Fluid Column Research Articles

Linear stability analysis of two fluid columns of different densities and viscosities in a gravity field

The linear stability of a vertical interface separating two miscible fluid columns of different densities and viscosities under the influence of gravity is investigated. This flow possesses a time-dependent reference state (each column accelerates at different rates owing to their different densities) and the interface thickness grows as the square root of time (by diffusion). Numerical integration of the linear initial-value problem is carried out and discussed in detail as a function of vertical and spanwise wavenumbers and the flow parameters. Adjoint-based optimization is performed in order to determine initial conditions that lead to maximum growth of disturbances in finite time. Results indicate that the rate of growth of the perturbation energy at small wavenumbers (less affected by viscosity initially) is dominated by two-dimensional modes (no spanwise variation). Substantial transient growth is observed at higher wave modes initially, followed by asymptotic decay of the perturbations at large time. Sensitivity of perturbation growth with respect to initial time, density and viscosity ratios is investigated. This work is complementary to previous inviscid analysis of this configuration, which showed that the interface was unconditionally unstable at all wave modes, even in the presence of surface tension, and that instability grew as the exponential of time squared.

\u2018Level-off\u2019 cement plugging method to cure lost circulation verified with case studies

Controlling lost circulation during drilling operations in a reservoir prone to fluid losses is typically remedied by cement squeezing or plug setting as the last resort. The aim being to minimize or stop drilling fluid losses and to regain full returns at surface, and to maintain wellbore integrity. Different placement methods of cement plugs have been discussed in detail in the literature, except for the ‘level-off’ method, which can be effective for curing complete loss circulation cases. Following modeling and calculations of this cement plug placement method, its design and execution procedures are discussed, together with two successful field cases in highly fractured carbonate reservoirs in the Middle East. Using drill pipe and a Retrievable-Test-Treat-Squeeze (RTTS) packer, set with some spacing from the loss zone, the method entails that the cement slurry is allowed to drop by gravity in order to cure lost circulation. As the column of fluid, mud and slurry in the well exceeds formation pore pressure, i.e., overbalanced conditions, a volume of acid-soluble cement slurry is allowed to slowly drop and freely penetrate the formation, i.e., through its fractures or caverns. During the penetration of this viscous slurry into the loss zone, the cement slurry can set and the fracture or fissure openings are plugged. Presented are detailed design calculations for the level-off placement technique, determination of required cement slurry and displacement volumes, and recommended displacement and RTTS packer setting depths. The expected depth of the top of cement plug is estimated. The design parameters are compared with field cases and explanations are given for possible discrepancies. Success of the operation is discussed in terms of final mud loss after cement plugging and Non-Productive Time mitigation. Detailed field procedures and execution are also presented. The level-off job is already practiced by the industry, but it is not published in the literature, in some cases they have different methods with causing some errors. To the best of authors’ knowledge, this is the first detailed description and stepwise calculation of the level-off cement placement technique in the literature.

Thermodynamic equilibrium of a fluid column under the influence of gravity

Abstract According to classical and statistical thermodynamics, a well-mixed fluid mass has a uniform temperature and is at thermodynamic equilibrium. It independent of the gravitational field. However, large well-mixed fluid masses, for example, in atmospheres and oceans, often are isentropic. One has attributed that to the influence of gravity. This has raised the question if such masses can go to a different restricted thermodynamic equilibrium with uniform entropy. Discussions on this issue have continued for three centuries without finding a final answer. This paper presents another analysis of the question if a fluid mass under the influence of gravity may go to a restricted equilibrium state with uniform entropy. At first, it analyses previous work as a background study. Then, it describes a kinetic model for the motion of fluid parcels in a vertical fluid column. This model is the tool for studying if and how the column may go to an isentropic equilibrium. The kinetic model supports the hypothesis that a fluid column under the influence of gravity may go to a restricted equilibrium state with an isentropic temperature profile. A statically unstable column can reach that state spontaneously, while its entropy increases and gravitational potential energy decreases. The latter energy is the source of the kinetic energy for the motion of its fluid parcels, driving the internal heat transfer that results in the isentropic profile.

