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  • 1
    Electronic Resource
    Electronic Resource
    Chaotic advection in a complex annular geometry (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1063-1067 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dynamics of Lagrangian particles in a complex geometry is studied, both experimentally and through a full numerical simulation of the Navier–Stokes equations. The geometry is an annulus whose walls can be rotated independently. Stationary cylindrical rods can be positioned within the annulus in several arrangements. A variety of heteroclinic orbits are found at low Reynolds numbers, where the fluid flow is steady. As the flow becomes unsteady to a time-periodic (two-dimensional) state, it spontaneously gives rise to heteroclinic tangles that provide the organizing structure for the chaotic motion of fluid particles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858086
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  • 2
    Electronic Resource
    Electronic Resource
    Reynolds stress analysis of EMHD-controlled wall turbulence. Part I. Streamwise forcing (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 9 (1997), S. 788-806 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this work we investigate numerically turbulent flow of low electrical conductivity fluid subject to electro-magnetic (EMHD) forcing. The configuration is similar to the one considered in the experimental work of Henoch and Stace [Phys. Fluids 7, 1371 (1995)] but in a channel geometry. The lower wall of the channel is covered with alternating streamwise electrodes and magnets to create a Lorentz force in the positive streamwise direction. Two cases are considered in detail corresponding to interaction parameter values of 0.4 (case 1) and 0.1 (case 2). The effect of switching off and on the electrodes is also studied for the two cases. At the Reynolds number considered (Reτ(approximate)200), a drag increase was obtained for all cases, in agreement with the experiments of Henoch and Stace. A Reynolds stress analysis was performed based on a new decomposition of the gradients normal to the wall of the Reynolds stress −u′v′. It was found that the vortex stretching term w′w2′ and the spanwise variation of the stress component u′w′ are responsible for the drag increase. More specifically, the term ∂(u′w′)/∂x3 is associated with secondary vortical motions in the near-wall and becomes large and positive for large shear stress in regions where fluid is moving toward the wall. In contrast, negative values are associated with regions of lower shear where fluid is being lifted away from the wall. Unlike the unperturbed flow, in the controlled flow high speed near-wall streamwise jets are present (case 1) even in the time-averaged fields. Other changes in turbulence structure are quantified using streak spacing, vortex lines, vorticity quadrant analysis, and plots of the rms value of the vorticity angle. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.869210
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  • 3
    Electronic Resource
    Electronic Resource
    The crisis of transport measures in chaotic flow past a cylinder (1989)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 1 (1989), S. 628-630 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dynamics of the flow past a cylinder is drastically affected when external periodic disturbances are present in the flow. For a certain range of amplitude and frequency of disturbances the flow bifurcates to a chaotic state, even at low Reynolds numbers. In this Letter, the effect of these chaotic responses on transport measures (e.g., the heat transfer coefficient) by direct numerical simulations using spectral element methods is investigated. It is found that transport quantities oscillate aperiodically in time; their time-averaged value increases slightly from the undisturbed state. However, their amplitude increases dramatically from that of the undisturbed flow or any other excited, resonant, or quasiperiodic neighboring state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857435
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  • 4
    Electronic Resource
    Electronic Resource
    Vorticity transport in modeling three-dimensional unsteady shear flows (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 7 (1995), S. 688-690 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This Letter examines how intrinsic three-dimensionality influences the flow structure of unsteady shear flows, and models its effects by considering spanwise averages of vorticity fluctuations. In particular, uniform flow past a half-cylinder is considered: the two-dimensional wake is very different from its three-dimensional counterpart and the evolution equation for the spanwise spatially averaged vorticity is analyzed. It is shown that the vector flux which transports spanwise vorticity due to spatial vorticity fluctuations is of considerable magnitude (large in comparison with molecular diffusion) and is far from co-linear with the gradient of the spanwise averaged vorticity. Modeling this flux using classical eddy viscosity ideas would therefore seem inappropriate. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868592
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  • 5
    Electronic Resource
    Electronic Resource
    Computational reducibility of unsteady viscous flows (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 653-656 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: It is shown by direct numerical simulation of the incompressible Navier–Stokes equations that the vortex wakes of bluff objects can be reproduced by only knowing the time-average flow at one specific location behind the object, where it is most unstable according to linear instability theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.857716
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  • 6
    Electronic Resource
    Electronic Resource
