Non-uniform Pressure Distribution Research Articles

An Analytical Model for Contact Pressure Prediction Considering Dimensional Error of Stamped Bipolar Plate and Gas Diffusion Layer in Proton Exchange Membrane Fuel Cell Stack Assembly

Contact pressure distribution between bipolar plate (BPP) and gas diffusion layer (GDL) has significant impact on performance and life time of proton exchange membrane (PEM) fuel cell. Most current studies for contact pressure prediction are based on finite-element analysis (FEA), requiring huge computation for the whole fuel cell assembly. Comparatively speaking, the more generalized and well-developed analytical methods are deficient in this field. The objective of this study is to propose a full-scale continuous equivalent model to predict GDL contact pressure effectively in the PEM fuel cell. Using the model, the nonuniform pressure distribution resulted from dimensional errors of metallic BPP and GDL could be obtained. First, a parameterized theoretical model of BPP/GDL assembly is established based on equivalent stiffness analysis of components, and definition methods of dimensional errors are proposed according to actual measurements and Monte Carlo simulation (MCS). Then, experiments are carried out to obtain the actual GDL contact pressure and the model results show good agreement with experimental results. At last, effects of dimensional errors are investigated. Acceptable assembly pressure for a given fuel cell is suggested based on the model. This model is helpful to understand the effect of the dimensional errors, and it also could be adopted to guide the manufacturing of BPP, GDL, and the assembling of PEM fuel cell.

Measurement of effective bulk and contact resistance of gas diffusion layer under inhomogeneous compression – Part I: Electrical conductivity

This paper describes a measurement technique developed for the determination of the effective electrical bulk resistance of the gas diffusion layer (GDL) and the contact resistance distribution at the interface of the GDL and the bipolar plate (BPP). The novelty of this study is the measurement and separation of the bulk and contact resistance under inhomogeneous compression, occurring in an actual fuel cell assembly due to the presence of the channels and ribs on the bipolar plates. The measurement of the electrical contact resistance, contributing to nearly two-third of the ohmic losses in the fuel cell assembly, shows a non-linear distribution along the GDL/BPP interface. The effective bulk resistance of the GDL under inhomogeneous compression showed a decrease of nearly 40% compared to that estimated for homogeneous compression at different compression pressures. Such a decrease in the effective bulk resistance under inhomogeneous compression could be due to the non-uniform distribution of pressure under the ribs and the channels. This measurement technique can be used to identify optimum GDL, BPP and channel-rib structures based on minimum bulk and contact resistances measured under inhomogeneous compression.

Preliminary Development of a Free Piston Expander–Linear Generator for Small-Scale Organic Rankine Cycle (ORC) Waste Heat Recovery System

A novel free piston expander-linear generator (FPE-LG) integrated unit was proposed to recover waste heat efficiently from vehicle engine. This integrated unit can be used in a small-scale Organic Rankine Cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electric energy. The conceptual design of the free piston expander (FPE) was introduced and discussed. A cam plate and the corresponding valve train were used to control the inlet and outlet valve timing of the FPE. The working principle of the FPE-LG was proven to be feasible using an air test rig. The indicated efficiency of the FPE was obtained from the p–V indicator diagram. The dynamic characteristics of the in-cylinder flow field during the intake and exhaust processes of the FPE were analyzed based on Fluent software and 3D numerical simulation models using a computation fluid dynamics method. Results show that the indicated efficiency of the FPE can reach 66.2% and the maximal electric power output of the FPE-LG can reach 22.7 W when the working frequency is 3 Hz and intake pressure is 0.2 MPa. Two large-scale vortices are formed during the intake process because of the non-uniform distribution of velocity and pressure. The vortex flow will convert pressure energy and kinetic energy into thermodynamic energy for the working fluid, which weakens the power capacity of the working fluid.

Punching shear resistance of steel–concrete–steel sandwich composite shell structure

A novel concept of steel–concrete–steel (SCS) sandwich conical structure has been developed for the Arctic offshore platforms. In this structure, punching shear resistance to localized patch loading is the main concern that considers the high pressure zones due to nonuniform distribution of the ice contact pressure. In this paper, quasi-static tests on nine large scale SCS sandwich shell structure were firstly carried out to investigate the ultimate strength behavior of the SCS sandwich shell structure under patch loading. Based on these test results, the influences of different parameters on the ultimate resistance were discussed and analyzed. These studied parameters are composite action, steel shell thickness, spacing of connector, strength of concrete core, and curvature of the sandwich shell. Theoretical models were developed to predict the shear resistance of the SCS sandwich shell structure. The innovations of the developed models include developing formulae to predict the resistances of the connector used in the sandwich shell structure, redefining the critical perimeter to analyze the punching shear resistance of SCS sandwich shell, and modifying the formulae in Eurocode 2 to calculate the punching shear resistance. The accuracy of the developed prediction models were checked and confirmed by nine reported tests and 11 tests in the literature. Finally, design recommendations on the punching shear resistance of the SCS sandwich shell were offered based on the discussions and validations.

Detailed Structural and Mechanical Response of Wet Foam to the Settling Particle.

