Local Fluid Flow Research Articles

Growth and characterization of complex mineral surfaces

Precipitation of mineral aggregates near the Earth's surface or in subsurface fractures and cavities often produces complex microstructures and surface morphologies. Here we demonstrate how a simple surface normal growth (SNG) process may produce microstructures and surface morphologies very similar to those observed in some natural carbonate systems. A simple SNG model was used to fit observed surfaces, thus providing information about the growth history and also about the frequency and spatial distribution of nucleation events during growth. The SNG model can be extended to systems in which the symmetry of precipitation is broken, for example by fluid flow. We show how a simple modification of the SNG model in which the local growth rate depends on the distance from a fluid source and the local slope or fluid flow rate, produces growth structures with many similarities to natural travertine deposits.

Study of gas biogeochemical parameters of gaseous fluids from the bottom of the Black Sea

Gaseous fluid flows from sediments into bottom water of seas and oceans attract attention of oceanographers, particularly specialists in gas geochemistry [1]. Fluid flows from the Black Sea bottom were the main study object in Cruise 61 of the R/V Professor Vodyanitskii , carried out in the Ukrainian segment of the Black Sea in June‐July, 2004. The expedition was organized and conducted by the Division of Marine Geology and Sedimentary Ore Formation, National Academy of Sciences of Ukraine. Gas biogeochemical parameters were investigated at the Institute of Oceanology (Moscow). These works were aimed at studying spatial variations in gas biochemical parameters in seawater and bottom sediments in the zone of local fluid flows. Bottom water was sampled in fluid discharge areas detected by multibeam sonar as a gaseous plume. Samples were taken closest to the fluid discharge site in the depth interval of 200‐2000 m. Specially designed equipment and methods were used for the study of gas biogeochemical parameters [2]. Water samples were taken with a hermetic bottom bathometer from the bottom layer of 1.5‐2.0 m above the seafloor, with the bathometer cartridge at several meters above the bottom, and by integration of the bathometer from the water column located 100‐150 m above the bottom. Bottom sediments were sampled by a gravity corer with a plastic container that fills with sediments in situ. In the case of spontaneous degassing of gas-saturated sediment after dismantling of the corer on deck, the released gas mixture inflated the container and the gas sample was taken by syringe for chromatographic analysis. Then, sediment samples were taken at different levels of the core. The content of gas components (hydrocarbons, helium, hydrogen, carbon dioxide, and permanent gases) and biochemical parameters, such as the ATP content and alkaline phosphomonoesterase activity (APHA), were simultaneously determined in water and sediment samples. The article presents new factual data on underwater fluid discharge areas. Methane and carbon dioxide appeared to be most informative among gas components. Hydrogen, helium, and homologues of methane were not detected, because their concentration in the analyzed gas mixture was below the sensitivity of the chromatograph. The gas mixture was either sampled immediately from the plastic container of the gravity corer (or hermetic core catcher) or was extracted by phase-equilibrium degassing of seawater and sediment samples. In both cases, air was the main gas carrier. Therefore, oxygen and nitrogen were also measured methodologically. In total, approximately 100 samples were analyzed. The gas mixture contained methane (0.0‐89.8 vol %) and carbon dioxide (0.0‐15.5 vol %). The total content of oxygen and nitrogen varied from 8.65 to 100 vol %. Table 1 presents data on various gas mixtures obtained from the gravity corer container and hermetic core catcher or extracted from water and sediment samples. Biochemical parameters were measured in the same water and sediment samples. In addition, certain intervals of the core and bottom and surface water samples were analyzed to estimate biological activity of seawater medium in study areas. Table 2 presents gasometric and biochemical data. The data from Tables 1 and 2 make it possible to disclose some peculiarities in the distribution of gas biogeochemical parameters. In the study areas, bottom water is saturated with methane and carbon dioxide. The biological activity of water is sufficiently high and

Ultrasonic properties of sedimentary rocks: effect of pressure, saturation, frequency and microcracks

