Flow Regime Analysis Research Articles

Heat transfer measurement and flow regime visualization of two-phase pulsating flow in an evaporator

Heat transfer and flow regime of two-phase R134a pulsating flow in an evaporator have been studied experimentally in this work. Heat transfer coefficient was measured and liquid-vapor two-phase flow regimes was observed under different mass flux, inlet vapor quality and pulsating period. Results show that heat transfer can be enhanced by pulsating flow in short periods (<16 s) with a maximum enhancement of 28% compared with the continuous flow; a deterioration of heat transfer is also observed for flow under long pulsating periods for some conditions, which could be as much as 30%. The effect of mass flux and inlet vapor quality is complex and coupled with pulsation period, which might result from the difference of flow developing between different pulsating periods. The instantaneous flow regime is recorded by a high speed camera and statistically analyzed to explore the mechanism of pulsating flow on heat transfer performance. The statistical analysis of flow regime from the recorded video shows that the flow regime of pulsating flow at on-time remains the same as that of continuous flow with the same instant mass flux, and the flow regime at off-time is mostly stratified, stratified wavy flow depending on the pulsating period. The transition of stratified-wavy flow regime to intermittent, annular flow regime of pulsating flow is the main contribution to the enhancement of heat transfer while dry-out during off-time plays an important role in heat transfer deterioration.

An innovative model to evaluate fracture closure of multi-fractured horizontal well in tight gas reservoir based on bottom-hole pressure

Due to formation damage and fracture closure, the effective fracture half-length is usually much shorter than the designed half-length. However, available pressure transient analysis (PTA) models of multi-fractured horizontal wells (MFHWs) hardly consider the effect of non-uniform fracture closure of double-segment fractures (DSF) on transient pressure characteristics, which could bring about incorrect results since conductivity and flux density of fracture segment near wellbore are much larger than those of fracture segment far from wellbore. To fill this gap, this paper aims at presenting a novel approach to evaluate effective fracture properties through PTA more accurately. This new model allows each hydraulic fracture of MFHW consists of two individual segments with their own properties (e.g. length, conductivity and flux density, etc). Pressure and its derivative curves are developed for flow-regime analysis. The solution is validated with numerical results in Saphir. Sensitivity analysis further seek the feasible application on interpretation of effective fracture properties. The field application demonstrates the practical use of the proposed model in estimating fracture half-length with different conductivity during production stage to identify the extent of fracture closure using pressure data.

A semi-analytical approach for analysis of the transient linear flow regime in tight reservoirs under three-phase flow conditions

The majority of current techniques for production data analysis of hydraulically-fractured horizontal wells only consider single- or two-phase flow conditions. Three-phase flow is expected to occur in many practical scenarios of low-permeability reservoir production when water (frac fluid and/or formation) production is occurring along with hydrocarbon phases. In this paper, a new semi-analytical method is developed to allow production data analysis of wells producing from low-permeability (tight) reservoirs exhibiting three-phase during the transient linear flow period.The proposed technique is based on the modified black oil formulation and is developed by employing the Boltzmann transformation. The modified black oil formulation comprises three highly nonlinear Partial Differential Equations (PDE's) that describe the flow of water, oil and gas in the reservoir. The use of the Boltzmann transformation allows conversion of this set of governing fluid flow PDE's to a set of Ordinary Differential Equations (ODE's) that can be easily solved using numerical techniques such as the Runge-Kutta method. In this paper, oil relative permeability under three-phase flow conditions is modelled using Stone's well-known first model.A theoretical basis for the occurrence of straight lines on a plot of 1/q vs √t for any producing phase is provided herein. The slope of the line is a function of the flowing fluid properties, and the reservoir and fracture characteristics. One important advantage of using the Boltzmann transformation is that the linear flow parameter, xf√k, can be directly estimated as a part of the solution process without the need for defining any complex pseudo-variable transformations. The results demonstrate that the relative error in xf√k estimation is generally less than 10% for different fluid models and relative permeability curves. Moreover, the simultaneous solutions of the ODE's provide the pressure and phase saturations as unique functions of the Boltzmann variable; these can be used to find relationships between pressure and phase saturations that in turn can be utilized for other rate transient analysis applications. The derived pressure-saturation relationships are in excellent agreement with numerical simulation for different three-phase relative permeability models.The significance of this paper is that a new theoretical and practical framework is introduced for the first time to analyze three-phase production data for transient linear flow under constant flowing bottomhole pressure conditions. These conditions are of practical importance for many tight production scenarios where water, in addition to hydrocarbon phases, are mobile in the reservoir. The results are validated using numerical simulation.