    Chaotic transport in two- and three-dimensional flow past a cylinder (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1051-1062 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The response of transport measures (Nusselt number, drag and lift force) for two- and three-dimensional flow past a heated cylinder reaching a chaotic state is investigated numerically using a spectral element discretization at a Reynolds number Re=500. The undisturbed two-dimensional flow remains periodic at this Reynolds number, unless a suitable forcing is applied on the naturally produced system. Three-dimensional simulations establish that three-dimensionality sets in at Re≈200. Successive supercritical states are established through a series of period-doublings, before a chaotic state is reached at a Re≈500. For the two-dimensional forced flow, all transport measures oscillate aperiodically in time and undergo a "crisis,'' i.e., a sudden and dramatic increase in their amplitude. The corresponding three-dimensional, naturally produced chaotic state corresponds to a less drastic change of the transport quantities with both rms and mean values lower than their two-dimensional counterparts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858085
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  • 7
    Electronic Resource
    Electronic Resource
    Hybrid spectral-element-low-order methods for incompressible flows (1991)
    Springer
    Journal of scientific computing 6 (1991), S. 79-100 
    Details
    ISSN: 1573-7691
    Keywords: Incompressible flows ; complex geometries ; spectral elements ; finite differences ; finite elements
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science
    Notes: Abstract In this article we present a new formulation for coupling spectral element discretizations to finite difference and finite element discretizations addressing flow problems in very complicated geometries. A general iterative relaxation procedure (Zanolli patching) is employed that enforcesC 1 continuity along the patching interface between the two differently discretized subdomains. In fluid flow simulations of transitional and turbulent flows the high-order discretization (spectral element) is used in the outer part of the domain where the Reynolds number is effectively very high. Near “rough” wall boundaries (where the flow is effectively very viscous) the use of low-order discretizations provides sufficient accuracy and allows for efficient treatment of the complex geometry. An analysis of the patching procedure is presented for elliptic problems, and extensions to incompressible Navier-Stokes equations are implemented using an efficient high-order splitting scheme. Several examples are given for elliptic and flow model problems and performance is measured on both serial and parallel processors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01062115
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  • 8
    Electronic Resource
    Electronic Resource
    Parallel spectral-element—Fourier simulation of turbulent flow over riblet-mounted surfaces (1992)
    Springer
    Theoretical and computational fluid dynamics 3 (1992), S. 219-229 
    Details
    ISSN: 1432-2250
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract The flow in a channel with its lower wall mounted with streamwise V-shaped riblets is simulated using a highly efficient spectral-element—Fourier method. The range of Reynolds numbers investigated is 500 to 4000, which corresponds to laminar, transitional, and turbulent flow states. Our results suggest that in the laminar regime there is no drag reduction, while in the transitional and turbulent regimes drag reduction up to 10% exists for the riblet-mounted wall in comparison with the smooth wall of the channel. For the first time, we present detailed turbulent statistics in a complex geometry. These results are in good agreement with available experimental data and provide a quantitative picture of the drag-reduction mechanism of the riblets.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00417914
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  • 9
    Electronic Resource
    Electronic Resource
    A new triangular and tetrahedral basis for high-order (hp) finite element methods (1995)
    Chichester [u.a.] : Wiley-Blackwell
    International Journal for Numerical Methods in Engineering 38 (1995), S. 3775-3802 
    Details
    ISSN: 0029-5981
    Keywords: finite-elements ; high-order ; spectral expansions ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics , Technology
    Notes: In this paper we describe the foundations of a new hierarchical modal basis suitable for high-order (hp) finite element discretizations on unstructured meshes. It is based on a generalized tensor product of mixed-weight Jacobi polynomials. The generalized tensor product property leads to a low operation count with the use of sum factorization techniques. Variable p-order expansions in each element are readily implemented which is a crucial property for efficient adaptive discretizations. Numerical examples demonstrate the exponential convergence for smooth solutions and the ability of this formulation to handle easily very complex two- and three-dmensional computational domains employing standard meshes.
    Additional Material: 16 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/nme.1620382204
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  • 10
    Electronic Resource
    Electronic Resource
    Spectral element-FCT method for scalar hyperbolic conservation laws (1992)
    Chichester : Wiley-Blackwell
    International Journal for Numerical Methods in Fluids 14 (1992), S. 707-727 
    Details
    ISSN: 0271-2091
    Keywords: Spectral element ; Flux-corrected transport ; Shock capturing ; Huperbolic discontinuous problems ; Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: A new algorithm based on spectral element discretizations and flux-corrected transport (FCT) ideas is developed for the solution of discontinuous hyperbolic problems. A conservative formulation is proposed, based on cell averaging and reconstruction procedures, that employs a staggered grid of Gauss-Chebyshev and Gauss-Lobatto-Chebyshev discretizations. In addition, high-order time-differencing schemes, a flux limiter and a general spectral filter are employed to improve the quality of the solution. It is demonstrated through model problems of linear advection and examples of one-dimensional shock formation that the proposed algorithm leads to stable, non-oscillatory solutions of high accuracy away from discontinuities. Typically, spectral or spectral element methods perform very poorly in the presence of even weak discontinuities, although they produce only exponentialy small errors for smooth solutions. Spectral element-FCT methods can provide spectral properties (i.e. minimum dispersion and diffusion errors) as well as great flexibility in the discretization, since a variable number of macroelements or collocation points per element can be employed to accommodate both accuracy and geometric requirements.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/fld.1650140605
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