Unstable Flow Regime Research Articles

Hybrid Multiscale Finite Volume method for two-phase flow in porous media

We present a novel hybrid (or multiphysics) algorithm, which couples pore-scale and Darcy descriptions of two-phase flow in porous media. The flow at the pore-scale is described by the Navier–Stokes equations, and the Volume of Fluid (VOF) method is used to model the evolution of the fluid–fluid interface. An extension of the Multiscale Finite Volume (MsFV) method is employed to construct the Darcy-scale problem. First, a set of local interpolators for pressure and velocity is constructed by solving the Navier–Stokes equations; then, a coarse mass-conservation problem is constructed by averaging the pore-scale velocity over the cells of a coarse grid, which act as control volumes; finally, a conservative pore-scale velocity field is reconstructed and used to advect the fluid–fluid interface. The method relies on the localization assumptions used to compute the interpolators (which are quite straightforward extensions of the standard MsFV) and on the postulate that the coarse-scale fluxes are proportional to the coarse-pressure differences. By numerical simulations of two-phase problems, we demonstrate that these assumptions provide hybrid solutions that are in good agreement with reference pore-scale solutions and are able to model the transition from stable to unstable flow regimes. Our hybrid method can naturally take advantage of several adaptive strategies and allows considering pore-scale fluxes only in some regions, while Darcy fluxes are used in the rest of the domain. Moreover, since the method relies on the assumption that the relationship between coarse-scale fluxes and pressure differences is local, it can be used as a numerical tool to investigate the limits of validity of Darcy’s law and to understand the link between pore-scale quantities and their corresponding Darcy-scale variables.

Morphological analysis and evolution of buried tunnel valleys in northeast Alberta, Canada

Tunnel valleys are large elongated depressions eroded into unconsolidated sediments and bedrock. Tunnel valleys are believed to have been efficient drainage pathways for large volumes of subglacial meltwater, and reflect the interplay between groundwater flow and variations in the hydraulic conductivity of the substrate, and basal meltwater production and associated water pressure variations at the ice-bed interface. Tunnel valleys are therefore an important component of the subglacial hydrological system.Three-dimensional modelling of geophysical and lithological data has revealed numerous buried valleys eroded into the bedrock unconformity in northeast Alberta, many of which are interpreted to be tunnel valleys. Due to the very high data density used in this modelling, the morphology, orientation and internal architecture of several of these tunnel valleys have been determined.The northeast Alberta buried tunnel valleys are similar to the open tunnel valleys described along the former margins of the southern Laurentide Ice Sheet. They have high depth to width ratios, with undulating, low gradient longitudinal profiles. Many valleys start and end abruptly, and occur as solitary, straight to slightly sinuous incisions, or form widespread anastomosing networks. Typically, these valleys are between 0.5 and 3 km wide and 10 and 30 m deep, although the depth of incision along some thalwegs exceeds 100 m. Several valleys extend for up to 60 km, but most are between 10 and 30 km long.Valley fills comprise a range of lithofacies, including stacked sequences of diamict, glaciofluvial sands and gravels and glaciolacustrine silts and clays. Displaced bedrock, presumably of glaciotectonic origin, also occurs within several anastomosing valleys. Several channel bodies are exposed along a number of valley sections suggesting progressive valley development through repeated cycles of sediment discharge. Cut-and-fill structures that are capped by fine-grained sequences of rippled sand and mud-rich drapes within these channel bodies suggest unstable flow regimes within the valley and the discharge of sediment-laden basal meltwater under flood-like conditions followed by wane flow events or periods of lower meltwater discharge, likely concomitant with localized modification by glacial ice. Basal meltwater is inferred to have been released as episodic jökulhlaups beneath the western Laurentide Ice Sheet, which at times re-used existing valley systems, which were spatially and temporally stable features, and at other times incised new valleys.