Development of rating curve for Ofu River at Oforachi Hydrometric Station

The stage-discharge rating curve is widely used in the research for analysis of flow regime and design of hydraulic structure and river channel where the measurement of discharge is difficult especially in developing countries like Nigeria. This study sought to develop a stage-discharge relationship for Ofu River in Nigeria where water resources project planning has been limited by the unavailable of stream flow data. Discharge Measurement was carried out using Valeport Current meter while stage was measured using staff gauge at Oforachi Bridge hydrometric station from February, 2016 to January, 2017. The Stage and discharge results were used to determine the curve coefficient via regression analysis and finally the rating curve equation for the river at this station. The Oforachi Bridge serves as the permanent control, thus the curve is reliable.Keywords: Discharge, Efficiency, Ofu River, Rating curve, Hydrometric station

The cyclical nature of hydrological regime of a mountain and upland river in the upper Vistula catchment in the multi-year period of 1984–2012: A potential tool for paleohydrology analysis

The study assesses the cyclical nature of occurrences of wet and dry years in the catchment of the Dunajec – to the Nowy Targ-Kowaniec section and the Rudawa – section Balice. The analysis was based on the data of mean daily flows from the multi-year period of 1984–2012. Student's t-test was used in order to demonstrate the differences between the values of flows in dry and wet years. The relationship between the Mean Annual Flow (MAF) flows was determined in subsequent years by using the test of series. The cyclical nature of mean annual flows was examined by using a Fourier series. The probability of occurrence of wet and dry years was estimated by the conditional probability. The analysis showed that the mean annual flows in both catchments varied between dry and wet years. The greatest periodicity of MAF flows occurs in the Rudawa catchment; this can be the result of changing weather conditions determined by the predominant types of atmospheric circulation, as well as by the greater retention capacity of the catchment. The conditional probabilities of the occurrence of dry and wet years varied from 54% to 88%. The highest probabilities occur in the Rudawa catchment, where there is the greatest cyclicality of flows. Another study goal was to conduct an example study for paleohydrologists in analyzing long period times for flood observations of different tributaries, which are feeding the same river treating a time series analysis of flow regimes as a simple paleohydrological tool. As postulated by Gregory and Walling (1973), any study of water and sediment dynamics in the past introduces a promise for future investigations. In that sense, if have a similar case and data used here might be possible to analyze hydrological trends for catchments in the future.

Experimental study of vertical and horizontal two-phase pipe flow through double 90 degree elbows

This paper presents an experimental study of the characteristics of the two-phase flow through vertical and horizontal pipes connected using 90-degree elbows with an inner diameter of 101.6 mm. 66 flow conditions are performed in this experiment, covering bubbly, plug, slug, pseudo slug, and stratified flow for horizontal pipe, and bubbly, cap bubbly, churn turbulent, and falling film/annular flow for vertical pipe. A detailed flow regime analysis for both horizontal and vertical pipe flow are discussed. Flow regime maps for both horizontal flow and vertical downward flow agree with the existing maps (Mandhane et al., 1974; Qiao et al., 2017). The effect of the elbow on the flow regime transition is primarily discussed based on the experimental results and observations. The effect of elbow observed in this experiment mainly contributes to the large bubble breakup and the change of void distribution. Area averaged void fraction development is presented and the sharp drop of void fraction in vertical downward test section is observed which results from kinematic shock phenomenon. A frictional pressure drop analysis is also studied using Lockhart-Martinelli correlation and it is found that C = 40, 100, and 125 are correlated with the frictional pressure drop data for horizontal, vertical, and whole test sections with minimum differences. With the database, the drift flux model for vertical downward flow (Goda et al., 2003) is validated and the relative difference between model and data is 15.95%.

