Local Pressure Distribution Research Articles

Modeling of curving hydraulic fracture propagation from a wellbore in a poroelastic medium

Understanding near-wellbore hydraulic fracture behaviors is vital for hydraulic fracturing treatments and other injection-related operations in the petroleum industry. This paper presents a fully coupled fluid flow and geomechanics model for growth of hydraulic fractures in the near-wellbore region. The model is developed within the framework of the extended finite element method (XFEM). Fracture initiation and propagation, fracturing fluid flow, rock deformation, and pore fluid flow are coupled into the XFEM framework. The model is validated against experimental results in the literature. Capabilities of the proposed model for capturing fracture geometry, fluid flow, and local stress and pore pressure distributions are illustrated with numerical examples. A parametric study is carried out using the model to investigate a few operational parameters’ effects on near-wellbore fractures. Some recommendations are provided for reducing fracture tortuosity and breakdown pressure based on the results of the parametric study. The XFEM model proposed in this paper provides an efficient tool to predict arbitrary hydraulic fracture growth in the wellbore vicinity. It can be used to aid designs of various hydraulic fracturing related operations in the petroleum industry, such as fracturing stimulations, injectivity tests, waterfloods, and waste injections.

Effects of building parameters on wind loads on flat-roof-mounted solar arrays

Roof-mounted solar panels play an increasingly important role in developing renewable energy. Wind loading is a major concern for these systems and is affected by parameters related to both building and array dimensions. The effects of building side ratio (D/B=0.4 to 1), aspect ratio (H/B=0.8 to 2) and parapet height (hp/H=0 to 0.06) are investigated under the condition of a constant building height (H=20 m) in wind tunnel tests. The local wind pressure distributions and area-averaged net pressure coefficients of solar panels are examined. The results indicate that the most critical negative peak area-averaged net pressure coefficients considering all wind directions and all panels show an increasing tendency with increase in side ratio, but that the most critical positive peak values show no significant change. An increase in aspect ratio leads to decrease the most critical positive and negative peak net pressures. Curves fitting for the envelopes of equivalent values (GCpn)eq in the format of ASCE 7-10 are introduced based on all tested models. The most critical positive peak area-averaged net pressures decrease with increase in parapet height, and the most critical negative peak values of building models with a parapet are smaller than those without a parapet.

Suppressing Nanoscale Wear by Graphene/Graphene Interfacial Contact Architecture: A Molecular Dynamics Study.

Nanoscale wear is one of the key factors hindering the performance and lifetime of micro- and nanosystems, such as the scanning tip wear in atomic force microscopy (AFM), the head-disk interface in magnetic storage system, and the moving components in micro- or nanoelectromechanical systems (MEMS/NEMS). Here, we propose to construct the graphene/graphene interfacial architecture to suppress the nanoscale wear. Molecular dynamics simulations show that the atomic roughness of the sliding surfaces with either stepped or amorphous structure can lead to strong inhomogeneity of the local contact pressure distribution. By coating graphene on both sides of the frictional surfaces, the local contact pressure fluctuations due to the atomic roughness are suppressed. Moreover, this trend is more evident with the increasing layer number of the graphene coating. Furthermore, the nanoscratching simulation suggests that the rupture of graphene is driven by the inhomogeneous pressure distribution-induced lateral atomic interlocking between the rough tip and substrate and the consequent in-plane lattice deformation and C-C bond breaking during sliding. By coating graphene on the rough amorphous carbon tip, the critical normal load for wear failure of graphene is significantly increased, due to the weakening effect of the atomic interlocking by improving the contact conditions with atomically smooth graphene/graphene sliding interface. This investigation reveals a strategy for reducing nanowear by suppressing the local contact pressure fluctuations via graphene/graphene sliding interface architecture, which provides a theoretical guidance for designing wear-resistant coatings for the longevity of AFM probes and MEMS/NEMS systems.