Seismic velocities and attenuations are influenced by lithology, porosity and permeability as well as the kind and quantity of the pore fluids. The microstructure of rocks is another important factor influencing seismic properties. This influence can be used to investigate the pressure-dependent closure of microcracks. For this purpose, velocities and attenuations of P and S waves have been determined at ultrasonic frequencies in three different rocks (vacuum dry and partially saturated with water) as a function of hydrostatic pressure up to 200 MPa. A new combined model has been developed. With this model an explicit description of the microstructure [local fluid flow (LF)] and macrostructure (Gassmann effect, global fluid flow) is possible. Assuming a patchy saturation (i.e. inhomogeneous distribution of the fluid in the pores) and modulus reduction, both the saturation-dependent measurements and the pressure-dependent data can be explained. The modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears: in high-porosity sandstones (Bentheimer and Obernkirchner Sandstone) a strong closure of cracks already with a small increase of pressure must be concluded. In a low-porosity Harzer Greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0–100 MPa. It is shown, however, that there must be open cracks in all rocks even under elevated hydrostatic pressure, resulting in a measurable effect due to LF.

Evolution of fracture and fault-controlled fluid pathways in carbonates of the Albanides fold-thrust belt

The process of fracture and fault formation in carbonates of the Albanides fold-thrust belt has been systematically documented using hierarchical development of structural elements from hand sample, outcrop, and geologic-map scales. The function of fractures and faults in fluid migration was elucidated using calcite cement and bitumen in these structures as a paleoflow indicator. Two prefolding pressure-solution and vein assemblages were identified: an overburden assemblage and a remote tectonic stress assemblage. Sheared layer-parallel pressure-solution surfaces of the overburden assemblage define mechanical layers. Shearing of mechanical layers associated with folding resulted in the formation of a series of folding assemblage fractures at different orientations, depending on the slip direction of individual mechanical layers. Prefolding- and folding-related fracture assemblages together formed fragmentation zones in mechanical layers and are the sites of incipient fault localization. Further deformation along these sites was accommodated by rotation and translation of fragmented rock, which formed breccia and facilitated fault offset across multiple mechanical layers. Strike-slip faults formed by this process are organized in two sets in an apparent conjugate pattern. Calcite cement and bitumen that accumulated along fractures and faults are evidence of localized fluid flow along fault zones. By systematic identification of fractures and faults, their evolution, and their fluid and bitumen contents, along with subsurface core and well-log data, we identify northeast-southwest–trending strike-slip faults and the associated structures as dominant fluid pathways in the Albanides fold-thrust belt.

Comparison of heat transfer conditions in tube bundle cross-flow for different tube shapes

Detailed transient numerical simulations of fluid and heat flow were performed for a number of heat exchanger segments with cylindrical, ellipsoidal and wing-shaped tubes in a staggered arrangement. The purpose of the analysis was to get an insight of local heat transfer and fluid flow conditions in a heat exchanger and to establish widely applicable drag coefficient and Stanton number correlations for the heat exchanger integral model, based on average flow variables. The simulation results revealed much more complex flow behavior than reported in current literature. For each of the almost 100 analyzed cases, the time distributions of the Reynolds number, the drag coefficient and the Stanton number were recorded, and their average values calculated. Based on these average values, the drag coefficient and the Stanton number correlations were constructed as polynomial functions of the Reynolds number and the hydraulic diameter. The comparison of the collected results also allows more general conclusions on efficiency and stability of the heat transfer process in tube bundles.

Kübler Index and K-Ar Ages of Illite in the Yinshan Polymetallic Deposit, Jiangxi Province, South China: Analyses and Implications

Abstract. The Yinshan polymetallic deposit is a hydrothermal vein-type deposit closely related to Late Jurassic felsic-inter-mediate volcanic-subvolcanic activity in Jiangxi Province, South China. Illite is a major alteration mineral observed in the deposit. Our study shows that the Kübler index of the illite has a close relation to ore-forming fluids of different stages of hydrothermal alteration and mineralization. The early Pb-Zn-Ag mineralization dated at 130–136 Ma is characterized by relatively low water/rock ratios and diffusive fluid movement within phyllite, whereas the later Cu-Au-S mineralization at 122 -125 Ma was accompanied by higher water/rock ratios and localized fluid flow through fractures and channels. Illite formed in the early Pb-Zn-Ag mineralization stage contains swelling layers while the illite formed in the later Cu-Au-S mineralization stage has no swelling layers but was associated with intensive chloritization. The last stage of mineralization (at 104 Ma) was minor and did not produce significant amounts of illite.

