10 results
Now showing1 - 10 of 10
- 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 , Structure of a turbulent flow through plane channels with smooth and rough walls : an analysis based on high resolution DNS resultsTwo different DNS-approaches (direct numerical simulation of turbulent flow, i.e. without turbulence modeling) are used to determine the detailed structure of turbulent flow through channels with smooth and rough walls. They are a finite volume method (FVM) with grid refinement toward the walls and a Lattice Boltzmann method (LBM) with a uniform grid. With the LBM-approach high resolutions with up to 300. million grid points are possible. Compared to the FVM-approach with less grid points and grid refinement toward the wall much higher resolutions away from the walls occur in the LBM solutions. Due to these high resolutions hairpin vortices can be clearly identified for these flows. They are known to be important building blocks in boundary layers. In channel flows, however, their dense existence has only been detected by experimental studies, whereas numerically it still has not been clearly visualized, especially in rough wall channel flows. Our LBM results show that hairpin vortices and their organizations in large scale and very large scale motions (LSM, VLSM) prevail in both smooth wall and rough wall channel flows. The similarity of the vortical structures in the outer layers over smooth walls and over the present k-type rough walls supports Townsend's wall similarity hypothesis.Publicationtype: Journal Article172 - 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 , Fabrication and characterization of deformable porous matrices with controlled pore characteristics(Springer, 2014-12-09); ; ; ; The subject of this study concerns a method of manufacture of porous media for which the solid matrix is capable of experiencing deformation under the influence of the flow field. Conventionally, the matrix design parameters, elasticity and pore geometry, cannot be precisely controlled and the choice of parameters is limited to existing available media. Here a solution is provided that uses an indirect solid-free form fabrication process that combines 3D Printing with an infused Polydimethylsiloxane elastomer to provide a highly deformable matrix with controlled pore architecture. The manufacturing method is presented in detail. Local microscopy analysis of the manufactured matrix shows that the method has a high capability to accurately create pore structures at length scales as low as 0.75 mm. Experimental flow measurements further validate that the intended pore geometry is able to be reproduced in highly deformable matrices. The experimentally determined permeability of the deformable matrix is determined to agree with the intended within 95 %.Publicationtype: Journal Article78 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Molecular dynamics simulations of benchmark geometries representing a flow through porous media(American Society of Mechanical Engineers (ASME), 2014-08); ; ; Fluid Dynamics Simulations are often based on continuum assumptions that, for micro-or nano-flows, are extended by models that take into account special phenomena (e. g. wall slip) that occour in these dimensions. On the nano-scale not only the near wall layers show peculiarities. Especially for liquids the bulk flow deviates from smooth profiles and thus strongly differs from continuum theory. So far mainly the flow in straight channels has been investigated with Molecular Dynamics Simulations. In real porous media such perfectly flat channels do not appear. Thus, to analyze the applicability of the continuum assumption for the simulation of fluid flow on the pore-scale the type of geometries that are evaluated has to be extended. Therefore in this paper we present the results of Molecular Dynamics Simulations of a liquid in a Couette flow, the flow through a straight channel, a bend and through an array of cylinders. We present the results for varying wall-fluid potentials, shear rates, and temperatures and compare the results to the validated data from a finite volume solution of the Navier-Stokes-Equations for the same geometries. As a result we show the difficulties due to complex geometries for the simulation with Molecular Dynamics. Also the boundary conditions for Molecular Dynamics Simulations, like the thermostat or the wall model are discussed. We show that for some parameter sets, a critical behaviour can be observed for the more complex geometries. Such results are evaluated to show the differences between MD and FVM simulations. It turns out how-ever, that the limits of the continuum assumptions are very different for different geometries. In complex geometries the minimum characteristic size can be much smaller than in straight channels. Hence the scope of continuum methods for the simulation of fluid flow through porous media on the pore scale is larger than expected.Publicationtype: Conference Paper118 - 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 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A new partial slip boundary condition for the Lattice-Boltzmann method(American Society of Mechanical Engineers (ASME), 2013); ; In micro or nano flows a slip boundary condition is often needed to account for the special flow situation that occurs at this level of refinement. A common model used in the Finite Volume Method (FVM) is the Navier-Slip model which is based on the velocity gradient at the wall. It can be implemented very easily for a Navier-Stokes (NS) Solver. Instead of directly solving the Navier-Stokes equations, the Lattice-Boltzmann method (LBM) models the fluid on a particle basis. It models the streaming and interaction of particles statistically. The pressure and the velocity can be calculated at every time step from the current particle distribution functions. The resulting fields are solutions of the Navier-Stokes equations. Boundary conditions in LBM always not only have to define values for the macroscopic variables but also for the particle distribution function. Therefore a slip model cannot be implemented in the same way as in a FVM-NS solver. An additional problem is the structure of the grid. Curved boundaries or boundaries that are non-parallel to the grid have to be approximated by a stairlike step profile. While this is no problem for no-slip boundaries, any other velocity boundary condition such as a slip condition is difficult to implement. In this paper we will present two different implementations of slip boundary conditions for the Lattice-Boltzmann approach. One will be an implementation that takes advantage of the microscopic nature of the method as it works on a particle basis. The other one is based on the Navier-Slip model. We will compare their applicability for different amounts of slip and different shapes of walls relative to the numerical grid. We will also show what limits the slip rate and give an outlook of how this can be avoided.Publicationtype: Conference Paper184 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , A micro-sized generic demonstrator for studying synthetic biological processesIn the field of synthetic biology, enzymatic pathways are used to enhance the efficiency of chemical production processes. These pathways consist of micro reactors that are filled with porous media and enzymes that catalyze the partial reactions. In the present study we summarize the requirements of the micro reactors used for these applications and give an overview of the different problems that have to be solved. Furthermore we present the idea of a generic demonstrator as a research tool. In the interdisciplinary field of synthetic biology it can enhance the interaction between different groups. It will serve as a common base for numerically as well as for experimentally working groups. We propose a definition of a generic demonstrator for a micro fluidic reactor as it is typically used in this field. Besides, present work of our group is presented, that analyzes the flow field in the micro channels of the reaction zone that is filled with porous media. We show how computational costs can be saved by studying the entry lengths of these channels.Publicationtype: Conference Paper26