Liquid-liquid capillary rise

We study the dynamics of capillary rise of a liquid-liquid interface using two immiscible liquids. By adjusting the height of the less dense fluid column, we measure the rise of the ensemble in the tube. A theoretical model is proposed that captures the time dependence of the position of the interface. Particular attention is given to the dynamics of the oil-water interface. Different types of flow regimes are identified and compared with previous theoretical and experimental findings. The theoretical model was found to be in good qualitative agreement, even though the contact angles of the two interfaces do not remain constant during the elevation of the fluid.

Rock mechanics and wellbore stability of deep shale during drilling and completion processes

The seepage coupling effect between the borehole rock mass and borehole fluid and the effective stress field distribution around the wellbore are obviously different under different working conditions of drilling, fracturing and completion, which affect the stability of horizontal borehole of shale gas reservoir. The physical, chemical, and mechanical properties of the shale in Longmaxi formations in a block of China immersed with different working fluids were tested experimentally. Combined with the experimental data, a theoretical model was established to evaluate and analyze the wellbore stability of Longmaxi formations shale under different working conditions of drilling, fracturing and completion. The results show that the shale of the Longmaxi Formation is a typical hard and brittle shale, the bedrock is dense, with high mechanical strength and weak hydration and expansion ability. The immersion effect of different working fluids has little influence on the expansion and mechanical properties of shale bedrock. However, the mechanical strength of shale with relatively developed bedding fractures is low, and the immersion effect of different drilling fluid further weakens the mechanical strength, resulting in obvious anisotropy of mechanical strength of underground rock. The coupling effect of seepage between wellbore and formation has obvious influence on the dynamic distribution of borehole pore pressure and wellbore stability under different working conditions. The effective fluid column pressure at the bottom of the hole is the highest during the fracturing process, which leads to the increase of pore pressure near the wellbore, and then to the increase of collapse pressure around the borehole wall. The equivalent density of collapse pressure rises to the maximum of 1.93 g/cm3, and the wellbore stability is the worst in the fracturing process. The wellbore stability is secondary in the drilling process, and the wellbore stability is good when drilling along the direction of the maximum horizontal principal stress, and the equivalent density of collapse pressure is 1.69 g/cm3. During the completion process, the formation fluid flows into the wellbore, the pore pressure in the borehole is relieved, and the equivalent density of the collapse pressure is reduced to approximately 1.35 g/cm3, the wellbore stability is the best and open hole completion can be attempted in the horizontal well section. The results provide a certain theoretical reference for the selection of the drilling and completion schemes in shale formation.

Study For Water Shutoff Using Tandem Through Tubing Bridge Plugs Technique, Case Study

Increasing water production rate is from one of the well knownmain problems that face any oil producer in the world. Although this problem is most happen in old wells, this can also happen in new drilled wells as well. It causes differenteconomic problems for oil produced companies. First, increasing water production affectsproduced wells performance and reduced their production lifetime. The high percentage of the water in the wellbore increasing the hydrostatic pressure (weight of the fluid column) which cause increasing in the lifting power required. which increasingoil and gas production cost per barrel and causing toreduce the drawdown. As an example, if the well is gas lift well, the quantity of gas needs to be injected for lifting the fluid from the wellbore to the surface is increasing with the production of heigh water cut than without producing water. Increasing water cut percentage and Water production also increases the possibility of corrosion, scale and has negative effect on the field equipment starting from the wellbore itself to the surface facilities. Also another serious and important problem is increasing produced water separating cost, treatment, and disposal is a great challenge to oil producers budgets. It costs almost $1.1 billion/year in averageforseparation and disposal of the produced water. Found solution for that type of production help in minimizingcost for the operators and increasing the yearly profit of their operations. Then, water shutoff jobs and operations are very essential. Finally, bythe good understandingfor the formation characterizations and otherfield problems of the field, we can avoid un-necessary water production during the wellbore designing stage.Through tubing bridge plugs (TTBPs) as one of water shut-off (WSO) workovers in the Belayim Fields resulted in average gradualproduction increasing of 2450 bopd.The Average water cut percentage (WC) reduced from 54% to 15%. 87WSOworkoversjobs done since December 1991. Technical evaluation and economical income successful evaluation approaches 90%. Just below 4.5 million dollars companies spent for an average cost of 60,500 dollars per job. Costs spent in less than two days equal to using income from a13 dollars per barrel from crude oil price. Depend on achieved results during the last 4.5 years, these WSO (water shut off)workoversdone in Belyim Field as an effective and on-going cases history for running and set zone water production control. Water production probleminBelayim Field gradually increasing during the last 10 years.Which result indropping inoil production rates.Depend on the reservoir characteristics; lower water zone isolated using through tubing bridge plugs (TTBPs)usingSchlumberegerelectric line units. Dump bailer tools used to put a 14 feet cement cap above the TTBPs to have means of a permanent pressure seal. After 24-hourswaiting on cement time, wells returned back to production with a great change and higher oil rate and very good reduction in Water Cut. Rigless cost for TTBP water shut off workover becomes much less than conventional rig water shut offoperations which result in averaged more than 450,000 dollars per job. Before December 1990,conventional rig operationsWSOwasonly the method used in Belayim Field. now, riglessWSOworkovers becomesa very important for reducing cost or to control cost with financial language.Rigless WSOoperations becomes alsovery important reservoir control tool forincreasing oil production and reducing water production which helps to save reservoir energy.