Experiments and analysis of drainage displacement processes relevant to carbon dioxide injection

The motivation for this work is a dramatically improved understanding of the fluid mechanics of drainage processes with applications such as CO_{2} storage in saline aquifers and water-alternating-gas injection as an enhanced oil recovery method. In this paper we present in situ distributions of wetting and nonwetting fluids obtained during core-scale two-phase immiscible drainage experiments. The ratio of the viscosity of the resident fluid to that of the invading fluid varies across a range of 0.43 to 150. Saturation distributions observed during dynamic displacement experiments are surprisingly smooth and do not display only one or a few dominant fingers, contrary to the indications of the current literature. The analysis of the saturation distribution using the fractal dimensions of the dynamic three-dimensional saturation distributions suggests that the constitutive relationships for porous media, namely, the relative permeability functions, are history dependent. Accordingly, it is suggested that the nonlinear, unstable flow regime is the regime where efforts to improve physical understanding must be focused.

Numerical simulation of gas–solid flow in a conveying vessel

Conveying vessels, which are widely used in pneumatic systems, feature gas–solid flow with intensive inter-phase interactions and complex flow patterns. In this study, a two-fluid model was used to simulate flow behaviors in an experimental vessel. To model the gas phase, a k-ε turbulent model and the particle phase kinetic theory of granular flow were used. Three hydraulic stages were indicated: the rising, static and declining phases. Flow regimes similar to a spouted bed were formed near the wall, which became increasingly stable over time. Unstable flow regimes that consisted of bubbles and slugs were formed near the central part of the vessel. The velocity distributions of the gas and particles, the pressure profile and the secondary flow are described. A fitted curve of the output and input was obtained, and the influences of inlet conditions, such as velocity and pressure, were also investigated.

The energetics of error-growth and the predictability analysis in precipitation prediction

Sensitivity simulations are conducted in AREM (Advanced Regional Eta-Coordinate numerical heavy-rain prediction Model) for a torrential precipitation in June 2008 along South China to investigate the effect of initial uncertainty on precipitation predictability. It is found that the strong initial-condition sensitivity for precipitation prediction can be attributed to the upscale evolution of error growth. However, different modality of error growth can be observed in lower and upper layers. Compared with lower-level, significant error growth in the upper-layer appears over both convective area and high jet stream. It thus indicates that the error growth depends on both moist convection due to convective instability and the wind shear associated with dynamic instability. As heavy rainfall process can be described as a series of energy conversion, it reveals that the advection-term and latent heating serve as significant energy sources. Moreover, the dominant source terms of error-energy growth are nonlinearity advection (ADVT) and difference in latent heating (DLHT), with the latter being largely responsible for the rapid error growth in the initial stage. In this sense, the occurrence of precipitation and error-growth share the energy source, which implies the inherent predictability of heavy rainfall. In addition, a decomposition of ADVT further indicates that the flow-dependent error growth is closely related to the atmospheric instability. Thus the system growing from unstable flow regime has its intrinsic predictability.

Snow avalanche flow-regime transitions induced by mass and random kinetic energy fluxes

AbstractWe perform a linear-stability analysis of a system of ordinary differential equations describing the motion of dense flowing avalanches. The depth-averaged equations relate the mean translational velocity of the avalanche to the production of turbulent kinetic energy associated with the random motion of the snow granules. Flow friction is described by an extended Voellmy model, where both the Coulomb and turbulent resistance are related to the random kinetic energy. We identify two snow avalanche flow regimes: (1) an unstable frictional flow regime characterized by low random kinetic energy production and (2) a stable, collisional flow regime at the avalanche front. Flow-regime transitions are governed by the production of random kinetic energy, which is controlled by mass. The unstable regime is characterized by a saddle point which the avalanche encounters at the onset of motion and deposition. Depending on the release mass and/or mass growth, the avalanche head will either fluidize, leading to far-reaching avalanches, or starve and die out. At the tail of the avalanche this saddle point also controls the commencement of deposition and therefore the stopping behaviour of avalanches. We discuss the conditions for flow-regime transitions and the enhanced mobility of snow avalanches.

Shear Banding and Flow-Concentration Coupling in Colloidal Glasses

We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate γ(c) we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of γ(c) with concentration, and the fluctuations in the flow.