Influence of local fluid flow on properties of low permeability Cretaceous siltstones (South-Eastern France): Implications for a nuclear waste deep repository

Understanding the physical and chemical behaviour of a sedimentary sequence during post-depositional fluid flow is of prime importance in evaluating its potential in the context of a nuclear waste deep repository. The Cretaceous siltstones of the Marcoule area (Gard, south-eastern France), investigated by ANDRA (French nuclear waste management agency) as a potential site for such a storage facility but finally not selected for the construction of an underground laboratory, are intersected by two major faults which show evidence of fluid circulation through the sequence postdating deposition. Late-stage fluid circulation induced secondary mineral crystallisation (calcite, celestite, pyrite, and barite, referred here as “epigenetic” as opposed to paragenetically earlier calcite and pyrite development during burial and referred here as “diagenetic”). These epigenetic phases occur both in the fault and in sub-perpendicular fractures in the surrounding siltstones. Clay minerals appear to be unaffected by this fluid circulation, and both mineralogical and isotopic (K–Ar) results argue in favour of simple physical reworking of the surrounding clay material, without significant mobilization of the major and trace elements. Rare earth elements, which might be considered as natural analogues of actinides, exhibit any variation in content between fault and siltstones clay-size fractions. In agreement with mineralogical and isotopic results, this behaviour might traduce that REE remained bound to the clay minerals and were not significantly remobilized into other mineral phases. The effect of the fluid interaction can only be deduced from increase in Sr, Ba and Pb contents in the finest clay size fractions. These elements are probably incorporated into mineral phases associated with the clay particles. They appear to be of local origin, as suggested by the Sr-isotope ratio of the epigenetic calcites close to that of diagenetic calcite. A difference of Pb-isotope composition between diagenetic (formed in the matrix during early and burial diagenesis) and epigenetic (precipitated in the gouge during late-stage fluid flow) pyrite is thought to reflect the Pb-isotope evolution over time in the siltstones until the formation of pyrite in the gouge. Assuming that the diagenetic pyrite formed during very early diagenesis in the sequence (c. 97–98 Ma based on glauconite dating [Rousset, D., Leclerc, S., Clauer, N., Lancelot, J., Cathelineau, M., Aranyossy, J.F., 2004. Age and origin of Albian glauconites and associated clay minerals inferred from a detailed geochemical analysis. Journal of Sedimentary Research 74, 631–642.]), a modelled-evolution of the Pb-isotope composition leads to an estimate of the time elapsed between diagenesis and fluid flow of some 60 Ma. Consequently, fluid circulation which precipitated pyrite could have occurred 39 ± 7 Ma ago, during the Pyrenean thrusting.

Molecular dynamics study of tethered polymers in shear flow

Single macromolecules can now be isolated and characterized experimentally using techniques such as optical tweezers and videomicroscopy. An interesting and important single-molecule problem is that of the dynamics of a polymer chain tethered to a solid surface and subjected to a shear flow. An experimental study of such a system was reported by Doyle et al. (Phys. Rev. Lett. 84, 4769 (2000)), and their results showed a surprising recirculating motion of the DNA chain. We explore this problem using molecular dynamics computer simulations with explicit hydrodynamic interactions. The dynamical properties of a Freely Jointed Chain (FJC) with Finitely Extensible Nonlinear Elastic (FENE) links are examined in similar conditions (i.e., confined between two surfaces and in the presence of a Poiseuille flow). We see the remarkable cyclic polymer motion observed experimentally, and we show that a simple cross-correlation function can be used to measure the corresponding period of motion. We also propose a new empirical equation relating the magnitude of the shear flow to the amount of chain deformation, an equation that appears to apply for both weak and strong flows. Finally, we report on packing effects near the molecularly flat wall, an associated chain-sticking phenomenon, and the impact of the chain hydrodynamic drag on the local fluid flow.