Development of two-dimensional interdigitated center of pressure sensor

This paper presents the development of a two-dimensional (2D) flexible patch sensor to detect and monitor the center of pressure (CoP) location and the total magnitude of a spatially distributed pressure to the specific surface areas of engineering structures. The CoP sensor with the contact mode induced by a pressure distribution was formulated by force sensitive resistor technology and was mainly composed of a thin conductive polymer layer, adhesive spacers, and two interdigitated patterned electrode films with unique sensing aperture shadings. By properly mapping the interdigitated electrode patterns to the top and bottom surfaces of the conductive polymer, the proposed sensor ideally enables to measure an overall applied pressure level and its centroid location within a predetermined sensing region in real-time. The CoP sensor containing 36 sensing sections within a dimension of around 3 × 3 inches was prototyped and experimentally investigated to verify its capability to identify the CoP location and magnitude due to the presence of a permanent magnet-based local pressure distribution. Only five electric wires connected to the CoP sensor to inspect the pressure-sensing positions of 36 segments. The evaluation results of the measured sensor data demonstrate good agreements with the actual test parameters such as the total pressure and its centroid position with about 5% locational error. However, to provide accurate information on the overall pressure range, the compensation factors must be determined and applied to the individual sensing sections of the sensor.

On direct internal methane steam reforming kinetics in operating solid oxide fuel cells with nickel-ceria anodes

Major operating challenges remain to safely operate methane fuelled solid oxide fuel cells due to undesirable temperature gradients across the porous anode and carbon deposition. This article presents an experimental study on methane steam reforming (MSR) global kinetics for single operating SOFCs with Ni-GDC (gadolinium doped ceria) anodes for low steam to carbon (S/C) ratios and moderate current densities. The study points out the hitherto insufficient research on MSR global and intrinsic kinetics for operating SOFCs with complete Ni-ceria anodes. Further, it emphasizes the need to develop readily applicable global kinetic models as a subsequent step from previously reported state-of-art and complex intrinsic models. Two rate expressions of the Power law (PL) and Langmuir-Hinshelwood (LH) type have been compared and based on the analysis, limitations of using previously proposed rate expressions for Ni catalytic beds to study MSR kinetics for complete cermet anodes have been identified. Firstly, it has been shown that methane reforming on metallic (Ni) current collectors may not be always negligible, contrary to literature reports. Both PL and LH kinetic models predict significantly different local MSR reaction rate and species partial pressure distributions along the normalized reactor length, indicating a strong need for further experimental verifications.

The hydrodynamic forces on a circular cylinder in proximity to a wall with intermittent contact in steady current

We present a three-dimensional Large Eddy Simulations (LES) study on the hydrodynamics of a circular cylinder close to a wall. Intermittent gaps between the cylinder and wall, forming spanning and non-spanning sections along the span are considered. This geometry setting represents an idealized model of pipelines laid on an uneven seabed. As compared to that of the corresponding spanwise-uniform cylinders, the drag coefficient on spanning sections is generally smaller, while the drag on non-spanning sections is slightly larger. Large amplitudes of the sectional forces are observed, partially due to the variations of the gap flow under the spanned sections. Flow deflection from the non-spanning section to the spanning section induces changes in local force and pressure distributions. Despite these changes, the mean force on the cylinder with intermittent contact as a whole is shown to be reasonably estimated by pro rata of forces from spanwise-uniform cylinders over the parameter space considered in this study.

