analysis of our data lead us to
conclude that flame spreading can be reduced by simultaneously preventing
the formation of the convection zone and reducing the fuel surface temperature. 相似文献
4.
Summary We have shown through M?ssbauer spectroscopy that the electric field gradient (EFG), associated with an atomic defect, is
sensitive to the bending of ultrathin Si crystals. The changes in the EFG depend on the bending direction. A direct application
is the derivation of the orientation of defects that are located in the crystal’s surface region. We have used this observation
for determining the configuration of Co dimers in Si. Based on the bending experiments, the pair axis is found to point in
the <110> direction with respect to the host lattice.
Paper presented at ICAME-95, Rimini, 10–16 September 1995. 相似文献
5.
E. Degroote 《Journal of Thermal Analysis and Calorimetry》2006,84(1):67-70
Fire safety on fuel containers
can be improved at its initial stage if flame spreading can be controlled.
Therefore, the understanding of the fundamental processes that control flame
spreading will help us to determine a few control parameters that could be
useful to improve security in fuel deposits. A series of experiments have
been conducted in different fuel containers that helped to understand the
basic mechanisms involved. A new phenomenon of convection ahead of the flame
is observed in liquid fuels that do not appear in solid fuels. Finally, two
control factors have been found useful to control fire spread: the initial
fuel surface temperature and the convection zone observed in front of the
flame. The first experimental results observed controlling these two factors
led flame to spreading velocities of order 1 cm s–1
and, in some cases, flame extinguishes. 相似文献
6.
Nagy D. L. Bottyán L. Deák L. Degroote B. Dekoster J. Leupold O. Major M. Meersschaut J. Rüffer R. Szilágyi E. Vantomme A. 《Hyperfine Interactions》2002,141(1-4):459-464
The off-specular (diffuse) nuclear resonant reflectivity of synchrotron radiation is a sensitive measure of the lateral autocorrelation
of the magnetisation in thin films and multilayers. The width of the diffuse scattering peak measured at an electronically
forbidden reflection is inversely proportional to the in-plane correlation length of the magnetisation direction. The average
size of the in-plane antiferromagnetic domains is determined in different states of the same Fe/Cr superlattice. The hyperfine
magnetic fields in coexisting small and large domains are measured independently.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
7.
Steady free surface flows are of interest in the fields of marine and hydraulic engineering. Fitting methods are generally used to represent the free surface position with a deforming grid. Existing fitting methods tend to use time-stepping schemes, which is inefficient for steady flows. There also exists a steady iterative method, but that one needs to be implemented with a dedicated solver. Therefore a new method is proposed to efficiently simulate two-dimensional (2D) steady free surface flows, suitable for use in conjunction with black-box flow solvers. The free surface position is calculated with a quasi-Newton method, where the approximate Jacobian is constructed in a novel way by combining data from past iterations with an analytical model based on a perturbation analysis of a potential flow. The method is tested on two 2D cases: the flow over a bottom topography and the flow over a hydrofoil. For all simulations the new method converges exponentially and in few iterations. Furthermore, convergence is independent of the free surface mesh size for all tests. 相似文献
8.
E. Degroote P.L. Garcia-Ybarra 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):381-386
Flame spreading over liquid ethanol has been experimentally characterized for ethanol for subflash temperatures, in two different
channels. Three different spreading regimes have been observed. A uniform region (with flame velocities close to 10 cm/s)
appears for values of the initial surface liquid temperature above a critical value . For values an oscillatory regime occurs. For very low temperatures, , a new uniform regime appears with slow propagation velocities (close to 1 cm/s). The critical point has been described as a Hopf bifurcation, while resembles a homoclinic connection.
Received 16 October 1998 and Received in final form 23 June 1999 相似文献
9.
10.
Iron layers (0.15-10 ML thick) deposited on Ag (1 0 0) substrates were investigated by conversion electron Mössbauer spectrometry over a broad temperature range. The layers were characterized by scanning tunneling microscopy. Different forms of the layers, depending on their thickness, were observed. Minimum roughness of the layers were found at 0.15 and 10 ML thickness values. The Mössbauer spectra showed systematic thickness dependence. At low thickness values, broad doublets were observed, while above 6 ML, magnetic split spectra appeared at room temperature. At low temperatures, magnetically split spectra appeared with parameter values characteristic of Fe-Ag and Fe-Fe atomic interactions. The hyperfine split spectra indicated magnetic anisotropy and an enhanced saturation hyperfine magnetic field of ?40 T. The latter value is the highest ever measured for iron in thin layers. 相似文献