Investigation on the Controlling Factors of Pressure Wave Propagation Behavior Induced by Pulsating Hydraulic Fracturing

Summary Pulsating hydraulic fracturing (PHF) is a promising fracturing technology for unconventional reservoirs because it could improve the hydraulic fracturing efficiency through inducing the fatigue failure of reservoir rocks. Understanding of the pressure wave propagation behavior in wellbores and fractures plays an important role in PHF optimization. In this paper, a transient flow model (TFM) was used to describe the physical process of pressure wave propagation induced by PHF, and this model was solved by the method of characteristics (MOC). Combination of the TFM and MOC was validated with experimental data. The impacts of controlling factors on the pressure wave propagation behavior were fully discussed, and these factors include the frequency of input loading, an injection mode, an injection position, and friction. More than 10,000 sets of pressure wave propagation behaviors in different scenarios were simulated, and their differences were illustrated. In addition, the generation mechanisms of different pressure wave propagation behaviors were explained by the Fourier transform theory and the vibration theory. The important finding is that there is resonance phenomenon in the propagation of the pressure wave, and the resonance frequencies are almost equal to the natural frequencies of a fluid column. As a consequence of resonance phenomenon, the amplitudes of bottomhole pressure (BHP) and fracture tip pressure will increase sharply when the input loading frequency is close to the resonance frequency and less than 5 Hz; otherwise, the resonance phenomenon will disappear. Furthermore, an injection mode can alter the resonance frequency and the amplitude and frequency of the induced pressure wave. In addition, a friction effect can significantly decrease both the resonance frequency and the resonance amplitude. These findings indicate that the optimized input loading frequency should be close to the natural frequency of a fracturing fluid in a wellbore to enhance its BHP.

A Novel Technique for Donor Insertion and Unfolding in Descemet Membrane Endothelial Keratoplasty.

To describe a novel technique for insertion and unfolding of donor Descemet membrane (DM) scroll during DM endothelial keratoplasty (DMEK). DMEK was performed in 20 eyes. The donor DM scroll was loaded into a glass injector, and the fluid column was manipulated to achieve a double scroll configuration. After performing DM stripping and inferior iridectomy, an anterior chamber (AC) maintainer was placed in position. The glass injector was inserted into the AC and the tip rotated to ensure correct orientation of the DM scroll. After disconnecting the IV tubing from the AC maintainer, the donor DM scroll was delivered into the AC. Donor unfolding was performed using surface tapping techniques. An air bubble was injected under the graft to obtain a complete AC air fill. The outcomes of this surgical technique were analyzed. Donor DM insertion and unfolding were performed without any complications in all eyes. Double scroll configuration of donor DM in the injector could be achieved in 17 of 20 eyes (85%). A controlled delivery, maintaining correct scroll orientation, was possible in all eyes. Intraoperative duration from insertion to unfolding was 252.7 ± 45.4 seconds. A clear cornea was achieved in all eyes, with 75% of eyes (15/20) achieving corrected distance visual acuity of 20/20 or better, at average follow-up duration of 13.3 ± 1.3 months. Endothelial cell loss at 6 months follow-up was noted to be 16.7%. Partial donor detachment was noted in 2 eyes (10%), which resolved spontaneously. Our novel technique of donor DM insertion allows easy donor insertion and unfolding, thereby simplifying the DMEK procedure.