Flow behavior analysis of two-phase (gas/water) flowback and early-time production from hydraulically-fractured shale gas wells using a hybrid numerical/analytical model

In recent years, quantitative analysis of flowback and early-time production data for interpretation of hydraulic fracture properties in shale has received significant attention. With high-resolution phase rates and flowing pressures, analysis of flowback and early-time production data provides an early opportunity to determine key parameters for well performance forecasting. The main purpose of this paper is to quantify flow characteristics associated with two-phase flowback and early-time production of shale gas wells exhibiting both planar fractures and a complex fracture network using a rigorous and efficient flow model.To study flow characteristics of this complex system, a hybrid numerical/analytical model is developed in which matrix and hydraulic fracture flow is accounted for. The finite-difference method is used to numerically model unsteady-state two-phase fracture flow. The analytical matrix flow model, derived using the line-source function, is dynamically coupled with the fracture flow model by imposing continuity of pressure and flux on the fracture surface. The main advantage of the hybrid solution over more conventional numerical simulation is reduced model setup and run time, without loss of the important physics.Detailed flow regime analysis reveals that the gas/water flowback and early-production of a horizontal well with a planar fracture geometry may the following complex sequence of flow-regime: 1) radial flow of water in the fracture; 2) a first transient linear flow of water in the fracture; 3) a second transient linear flow of water in the fracture; 4) boundary-dominated flow of water in the fracture and gas linear flow in matrix. Water primarily depletes in the first three flow periods. Development of the water boundary-dominated flow and gas linear flow indicates the system indicate that the system is at the end of flowback period and proceeds early-time production. Specially, the second water linear flow period develops when both gas and water production are dominating the system, and is caused by the pressure maintenance associated with gas flux from matrix. A complex fracture network with the same total fracture length as a planar fracture system will exhibit the same sequence of flow regimes. Due to the accelerated water drainage caused by multiple contact points between fractures and the wellbore in complex fracture network, the system also exhibits a shortened second water transient linear flow period and earlier water boundary-dominated flow.Finally, the new model is also applied to match flowback data obtained from a multi-fractured horizontal well completed in the Marcellus Shale. The results demonstrate the practical application of the new model for deriving reservoir/fracture properties from flowback data and for forecasting.

Thread-based microfluidics: Flow patterns in homogeneous and heterogeneous microfiber bundles

Thread-based microfluidics has recently seen considerable developments in the domain of portable diagnostic systems, smart bandages and tissue engineering. Similarly to paper-based microfluidics, thread-based microfluidics uses the wicking of fibers to move fluids. It has the advantage of confining and guiding the fluid along the yarns in a one, two or three dimensional space.A global approach to the motion of fluids in yarns and fiber bundles has already been reported in the literature based on the Lucas–Washburn–Rideal law. However no detailed investigation of the flow pattern inside the bundle has been conducted, depending on the internal structure of the bundle. Especially when the bundle possesses heterogeneous wetting properties, such as two different wetting regions interior and exterior, different flow patterns may exist.In this work, we perform a theoretical and numerical analysis of the different flow regimes for homogenous and heterogeneous fiber bundles. It is demonstrated that a limited number of fibers is sufficient for thread-based capillary flows, and that a caging of the flow can be achieved by realizing a lyophobic envelope.

A Laplace-Domain Hybrid Model for Representing Flow Behavior of Multifractured Horizontal Wells Communicating Through Secondary Fractures in Unconventional Reservoirs