Effect of non-uniform pore pressure fields on hydraulic fracture propagation

In elastic materials, it is evident that the path of a propagating fracture is deflected from its normal course in presence of imperfections of the material or loading conditions. This paper aims at investigating hydraulic fracture deviation induced by non-uniformity of pore pressure fields with the use of a fully coupled poroelastic model based on the finite element method. The model includes a poroelastic domain in which pressurized hydraulic fractures are explicitly embedded, thus allowing to realistically model the fluid flow inside the fracture and to intrinsically consider the fracturing fluid load on the fracture walls as well as fluid leak-off into the formation. The latter process (fluid leak-off into the formation) controls both the length and the orientation of the fracture by changing the local pore pressure which in turn leads to a change in magnitude and direction of local principal stresses around the fracture tip. An innovative method, Mean Rotation Angle (MRA) is utilised for post-processing of evolving stress data at the vicinity of the fracture tip. The MRA predicts the potential growth path of pressurized fractures. In this paper pore pressure induced fracture reorientation is studied for a single fracture as well as closely spaced fractures. Results of this study indicate that presence of a pore pressure anomaly changes the growth path of a hydraulic fracture, towards or away from the anomaly. A higher than average pore pressure zone attracts the fracture while a lower pressure anomaly zone repulses the growing fracture. The fracture growth direction depends on the differential pressure and the distance between the anomaly and the fracture tip. Also in case of two simultaneously growing transverse fractures pressurized by injected fluid, it has been observed that the fluid leak-off controls the potential deviation angle of the fractures through changing the local pore pressure distribution pattern. It is shown that there are three distinct trends for the change of potential deviation angle due to fluid leak-off and that these three trends are linked to three corresponding stages of hydraulic communication between the two fractures. Furthermore, this study shows that change in matrix permeability, stress anisotropy, fracture half-length, spacing and the rate of leak-off influence the timing of each of the stages to the extent to which the corresponding stage of hydraulic communication of the two fractures are affected. This new understanding gives a better insight into the mechanism by which closely spaced hydraulic fractures interact and help optimize the design of multi-stage hydraulic fracture treatments.

Aerodynamic Testing of Helicopter Side Intake Retrofit Modifications

Aerodynamic characteristics of helicopter engine side air intakes are investigated. The experimental data set is obtained by wind tunnel testing of a full-scale helicopter fuselage section model. For the simulation of realistic engine operation, engine mass flow rates are reproduced. Five-hole pressure probe data of the aerodynamic interface plane as well as local surface pressure distributions are compared for different geometries and operation conditions. Previous investigations indicate that unshielded, sideways-facing air intakes yield lowest distortion and highest total pressure levels in low speed conditions. In fast forward flight condition, however, forward-facing intake shapes are more beneficial. On this basis, the current research assesses the optimization potential of retrofit modifications such as a rear spoiler (small scoop) and an intake guide vane. Two optimal configurations of retrofit modifications are identified, combining benefits of the various basic intake and plenum chamber shapes.

Optimization technique for rolled edge control process based on the acentric tool influence functions.

In the process of computer controlled optical surfacing (CCOS), the uncontrollable rolled edge restricts further improvements of the machining accuracy and efficiency. Two reasons are responsible for the rolled edge problem during small tool polishing. One is that the edge areas cannot be processed because of the orbit movement. The other is that changing the tool influence function (TIF) is difficult to compensate for in algorithms, since pressure step appears in the local pressure distribution at the surface edge. In this paper, an acentric tool influence function (A-TIF) is designed to remove the rolled edge after CCOS polishing. The model of A-TIF is analyzed theoretically, and a control point translation dwell time algorithm is used to verify that the full aperture of the workpiece can be covered by the peak removal point of the tool influence functions. Thus, surface residual error in the full aperture can be effectively corrected. Finally, the experiments are carried out. Two fused silica glass samples of 100 mm×100 mm are polished by traditional CCOS and the A-TIF method, respectively. The rolled edge was clearly produced in the sample polished by the traditional CCOS, while residual errors do not show this problem the sample polished by the A-TIF method. Therefore, the rolled edge caused by the traditional CCOS process is successfully suppressed during the A-TIF process. The ability to suppress the rolled edge of the designed A-TIF has been confirmed.

On the local pressure of the Navier-Stokes equations and related systems

In the study of local regularity of weak solutions to systems related to incompressible viscous fluids local energy estimates serve as important ingredients. However, this requires certain information on the pressure. This fact has been used by V. Scheffer in the notion of a suitable weak solution to the Navier-Stokes equations, and in the proof of the partial regularity due to Caffarelli, Kohn and Nirenberg. In general domains, or in case of complex viscous fluid models a global pressure does not necessarily exist. To overcome this problem, in the present paper we construct a local pressure distribution by showing that every distribution $ \partial _t \boldsymbol u +\boldsymbol F $, which vanishes on the set of smooth solenoidal vector fields can be represented by a distribution $ \partial _t \nabla p_h +\nabla p_0 $, where $\nabla p_h \sim \boldsymbol u $ and $ \nabla p_0 \sim \boldsymbol F$.