Home ranges and space use of muskrats Ondatra zibethicus in restricted linear habitats

AbstractModern farming practices in the midwestern United States have drastically altered the landscape. Most wetlands have been drained, and small streams are channelized to transport excess water away from tile‐drained agricultural fields. Loss of critical wetland habitat has shifted the distribution of muskrats Ondatra zibethicus, an economically important furbearer in the region, to highly altered riparian habitats with unstable flow regimes. However, information regarding home‐range size and space use for muskrats occurring in these linear habitats is lacking. We used location data from 26 radio‐marked muskrats to estimate home‐range size and space use in riparian habitats in an agroecosystem in east‐central Illinois, USA. Home ranges were highly linear and confined to stream bank edges. Contrary to our prediction, muskrats did not freely move to upland habitat (e.g. row‐crop agriculture) adjacent to stream edges to forage. Linear home ranges were longer for adults than for juveniles. Home‐range size also was related positively to number of burrows used by individuals. As expected, muskrats used space non‐randomly within linear home ranges with most movements aggregated around established bank burrows. Muskrats in riparian habitat are multiple central‐place foragers. Our study provides insight into how muskrats, a semi‐aquatic species affected by large‐scale landscape change, use space within highly restricted linear habitats.

Effects of flooding and riparian buffers on survival of muskrats (Ondatra zibethicus) across a flashiness gradient

Increased agricultural production within the Grand Prairie region, USA, has resulted in drainage of most natural wetlands within the landscape. Muskrats ( Ondatra zibethicus (L., 1766)) in this region have shifted much of their distribution to riparian habitats that have unstable flow regimes and flood inundation times that could be related to position within watersheds. We investigated predation risk of radio-marked riparian muskrats during flooding events in relation to landscape position. We used known-fate models and an information–theoretic approach to examine effects of age, season, hydrology, and riparian width on weekly survival rates. During flooding events, muskrats positioned farther from headwaters were displaced for longer, as well as exposed to predation from terrestrial predators for longer, than those positioned closer to headwaters. However, this increased exposure during floods did not translate into lower survival because most mortalities were due to predation by American mink ( Neovison vison (Schreber, 1777)) along stream edges during nonflooding periods. Weekly survival of muskrats was lower in winter (mean = 0.9377, SE = 0.1793) than in nonwinter (mean = 0.9770, SE = 0.0116) and was positively related to riparian width. Larger riparian buffers can increase muskrat survival in small streams and agricultural ditches within highly altered, human-dominated agroecosystems. Our study provides a rare example of linking riparian buffers to fitness for a stream-associated organism.

Finite element and sensitivity analysis of thermally induced flow instabilities

AbstractThis paper presents a finite element algorithm for the simulation of thermo‐hydrodynamic instabilities causing manufacturing defects in injection molding of plastic and metal powder. Mold‐filling parameters determine the flow pattern during filling, which in turn influences the quality of the final part. Insufficiently, well‐controlled operating conditions may generate inhomogeneities, empty spaces or unusable parts. An understanding of the flow behavior will enable manufacturers to reduce or even eliminate defects and improve their competitiveness. This work presents a rigorous study using numerical simulation and sensitivity analysis. The problem is modeled by the Navier–Stokes equations, the energy equation and a generalized Newtonian viscosity model. The solution algorithm is applied to a simple flow in a symmetrical gate geometry. This problem exhibits both symmetrical and non‐symmetrical solutions depending on the values taken by flow parameters. Under particular combinations of operating conditions, the flow was stable and symmetric, while some other combinations leading to large thermally induced viscosity gradients produce unstable and asymmetric flow. Based on the numerical results, a stability chart of the flow was established, identifying the boundaries between regions of stable and unstable flow in terms of the Graetz number (ratio of thermal conduction time to the convection time scale) and B, a dimensionless ratio indicating the sensitivity of viscosity to temperature changes. Sensitivities with respect to flow parameters are then computed using the continuous sensitivity equations method. We demonstrate that sensitivities are able to detect the transition between the stable and unstable flow regimes and correctly indicate how parameters should change in order to increase the stability of the flow. Copyright © 2009 John Wiley & Sons, Ltd.