Summary In a multiwell pad, the chance of interwell communication increases because of the creation of primary and secondary fractures during hydraulic-fracture stimulation. The flow behavior associated with communicating wells is significantly different from that of a single isolated well, because of interplay of flow caused by the interconnected fractures, complex connections, and multiple production conditions. The main purpose of this paper is to develop a rigorous and efficient flow model and quantify flow characteristics of multiple pad wells communicating through primary and secondary fractures. In the model, matrix and primary- and secondary-fracture flows are captured. Fractures are explicitly represented by discrete segments. The Laplace-transform finite-difference (LTFD) method is used to numerically model fracture flow, with sufficient flexibility to consider arbitrary fracture geometries and fracture-conductivity distributions. The analytical matrix-flow model, derived with the line-source function in the Laplace domain, is dynamically coupled with the fracture-flow model, by imposing the continuity of pressure and flux on the fracture surface. Thus, a hybrid model in the Laplace domain is constructed. The main advantage of the solution occurring in Laplace domain is that computations can be performed at predetermined, discrete times, and with grids only for fractures. Thus, stability and convergence problems caused by time discretization are avoided, and the burden of gridding and computation is decreased without loss of important fracture characteristics. The model is validated through comparison with a fully numerical simulator and a semi-analytical model. Detailed flow-regime analysis reveals that pressure interference caused by communication significantly alters the flow signature compared with single (isolated) wells. Before interference, the communicating wells behave as single isolated wells, and will exhibit a fracture linear-flow period and possibly even a matrix linear-flow period. After interference, the flow behavior of the system will vary largely with different production strategies. When the communicating wells all operate under the constant-rate condition, the transient responses of the wells will gradually merge to develop another matrix linear-flow period. If the wells are operated under the constant-bottomhole-pressure (BHP) conditions, the response deviation caused by interference will increase with production; therefore, one of the wells will undergo a rate loss. The results of a sensitivity analysis for a two-well system demonstrate that the time to well interference is primarily determined by secondary-fracture conductivity, number of connections, and communicating-well operating conditions. With larger contrasts in these properties, interference time is accelerated. However, for different production strategies, the effects on the flow behavior after interference are variable.

Analysis of pressure behaviors and flow regimes of naturally and hydraulically fractured unconventional gas reservoirs using multi-linear flow regimes approach

This paper targets the applicability of multi-linear flow regimes approach in gas reservoirs having multiple porous media: matrix, natural fractures, and hydraulic fractures. The approach attempts to understand pressure profiles and flow regimes developed in some of unconventional shale layers that have already been naturally fractured. In this paper, the main focus is given to fluid flow phenomenon from the matrix towards naturally induced fractures and from these fractures to the hydraulic fractures. Studying the impact of matrix, natural fracture and hydraulic fracture parameters on reservoir performance is another objective for this paper.Purposefully, several mathematical models have been developed to investigate transportation phenomenon of reservoir fluid in porous medium. The porous medium is assumed to be finite rectangular reservoir having different configurations. It is divided into two zones; inner porous medium close to the horizontal wellbore where multiple-hydraulic fractures propagate to some distance and outer porous medium where naturally induced fractures exist in the drainage area close to the outer boundary. Reservoir fluids, in the outer zone, are assumed to flow from the matrix to natural fractures and then from natural fractures to the inner zone. In the inner porous medium, fluids flow from matrix to natural fractures in addition to fluids flowing from the outer porous medium. Then all fluids flow by natural fractures to hydraulic fractures and finally from hydraulic fractures to the wellbore.The mathematical models for fluid flow in the two zones of the reservoir have been solved and three main flow regimes have been characterized. For each flow regime, an analytical model is used to describe its pressure profile with time and production performance. Bilinear flow regime, typically observed at early time, refers to simultaneous linear flow inside hydraulic fractures and flow from inner porous medium to the hydraulic fractures while linear flow describes fluids flow from inner zone to hydraulic fractures and late linear or boundary dominated flow regime describes fluids flow from the outer to the inner porous media.The main outcomes of this study are: 1) The ability to characterize reservoirs with triple porous media using multi-linear flow regimes approach. 2) Understanding the impact on pressure distribution of inner porous medium caused by the characteristics of hydraulic fractures such as fracture width and conductivity in addition to hydraulic fracture storativity. 3) Understanding the effect of reservoir configuration, matrix, and natural fractures characteristics on pressure distribution in the outer porous media. 4) Pressure distribution and flow regimes obtained by applying transient flow approach are not similar to those obtained by applying pseudo-steady state flow approach. 5) Characterization of different linear flow regimes, controlled by different parameters, with respect to production time.

