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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 422-427 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A model problem is proposed to investigate the steady response of a reacting, compressible laminar jet to Mach waves generated by wavy walls in a channel of finite width. The model consists of a two-dimensional jet of fuel emerging into a stream of oxidizer which are allowed to mix and react in the presence of the Mach waves. The governing equations are taken to be the steady parabolized Navier–Stokes equations which are solved numerically. The kinetics is assumed to be a one-step, irreversible reaction of the Arrhenius type. Two important questions on the Mach wave–flame interactions are discussed: (i) how is the flame structure altered by the presence of the Mach waves, and (ii) can the presence of the Mach waves change the efficiency of the combustion processes?
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 412-421 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct numerical simulations of turbulent Rayleigh–Bénard convection have been conducted in large aspect ratio boxes to examine the characteristics of convection in the range of Rayleigh numbers corresponding to the soft turbulence regime in the Chicago group classification. A measured exponent in the power law dependence of Nusselt number on Rayleigh number is 0.274, closer to the value 2/7 found in a small aspect ratio cell at the University of Chicago for the hard turbulence regime than the 1/3 obtained there for soft turbulence. The distributions of temperature fluctuation T' in the convective core do not exhibit a unique functional form. Gaussian, exponential, and mixed (Gaussian-exponential) forms are observed. Further, it is shown that a transition in the functional form of the T' pdf can be produced by changing the aspect ratio of the computational box without changing Rayleigh number at all. These results are in agreement with other studies, which have indicated that the hard/soft characterization of turbulent convection may partially reflect as yet not fully understood distinctions between convective turbulence in large and small aspect ratio cells.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3023-3025 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The influence of a "nonscale invariant'' forcing is investigated in the renormalization group theory of stirred fluids. It is shown that such forcings introduce a free parameter in the theory. However, the difficulties inherent in the use of the ε expansion remain when the results are applied to real turbulence.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3038-3048 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Wave formation in the gravity-driven low-Reynolds number flow of two liquid films down an inclined plane is studied by a linear stability analysis. Wavy motion can appear due to an instability of either the fluid–fluid interface or the fluid-air free surface. It is shown that the flow is always unstable and wavy motion can occur when the less viscous layer is in the region next to the wall for any Reynolds number and any finite interface and surface tensions. Stability can be achieved for the configuration with the more viscous component adjacent to the wall in the presence of interfacial tension when Reynolds number is small enough.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3085-3098 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A shear layer formed by two unidirectional gas streams of different velocities with a multisize (polydisperse) spray of evaporating droplets suspended in one of the gas streams is considered here. Similarity solutions are presented for the evolution in droplet size distributions across the shear layer and the effects of various initial droplet size distributions on the profiles of vapor concentrations are examined. A qualitative comparison between the present results for typical computed total mass distributions of the liquid phase and experimental data reported by Lazaro and Lasheras [Phys. Fluids A 1, 1035 (1989); Proceedings of the 22nd Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1988), pp. 1991–1998; J. Fluid Mech. 235, 143 (1992)] shows strong similarity between the two sets of profiles. This supports the assumptions and boundary conditions employed in the present theoretical study. The general behavior of the theoretical SMD (Sauter mean diameter) distribution of the spray across the shear layer also compares well with the reported experimental results of Lazaro and Lasheras.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3162-3166 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In Marangoni–Bénard convection with mass transfer from the air to the liquid, an experimental exploration of reflections of solitary waves at walls has been carried out. Mach stems are observed for small enough incident angles. Upon increasing the incident angle the stem disappears thus leading to regular reflections with, however, the angle of reflection being generally larger than the incident angle.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1939-1946 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical computations are performed to investigate the structure and evolution of flow in a two-dimensional axisymmetric rotating lid cavity. Frequency decompositions, phase diagrams, and flow visualization are used to monitor the changing kinematics of the flow as the speed of the rotating side wall is increased. For the flow regimes investigated, the approach to steady state is characterized by oscillatory instabilities, quasiperiodic flow, and chaotic flow leading to turbulence. A detailed study of the physical mechanisms involved in the computed flow field indicates the existence of at least two independent frequencies at a rotational Reynolds number Re in the range of 8×105 to 9×105. At Re(approximately-greater-than)106, a multiple of independent frequencies appear in the spectrum that indicate a transition to turbulence. This seems to be consistent with the experiments of Daily and Nece [ASME J. Basic Eng. 82, 217 (1960)] for the Reynolds number at which transition to weak turbulence is observed and with the theoretical scenario of Ruelle and Takens [Commun. Math. Phys. 20, 167 (1967)] for the transition to turbulence.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1947-1951 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A forced Landau–Stuart equation is studied in order to derive a low-dimensional model describing the temporal behavior of a paradigm open flow, the two-dimensional forced cylinder wake. Numerical results from the model exhibit several characteristics of circle maps, and compare qualitatively to previous experimental results for an oscillating cylinder wake. The low-dimensional model is also shown to reduce to a circle map in the limit of small forcing amplitudes. Observation of circle map dynamics in the forced Landau–Stuart equation strengthens the conjecture that globally unstable fluid flows are amenable to a dynamical systems approach focusing on the study of low-dimensional iterative maps. The established connection between the Landau–Stuart equation and the circle map unifies certain aspects of spatiotemporal stability and low-dimensional chaos theory.
    Type of Medium: Electronic Resource
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  • 9
    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.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1963-1970 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent experimental and computational observations demonstrate the occurrence of large-scale intermittency for diffusing passive scalars, as manifested by broader than Gaussian probability distribution functions. Here, a family of explicit exactly solvable examples is developed which demonstrates these effects of large-scale intermittency at any positive time through simple formulas for the higher flatness factors without any phenomenological approximations. The exact solutions involve advection–diffusion with velocity fields involving a uniform shear flow perturbed by a random fluctuating uniform shear flow. Through an exact quantum mechanical analogy, the higher-order statistics for the scalar in these models are solved exactly by formulas for the quantum-harmonic oscillator. These explicit formulas also demonstrate that the large time asymptotic limiting probability distribution function for the normalized scalar can be either broader than Gaussian or Gaussian depending on the relative strength of the mean flow and the fluctuating velocity field.
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