Tornado-like vortices in the quasi-cyclostrophic regime of Coriolis-centrifugal convection

Coriolis-centrifugal convection () in a cylindrical domain constitutes an idealised model of tornadic storms, where the rotating cylinder represents the mesocyclone of a supercell thunderstorm. We present a suite of direct numerical simulations, analysing the influence of centrifugal buoyancy on the formation of tornado-like vortices (TLVs). TLVs are self-consistently generated provided the flow is within the quasi-cyclostrophic (QC) regime in which the dominant dynamical balance is between pressure gradient and centrifugal buoyancy forces. This requires the Froude number to be greater than the radius-to-height aspect ratio, . We show that the TLVs that develop in our simulations share many similar features with realistic tornadoes, such as azimuthal velocity profiles, intensification of the vortex strength, and helicity characteristics. Further, we analyse the influence of the mechanical bottom boundary conditions on the formation of TLVs, finding that a rotating fluid column above a stationary surface does not generate TLVs if centrifugal buoyancy is absent. In contrast, TLVs are generated in the QC regime with any bottom boundary conditions when centrifugal buoyancy is present. Our simulations bring forth insights into natural supercell thunderstorm systems by identifying properties that determine whether a mesocyclone becomes tornadic or remains non-tornadic. For tornadoes to exist, a vertical temperature difference must be present that is capable of driving strong convection. Additionally, our predictions dimensionally imply a critical mesocyclone angular rotation rate of . Taking a typical mesocyclone height of , this translates to for centrifugal buoyancy-dominated, quasi-cyclostrophic tornadogenesis. The formation of the simulated TLVs happens at all heights on the centrifugal buoyancy time scale . This implies a roughly 1 minute, height-invariant formation for natural tornadoes, consistent with recent observational estimates.

Nonlinear hydromagnetic instability of oscillatory rotating rigid-fluid columns

The current paper is concerned with the nonlinear stability analysis of rotating magnetic fluid columns. The rotation sources are a mixture of both uniform and oscillating behavior. The motivation behind tackling this topic is the increasing interest in atmospheric and oceanic motions. The system consists of two magnetic phase fluid that fills two infinite vertical cylinders. An azimuthal uniform magnetic field is penetrated on the system. The governing equations of motion, in terms of the Coriolis force and reduced pressure, along with Maxwell’s equation in the quasi-static approximations are considered. Consequently, the disturbance of the interface has an azimuthal behavior. The fluids are fully saturated in porous media. In light of the implication of the nonlinear boundary conditions, the solutions of the linearized equations of motion resulted in a nonlinear characteristic dispersion equation. Utilizing the homotopy perturbation technique, this equation is analyzed. A modification of the latter equation is made to seem like a nonlinear Klein–Gordon equation. The stability criteria are realized in linear as well as nonlinear approaches. A set of diagrams is graphed to illustrate the effects of several non-dimensional numbers on the stability profile in resonance as well as non-resonance cases.

Combination of electrical submersible pump and gaslift for offshore wells in Vietnam

Artificial lifting systems have been in use for a long time all over the world. Artificial lifting systems act on the fluid column in the tubing to support and push the fluid column to the surface. Through the use of each method separately, we can analyze the advantages and disadvantages, the impacts and limitations of each method on the fluid column in the tubing, from which we can design an artificial lifting system to optimize production. In addition, practical limitations also have negative effects on the system such as injection pressure and injection gas available causing a restriction on the production when using GL, or pump horsepower and power supply causes limitations when using ESP. For those reasons, we can combine the two methods together to create an efficient production system. In this study paper, the effects on the fluid column in the tubing are analyzed when combining two methods, which are the GL method and the ESP method. Each method causes a different effect on the fluid column. That difference in this paper will be analyzed based on the "equivalent depth" method, which will represent the pressure points at each position in the tubing when using each artificial lifting method. In addition, we will use the "nodal analysis" method to find out the actual product of the production system when using a combination of the two methods. Based on the analysis of two methods "equivalent depth" and "nodal analysis", it has been proved that it is feasible and appropriate to combine the two methods GL and ESP for many special production wells that can be used for specific fields, such as deep wells. The systems of this combination can be used for wells for long time with respect to the pressure drop over time.