Acoustical characterization of fluid-saturated porous media with local heterogeneities: Theory and application

A linear dynamic model of fully saturated porous media with local (either microscopic or mesoscopic) heterogeneities is developed within the context of Biot’s theory of poroelasticity. Viscoporoelastic behavior associated with local fluid flow is characterized by the notion of the dynamic compatibility condition on the interface between the solid and the fluid. Complex, frequency-dependent material parameters characterizing the viscoporoelasticity are derived. The complex properties can be obtained through determining the quasi-static poroelastic parameters, the properties of individual constituents, and the relaxation time of the dynamic compatibility condition on the interface. Relationships among various quasi-static poroelastic parameters are developed. It is shown that local fluid flow mechanism is significant only in the porous media with local heterogeneities. The relaxation time of the compatibility condition on the interface depends upon the details of local structure of porous media that control local fluid pressure diffusion. The new model is used to describe the velocity dispersion and attenuation in fully saturated porous media. The proposed model provides a theoretical framework to simulate the acoustical behavior of fully saturated porous media over a wide range of frequencies without making any explicit assumption about the structure of local heterogeneities.

Contribution of vortex structures and flow separation to local and overall pressure and heat transfer characteristics in an ultralightweight lattice material

Ultralightweight lattice-frame materials (LFMs) with open, periodic microstructures are attractive multifunctional systems that can perform structural, thermal, actuation, power storage and other functions [A.G. Evans, J.W. Hutchinson, M.F. Ashby, Multifunctionality of cellular metal systems, Prog. Mater. Sci. 43 (1999) 171–221]. This paper presents experimental and numerical studies of local fluid flow behaviour and its contribution to local and overall pressure and heat transfer characteristics of such a lattice material with tetrahedral unit cells. A single layer of the LFM with porosity of 0.938 is sandwiched between impermeable endwalls that receive uniform heat flux and the heat transfer is subjected to forced air convection. Experimental measurements with particle image velocity (PIV) and thermochromic liquid crystal (TLC), backed by computational fluid mechanics (CFD) simulations, revealed two dominant local flow features in the LFM. Distinctive vortex structures near the vertices where the LFM meets the endwalls and flow separation on the surface of LFM struts were observed. The vortex structures formed around the vertices include horseshoe vortices and arch-shaped vortices. The horseshoe vortex increases local heat transfer on the endwall region up to 180% more than that in regions where the least influence of the horseshoe vortex is present. The arch-shaped vortex behind the vertices creates regions of flow recirculation and reattachment, leading to relatively high heat transfer. The location of flow separation along the struts varies with the spanwise position due to the presence of vertices (or endwalls). The regions on the strut surface before flow separation contribute approximately 40% of the total heat transfer in the LFM. The delay of the flow separation leads to an increase in the overall heat transfer. Comparisons with foams and other heat dissipation media such as packed beds, louvered fins and microtruss materials suggest that the LFMs compete favourably with the best available heat dissipation media.

Analysis of Laminar Forced Convection of Air Crossflow in In-Line Tube Banks with NonSquare Arrangements

ABSTRACT Numerical analysis is made of forced-convection heat transfer in laminar two-dimensional steady crossflow in banks of plain tubes in square and nonsquare in-line arrangements. A finite-volume method with a nonorthogonal, boundary-fitted grid and co-located variable storage is used to solve the Navier–Stokes equations and energy conservation equation for a tube bundle with five longitudinal rows, including inlet and outlet sections. Local and overall heat transfer and fluid flow results are presented at combinations of transverse and longitudinal pitch-to-diameter ratios of 1.25, 1.5, and 2.0 at Reynolds numbers of 100 and 300 for a Prandtl number of 0.71. A comparison of the present study results with well-established experiments and empirical correlations showed good overall agreement. New equations are proposed for a correction factor for the effects of nonsquare arrangements on average friction factor.

Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition

Abstract We present a novel mechanical model for the extraction of outer core material upwards across the CMB into the mantle side region of D” and subsequent interaction with the post-perovskite (ppv) phase transition. A strong requirement of the model is that the D” region behaves as a poro-viscoelastic granular material with dilatant properties. Using new ab-initio estimates of the ppv shear modulus, we show how shear-enhanced dilation promoted by downwelling mantle sets up an instability that drives local fluid flow. If loading rates locally exceed c. 10−12 s−1, calculated core metal upwelling rates are >10−4 m/s, far in excess of previous estimates based on static percolation or capillary flow. Associated mass flux rates are sufficient to deliver 0.5% outer core mass to D” in < 106 yr, provided the minimum required loading rate is maintained. Core metal transported upwards into D” may cause local rapid changes in electrical and thermal conductivity and rheology that if preserved, may account for some of the observed small wavelength heterogeneties (e.g. PKP scattering) there.

Modeling Deformation-Induced Fluid Flow in Cortical Bone?s Canalicular?Lacunar System

To explore the potential role that load-induced fluid flow plays as a mechano-transduction mechanism in bone adaptation, a lacunar-canalicular scale bone poroelasticity model is developed and implemented. The model uses micromechanics to homogenize the pericanalicular bone matrix, a system of straight circular cylinders in the bone matrix through which bone fluids can flow, as a locally anisotropic poroelastic medium. In this work, a simplified two-dimensional model of a periodic array of lacunae and their surrounding systems of canaliculi is used to quantify local fluid flow characteristics in the vicinity of a single lacuna. When the cortical bone model is loaded, microscale stress, and strain concentrations occur in the vicinity of individual lacunae and give rise to microscale spatial variations in the pore fluid pressure field. Furthermore, loading of the bone matrix containing canaliculi generates fluid pressures in the contained fluids. Consequently, loading of cortical bone induces fluid flow in the canaliculi and exchange of fluid between canaliculi and lacunae. For realistic bone morphology parameters, and a range of loading frequencies, fluid pressures and fluid-solid drag forces in the canalicular bone are computed and the associated energy dissipation in the models compared to that measured in physical in vitro experiments on human cortical bone. The proposed model indicates that deformation-induced fluid pressures in the lacunar-canalicular system have relaxation times on the order of milliseconds as opposed to the much shorter times (hundredths of milliseconds) associated with deformation-induced pressures in the Haversian system.

Modelling Sand Production and Erosion Growth Under Combined Axial and Radial Flow

Abstract The paper investigates the phenomenon of sand production in both axial and radial flow conditions using a coupled particle erosion-fluid transport-stress model that has been developed recently by the authors. The motivation for such a study stems from the fact that perforations in a slotted oil well can modify both the local fluid flow and stress regimes, and hence influence sand production. Based on available lab test information, the authors examine sand production in a hollow cylinder test in which sand is being produced under combined axial and radial flow of oil in a sandstone specimen. Aspects such as sand flux and porosity evolutions, as well as erosion growth around the inner wall of the cylinder, are investigated using the numerical model in view of understanding the physics of sand production. Introduction Sand is produced in a porous granular material whenever sand particles are being dislodged from the matrix as a result of very high fluid pressure gradients, thereby leaving behind mechanically damaged zones. From a broader perspective, this phenomenon can be viewed as a particle fluidization with erosion process by which a sand matrix is disaggregated due to a combination of stress changes and fluid flow when fluid is aggressively pumped from a porous medium. By virtue of the complexity of the physics of the problem, several challenges are encountered in any numerical modelling endeavour. Various researchers, for instance Jensen et al.(1), have attempted to model the above mentioned physical phenomenon using numerical techniques based on the discrete element method. Alternatively, a continuum mechanics approach can also be adopted in which mass balance is applied to a three-phase system comprised of solid, fluid, and fluidized solid, as was first proposed by Vardoulakis et al.(2). This approach was subsequently extended by Wan and Wang(3, 4) to include the deformation of the solid matrix and address general initial boundary value problems. As such, a standard finite element technique combined with Newton-Raphson method was used with some success for the solution of resulting non-linear equations which involve fluidized solid concentration, fluid pressure, and porosity as primary variables. It was found that numerical results were corrupted with instabilities in the form of node-tonode oscillations or wiggles whenever the solved field variables suffered tremendous distortions with high gradients during sand production. In view of addressing the above mentioned numerical difficulty, Wan and Wang(5) have recently introduced new numerical techniques which are akin to stabilization methods, known as Streamline Upwind/Petrov-Galerkin (SUPG) and Galerkin Least Squares (GLS) formulations [see Brooks and Hughes(6) and Hughes et al.(7)]. In Wan and Wang(5), local field variables found in the governing equations, such as density, flux, and stress, are expanded into a Taylor series for a finite size domain. An optimized local mean technique was introduced based on concepts of Finite Increment Calculus(8) and second gradient theories(9). As such, the original form of the governing equations describing the physics of the problem is preserved, while additional terms leading to numerical stabilization naturally emerge during the numerical process.

