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Item type:Publication, Access status: with files , Determination of the applied thermal tortuosity of porous media with macroscopic geometric parameters using a neural network modelThe tortuosity of a porous medium has a significant effect on heat transfer through it, but this effect is difficult to quantify. In this study, we have developed the concept of applied thermal tortuosity and the thermal tortuosity function which characterizes the effect of porous medium geometry and the thermal conductivity ratio on the averaged path length of heat conduction in a porous medium. We have also proposed new macroscopic geometric parameters of porous media that can better describe the complexity of the porous media geometry. The relationship between the macroscopic geometric parameters and the developed thermal tortuosity function was established by using a neural network model. We used the developed thermal tortuosity function to calculate effective thermal conductivity, which can be applied in the thermal energy equation. Computations were performed for arbitrary two-dimensional porous media. Despite the model’s uncertainties, the developed neural network model is significantly more accurate than empirical correlations for determining the effective thermal conductivity. Optimization of the neural network architecture can further improve the accuracy of the model, but the problem of uncertainty cannot be completely solved. The study shows the importance of embedding the established knowledge of transport in porous media into the neural network model to improve its accuracy.Publicationtype: Journal Article40 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Multiscale modeling and simulation of turbulent flows in porous mediaNumerical simulation is an important tool for understanding the physics of flows in porous media and for making predictions. The state of the art of multiscale modeling and simulation of turbulent flows in porous media is reviewed in this paper. Numerical simulations of flows in porous media can be classified as microscopic simulations, in which both macroscopic and pore-scale flows are directly resolved, and macroscopic simulations, in which the pore-scale motions are modeled while the volume-averaged equations are solved. Studies in the past few years have shown that microscopic simulations improve the understanding of turbulent flows in porous media considerably; this motivates the development of more efficient and more accurate turbulence models for macroscopic simulations. On the basis of this review, we believe that simulation of flows with higher Reynolds numbers, understanding the transport of macroscopic turbulence, modeling of turbulent flows in inhomogeneous and anisotropic porous media, simulation of compressible and multiphase turbulent flows in porous media, and fluid–structure interaction in deformable porous matrices are important topics to be studied in the future.Publicationtype: Journal Article13 12 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Prediction of pore-scale-property dependent natural convection in porous media at high Rayleigh numbers(2022-09); ; Natural convection in porous media has received increasing attention in recent years due to its significance in engineering applications. This process is traditionally analyzed by the solution of the classical Darcy-Oberbeck-Boussinesq (DOB) equations. According to the DOB equations, natural convection in porous media is exclusively dependent on the Rayleigh-Darcy number, Ra, while the Sherwood number, Sh, has a linear relationship with Ra at high Rayleigh numbers. However, these predictions conflict with experimental observations. In this study, we have performed a pore-scale resolved direct numerical simulation (DNS) study of natural convection in periodic porous media composed of two-dimensional square and circular obstacles. Based on our analysis, a new correlation of Sh for large Rayleigh numbers (Ra≥1000), low Darcy numbers Da, and high Schmidt numbers Sc (Da/Sc≤2×10−8) has been proposed, expressed as Sh=aRa1−0.2φ2+1, where a=0.011±0.002 is a pore-scale geometric parameter. The new correlation has been validated over a wide range of Rayleigh numbers, porosity values, and pore-scale geometries. Our DNS results also show that, with a decrease of porosity, it becomes more difficult for mega-plumes with low wavenumbers to enter the boundary layer. Low wavenumber motions decay much faster with a decrease of Da than the pore-scale motions near the wall. The volume-averaged dissipation rate nondimensionalized using the pore size εˆi has the scaling law εˆi∼Da in the internal region and εˆi∼Da1/2 in the near-wall region. We expect that these characteristics obtained from DNS also apply to natural convection in porous media with much lower Darcy numbers.Publicationtype: Journal Article55 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Turbulent forced convection in porous media: A direct numerical simulation studyForced convection in porous media has many important applications, one of which is in thermal energy storage systems that use low cost materials, such as stones or brick, as heat storage materials. Turbulence is welcomed in such thermal energy storage systems since it efficiently enhances heat transfer. Forced convection in porous media can be described by the macroscopic momentum and energy equations. In order to close the macroscopic equations, we analyzed the microscopic flow and temperature fields in porous media by direct numerical simulation (DNS) methods. Two DNS methods were adopted in our study to compare and verify the results. They are a finite volume method (FVM) and a Lattice-Boltzmann method (LBM). The porous matrix is made of a large number of periodically arranged 3-dimensional spheres. Based on our DNS results, we proposed a macroscopic model for calculating forced convection in porous media.Publicationtype: Conference Paper141 - Some of the metrics are blocked by yourconsent settings
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); ; ; 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 Article76 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Turbulent flow in the micro structures of porous media(American Society of Mechanical Engineers (ASME), 2015-07); ; ; 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 Paper66 - Some of the metrics are blocked by yourconsent settings
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); ; ; 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 Article142 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Turbulente Strömungen durch poröse Materialien : wie sehen die aus?• 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 müssenPublicationtype: Journal Article84 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Turbulence in porous media : some fundamental questions addressed by DNS solutions(2014-06); ; ; 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 Part35