An apparent permeability model of shale gas under formation conditions

There are various types of pores in shale, mainly consisting of nanopores and micropores, causing flow regime variations and apparent permeability changes during the development of the reservoir. In this paper, a Knudsen number calculation model is proposed with the actual conditions of gas in a shale reservoir. Then, based on the distribution of pores in shale, an apparent permeability model is established using hydrodynamics, and a calculation method is put forward for the actual permeability of a reservoir. Finally, the Knudsen number model and the permeability correction coefficient model are used to analyze the flow regime and permeability correction coefficients in pores during the development of the shale gas reservoir. Results show that with a decreasing of pressure, the Knudsen number increases, the flow regime changes from continuous flow and slip flow to transition flow or free molecular flow. When the Knudsen number is Kn > 0.1, and with a further increasing of Kn, gas molecule slippage greatly intensifies and the permeability correction coefficient Kapp/Kd significantly increases. While the Knudsen number increases, the permeability correction coefficient significantly increases in the micropores and the small pores, but this does not appear in the macropores and the mesopores. These results can be used to guide flow regime analysis and production forecasting in shale gas reservoirs.

Evaluation of steady flow torques and pressure losses in a rotary flow control valve by means of computational fluid dynamics

In this paper, a novel design of a rotary hydraulic flow control valve has been presented for high flow rate fluid power systems. High flow rates in these systems account for substantial flow forces acting on the throttling elements of the valves and cause the application of mechanically sophisticated multi-staged servo valves for flow regulation. The suggested design enables utilisation of single-stage valves in power hydraulics operating at high flow rates regimes. A spool driver and auxiliary mechanisms of the proposed valve design were discussed and selection criteria were suggested. Analytical expressions for metering characteristics as well as steady flow torques have been derived. Computational fluid dynamics (CFD) analysis of steady state flow regimes was conducted to evaluate the hydraulic behaviour of the proposed valve. This study represents a special case of an independent metering concept applied to the design of power hydraulic systems with direct proportional valve control operating at flow rates above 150 litres per minute. The result gained using parametric CFD simulations predicted the induced torque and the pressure drops due to a steady flow. Magnitudes of these values prove that by minimising the number of spool's mobile metering surfaces it is possible to reduce the flow-generated forces in the new generation of hydraulic valves proposed in this study. Calculation of the flow jet angles was analytically verified by measuring the deflection of the velocity vector using flow velocity field distribution, obtained during visualisation of the results of CFD simulations. The derived calculation formulas can predict metering characteristics, values of steady flow torques and jet angles for the specified design and geometry of the suggested valve. The proposed novel structure of the flow control valve promises to attain improved controllability, reliability and efficiency of the hydraulic control units of heavy mobile machinery operating at high flow rates regimes.

On the CFD Analysis of a Stratified Taylor-Couette System Dedicated to the Fabrication of Nanosensors

Since the pioneering work of Taylor, the analysis of flow regimes of incompressible, viscous fluids contained in circular Couette systems with independently rotating cylinders have charmed many researchers. The characteristics of such kind of flows have been considered for some industrial applications. Recently, Taylor-Couette flows found an innovative application in the production of optical fiber nanotips, to be used in molecular biology and medical diagnostic fields. Starting from the activity of Barucci et al., the present work concerns the numerical analysis of a Taylor-Couette system composed by two coaxial counter-rotating cylinders with low aspect ratio and radius ratio, filled with three stratified fluids. An accurate analysis of the flow regimes is performed, considering both the variation of inner and outer rotational speed and the reduction of fiber radius due to etching process. The large variety of individuated flow configurations provides useful information about the possible use of the Taylor-Couette system in a wide range of engineering applications. For the present case, the final objective is to provide accurate information to manufacturers of fiber nanotips about the expected flow regimes, thus helping them in the setup of the control process that will be used to generate high-quality products.

Multiphase fluid flow and heat transfer characteristics in microchannels

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently, which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design.