Динамика магнитных жидкостей, подвергающихся колебательному сдвигу

The purpose of the work: to study the dynamics of the volume of a magnetic fluid levitating in an external magnetic field experiencing an oscillatory shift. Metods. The experiments were carried out on installations developed on the basis of known methods and equipment for magnetic measurements and manufactured independently. We study samples of magnetic fluid based on magnetite Fe3O4 stabilized with oleic acid. Kerosene was used as a carrier fluid. Results. Samples of a magnetic fluid with different physical parameters are considered, and the dependence of the viscosity on the magnetization of the magnetic field is investigated. The increase in viscosity in a magnetic field upon the action of a levitating MF column on the interface can be explained by an increase in particle interactions, which lead to MF microstructuring in the near-wall layer. The microstructure of the sample and the presence of large magnetic particles exert the greatest influence on the dynamics of a magnetic fluid, which undergoes an oscillatory shift and magnetoviscous effect. Conclusion. An assessment of the viscous magnetism effect in a thin near-wall layer of a column of magnetic fluid vibrating in a tube is made when a strong magnetic field is applied. The calculation of the viscosity value is carried out according to the formula obtained on the basis of two different theoretical approaches. The results of the work can be used to develop a method for express testing of magnetic fluid samples, as well as to develop acceleration and vibration sensors based on magnetic fluids. This method can be used to study magnetophoresis and aggregation of nanoparticles. Information on the viscous magnetic effect will be valuable for microfluidic technologies, where magnetic fluids with anisotropic particles, functionalized with various specific surfactants and flowing in microchannels, are used.

Borehole wall tensile caving instability in the horizontal well of deep brittle shale.

With the increasing drilling depth of shale formation, downhole collapse is a frequent occurrence, which often manifests as borehole wall caving. We used the deep shale of the Longmaxi Formation to conduct the mechanical loading and unloading experiments under different downhole working conditions and a theoretical evaluation method of borehole wall caving and instability was proposed. As the confining pressure and axial load increased, the acoustic velocity increased. When a certain value was reached, the acoustic velocity of the rock mass had minimal changes. As the confining pressure continued to unload and decrease, the acoustic velocity decreased. At the moment of core failure, the acoustic velocity suddenly dropped. When the axial force of loading was constant, the unloading speed of confining pressure increased, and shale could easily be destroyed. The pressure at the well bottom changed rapidly, the likelihood of borehole wall failure increased. The deep shale has high brittleness. Under the bottom-hole pressure, the borehole wall rock was prone to brittle fracture failure along the parallel bedding surface. Under different working conditions, obvious changes could be observed in the pressure of the effective fluid column at the well bottom. The pressure changed rapidly, which, in turn, caused the rock at the well bottom to break down, thereby resulting in borehole wall caving. After tripping out and turning the pump off, the shale tensile stress in the upper and lower sidewalls of the horizontal well section was responsible for tensile caving.

Magnetorheological cements for plug setting control in oil and gas wells

Magnetorheological-blended cements (MRBC) are promising materials for plugging and abandonment applications in oil and gas wells because their viscosity can be reversibly controlled through the addition of magnetic particles and application of an external magnetic field. In this study, the fresh state and hardened state properties of MRBC with varied amounts of additives were investigated, including fresh slurry viscosity, thickening time, and compressive strength. A new test method was developed to determine the pressure a liquid MRBC column can hold while activated by a magnetic field. Results showed that the apparent viscosity of MRBC formulations increased as the content of magnetic particles increased, while thickening time was reduced. Compressive strengths of MRBC were found to be similar to standard neat slurries, with MRBC showing slightly higher early strength. The newly developed pressure testing method demonstrated that the MRBC fluid columns tested were able to hold more than the minimum goal of 345 kPa (50 psi) of pressure before movement was indicated. This MRBC characteristic can be exploited to form temporary and permanent cement barriers both inside casing and open hole, as well as in casing annuli. For the latter application, it was shown that sufficient magnetic field strength (in the range of 0.15T–0.25T) can be generated in the annular space outside of a stainless steel casing with the magnet located inside the casing, to still effectively magnetize the MRBC slurry.

