Bibliothek

X
feed icon rss

Ihre E-Mail wurde erfolgreich gesendet. Bitte prüfen Sie Ihren Maileingang.

Leider ist ein Fehler beim E-Mail-Versand aufgetreten. Bitte versuchen Sie es erneut.

Vorgang fortführen?

Exportieren
  • 1
    Digitale Medien
    Digitale Medien
    On the generation of turbulent wall friction (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 634-641 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The formation of streaky velocity structures in the near wall region of turbulent boundary layers is studied through a simplified two-dimensional computational model in the plane normal to the average velocity. It is shown that the redistribution of the longitudinal velocity by streamwise vortices produces features very similar to those observed in the experiments, and that compact streamwise vortices form naturally from more general vorticity distributions. It is also shown, both numerically and analytically, that one effect of the formation of the streaks is to increase the average wall friction, and it is suggested that this effect is responsible for the higher friction in turbulent boundary layers, as opposed to laminar ones. An approximate quantitative analysis of the process supports this assumption.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.868303
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 2
    Digitale Medien
    Digitale Medien
    On the structure and control of near wall turbulence (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 944-953 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A simplified self-sustaining cycle is proposed for the events in the near wall region of a turbulent boundary layer. An approximate quantitative analysis of the resulting model predicts the right dimensions for the sublayer streaks and for other flow structures. The model is checked further by applying it to a set of numerical simulations in which the longitudinal and transverse no-slip conditions are applied at different positions with respect to the wall, and to the analysis of flow over riblets. Substantial changes in the flow statistics are obtained, including drag reductions, and the resulting trends are also predicted correctly by the model.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.868327
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 3
    Digitale Medien
    Digitale Medien
    Fractal interfaces and product generation in the two-dimensional mixing layer (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1261-1268 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The dependence of product generation on Péclet and Reynolds numbers in a numerically simulated, reacting, two-dimensional, temporally growing mixing layer is related theoretically to the fractal dimension of the passive scalar interfaces. This relation is verified using product generation measurements and fractal dimensions derived from the box counting technique. A transition from a low initial dimension to a higher one of approximately (5)/(3) is identified and shown to be associated to the kinematic distortion of the flow field during the first pairing interaction. It is suggested that the structures responsible for this transition are nondeterministic, nonrandom, inhomogeneous fractals. In the range of Schmidt numbers investigated (0.25–4), only the large scales are involved. No further transitions, either in the spectra of the vorticity field or in the mixing behavior, are found for Reynolds numbers up to 90 000.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.858054
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 4
    Digitale Medien
    Digitale Medien
    Bifurcations and bursting in two-dimensional Poiseuille flow (1987)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 3644-3646 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The bifurcations to two-dimensional unsteady behavior of a large amplitude equilibrium shear wave train in two-dimensional Poiseuille flow are studied by direct simulation of the time evolution of the full Navier–Stokes equations. It is found that the wave train becomes unstable at Re∼5600, and sheds a limit cycle which, at higher Re, seems to undergo further transitions to more complex behaviors. It is shown that the site of the original bifurcation is in the neighborhood of the walls and that it shows some characteristics suggestive of the burst generation mechanism in the boundary layer.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.866450
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 5
    Digitale Medien
    Digitale Medien
    Kinematic alignment effects in turbulent flows (1992)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 652-654 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It is shown that, in the neighborhood of a vortex whose maximum vorticity is large with respect to that in the surrounding flow, the two principal strains with the largest absolute values lie in the equatorial plane, so that the vorticity is automatically aligned to the intermediate eigenvector. This purely kinematic effect is offered as an explanation for the alignment properties recently reported for the velocity and pressure derivatives in turbulent flows. The model is compared with experimental evidence from numerical simulations. The observed ratio between the principal strains is also related to the properties of a two-dimensional Burgers' vortex.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.858282
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 6
    Digitale Medien
    Digitale Medien
