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Item type:Publication, Access status: without files , The physics of stripe patterns in turbulent channel flow determined by DNS resultsThe turbulent flow in an infinitely extended plane channel is analysed by solving the Navier-Stokes equations with a DNS approach. Solutions are obtained in a numerical solution domain of finite size in the streamwise as well as in the lateral direction setting periodic boundary conditions in both directions. Their impact on large scale structures in the turbulent flow field is analysed carefully in order to avoid their suppression. When this is done appropriately well known stripe patterns in these flows can be observed and analysed especially with respect to their relative motion compared to the mean flow velocity. Various details of this stripe pattern dominated velocity field are shown. Also global parameters like the friction factor in the flow field and the Nusselt number in the temperature field are determined based on the statistics of the flow and temperature data in a very large time period that guarantees fully developed turbulent flow and heat transfer.Publicationtype: Journal Article99 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Turbulent flow in rough wall channels : validation of RANS modelsTwo kinds of a generic roughness on plane channel walls have been investigated by standard RANS turbulence models. The objective was to accurately predict mean flow quantities for steady incompressible flows through rough wall channels. Seven turbulence models have been used in a special validation procedure based on direct numerical simulation (DNS) results for several values of the roughness spacing and the Reynolds number (which here are the free parameters). In addition to single RANS solutions also a parameter expansion method (PEM) is examined with respect to its capability to accurately predict rough wall channel flows for an extended range of parameters, i.e. for various roughness heights and Reynolds numbers. Those methods that could be validated were then used to predict results for parameters beyond the validation range. Their accuracy can be assessed by comparison with reasonably extended DNS results. The final conclusion is that RANS solutions only with certain turbulence models and over some parameter ranges can be used to predict turbulent channel flows influenced by wall roughness.Publicationtype: Journal Article77 - 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 , The head change coefficient for branched flows : why "losses" due to junctions can be negativeIn this study, the phenomenon of negative "loss"-coefficients reported in various studies about turbulent branched flows through combining junctions is investigated systematically. It turns out, that the "loss" for one branch of a junction and its adjacent ducts only in parts is due to devaluation of mechanical energy, which would be a real loss, so that the term energy change is more appropriate. The other part of the energy change which is not due to a loss is due to a mutual energy transfer between the two partial flows. As a consequence, non-dimensional coefficients should be called energy change coefficients rather than head loss coefficients whenever branching of flows occurs. Furthermore, a method is introduced by which losses and the energy transfer are determined directly in the flow field. The field values allow a quantification and visualization of the local loss and work transfer, while their integral values can be used to quantify the individual contributions of losses and energy transfer within the energy change of a flow through a junction.Publicationtype: Journal Article156 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , 189 235 - 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 , Strömungsmechanik : Physik - mathematische Modelle - thermodynamische Aspekte (3., ergänzte Auflage)Probleme in der Strömungsmechanik werden immer häufiger durch den Einsatz von kommerziellen Computerprogrammen gelöst. Eine solche Vorgehensweise setzt aber voraus, dass die Physik des Problems wirklich verstanden ist. Das Buch trägt zum grundlegenden Verständnis der Zusammenhänge bei, indem es die Physik verschiedener Strömungsformen anschaulich darstellt. · Die mathematischen Grundgleichungen, insbesondere die Navier-Stokes-Gleichungen und der Energiesatz, werden zunächst in allgemeiner Form bereitgestellt und in ihrer mathematischen Bedeutung erläutert. · Die physikalisch/mathematische Modellierung einzelner wichtiger Strömungen bzw. Strömungsformen wird anschließend konsequent aus diesen Grundgleichungen abgeleitet. Die Autoren verfolgen dabei systematisch das Konzept der deduktiven Herleitung. · Thermodynamische Überlegungen werden herangezogen, insbesondere um Verluste bei Strömungen physikalisch interpretieren zu können. Dimensionsanalytische Überlegungen spielen eine wichtige Rolle. Das Buch enthält zahlreiche Beispiele und Übungsaufgaben mit vollständigem Lösungsweg. Neu an der 3. Auflage ist das Kapitel „Strömungen aus thermodynamischer Sicht“. In diesem Teil wird gezeigt, wie wichtige strömungsmechanische Größen (z.B. die Verlustbeiwerte) mit thermodynamischen Überlegungen ermittelt und interpretiert werden können.Publicationtype: Book108 - 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