Flow Analysis of a Helicopter Engine Side Air Intake

A detailed experimental and numerical investigation on the flow characteristics of an engine side air intake of a lightweight helicopter is conducted. The experimental dataset is obtained in a comprehensive wind-tunnel campaign on a novel full-scale model of a helicopter fuselage section. In the wind-tunnel tests, engine mass flow rates corresponding to realistic helicopter operating conditions are reproduced. For fast forward-flight conditions, five-hole pressure probe data in the aerodynamic interface plane as well as local surface pressure distributions are compared with the data obtained by unsteady Reynolds-averaged Navier–Stokes simulations. Regarding the corresponding numerical sectional fuselage case, all components of the wind tunnel model and the essential wind tunnel parts are comprised. To assess the influence of the truncation of the fuselage, an additional full fuselage case is investigated. A good agreement of the numerical simulations and the wind-tunnel data is achieved. The investigation of the side intake geometry yields a distorted total pressure distribution in the aerodynamic interface plane with regions of high and low total pressures. The origin of the aerodynamic interface plane total pressure distribution is explained in detail, analyzing the flow topology in the inlet region and the engine intake duct.

Modelling of friction stir welding of DH36 steel

A 3-D computational fluid dynamics (CFD) model was developed to simulate the friction stir welding of 6-mm plates of DH36 steel in an Eulerian steady-state framework. The viscosity of steel plate was represented as a non-Newtonian fluid using a flow stress function. The PCBN-WRe hybrid tool was modelled in a fully sticking condition with the cooling system effectively represented as a negative heat flux. The model predicted the temperature distribution in the stirred zone (SZ) for six welding speeds including low, intermediate and high welding speeds. The results showed higher asymmetry in temperature for high welding speeds. Thermocouple data for the high welding speed sample showed good agreement with the CFD model result. The CFD model results were also validated and compared against previous work carried out on the same steel grade. The CFD model also predicted defects such as wormholes and voids which occurred mainly on the advancing side and are originated due to the local pressure distribution between the advancing and retreating sides. These defects were found to be mainly coming from the lack in material flow which resulted from a stagnant zone formation especially at high traverse speeds. Shear stress on the tool surface was found to increase with increasing tool traverse speed. To produce a “sound” weld, the model showed that the welding speed should remain between 100 and 350 mm/min. Moreover, to prevent local melting, the maximum tool’s rotational speed should not exceed 550 RPM.

The role of fouling in optimizing direct-flow filtration module design

The local transmembrane pressure (TMP) distribution plays a crucial role in the performance of hollow-fiber microfiltration and ultrafiltration membrane devices. A direct-flow filter comprises an array of hollow fibers encased within a single module. The TMP in each hollow fiber is dependent on the spacing between neighboring fibers, and evolves with time due to pore clogging by contaminants (fouling). We consider an idealized set-up in which the fibers are undeformable and equally spaced within the device, and study the impact of the pore-blocking phenomena on the TMP during the filtration process. The model is used to evaluate the optimum inter-fiber spacing that maximizes the fluid processed either after a prescribed time or before the filter blocks and its dependence on the membrane permeability and the fouling rate. We show that significant improvements can be made on the operating efficiency of a direct-flow device through careful choice in the fiber spacing during fabrication.

スパッタリングプロセスにおける静的圧力分布，および動的圧力変化の実測

It is extremely important to measure and control the dynamic behavior of the pressure distribution and gas flows near the substrates inside the vacuum process facility. However, it has been rarely accomplished with existing vacuum sensing technology. The authors developed an ultra-small thin film micro pressure sensor using TaAl-N thin film as sensing element, and measured the local pressure distribution at steady state near the target and near the substrate in the sputtering system. Furthermore, continuous measurement of the dynamic pressure variations during the reactive sputtering process was successfully conducted to improve the fabrication quality and productivity in the vacuum fabrication systems.

