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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2083-2085 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The concept of eddy viscosity is generalized to include an "eddy β-term'' that accounts for the effect of unresolved turbulence and Rossby waves on the resolved modes in the subgrid-scale representation of β-plane turbulence.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2104-2116 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The hydrodynamic force on a body that undergoes translational acceleration in an unbounded fluid at low Reynolds number is considered. The results extend the prior analysis of Lovalenti and Brady [to appear in J. Fluid Mech. (1993)] for rigid particles to drops and bubbles. Similar behavior is shown in that, with the inclusion of convective inertia, the long-time temporal decay of the force (or the approach to steady state) at finite Reynolds number is faster than the t−1/2 predicted by the unsteady Stokes equations.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2229-2233 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The instability of the von Kármán vortex streets and the existence of a global Lyapunov function at the special aspect ratio h/l=(1/π)sinh−1(1), are some of the difficulties with the well-known von Kármán model. By consistently applying the principle of genericity, its shown that a new family of near-equilibrium periodic solutions of the von Kármán model for aspect ratios near 0.281... supplies numerous theoretical candidates for observed vortex trails. This set of solutions implies that there is no global Lyapunov functions when h/l≠(1/π)sinh−1(1) which in turn leads to a rich variety of near-equilibrium solutions for the model.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2298-2300 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Disturbance structures that achieve maximum growth over a specified interval of time have recently been obtained for unbounded constant shear flow making use of closed-form solutions. Optimal perturbations have also been obtained for the canonical bounded shear flows, the Couette, and plane Poiseuille flows, using numerical solution of the linearized Navier–Stokes equations. In this note it is shown that these optimal perturbations have similar spectra and structure indicating an underlying universality of shear flow dynamics that is not revealed by traditional methods based on modal analysis.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2301-2303 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental data on the mean axial velocity and the intensity of the radial and axial velocity components on the axis of counterflowing turbulent streams are compared with a previously published Reynolds stress theory. Agreement is found to be satisfactory.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 2306-2308 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Smagorinsky subgrid model is revised to properly account for grid anisotropy, using energy equilibrium considerations in isotropic turbulence. For moderate resolution anisotropies, Deardorff's estimate involving an equivalent grid scale Δeq=(Δ1Δ2Δ3)1/3 is given a rigorous basis. For more general grid anisotropies, the Smagorinsky eddy viscosity is recast as νT=[csΔeqf(a1, a2)]2||S˜||, where f(a1,a2) is a function of the grid aspect ratios a1 and a2, and ||S˜|| is the resolved strain rate magnitude. The asymptotic behavior of νT at several limits of the aspect ratios are examined. Approximation formulas are developed so that f(a1,a2) can easily be evaluated in practice, for arbitrary values of a1 and a2. It is argued that these results should be used in conjunction with the dynamic model of Germano et al. whenever the anisotropy of the test-filter differs significantly from that of the basic grid.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1879-1882 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An expanding spherical gas bubble moves in an incompressible, viscous fluid under gravity. The nonlinear equations of motion are solved numerically. The growth of the radius of the bubble, the velocity of its center, and the drag experienced by the bubble are obtained with the progress of time. The gravity is found to influence the results significantly.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1914-1927 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thermocapillary convection is studied in two immiscible liquid layers with one free surface, one liquid/liquid interface, and differential heating applied parallel to the interfaces. An analytical solution is introduced for infinite horizontal layers. The defining parameter for the flow pattern is λ, the ratio of the temperature coefficient of the interfacial tension to that of the surface tension. Four different flow patterns exist under zero gravity conditions. "Halt'' conditions which halt the fluid motion in the lower encapsulated liquid layer have been found. A numerical experiment is carried out to study effects of vertical end walls on the double layer convection in a two-dimensional cavity. The halt condition obtained from the analytical study is found to be valid in the limit of small Reynolds numbers. The flow in the encapsulated liquid layer can be suppressed substantially.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1933-1938 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: How to describe vorticity creation from a moving wall is a long standing problem. This paper discusses relevant issues at the fundamental level. First, it is shown that the concept of "vorticity flux due to wall acceleration'' can be best understood by following fluid particles on the wall rather than observing the flow at fixed spatial points. This is of crucial importance when the time-averaged flux is to be considered. The averaged flux has to be estimated in a wall-fixed frame of reference (in which there is no flux due to wall acceleration at all); or, if an inertial frame of reference is used, the generalized Lagrangian mean (GLM) also gives the same result. Then, for some simple but typical configurations, the time-averaged vorticity flux from a harmonically oscillating wall with finite amplitude is analyzed, without appealing to small perturbation. The main conclusion is that the wall oscillation will produce an additional mean vorticity flux (a fully nonlinear streaming effect), which is partially responsible for the mechanism of vortex flow control by waves. The results provide qualitative explanation for some experimentally and/or computationally observed phenomena.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1952-1962 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tracer dispersion is studied in a plane fracture geometry with a radial flow between closely spaced parallel walls with either two smooth surfaces or one smooth and one rough surface. An echo dispersion technique in which the fluid is first injected into the fracture during a time Tinv and then pumped back through a detector is used in the experiments and is complemented by Monte-Carlo-type numerical simulations. For the smooth wall case, the Taylor dispersion mechanism is dominant when longitudinal molecular diffusion is negligible: it is verified numerically and experimentally that its transition to irreversibility only depends on the ratio of Tinv to the transverse diffusion time τm across the fracture thickness. In addition, the variation of Δ¯T¯2/(4Tinvτm) with respect to Tinv/τm is the same as for a flow of parallel geometry (Δ¯T¯2 being the mean square deviation of the transit time). Longitudinal molecular diffusion increases the global dispersion like Tinv/(PeQτm) at low Péclet numbers PeQ and long times due to the longitudinal velocity gradient. When one of the walls is rough, one expects to have a geometrical dispersion locally proportional to the velocity. A corresponding linear variation of Δ¯T¯2/(4Tinvτm) with respect to (square root of)Tinv/PeQ is observed experimentally.
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