Solid-Particles Flow Regimes in Air/Water Stratified Flow in a Horizontal Pipeline

Summary The entrainment of solid particles in crude oil occurs during production from reservoirs with low formation strength. The stationary solid-particles bed at the horizontal pipe bottom can cause operational problems such as production decline, excessive pressure loss, equipment failure, erosion, and corrosion. Solid-particles deposition can be managed by operating above the critical solid-particles-deposition velocity, which is the velocity that maintains the continuous movement of particles at the pipe bottom. Here, a comprehensive analysis of solid-particle flow regimes in stratified flow in a horizontal pipeline is presented, which is a novel contribution because it is applied to multiphase flow. The effect of concentration on the solid-particle flow regimes and identification of the critical solid-particles-deposition velocities for various particle concentrations are also investigated. The understanding of solid-particle flow regimes in pipelines for any given set of operational conditions is important for identifying the nature of particle interaction and movement. Experimental studies are conducted in a 4-in. horizontal pipeline for a stratified flow regime that uses air, water, and glass beads at relatively low solid-particles concentrations (&amp;lt;10,000 ppm). The effects of different experimental conditions, such as gas velocity, solid-particles concentration, and particle size, are investigated in this study. Six main solid-particles flow regimes in horizontal air/water flow are identified, and can be distinguished visually: fully dispersed solid flow, dilute solids at wall, concentrated solids at wall, moving dunes, stationary dunes, and stationary bed. Therefore, the critical solid-particles-deposition velocities are determined on the basis of the transition between moving (concentrated solids at wall/moving dunes, as appropriate) and stationary (stationary dunes/bed, as appropriate) solid particles. The experimental data show that with small particle size, the critical solid-particles-deposition velocity is almost independent of concentration, while with larger particle sizes, the critical velocity increases with the concentration.

CFD analysis of flow regimes in airlift reactor using Eulerian‐Lagrangian approach

AbstractThe hydrodynamic aspects of the concentric tube airlift reactor have been studied using two‐phase CFD simulations with a Eulerian‐Lagrangian approach. The three‐dimensional CFD simulations are carried out with the experimental geometry of airlift reactor (H/D = 12) reported in the published literature. The standard k‐ϵ turbulence model is used with additional consideration of production and dissipation of buoyancy‐induced turbulence. Two different bubble size distributions (BSDs) and their Sauter mean diameters have been considered to represent the gas distribution at sparger in the reactor. This study shows that the BSD1 (1–5–10 mm) containing a high fraction of small bubbles (≤ 5 mm) represents the hydrodynamics of flows appropriately as compared to the BSD2 (5–10–15 mm) containing a high fraction of large bubbles (≥ 10 mm) or the single size bubble diameters (5.25 mm and 10 mm). Further, three regimes of operation such as no gas bubbles in the downcomer (regime I), stationary gas bubbles in the downcomer (regime II), and gas bubble recirculation from the downcomer section to the riser section (regime III), are verified using CFD simulations with Lagrangian particle tracking. Satisfactory agreement (within 15 % deviation) with the experimental data was observed for parameters such as the gas holdups in the riser and downcomer and the liquid circulation velocity in the flow regimes I and II for BSD1.

Geological and geotechnical assessment of some derelict barite fields in the Abakaliki Basin, Nigeria

The proliferation of abandoned surface excavations and subsequent derelict of ore fields not only contribute to land degradation and landscape disruption but also result in loss of economic deposits, dearth of mining data and generation of unprotected spoils with known potential to contaminate ground and surface water sources. This study adopted field description and measurements, laboratory analysis and numerical simulations techniques to evaluated some abandoned barite fields in Nigeria with the aim of factoring geological and geotechnical issues that render excavations derelict. The modes of occurrence, structural styles and lithofacies constitute a complex and highly varied geologic setting for ore incidences and lead to significant errors in interpreting their geophysical signatures and obtaining reliable ore quantity estimates. Ore grades vary widely and groundwater flow is driven by structural dips and topographic gradient with the ores and intrusive rocks forming as seals. The extraction of the permeable barriers using surface excavations leads to modification of hydraulic gradient distribution around the excavations leading to groundwater incursion and associated slope failures. The use of diversion ditches, sump and pump technique and slope unloading will achieve stable excavations. Unreliable reserve estimates, ore grade migration, groundwater invasion and resulting slope distress are the major impediments that render surface excavations derelict. Optimum location of excavations should be preceded by exploratory drilling and analysis of groundwater flow regime.