Prediction of S-shaped characteristics in reversible pump-turbines using different numerical approaches

The prediction of characteristics and flow phenomena in reversible pump-turbines becomes increasingly important, since operations under off-design conditions are required to respond to frequency fluctuations within the electrical grid as fast as possible. Fulfilling the requirements of a stable and reliable operation under continuously expanding operating ranges challenges the hydraulic design and requires ambitious developments. Beyond that, precise estimations of occurring flow phenomena combined with a detailed understanding of their causes and mechanisms are essential. This study aims at predicting the S-shaped characteristics of two reversible pump-turbines by using different numerical approaches. Therefore, measurements at a constant wicket-gate opening of Δγ = 10° were done. Based on these experimental data, unsteady flow simulations are performed under steady and transient operating conditions respectively: Starting from the best efficiency point in generating mode, through the runaway, along the S-curve, down to operation in reverse pump mode. The hydraulic machines are spatially discretized in model size with a near-wall refinement of y+mean ≤ 5 and y+mean ≥ 30. The application of two different solvers discloses deviations in underlying methods. The turbulence modeling is basically executed by the k-ω-SST and the standard k-ϵ model. Focusing on higher order numerics, the Explicit Algebraic Reynolds Stress Model (EARSM) is selected in the commercial code and extended with an approach for curvature correction (EARSM- CC). In the open-source software, the four-equation v2-f model assumes the role of higher order numerics. The temporal discretization errors are observed using three different time-step sizes. As a supplement, experimental data obtained from the HydroDyna pump-turbine are used as additional validation, providing integral quantities and local pressure distributions at an operating point set on the S-curve. To sum this work up, a methodology is developed to approximate S-shaped characteristics and local effects within the hydraulic machinery properly.

A comprehensive computational human lung model incorporating inter-acinar dependencies: Application to spontaneous breathing and mechanical ventilation.

In this article, we propose a comprehensive computational model of the entire respiratory system, which allows simulating patient-specific lungs under different ventilation scenarios and provides a deeper insight into local straining and stressing of pulmonary acini. We include novel 0D inter-acinar linker elements to respect the interplay between neighboring alveoli, an essential feature especially in heterogeneously distended lungs. The model is applicable to healthy and diseased patient-specific lung geometries. Presented computations in this work are based on a patient-specific lung geometry obtained from computed tomography data and composed of 60,143 conducting airways, 30,072 acini, and 140,135 inter-acinar linkers. The conducting airways start at the trachea and end before the respiratory bronchioles. The acini are connected to the conducting airways via terminal airways and to each other via inter-acinar linkers forming a fully coupled anatomically based respiratory model. Presented numerical examples include simulation of breathing during a spirometry-like test, measurement of a quasi-static pressure-volume curve using a supersyringe maneuver, and volume-controlled mechanical ventilation. The simulations show that our model incorporating inter-acinar dependencies successfully reproduces physiological results in healthy and diseased states. Moreover, within these scenarios, a deeper insight into local pressure, volume, and flow rate distribution in the human lung is investigated and discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Intratidal Overdistention and Derecruitment in the Injured Lung: A Simulation Study.

Ventilated patients with the acute respiratory distress syndrome (ARDS) are predisposed to cyclic parenchymal overdistention and derecruitment, which may worsen existing injury. We hypothesized that intratidal variations in global mechanics, as assessed at the airway opening, would reflect such distributed processes. We developed a computational lung model for determining local instantaneous pressure distributions and mechanical impedances continuously during a breath. Based on these distributions and previous literature, we simulated the within-breath variability of airway segment dimensions, parenchymal viscoelasticity, and acinar recruitment in an injured canine lung for tidal volumes( VT ) of 10, 15, and 20mL·kg-1 and positive end-expiratory pressures (PEEP) of 5, 10, and 15cm H2O. Acini were allowed to transition between recruited and derecruited states when exposed to stochastically determined critical opening and closing pressures, respectively. For conditions of low VT and low PEEP, we observed small intratidal variations in global resistance and elastance, with a small number of cyclically recruited acini. However, with higher VT and PEEP, larger variations in resistance and elastance were observed, and the majority of acini remained open throughout the breath. Changes in intratidal resistance, elastance, and impedance followed well-defined parabolic trajectories with tracheal pressure, achieving minima near 12 to 16cm H2O. Intratidal variations in lung mechanics may allow for optimization of ventilator settings in patients with ARDS, by balancing lung recruitment against parenchymal overdistention. Titration of airway pressures based on variations in intratidal mechanics may mitigate processes associated with injurious ventilation.