LARGE-EDDY SIMULATION OF OPPOSING-JET-PERTURBED SUPERSONIC FLOWS PAST A HEMISPHERICAL NOSE

A supersonic flow past a hemispherical nose with an opposing jet placed on its axis has been investigated using large eddy simulation. We find that the flow behaviors depend mainly on the jet total pressure ratio and can be classified into three typical flow regimes of unstable, stable and transition. The unstable flow regime is characterized by an oscillatory bow shock with a multi-jet-cell structure and the stable flow regime by a steady bow shock with a single jet cell. The transition regime lies between the unstable and stable ones with a complex flow evolution. Turbulence statistics are further analyzed to reveal the relevant turbulent behaviors in the three flow regimes. The results obtained in this study provide a physical insight into the understanding of the mechanisms underlying this complex flow.

Two-phase flow instabilities in a silicon microchannels heat sink

Two-phase flow instabilities are highly undesirable in microchannels-based heat sinks as they can lead to temperature oscillations with high amplitudes, premature critical heat flux and mechanical vibrations. This work is an experimental study of boiling instabilities in a microchannel silicon heat sink with 40 parallel rectangular microchannels, having a length of 15 mm and a hydraulic diameter of 194 μm. A series of experiments have been carried out to investigate pressure and temperature oscillations during the flow boiling instabilities under uniform heating, using water as a cooling liquid. Thin nickel film thermometers, integrated on the back side of a heat sink with microchannels, were used in order to obtain a better insight related to temperature fluctuations caused by two-phase flow instabilities. Flow regime maps are presented for two inlet water temperatures, showing stable and unstable flow regimes. It was observed that boiling leads to asymmetrical flow distribution within microchannels that result in high temperature non-uniformity and the simultaneously existence of different flow regimes along the transverse direction. Two types of two-phase flow instabilities with appreciable pressure and temperature fluctuations were observed, that depended on the heat to mass flux ratio and inlet water temperature. These were high amplitude/low frequency and low amplitude/high frequency instabilities. High speed camera imaging, performed simultaneously with pressure and temperature measurements, showed that inlet/outlet pressure and the temperature fluctuations existed due to alternation between liquid/two-phase/vapour flows. It was also determined that the inlet water subcooling condition affects the magnitudes of the temperature oscillations in two-phase flow instabilities and flow distribution within the microchannels.

Properties of protein fibrils in whey protein isolate solutions: Microstructure, flow behaviour and gelation

We studied how protein fibrils (∼1 μm in length, prepared by heating at pH 2) can be used to modify structural properties of concentrated whey protein isolate (WPI) solutions. Using transmission electron microscopy, flow-induced birefringence and Thioflavin T fluorescence, we observed that the fibrils become shorter upon increasing the pH, and clusters were present at pH 5–7. For pH 3.5 and 7, the effect of adding fibrils to WPI solutions was studied. Different behaviour was observed, which makes this system rather complex. Rheological measurements showed that the presence of fibrils induced shear thickening (only at pH 3.5) and shear thinning behaviour, and resulted in an increase in viscosity and gel strength. Unstable flow regimes were observed for both pH values, which hint in the direction of phase separation. For pH 7, a phase separated dispersion was observed in the macrostructure after gelation using scanning electron microscopy.