Rectangular-Plate Turbulator Effects on Heat Transfer and Near-Wall Flow Characteristics in Fan Flows

ABSTRACTHeat transfer and near-wall flow characteristics in an inherently swirling fan flow, containing rectangular-plate turbulators with 45° and 90° angles of attack, were experimentally investigated. The heat transfer characteristics for uniform flows with the turbulators were also investigated for comparison. Eight heated aluminum plates, installed along a bottom duct-wall, were used as the heat transfer surfaces, which allows the studies of heat transfer variations along the duct and the studies of the relations between the local fluid flows and heat transfer variations. Three-component mean and fluctuating velocities were measured using a laser Doppler velocimetry to obtain the near-wall flow parameters, including the axial mean velocity, axial vorticity and turbulent kinetic energy. The temperatures on the eight heat transfer surfaces were measured using thermocouples to obtain the Stanton number distributions. Results suggest that the rectangular-plate turbulators in fan flows may cause the increases in the near-wall flow parameters and, consequently, augment the heat transfer, especially around the flow reattachment regions. Also, the rectangular-plate turbulator effect on heat transfer augmentation in fan flows may be as attractive as that in uniform flows at the investigated X/H ranges.

A new perspective on the coarse-grained dynamics of fluids.

A computational methodology is presented that is designed to model, at a coarse-grained level, the mesoscale dynamics of fluids and potentially other forms of soft matter. Within a molecular dynamics simulation, "ghost" particles of a specific size, corresponding to the fundamental length-scale of coarse-graining, are used as micro-probes designed to respond to local mesoscale fluid flows and stress gradients. A subsequent coarse-grained model is then developed that incorporates both the coarse-grained mesoscale dynamics and isothermal compressibility of the original microscopic system. The method is applied to water and methanol. A contrast with dissipative particle dynamics (DPD) is also presented.

The St Ives mesothermal gold system, Western Australia—a case of golden aftershocks?

The late orogenic evolution of the Archaean greenstone sequence in the Eastern Goldfields Province of the Yilgarn Craton in Western Australia is characterised by strike-slip tectonics and locally very high, fault-controlled, fluid fluxes. Fluid flow was associated with the formation of many fault-hosted and shear-zone-hosted gold deposits, which are commonly clustered adjacent to high displacement faults or shear zones. In the St Ives goldfield, near Kambalda, fluid flow in a gold-producing hydrothermal system was localised within arrays of low displacement faults and shear zones, which form part of the NNW trending, crustal-scale, Boulder–Lefroy fault system (BLFS). The numerous ore-hosting structures are kinematically related to sinistral to sinistral–oblique slip on the Playa Fault, which is a 20-km-long splay of the 200-km-long Boulder–Lefroy Fault. Most of the known gold mineralisation at St Ives occurs within an area of 20 km 2 immediately south-west of the Playa Fault. The distribution of low displacement faults and shear zones that host gold mineralisation is related to the presence of a kilometre-scale contractional jog (the Victory jog) and an associated imbricate thrust array on the Playa Fault. By analogy with modern seismogenic systems, the low displacement structures that localised fluid flow and gold mineralisation in the St Ives goldfield are interpreted as aftershock structures whose development was driven by major slip events on the BLFS. For large slip events on the BLFS, and mainshock rupture arrest at the Victory jog, modelling of co-seismic static stress changes indicates that most ore-hosting structures are localised within a crustal volume whose stress state was closer to failure as a consequence of Coulomb stress transfer. The modelling supports an interpretation that aftershock networks can form a high permeability damage zone that localises fluid flow and gold mineralisation within particular parts of crustal-scale fault systems. Both co-seismic stress transfer and time-dependent changes in fluid pressures, during post-seismic fluid redistribution, are implicated in driving the growth of low displacement, gold-hosting fault networks in the St Ives goldfield. Stress transfer modelling has application for area selection in exploration programs targeting mesothermal gold systems. Clustering of deposits hosted by aftershock fracture networks is favoured by the presence of major, long-lived jogs or bends that can repeatedly arrest ruptures propagating along high displacement faults.