Flow regime analysis for geologic CO2 sequestration and other subsurface fluid injections

Carbon dioxide (CO2) injection into a confined saline aquifer may be modeled as an axisymmetric two-phase flow problem. Assuming the CO2 and brine segregate quickly in the vertical direction due to strong buoyancy, and neglecting capillary pressure and miscibility, the lubrication approximation leads to a one-dimensional nonlinear advection-diffusion equation that describes the evolution of the sharp CO2-brine interface. The interface evolution is driven by two forces: the force from fluid injection (the lateral pressure gradient due to injection), and buoyancy. Analytical solutions can be derived when one of the two forces is dominant. The solutions depend on the viscosity ratio (M) between the displaced and injected fluids, and a buoyancy parameter (Γ) that measures the relative importance of buoyancy compared to the driving force from injection. Different combinations of these two parameters give different forms of the solutions. But for all the solutions, the radius of the lateral spreading of the injected fluid follows,r∝t1/2 with the proportionality coefficient differing for the different solutions. In this paper, we identify the kinds of solutions appropriate for practical CO2 injection projects as well as other subsurface fluid injection applications. We use data from eight CO2 injection projects, twenty-four acid gas injection projects, two liquid waste disposal projects, and one CO2-WAG enhanced oil recovery project. The solutions provide guidance for the expected behavior of fluid spreading under the different injection operations while providing general insights into overall fluid flow behavior.

THE WIND OF ROTATING B SUPERGIANTS. I. DOMAINS OF SLOW AND FAST SOLUTION REGIMES

ABSTRACT In the scenario of rotating radiation-driven wind theory for massive stars, three types of stationary hydrodynamic solutions are currently known: the classical (fast) m-CAK solution, the Ω-slow solution that arises for fast rotators, and the so-called δ-slow solution if high values of the δ line-force parameter are allowed independently of the rotation speed. Compared to the fast solution, both “slow solutions” have lower terminal velocities. As the study of the parameter domain for the slow solution is still incomplete, we perform a comprehensive analysis of the distinctive flow regimes for B supergiants that emerge from a fine grid of rotation values, Ω, and various ionization conditions in the wind (δ) parameter. The wind ionization defines two domains: one for fast outflowing winds and the other for slow expanding flows. Both domains are clear-cut by a gap, where a kink/plateau structure of the velocity law could exist for a finite interval of δ. The location and width of the gap depend on T eff and Ω. There is a smooth and continuous transition between the Ω-slow and δ-slow regimes, a single Ω δ-slow regime. We discuss different situations where the slow solutions can be found and the possibility of a switch between fast and slow solutions in B supergiant winds. We compare the theoretical terminal velocity with observations of B and A supergiants and find that the fast regime prevails mostly for early B supergiants while the slow wind regime matches better for A and B mid- and late-type supergiants.

A Comparison of the Wake Structures of Scale and Full-scale Pedalling Cycling Models

This paper presents a novel approach to better understand the unsteady aerodynamics associated with a dynamically pedalling cyclist. Using high resolution Particle Image Velocimetry (PIV) in a water channel, the large-scale wake structure is analysed for various phases of the crank cycle of a 1:4.5 scale-model cyclist/bicycle under both static and pedalling conditions. Both quasi-steady and dynamic pedalling leg results are compared with detailed velocity field surveys made in the wake of a full-scale pedalling cyclist mannequin of similar geometry and position in a wind tunnel. A time-averaged and phase-averaged analysis of the various flow regimes that occur throughout the pedal stroke shows good agreement between scale-model and full-scale mannequin investigations. This highlights the robustness of the formation of the primary wake flow structures when subjected to varying Reynolds number, bicycle/rider geometry and quasi-steady/dynamic pedalling conditions.