Vent area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (Campi Flegrei, Italy): new insights from directional fabric through image analysis

In order to provide new information about the source area and depositional mechanisms of the Upper Member of the Neapolitan Yellow Tuff (NYT), a prominent pyroclastic deposit of the Campi Flegrei Volcanic District (southern Italy), statistics on directional fabric, by means of computer-assisted image analysis on 32 rock samples, were compiled. Seventeen samples were collected along vertical direction on two selected exposures and fifteen were taken from outcrops widely distributed all around the Campi Flegrei Volcanic District. Fabric measurements within the investigated successions reveal a vertically homogeneous direction of the mean particle iso-orientation, with considerable variability in the strength of particle iso-orientation even at cm-scale. The existence of particle iso-orientation can be related to continuous sedimentation from a concentrated bedload region beneath suspension currents, producing massive or inversely graded beds by traction carpet sedimentation. The considerable vertical variability in the strength of iso-orientation is the result of very unstable flow regimes, up to the extreme condition of discrete depositional events, with a variable combination of traction carpet and/or direct suspension sedimentation. The vertical homogeneity in the mean orientation values, found in the investigated sections, may derive from the sequential deposition of laminae to thin beds, whose relatively flat upper surfaces were unable to significantly deflect the depositional system of the following currents. According to the observed homogeneous mean particle orientation values along the investigated vertical profiles, samples collected through areal distribution are considered representative of the local paleo-flow directions of the whole deposit. The mean directions of the samples collected areally show two different coherent patterns which point to the existence of two different source areas. The first, which includes all samples from the northern outcrops, appears to converge in a narrow area about 2 km NE of the town of Pozzuoli, largely in coincidence with the inferred area on the basis of the pumice fall distribution. The second, which includes samples from Capo Miseno and Posillipo areas, points to the central part of the Pozzuoli Bay, about 4 km offshore the town of Pozzuoli.

Lane changing analysis for two-lane traffic flow

In this paper, the two-lane traffic are studied by using the lane-changing rules in the car-following models. The simulation show that the frequent lane changing occurs when the lateral distance in car following activities is considered and it gives rise to oscillating waves. In contrast, if the lateral distance is not considered (or considered occasionally), the lane changing appears infrequently and soliton waves occurs. This implies that the stabilization mechanism no longer functions when the lane changing is permitted. Since the oscillating and soliton waves correspond to the unstable and metastable flow regimes, respectively, our study verifies that a phase transition may occur as a result of the lane changing.

Management of karst aquifers in Serbia for water supply

Karst aquifers represent one of the main sources of water supply in Serbia. Wide distributions of karstic areas, abundant reserves, and excellent quality of karst groundwater have been the reasons for its extensive use in water supply systems throughout the country. In Serbia, 70 karstic sources have been tapped for centralized water supply, with the assessed minimal capacity exceeding 4.5 m3/s. Most of the large cities of Eastern and Western Serbia currently use karst groundwater for water supply; however, due to unstable flow regime when only natural springflow is tapped, numerous problems arise during the recession period (summer–autumn). During the last two decades, after favorable conditions had been confirmed through hydrogeological survey and feasibility studies, several newly constructed systems for artificial control of karst aquifers (mostly in Eastern Serbia) resulted in significantly improved water supply.

GENETIC STRUCTURE OF THE WESTERN NORTH AMERICAN AQUATIC GASTROPOD GENUS TAYLORCONCHA AND DESCRIPTION OF A SECOND SPECIES

The Bliss Rapids Snail (Taylorconcha serpenticola ) is a threatened species that ranges along a short reach of the middle Snake River in southern Idaho. Additional Taylorconcha populations of uncertain taxonomic status have recently been discovered in other portions of the Snake River basin (Owyhee River, lower Snake River). We investigated the phylogenetic relationships and population structure of these snails, together with two outgroups, using cytochrome c oxidase subunit I (COI) of mitochondrial DNA and the first internal transcribed spacer region between the 18S and 5.8S ribosomal DNA. These data show no sharing of haplotypes or genotypes among T. serpenticola and the Owyhee-Lower Snake populations, with both depicted as monophyletic units within the Taylorconcha clade. Both of these datasets and morphological evidence suggest that the Owyhee-Lower Snake populations are a distinct species, which we describe herein (T. insperata new species). Application of an available COI molecular clock suggests that Taylorconcha arose in the late Miocene, when ancestral Snake River drainage was impounded in an extensive lacustrine system (‘Lake Idaho’) in western Idaho. The shallow population structuring of T. insperata suggests that the lower Snake River was only recently colonized subsequent to incision of Hells Canyon, draining of Lake Idaho, and development of a through-going river in the late Neogene. The absence of significant genetic structure in T. serpenticola, which is attributed to the unstable course and flow regime of the middle Snake River during the Quaternary, suggests that this species can be treated as a single management unit.

