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  • 1
    Electronic Resource
    Electronic Resource
    On the streak spacing and vortex roll size in a turbulent channel flow (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 2439-2443 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Streamwise high vorticity rolls and streaks in the turbulent channel flows have been the subject of many studies due to their important role in turbulence production, as a result of sweeping, ejection, and bursting of these structures. Understanding the physics of these streamwise structures is important in controlling drag producing events. Investigations of the average streak spacing of the low-speed streaks have resulted in the generally accepted range of λ+=λ¯uτ/ν=100±20, where λ¯ is the mean spanwise spacing between streaks, normalized to the viscous length ν/uτ. It is also reported, for y+≤30, that the streak spacing grows nearly linearly with distance from the wall. The previous studies mostly have focused on distances close to the wall. Here we report on correlation measurements extended into the log layer, which show that the linear growth of the vortex diameter and the streak spacing extends well in the log layer. Arguments are presented to distinguish these two measures. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868688
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  • 2
    Electronic Resource
    Electronic Resource
    Drag reduction in turbulent channel flow by phase randomization (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 686-694 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results of numerical simulations of plane turbulent channel flow are presented in which a forcing is introduced which derives from the randomization of selected Fourier modes. In all cases, the randomization is introduced uniformly throughout the channel. The properties of the resulting turbulence are strongly dependent on both the wave numbers whose phases are randomized and the forcing frequency. Two principal wave-number bands have been selected. The first includes a selected subset of the largest length scales of the turbulence. Forcing in this band results in a fully sustained maximum mass flux increase above that of normal turbulence of 30%, which translates into a drag reduction of 58%. Many of the statistical properties of the simulated drag-reduced turbulence generated in this manner are in good qualitative agreement with the statistical properties of turbulence observed in experiments in which drag reduction is achieved through the introduction of small concentrations of long-chained polymers into the flow. In a second set of simulations, the phases of the intermediate and smallest wavelengths were randomized. Forcing at these scales of motion results in a drag increase. Speculations on the mechanism of the drag reduction by phase randomization are offered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858652
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  • 3
    Electronic Resource
    Electronic Resource
    Probability distribution functions in turbulent convection (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 919-927 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results of an extensive investigation of probability distribution functions (pdf's) for Rayleigh–Bénard convection, in the hard turbulence regime, are presented. It is seen that the pdf's exhibit a high degree of internal universality. In certain cases this universality is established within two Kolmogorov scales of a boundary. A discussion of the factors leading to universality is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857968
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  • 4
    Electronic Resource
    Electronic Resource
    An eigenfunction approach to large scale transitional structures in jet flow (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 127-136 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Karhunen–Loève procedure is applied to the analysis of digitally imaged two-dimensional gas concentration fields obtained for a seeded axisymmetric jet. Large scale structures observed in the transitional region of a low Reynolds number jet are characterized in terms of an optimal basis. The essential features are captured by retaining roughly 30 parameters in the expansion. By retaining more terms smaller scales of the flow can be resolved.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857815
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  • 5
    Electronic Resource
    Electronic Resource
    An investigation of chaotic Kolmogorov flows (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 681-696 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional flow governed by the incompressible Navier–Stokes equations with a steady spatially periodic forcing (known as the Kolmogorov flow) is numerically simulated. The behavior of the flow and its transition states as the Reynolds number Re varies is investigated in detail, as well as a number of the flow features. A sequence of bifurcations is shown to take place in the flow as Re varied. Two main regimes of the flow have been observed: small and large scale structure regimes corresponding to different ranges of Re. Each of the regimes includes a number of periodic, chaotic, and relaminarization windows. In addition, each range contains a chaotic window with nonunique chaotic attractors. Spatially disordered, but temporally steady states have been discovered in the large scale structure regime. Features of the diverse cases are displayed in terms of the temporal power spectrum, Poincaré sections and, where possible, Lyapunov exponents and Lyapunov dimension.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858074
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  • 6
    Electronic Resource
    Electronic Resource
    Plane waves and structures in turbulent channel flow (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2217-2226 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A direct simulation of turbulent flow in a channel is analyzed by the method of empirical eigenfunctions (Karhunen–Loève procedure, proper orthogonal decomposition). This analysis reveals the presence of propagating plane waves in the turbulent flow. The velocity of propagation is determined by the flow velocity at the location of maximal Reynolds stress. The analysis further suggests that the interaction of these waves appears to be essential to the local production of turbulence via bursting or sweeping events in the turbulent boundary layer, with the additional suggestion that the fast acting plane waves act as triggers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857808
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  • 7
    Electronic Resource
    Electronic Resource
    Simulations of turbulent thermal convection (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 1911-1914 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A direct numerical simulation of thermal convection between horizontal plane boundaries has been performed, at a Rayleigh number Ra=9800 Rac, where Rac is the critical Rayleigh number for the onset of convection (Pr=0.72). The flow is found to be fully turbulent and analysis of the probability distributions for temperature fluctuations indicates that this is within the "hard turbulence'' regime, as defined by the Chicago group. Good agreement is shown to exist between their experiments and the present simulation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857516
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  • 8
    Electronic Resource
    Electronic Resource
    Reply to "Observations regarding ‘Coherence and chaos in a model of turbulent boundary layer' by X. Zhou and L. Sirovich [Phys. Fluids A 4, 2855 (1992)]'' (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 1579-1582 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In view of the "observations'' of the Cornell group, reassessment of their and our models for wall-bounded turbulence has been made. Wide ranging evidence is presented for the existence and key role of propagating modes (streamwise dependent modes), absent in the original Cornell model but present in some of their later models. Evidence that the heteroclinic orbit (the bursting mechanisms) found in the original Cornell model is most likely an artifact of their Galerkin projection is presented. A thorough discussion detailing the physical and mathematical soundness, as well as the universality, of our models is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868271
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  • 9
    Electronic Resource
    Electronic Resource
    Coherence and chaos in a model of turbulent boundary layer (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2855-2874 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dynamics of coherent structures in the wall region of a turbulent channel flow is considered. The Karhunen–Loève eigenfunctions and Galerkin procedure are used to derive the dynamical description. In a previous treatment of this problem by Aubry et al. [J. Fluid Mech. 192, 115 (1988)] the analysis required an inhomogeneous pressure term to be supplied from outside the theory. In the first part of this paper this theory is reconsidered on the basis of the construction of wall eigenfunctions that have a full channel validity. As a result of the methods developed here, a well-posed Hermitian theory is developed and convergence questions do not arise. Among a number of important consequences is the fact that no exterior pressure is required by the present theory. In the second part of this paper it is shown that the behavior of the resulting model equations include intermittency, quasiperiodic, and chaotic solutions. In the final part of this paper three-dimensional effects are introduced into the dynamics in order to produce a physically more realistic dynamical theory. It is felt that the bursting and ejection events in turbulent boundary layers is given a more satisfactory explanation within this framework.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858340
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  • 10
    Electronic Resource
    Electronic Resource
    Empirical and Stokes eigenfunctions and the far-dissipative turbulent spectrum (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 464-467 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown that the Stokes eigenfunctions and their corresponding spectra, frequently used in mathematical investigations of the Navier–Stokes equations, provide estimates on the spectrum of the two-point spatial covariance tensor. This, in turn, is used to estimate the far-dissipative turbulent spectrum. An exponential falloff is predicted and evidence given which implies that this is a sharp estimate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857744
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