Necking, beading, and bulging in soft elastic cylinders

Due to surface tension, a beading instability takes place in a long enough fluid column that results in the breakup of the column and the formation of smaller packets with the same overall volume but a smaller surface area. Similarly, a soft elastic cylinder under axial stretching can develop an instability if the surface tension is large enough. This instability occurs when the axial force reaches a maximum with fixed surface tension or the surface tension reaches a maximum with fixed axial force. However, unlike the situation in fluids where the instability develops with a finite wavelength, for a hyperelastic solid cylinder that is subjected to the combined action of surface tension and axial stretching, a linear bifurcation analysis predicts that the critical wavelength is infinite. We show, both theoretically and numerically, that a localized solution can bifurcate sub-critically from the uniform solution, but the character of the resulting bifurcation depends on the loading path. For fixed axial stretch and variable surface tension, the localized solution corresponds to a bulge or a depression, beading or necking, depending on whether the axial stretch is greater than a certain threshold value that is dependent on the material model and is equal to 23 when the material is neo-Hookean. At this single threshold value, localized solutions cease to exist and the bifurcation becomes exceptionally supercritical. For either fixed surface tension and variable axial force, or fixed axial force and variable surface tension, the localized solution corresponds to a depression or a bulge, respectively. We explain why the bifurcation diagrams in previous numerical and experimental studies look as if the bifurcation were supercritical although it was not meant to. Our analysis shows that beading in fluids and solids are fundamentally different. Fluid beading resulting from the Plateau–Rayleigh instability follows a supercritical linear instability whereas solid beading in general is a subcritical localized instability akin to phase transition.

Numerical Model for Heat Transfer in Saturated Layered Soil with Effective Porosity

A numerical model, called HT1, is presented for one-dimensional (1D) heat transfer in saturated incompressible layered soil with effective porosity and steady fluid flow. The model uses a series-parallel approach for heat transfer and accounts for advection, conduction, and thermal mechanical dispersion assuming local thermal equilibrium between solid and fluid phases. The key to HT1 is the definition of separate columns for the solid matrix and mobile pore fluid. The solid matrix column includes the solid phase and immobile pore fluid and consists of fixed elements. The mobile pore fluid column uses Lagrangian element-tracking to follow the fluid motion, which reduces numerical dispersion and simplifies heat transfer to that of dispersive flux between contiguous elements. Development of the HT1 model is first presented, followed by verification checks using available analytical and numerical solutions. Simulation results for several numeric examples are used to demonstrate model performance and the effects of various parameters, including effective porosity and multiple soil layers, on heat transfer behavior for saturated incompressible soil.

High-speed imaging of transition from fluid breakup to phase explosion in electric explosion of tungsten wires in air

High-speed visible imaging of sub-microsecond electric explosion of wires at the low specific energy deposition threshold reveals three distinct modes of wire failure as capacitor charge voltage and energy deposition are increased. For 100 μm diameter gold-plated tungsten wires of 2 cm length, deposited energies of 1.9 eV/atom produce a liquid column that undergoes hydrodynamic breakup into droplets with radii of the order of wire diameter on timescales of 200 μs. Instability growth, column breakup, and droplet coalescence follow classical Rayleigh–Plateau predictions for instability of the viscous fluid column. Above a deposited energy of 3.2 eV/atom, wires are seen to abruptly transition to an expanding mixture of micrometer scale liquid-droplets and vapor within one frame (less than 3.33 μs), which has been termed “phase explosion” in the literature. Between these two limits, at a deposited energy of 2.5 eV/atom, the wire radius remains unchanged for the first 10 μs before the onset of a rapid expansion and disintegration that resembles homogenous nucleation of mechanically unstable bubbles. Thermodynamic calculations are presented, which separate cases by temperature obtained during heating: below the boiling point, near the boiling point, and exceeding the boiling point.