    Bubble formation in dense fluidized beds (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 1457-1457 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: It has been known for a long time that fluidized beds of solid powder in gas may become unstable for certain values of relative and mass fraction, and that this behavior is important in determining the mixing properties of the bed. Several different mechanisms have been proposed for this instability, but no broad consensus is available on which are the dominant ones. That question is studied here by means of a linear stability analysis and a fully nonlinear spectral numerical simulation of a two-fluid model for the bed. Two instabilities are identified, both of which depend on the viscous interaction between phases. A first, one-dimensional, instability breaks the bed into alternating bands of high and lower concentrations, and is due to the interaction of the volumetric coupling between phases and the interphase friction. Another, secondary, two-dimensional instability deforms these slugs, and results in the formation of large bubbles that later coalesce into longitudinal streamers, which seem to be the final state of the bed. Both mechanisms are different from a third one that had been proposed by other investigators, and which depends on the interaction of volumetric coupling and buoyancy forces. This latter mechanism was also identified but found to be unimportant for these flows. Examples of the temporal evolution of slightly perturbed initial fields into the streamer state are given, and the relevant parameter ranges are identified. The similarities and differences with other two-phase flows with comparable volume fractions including bubbly flows, are explained. [Supported by Spain–U.S. Technological Cooperation Committee.]
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.858213
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 7
    Digitale Medien
    Digitale Medien
    Ejection mechanisms in the sublayer of a turbulent channel (1988)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 31 (1988), S. 1311-1313 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The structure of the vorticity field in the viscous wall layer of a turbulent channel is studied by examining the results of a fully resolved direct numerical simulation. It is shown that this region is dominated by intense three-dimensional shear layers in which the dominant vorticity component is spanwise. The advection and reproduction processes of these structures are examined and shown to be consistent with the classical generation mechanism for two-dimensional Tollmien–Schlichting waves. This process is fundamentally different from the usually accepted mechanism involving hairpin vortices.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.866721
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 8
    Digitale Medien
    Digitale Medien
    TURBULENT FLOWS OVER ROUGH WALLS (2004)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Fluid Mechanics 36 (2004), S. 173-196 
    Details
    ISSN: 0066-4189
    Quelle: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Thema: Maschinenbau , Physik
    Notizen: We review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number ks+ , which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low ks+ depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with delta/k 〈 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can perhaps be better described as flows over obstacles. We also review the evidence for the phenomenon of d-roughness. The theoretical arguments are sound, but the experimental evidence is inconclusive. Finally, we discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness elements themselves.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1146/annurev.fluid.36.050802.122103
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 9
    Digitale Medien
    Digitale Medien
    On the Role of Weight Restrictions in Data Envelopment Analysis (1997)
    Springer
    Journal of productivity analysis 8 (1997), S. 215-230 
    Details
    ISSN: 1573-0441
    Schlagwort(e): data envelopment analysis ; weight restrictions efficiency
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Wirtschaftswissenschaften
    Notizen: Abstract This paper examines the role that weight restrictions play in Data Envelopment Analysis (DEA). It is argued that the decision to include a factor (input or output) in a DEA model represents an implicit judgement that the factor has a non-trivial weight. It therefore seems perverse to allow DEA to assign a trivial weight to that factor in assessing the efficiency of a unit. There is therefore a strong case for imposing restrictions on factor weights. However, many existing methods of weight restriction are in practice unwieldy. This paper proposes an alternative approach we term contingent weight restriction which is both practical and intellectually consistent with the DEA philosophy. The paper explores the implications of alternative methods of weight restriction using simulated data from a well known production process.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1007715912664
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
  • 10
    Digitale Medien
    Digitale Medien
    Data envelopment analysis applied to quality in primary health care (1996)
    Springer
    Annals of operations research 67 (1996), S. 141-161 
    Details
    ISSN: 1572-9338
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Mathematik , Wirtschaftswissenschaften
    Notizen: Abstract The performance of primary care should ultimately be judged on its effect on the health outcome of individual patients. However, for the foreseeable future, it is inconceivable that the outcome data necessary to come to a judgement on performance will be available. And in any case, specification of the statistical model necessary to analyze outcome is fraught with difficulty. This paper therefore sets out a model of primary care performance which is based on the premise that certain measurable quality indicators can act as proxies for outcome. This being the case, a model of performance can be deduced which takes into account the effect of resources and patient characteristics on outcome. The most appropriate analytic technique to make this model operational is data envelopment analysis (DEA). It is argued that DEA can handle multiple dimensions of performance more comfortably, and is less vulnerable to the misspecification bias that afflicts statistically based models. The issues are illustrated with an example from English Family Health Service Authorities.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02187027
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
    Artikel (Nationallizenzen)
    Volltext
Schließen ⊗
Diese Webseite nutzt Cookies und das Analyse-Tool Matomo. Weitere Informationen finden Sie hier...