Fault-segment rupture, aftershock-zone fluid flow, and mineralization

We propose that zones of transient high permeability around ancient fault systems can be predicted if fault segments and likely locations for paleo-rupture arrest are identified. Lode gold deposits in the Kalgoorlie terrane, Western Australia, are the products of focused fluid flow through faulted crust. Deposits in the Mount Pleasant area are clustered on small-displacement structures over ∼10 km of the >50-km-long Black Flag fault. Field relationships and net slip distribution along the fault indicate that the deposits are adjacent to, but not within, a kilometer-scale dilatant jog, where two segments of the fault are linked. On this basis we infer that the dilatant jog was a long-term rupture-arrest site. The observations are compatible with rupture on segments of the Black Flag fault changing stress in the surrounding crust and bringing specific zones closer to failure. By analogy with active seismogenic fault systems, those zones correspond to regions where aftershocks occur preferentially after failure. Stress-transfer modeling of the system helps explain the location of mineralized small-displacement structures around the Black Flag fault and indicates that gold deposits in the area are located on structures that became transiently permeable and localized fluid flow during repeated aftershock ruptures. Thus, localized through-flow, or mixing of fluids within fault systems, is likely to be controlled by the distribution of aftershocks following rupture events; this distribution is predictable.

Elastic wave velocities and permeability of cracked rocks

Cracks play a major role in most rocks submitted to crustal conditions. Mechanically, cracks make the rock much more compliant. They also make it much easier for fluid to flow through any rock body. Relying on Fracture Mechanics and Statistical Physics, we introduce a few key concepts, which allow to understand and quantify how cracks do modify both the elastic and transport properties of rocks. The main different schemes, which can be used to derive the elastic effective moduli of a rock, are presented. It is shown from experimental results that an excellent approximation is the so-called non-interactive scheme. The main consequences of the existence of cracks on the elastic waves is the development of elastic anisotropy (due to the anisotropic distribution of crack orientations) and the dispersion effect (due to microscopic local fluid flow). At a larger scale, macroscopic fluid flow takes place through the crack network above the percolation threshold. Two macroscopic fluid flow regimes can be distinguished: the percolative regime close to the percolation threshold and the connected regime well above it. Experimental data on very different rock types show both of these behaviors.

Simulating the hydrodynamic conditions in the United States Pharmacopeia paddle dissolution apparatus.

The objective of this work was to examine the feasibility of developing a high-performance computing software system to simulate the United States Pharmacopeia (USP) dissolution apparatus 2 (paddle apparatus) and thus aid in characterizing the fluid hydrodynamics in the method. The USP apparatus was modeled using the hydrodynamic package Fluent. The Gambit program was used to create a "wireframe" of the apparatus and generate the 3-dimensional grids for the computational fluid dynamics solver. The Fluent solver was run on an IBM RS/6000 SP distributed memory parallel processor system, using 8 processors. Configurations with and without a tablet present were developed and examined. Simulations for a liquid-filled vessel at a paddle speed of 50 rpm were generated. Large variations in fluid velocity magnitudes with position in the vessel were evident. Fluid velocity predictions were in good agreement with those previously published, using laser Doppler velocity measurements. A low-velocity domain was evident directly below the center of the rotating paddle. The model was extended to simulate the impact of the presence of a cylindrical tablet in the base of the dissolution vessel. The presence of the tablet complicated the local fluid flow, and large fluid shear rates were evident at the base of the compact. Fluid shear rates varied depending on the tablet surface and the location on the surface and were consistent with the reported asymmetrical dissolution of model tablets. The approach has the potential to explain the variable dissolution results reported and to aid in the design/prediction of optimal dissolution conditions for in vitro--in vivo correlations.