PEMERIKSAAN STATIC FLUID MAGNETIC RESONANCE UROGRAPHY PADA ANAK DENGAN DUGAAN OBSTRUKSI TRAKTUS URINARIUS

Background : Static fluid Magnetic Resonance Urography (MRU) treats the urinary tract as a static column of fluid, using one of a variety of T2-weighted sequences that exploit the long T2 relaxation time of urine. It does not require the excretion of contrast material and is therefore useful for demonstrating the collecting system and ureter of an obstructed urinary tract. Static fluid MRU provides good morphology visualization and multiplanar three-dimensional reconstruction capability.
 Aim of study : To display the excellence of static fluid MRU in visualization of dilatated urinary tract segments in pediatric cases with suspectedurinary obstruction.
 Methods : Nine pediatric patients with suspected obstructive uropathy (as suggested by ultrasound) were included in this case study. The examinations were performed with 1.5T MR scanners using static fluid T2-weighted MRU sequence. Ultrasound examinations were done forall patients. Voiding cystourethrogram was done for 3 patients.
 Results : Eight studied patients had dilated collecting systems. Static fluid MRU can provide detailed structural assessment of dilatatedcollecting systems. It was superior to ultrasound in evaluation of obstructive double collecting system, ureteral ectopia, ureteral obstructionand massive hydroureteronephrosis.

Axial variability of pattern formation in Rayleigh-Bénard convection: MRI velocimetry in a low aspect ratio cylinder

Natural convection is a fundamental hydrodynamic instability and a template process for the study of pattern formation. It is also an important mode of energy transport in natural geophysical and environmental systems, as well as technological applications. In solar heating devices, pasteurization processes, industrial mixing, and pebble-bed nuclear reactors, low aspect ratio systems of greater depth than width are widely used for their favorable convection properties. Here, magnetic resonance imaging was used to quantify the axial spatial variation of flow patterns in a narrow cylinder. Fluid velocity maps were acquired in multiple transverse and longitudinal planes. It is shown that the flow patterns vary significantly as a function of the height above the heated bottom boundary. The large scale circulation varies among predicted azimuthal mode patterns. Azimuthal 180o reorientation of the upward and downward flow cells was observed over short axial distances. The complex circulation patterns observed suggest acquisition of velocity data in only one plane could lead to mischaracterization of flow patterns in low aspect ratios systems, an aspect of the current literature which is lacking quantification. A preliminary computational fluid dynamics model in STAR-CCM+, corroborated by the experimental results, provided velocity and temperature data for the full fluid column height. Together these results indicate the need for further investigation into circulation characteristics in long narrow cylinders. Enhanced understanding of these rapid axial variations in flow pattern will allow better modeling of heat and mass transport in low aspect ratio natural convection flows.

Floating under a levitating liquid.

When placed over a less dense medium, a liquid layer will typically collapse downwards if it exceeds a certain size, as gravity acting on the lower liquid interface triggers a destabilizing effect called a Rayleigh-Taylor instability1,2. Of the many methods that have been developed to prevent the liquid from falling3-6, vertical shaking has proved to be efficient and has therefore been studied in detail7-13. Stabilization is the result of the dynamical averaging effect of the oscillating effective gravity. Vibrations of liquids also induce other paradoxical phenomena such as the sinking of air bubbles14-19 or the stabilization of heavy objects in columns of fluid at unexpected heights20. Here we take advantage of the excitation resonance of the supporting air layer to perform experiments with large levitating liquid layers of up to half a litre in volume and up to 20 centimetres in width. Moreover, we predict theoretically and show experimentally that vertical shaking also creates stable buoyancy positions on the lower interface of the liquid, which behave as though the gravitational force were inverted. Bodies can thus float upside down on the lower interface of levitating liquid layers. We use our model to predict the minimum excitation needed to withstand falling of such an inverted floater, which depends on its mass. Experimental observations confirm the possibility of selective falling of heavy bodies. Our findings invite us to rethink all interfacial phenomena in this exotic and counter-intuitive stable configuration.