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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 10 (1998), S. 958-973 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The description of turbulent mixing and chemical reactions by Lagrangian probability density function methods offers some significant advantages over other methods, mainly due to the simulation of mixing processes and the exact treatment of chemical transformations. A key problem of such methods is the information on the time scales of processes, because they determine the dynamics and intensity of mixing. This question is considered for stratified flow. Different models are presented for the development of these time scales in time and their stationary spatial patterns in dependence on shear and stratification. The model predictions are shown to be in agreement with large-eddy simulations of stratified homogeneous shear flow. Two further applications of these models are considered: the description of transitions between flow regimes (characterized by different scaling quantities) in the stationary atmospheric surface layer and, second, the simulation of buoyant plume rise. It is shown that the predictions of the stationary frequency model agree with measured data. The consideration of limit cases of this model leads to connections between second-order closure parameters and (critical) flow numbers that characterize these transitions. These relationships are shown to be very advantageous for the application of closure models. A new flow number that characterizes the transition to free convective flow under unstable stratification is introduced here in analogy to the critical gradient Richardson number, which characterizes the onset of turbulence in stably stratified flow. The second application provides a new theory for buoyant plume rise. Two parameters that describe the turbulent mixing in the entrainment and extrainment stages of plume rise are explained as ratios of the relevant time scales. The two-thirds power law of buoyant plume rise, which is observed for nonturbulent and neutrally stratified flow, is obtained without having to make ad hoc assumptions. For turbulent flow, the plume's leveling-off is calculated in accord with measurements. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 703-716 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Linear and nonlinear Lagrangian equations are derived for stochastic processes that appear as solutions of the averaged hydrodynamic equations, since their moments satisfy the budgets given by these equations. These equations include the potential temperature, so that non-neutral flows can be described. They will be compared with nonlinear and non-Markovian equations that are obtained using concepts of nonequilibrium statistical mechanics. This approach permits the description of turbulent motion and buoyancy, where memory effects and driving forces with arbitrary colored noise may occur. The equations depend on assumptions that concern the dissipation and pressure redistribution. In the approximations of Kolmogorov and Rotta for these terms, the dissipation time scale remains open, which can be determined by the calculation of the production–dissipation ratio of turbulent kinetic energy. The features of these equations are illustrated by the calculation of turbulent states in the space of invariants. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Boundary layer meteorology 81 (1996), S. 147-166 
    ISSN: 1573-1472
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Non-Gaussianity effects, first of all the influence of the third and fourth moments of the velocity probability density function, have to be assessed for higher-order closure models of turbulence and Lagrangian modelling of turbulent dispersion in complex flows. Whereas the role and the effects of the third moments are relatively well understood as essential for the explanation of specific observed features of the fully developed convective boundary layer, there are indications that the fourth moments may also be important, but little is known about these moments. Therefore, the effects of non-Gaussianity are considered for the turbulent motion of particles in non-neutral flows without fully developed convection, where the influence of the fourth moments may be expected to be particularly essential. The transport properties of these flows can be characterized by a diffusion coefficient which reflects these effects. It is shown, for different vertical velocity distributions, that the intensity of turbulent transport may be enhanced remarkably by non-Gaussianity. The diffusion coefficient is given as a modification of the Gaussian diffusivity, and this modifying factor is found to be determined to a very good approximation by the normalized fourth moment of the vertical velocity distribution function. This provides better insight into the effect of fourth moments and explains the varying importance of third and fourth moments in different flows.
    Type of Medium: Electronic Resource
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  • 4
    Title: ¬The¬ Online Target Date Assignment Problem /; ZIB-Report 05-61
    Author: Heinz, Stefan
    Contributer: Krumke, Sven O. , Megow, Nicole , Rambau, Jörg , Tuchscherer, Andreas , Vredeveld, Tjark
    Publisher: Berlin :Konrad-Zuse-Zentrum für Informationstechnik,
    Year of publication: 2005
    Pages: 16 S.