Pressure effects on viscosity and flow stability of polyethylene melts during extrusion

In the present work, the effects of pressure on the viscosity and flow stability of four commercial grade polyethylenes (PEs) have been studied: linear-low-density polyethylene copolymer, high-density polyethylene, metallocene polyethylenes with short-chain branches (mPE-SCB), and metallocene polyethylenes with long chain branching (mPE-LCB). The range of shear rates considered covers both stable and unstable flow regimes. “Enhanced exit-pressure” experiments have been performed attaining pressures of the order of 500×105 Pa at the die exit. The necessary experimental conditions have been clearly defined so that dissipative heating can be neglected and pressure effects isolated. The results obtained show an exponential increase in both shear and entrance-flow pressure drop with mean pressure when shear rate is fixed and as long as flow is stable. These pressure effects are described by two pressure coefficients, βS under shear and, βE under elongation, that are calculated using time–pressure superposition and that are independent of mean pressure and flow rate. For three out of four PE, pressure coefficient values can be considered equal under shear and under elongation. However, for the mPE-LCB, the pressure coefficient under elongation is found to be about 30% lower than under shear. Flow instabilities in the form of oscillating flows or of upstream instabilities appear at lower shear rates as mean pressure increases. Nevertheless, the critical shear stress at which they are triggered remains independent of mean pressure. Moreover, it is found that the βS values obtained for stable flows do not differ much from the values obtained during upstream instability regimes, and differ really from pressure effects observed under oscillating flow and slip conditions.

Anti-Slug control of gas-lift wells - Experimental results

Unstable flow regimes occurring for multiphase flow in gas-lift wells have successfully been stabilized. using conventional linear control techniques. However, the control systems rely on downhole measurements which are at best unreliable, if at all available. In this paper, we employ a nonlinear observer for the states of the multi phase flow in the gas-lift well. The observer relies on topside measurements only, and downhole pressure for feedback control is estimated. Combining the non linear observer with conventional PI control of the downhole pressure, we demonstrate in laboratory experiments the potential for increasing production from gas-lift oil wells by stabilizing the multiphase flow.

Velocity field for polymer melts extrusion using particle image velocimetry: Stable and unstable flow regimes

Extrusion defects and flow instabilities are an important limitation in most polymer processing operations. Observation of the fluid kinematics to deduce the dynamical response of the polymer can be very useful to characterise these instabilities and understand the mechanisms involved in their triggering and enhancement. To do so, the flow of two linear polydimethylsiloxanes (PDMS) through an axisymmetric sudden contraction has been studied using pressure drop measurements as well as particles image velocimetry (PIV). This optical method for measuring flow velocities provides quantitative whole-field information even when flow conditions are non-stationary. It was shown that, for the two polymer melts studied, the flow is symmetric under stable conditions whereas non-symmetric and non-stationary flows are obtained under unstable conditions. These instabilities arise in the upstream part of the contraction. The extruded rod is then excited by these instabilities which generates the phenomenon of melt fracture at the die exit. In addition, the flow is found to be purely elongationnal on the axis during stable and unstable flow regimes. Under the experimental conditions investigated here, the resulting stretch rate on the centreline varies as x −3, x being the distance above the contraction plane. Moreover flow through contractions becomes unstable first around the centreline and close to the orifice plane. The instabilities then develop in the radial direction and invade progressively the entire upstream flow which completely looses its properties of symmetry: the flow is expected to have a spiralling motion and its two-dimensional (2D) projection looks like a knitting system. The frequency of the instabilities remains approximately the same while their amplitude increases with flow regimes. On the flow axis, instantaneous fluctuations of the velocity can reach ±30% of a mean value characteristic of a mean flow. These fluctuations are more important on each side of the axis where they can reach ±105% of a mean value.