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    Item type:Publication, Access status: without files ,
    A direct numerical simulation study on the possibility of macroscopic turbulence in porous media: Effects of different solid matrix geometries, solid boundaries, and two porosity scales
    (2016-06-01)
    Uth, M. F.
    ;
    Jin, Yan
    ;
    Kuznetsov, Andrey V.
    ;
    Herwig, Heinz
    In this study, we address the question of whether turbulent structures in a porous medium are restricted in size by the pore scale or whether the size of eddies may exceed the pore scale, leading to the formation of macroscopic coherent structures. Based on direct numerical simulations in porous media, we conclude that the size of turbulent eddies is restricted by the pore size, leading to the pore scale prevalence hypothesis (PSPH). We prove this hypothesis by considering four different porous matrices. In particular, we simulated turbulent flow in a two-dimensional matrix, a three-dimensional unbounded matrix, a three-dimensional matrix bounded by two parallel solid walls, and a three-dimensional matrix with two characteristic pore scales. The obtained results for the four simulated porous matrices support the PSPH. However, there is a partly open question of whether turbulent structures can reach the size of the largest pore scale if the solid matrix is characterized by more than one length scale.
    Publicationtype: Journal Article
      76
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    Item type:Publication, Access status: without files ,
    Turbulent flow in the micro structures of porous media
    (American Society of Mechanical Engineers (ASME), 2015-07)
    Herwig, Heinz
    ;
    Jin, Yan
    ;
    Uth, Marc-Florian
    ;
    Kuznetsov, Andrey V.
    Some fundamental issues with respect to turbulent flows through a porous matrix are addressed by analyzing DNS results (DNS: direct numerical simulation, i.e. no turbulence modeling). In a porous matrix with pore sizes of micro or even nano dimensions turbulent flow may occur when the (local) Reynolds number is sufficiently large. An open question, however, is whether turbulence structures are restricted in size by the pore size dimensions or not. This is an important aspect that immediately affects the way turbulence has to be modelled. In order to find out which influence the solid matrix has on the turbulence a generic matrix built from a large number of bars with square cross sections is investigated. Two different DNS approaches are used, a finite volume one and a Lattice-Boltzmann approach. From both DNS calculations detailed flow field information about the influence of the solid matrix on the turbulence structure are obtained. Finally the extension of Darcy's friction law by the Forchheimer term is investigated with respect to the question whether this extended law may be used in the fully turbulent flow regime.
    Publicationtype: Conference Paper
      66
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    Item type:Publication, Access status: without files ,
    Numerical investigation of the possibility of macroscopic turbulence in porous media : a direct numerical simulation study
    (Cambridge University Press (CUP), 2015-02-02)
    Jin, Yan
    ;
    Uth, Marc-Florian
    ;
    Kuznetsov, Andrey V.
    ;
    Herwig, Heinz
    When a turbulent flow in a porous medium is determined numerically, the crucial question is whether turbulence models should account only for turbulent structures restricted in size to the pore scale or whether the size of turbulent structures could exceed the pore scale. The latter would mean the existence of macroscopic turbulence in porous media, when turbulent eddies exceed the pore size. In order to determine the real size of turbulent structures in a porous medium, we simulated the turbulent flow by direct numerical simulation (DNS) calculations, thus avoiding turbulence modelling of any kind. With this study, which for the first time uses DNS calculations, we provide benchmark data for turbulent flow in porous media. Since perfect DNS calculations require the resolution of scales down to the Kolmogorov scale, often only approximate DNS solutions can be obtained, especially for high Reynolds numbers. This is accounted for by using and comparing two different DNS approaches, a finite volume method (FVM) with grid refinement towards the wall and a lattice Boltzmann method (LBM) with equal grid distribution. The solid matrix was simulated by a large number of rectangular bars arranged periodically. The number of bars in the solution domain with periodic boundary conditions was reduced systematically until a minimum size was found that does not suppress any large-scale turbulent structures. Two-point correlations, integral length scales and energy spectra were determined in order to answer the question of whether or not macroscopic turbulence can be found in porous media.
    Publicationtype: Journal Article
      142
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    Item type:Publication, Access status: without files ,
    Turbulente Strömungen durch poröse Materialien : wie sehen die aus?
    (HLRN, 2015)
    Jin, Yan
    ;
    Uth, Marc-Florian
    ;
    Kuznetsov, Andrey V.
    ;
    Herwig, Heinz
    • Strömungen durch poröse Medien kommen in Natur und Technik häufig vor (Grundwasserströmungen, Strömung durch Kiesschüttungen, Filtermaterialien, ...) • Bei entsprechend hohen Geschwindigkeiten sind diese Strömungen (wie Strömungen allgemein) turbulent, gekennzeichnet durch Turbulenzstrukturen, deren maximale Größe durch die Geometrie begrenzt ist, die durchströmt wird. • Es ist eine bis heute ungeklärte Frage, ob diese geometrische Begrenzung der Strukturgröße der Turbulenz durch die Geometrie insgesamt gegeben ist, oder durch die Porengröße des porösen Materials. • Für die Berechnung solcher Strömung ist diese Frage von großer Bedeutung, weil davon abhängt, wie stets erforderliche Turbulenzmodelle aussehen müssen. • Der einzige Weg, dies theoretisch und systematisch zu untersuchen besteht darin, eine solche Strömung in allen Einzelheiten zu berechnen, ohne dabei aber Turbulenzmodelle benutzen zu müssen
    Publicationtype: Journal Article
      84
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    Item type:Publication, Access status: without files ,
    Turbulence in porous media : some fundamental questions addressed by DNS solutions
    (2014-06)
    Uth, Marc-Florian
    ;
    Jin, Yan
    ;
    Kuznetsov, Andrey V.
    ;
    Herwig, Heinz
    In a generic porous matrix built by a large number of rectangular bars the flow is determined numerically by a DNS approach. Turbulent flow is thus simulated avoiding modelling in order to decide whether turbulent structures with scales much larger than the pore scale exist. So far only under-resolved DNS solutions are determined from which, however, definite conclusions with respect to the maximum turbulent length scale can be drawn.
    Publicationtype: Book Part
      35