Experimental investigation of extreme wave impacts on a rigid TLP model in cyclonic conditions

This paper describes a series of model tests of a rigidly mounted tension leg platform (TLP) subjected to extreme wave events corresponding to long-crested irregular wave trains of a 10,000-year cyclonic sea state. The experimental setup was instrumented to simultaneously measure wave surface elevations in the vicinity of the model, global wave impact forces and local pressure distribution on the underside of the model's topside deck. Model accelerations were also monitored for each wave impact event so that the inertial force due to structural dynamic response could be identified. The deck–column intersection areas were found to experience large wave-in-deck slamming pressures, in particular around the aft columns. A reduction of the deck clearance was found to increase the magnitude of the global horizontal forces; however, the global vertical forces and local wave-in-deck slamming pressures did not follow this trend.

CFD simulation of blood flow inside the corkscrew collaterals of the Buerger’s disease

Introduction: Buerger’s disease is an occlusive arterial disease that occurs mainly in medium and small vessels. This disease is associated with Tobacco usage. The existence of corkscrew collateral is one of the established characteristics of the Buerger’s disease. Methods: In this study, the computational fluid dynamics (CFD) simulation of blood flow within the corkscrew artery of the Buerger’s disease is conducted. The geometry of the artery is constructed based on the actual corkscrew artery of a patient diagnosed with the Buerger’s disease. The blood properties are the same as the actual blood properties of the patient. The blood flow rate is taken from the available experimental data in the literature. Results: The local velocity patterns, pressure and kinematic viscosity distributions in different segments of the corkscrew collateral artery was demonstrated and discussed for the first time for this kind of artery. The effects of non-Newtonian consideration for the blood viscosity behavior were investigated in different segments of the artery. Moreover, the variations of the blood flow patterns along the artery were investigated in details for each segment. Conclusion: It was found that the flow patterns were affected by the complex geometry of this artery in such a way that it could lead to the presence of sites that were prone to the accumulation of the flowing particles in blood like nicotine. Furthermore, due to the existence of many successive bends in this artery, the variations of kinematic viscosity along this artery were significant, therefore the non-Newtonian behavior of the blood viscosity must be considered.

Numerical Investigation of magnetohydrodynamic flow through Sudden expansion pipes in Liquid Metal Blankets

In fusion liquid metal blanket, sudden expansions and sudden contractions are very common geometries. Changing of the cross-section causes 3-D magnetohydrodynamic (MHD) effects, which will affect the flow pattern, current distribution and pressure drop. In this paper the numerical code based on OpenFOAM platform developed by University of Science and Technology of China was used to investigate and optimize the sudden expansion pipe. The code has been validated by the recommended benchmark cases including Shercliff, Hunt, ALEX experiments (rectangular duct and round pipe) and KIT experiment cases. The obtained numerical results agreed well with those of all the benchmark cases. Previous and valuable analytical and experimental works have been done by L. Buhler, et. el. Based on these works, in the present paper, further investigation of different expansion lengths between the upstream pipe and downstream pipe at high Hartmann number and Reynolds number were conducted. Besides, different expansion ratios with a specific expansion length were conducted. The numerical results showed that with the increasing of expansion length, the 3D MHD effects gradually weakened. Especially, the 3D pressure drop decreases with the increasing of expansion length. Whereas, the expansion ratio factor shows no obvious influences on the total MHD pressure drop but greatly influence the local pressure distribution. These numerical simulations can be used to evaluate the MHD flow inside the expansion and contraction pipes.