    Series Statement: ZIB-Report ZIB-Report 05-61
    ISSN: 1438-0064
    Type of Medium: Book
    Language: English
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  • 5
    Title: LP-Based Local Approximation for Markov Decision Problems /; ZIB-Report 06-20
    Author: Heinz, Stefan
    Contributer: Kaibel, Volker , Peinhardt, Matthias , Rambau, Jörg , Tuchscherer, Andreas
    Publisher: Berlin :Konrad-Zuse-Zentrum für Informationstechnik,
    Year of publication: 2006
    Pages: 16 S.
    Series Statement: ZIB-Report ZIB-Report 06-20
    ISSN: 1438-0064
    Type of Medium: Book
    Language: English
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  • 6
    Book
    Book
    Berlin :Konrad-Zuse-Zentrum für Informationstechnik,
    Title: Competitive Online Multicommodity Routing /; ZIB-Report 06-27
    Author: Harks, Tobias
    Contributer: Heinz, Stefan , Pfetsch, Marc. E.
    Publisher: Berlin :Konrad-Zuse-Zentrum für Informationstechnik,
    Year of publication: 2006
    Pages: 16 S.
    Series Statement: ZIB-Report ZIB-Report 06-27
    ISSN: 1438-0064
    Type of Medium: Book
    Language: English
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  • 7
    Publication Date: 2020-08-05
    Keywords: ddc:080
    Language: English
    Type: masterthesis , doc-type:masterThesis
    Format: application/pdf
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  • 8
    Publication Date: 2020-08-05
    Description: Testing is the process of stimulating a system with inputs in order to reveal hidden parts of the system state. In the case of non-deterministic systems, the difficulty arises that an input pattern can generate several possible outcomes. Some of these outcomes allow to distinguish between different hypotheses about the system state, while others do~not. In this paper, we present a novel approach to find, for non-deterministic systems modeled as constraints over variables, tests that allow to distinguish among the hypotheses as good as possible. The idea is to assess the quality of a test by determining the ratio of distinguishing (good) and not distinguishing (bad) outcomes. This measure refines previous notions proposed in the literature on model-based testing and can be computed using model counting techniques. We propose and analyze a greedy-type algorithm to solve this test optimization problem, using existing model counters as a building block. We give preliminary experimental results of our method, and discuss possible improvements.
    Keywords: ddc:510
    Language: English
    Type: reportzib , doc-type:preprint
    Format: application/pdf
    Format: application/postscript
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  • 9
    Publication Date: 2020-08-05
    Description: Constraint Integer Programming (CIP) is a generalization of mixed-integer programming (MIP) in the direction of constraint programming (CP) allowing the inference techniques that have traditionally been the core of \P to be integrated with the problem solving techniques that form the core of complete MIP solvers. In this paper, we investigate the application of CIP to scheduling problems that require resource and start-time assignments to satisfy resource capacities. The best current approach to such problems is logic-based Benders decomposition, a manual decomposition method. We present a CIP model and demonstrate that it achieves performance competitive to the decomposition while out-performing the standard MIP and CP formulations.
    Language: English
    Type: reportzib , doc-type:preprint
    Format: application/pdf
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  • 10
    Publication Date: 2022-03-14
    Description: Pseudo-Boolean problems lie on the border between satisfiability problems, constraint programming, and integer programming. In particular, nonlinear constraints in pseudo-Boolean optimization can be handled by methods arising in these different fields: One can either linearize them and work on a linear programming relaxation or one can treat them directly by propagation. In this paper, we investigate the individual strengths of these approaches and compare their computational performance. Furthermore, we integrate these techniques into a branch-and-cut-and-propagate framework, resulting in an efficient nonlinear pseudo-Boolean solver.
    Keywords: ddc:510
    Language: English
    Type: reportzib , doc-type:preprint
    Format: application/pdf
    Format: application/postscript
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