Zur Strömungssimulation in Einzelklüften: Gegenüberstellung von numerischen Methoden und Experiment

Scientific research in the field of flow and transport processes in fractured rock is of high relevance due to the frequent occurrence of fractures in geological formations. For this reason, fracture flow problems are addressed in many fields of engineering tasks. Based on results of numerical simulations and experiments, the scope of this work is to gain deeper insights into the flow in rough single fractures. Moreover, the applicability of simplified models of fracture flow for variable geometric and hydraulic parameters is evaluated. For the simulations, a measured aperture field of an epoxy replica of a natural granite single fracture is used. To obtain three different geometric flow configurations, the upper and lower side of the fracture are shifted in flow direction relative to each other. The numerical results of the two- and three-dimensional simulations of the Navier-Stokes equations are obtained with the commercial software package STAR-CD™. Due to their high resolution, the two-dimensional simulations in vertical sections of these aperture fields are used to show the variability of effective local apertures which evolve from inertial effects, stagnation and recirculation zones. The three-dimensional simulations are validated using the experimental results of SPILLER (2005). They allow for detailed insights into the evolution of flow through a single fracture as well as into the evolution of characteristic parameters such as effective apertures, streamlines and tortuosity. The three-dimensional simulations in the whole aperture field are supplemented by studies in parts of the fracture where the aperture is varied in vertical direction. Those studies target on the identification and characterization of the main parameters of influence and on conclusions for the application of simpler two- and one-dimensional simulation models. Besides analysing the influence of the relative roughness on the flow, the parameter studies allow a detailed evaluation of one-dimensional and depth-averaged two-dimensional simplified flow laws. As these laws predict flow proportional to the cube of the aperture, the one-dimensional flow law is denominated Cubic Law in literature. The two-dimensional depth-averaged law is called Local Cubic Law. The experimental as well as the numerical results show that flow in the fracture under study can be described by the law of FORCHHEIMER (1901). It is in accordance with the Local Cubic Law but involves an additional term that takes into account inertial effects proportional to the square of the flow velocity and the empirical Forchheimer coefficient. This flow law that is implemented in the research code BIGFLOW is applied in this work to carry out two-dimensional depth-averaged flow simulations. The evaluation of the Forchheimer model shows that the transmissivity in the experiments as well as in the 3D simulations can be represented well by the Forchheimer equation provided that alternative definitions of the aperture field are used even for a small value of relative roughness. In principle, the Forchheimer approach is valid for the simulation of flow in fractures as the discharge is well represented depending on the roughness and the Reynolds number. The alteration of streamlines over the Reynolds number can not be captured by the model and is underestimated compared to the 3D-Navier-Stokes simulation. Breakthrough curves that are generated from the simulated flow fields show significant differences between the two models with respect to conservative advective transport. Different approaches for a better flow prediction of the Cubic Law and the Local Cubic Law accounting for geometric, hydraulic and empirical influencing factors are summarized and applied to the given aperture fields. Based on the findings, a new Cubic Law approach is found that represents well the evolution of the effective apertures over the Reynolds number. It is not clear to what extent an upscaling of the findings for the small scale of the single fracture on larger scales can be accomplished. For ongoing research work it is recommended to apply the findings and methods of this work to different fracture types with varying boundary conditions as well as for flow simulations in fracture networks with distinctive single fractures or small scale fracture networks. The application is promising to achieve further knowledge in the equivalent modelling of fractures and fracture networks.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2008

Author dc:creator

• Becker, Torsten Arnold

Contributors dc:contributor

• Köngeter, Jürgen

• info:eu-repo/classification/ddc/620
• Kluftwasser
• Grundwasserstrom
• Numerische Strömungssimulation
• Strömung
• Numerisches Modell
• Hydrogeologie
• Ingenieurwissenschaften
• Einzelkluft
• Forchheimer
• Grundwassermodellierung
• single fracture
• flow simulation
• groundwater
• hydrogeology
• numerical simulation

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112959%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50413