Liquid foam, as a complex fluid, provides an observable prototype for studying a discrete fluid system. In this work, a numerical study on the settling behavior of a round particle in wet polydisperse foam has been conducted on the bubble scale. The local and nonuniform distribution of bubble pressure, as well as the localized plastic events, is presented. It shows a foam region of higher pressure in front of the settling particle due to the extrusion deformation of the bubbles applied by the particle. Additionally, the forces exerted on the particle by the disordered wet foam are measured during the sedimentation. It exhibits in particular a power-law dependence of the drag force caused by the bubble as a function of the foam quality. Moreover, sedimentation experiments are demonstrated to verify this power-law relation. The evolution of the components of drag force is demonstrated when a plastic event occurs in front of the settling particle. The result shows that both the contributions of the pulling force of foam films and the bubble pressure to the drag force decrease in that case. Likewise, the variation of both these contributions to the drag force is illustrated as well when a bubble in the wake detaches from the particle. These results assist in understanding the mesoscopic response of wet foam to a settling particle.

컴퓨터 제어를 통한 광학 가공에서의 다양한 툴 영향 함수의 모델링

The computer controlled optical surfacing (CCOS) technique provides superior fabrication performance for optical mirrors when compared to the conventional method, which relies heavily on the skill of the optician. The CCOS technique provides improvements in terms of mass production, low cost, and short polishing time, and are achieved by estimating and controlling the moving speed of the tool and toolpath through a numerical analysis of the tool influence function (TIF). Hence, the exact estimation of various TIFs is critical for high convergence rates and high form accuracy in the CCOS process. In this paper, we suggest a new model for TIFs, which can be applied for various tool shapes, different velocity distributions, and non-uniform tool pressure distributions. Our proposed TIFs were also verified by comparisons with experimental results. We anticipate that these new TIFs will have a major role in improving the form accuracy and shortening the polishing time by increasing the accuracy of the material removal rate.

A new insight into foaming mechanisms in injection molding via a novel visualization mold

The complex mechanisms of bubble nucleation and dynamics in foam injection molding have not been uncov- ered despite many previous efforts due to the non-steady stop-and-flow nature of injection molding and the non-uniform temperature and pressure distributions in the mold. To this end, a new visualization mold was designed and manufactured for the direct observation of bubble nucleation and growth/collapse in foam injection molding. A reflective prism was incor- porated into the stationary part of the injection mold with which the nucleation and growth behaviors of bubbles were suc- cessfully observed. The mechanisms of bubble nucleation in low- and high-pressure foam injection molding, with and without the application of gas-counter pressure, was investigated. We identified how the inherently non-uniform cell structure is devel- oped in low-pressure foam injection molding with gate-nucleated bubbles, and when and how cell nucleation occurs in high-pressure foam injection molding with a more uniform pressure drop.

Effect of roll compactor sealing system designs: A finite element analysis

In the pharmaceutical industry, the roll compaction is part of the dry granulation process, densifying fine powders into ribbons that will be later milled to produce granules with good flowability for subsequent die compaction process. Roll compactors are constructed with a sealing system, limiting the loss of powder from the sides. However, the sealing system may result in unwanted non-uniformity of the ribbon's properties. In this work, a 3D Finite Elements Method (FEM) modeling is used to analyze the roll compaction process and the effect of sealing system designs on the compacted ribbon's density distribution. A density dependent Drucker–Prager Cap (DPC) constitutive model for microcrystalline cellulose (Avicel PH-101) was calibrated and implemented in Abaqus/Explicit. Two different FEM models were investigated, one with a fixed side sealing called cheek plates and another where the side sealing is integrated with the bottom roll called rimmed-roll. Both numerical and experimental results clearly show the non-uniform roll pressure and density distribution for the cheek plates assembly, whereas the rimmed-roll shows an overall more uniformly distributed resultant pressure and density distribution. These results demonstrate the capability of FEM modeling to provide insight and help achieving a better understanding of the roll compaction process.

Pressure Generated from Head Garment with Padding Insert Used for Hypertrophic Scar Treatment

Pressure garment is a standard non-surgical treatment for hypertrophic scar caused by burn injury. However, the main problem identified was the non-uniform pressure distribution due to the complexity of the contour at the facial area. Therefore, padding was developed to produce more efficient pressure distribution. This study used 3D scanning apparatus to obtain the cross sections of the facial area. The padding was developed using 3D modeling software which act as an insert to fill the gaps at the contact area of the garment and facial areas. The result shows that by inserting the padding underneath the garment, the pressure outputs indicate an acceptable pressure range with the suitable reduction factor for the head garment fabrication.

A role of friction-induced torque in sliding friction of rubber materials

Herein, the role of friction-induced torque in sliding friction of rubber materials is investigated. A friction tester was developed to monitor the friction force between a silicon rubber block and a surface made of PMMA11PMMA: poly(methyl methacrylate).. Based on the experimental results, a simplified model was developed to express the nonuniform distribution of normal pressure developed due to the friction-induced torque, and the normal pressure dependence of the real contact area based on Persson׳s contact theory. Based on the modeling, the mechanism of the effect of the friction-induced torque on frictional force reductions was clarified.

Modeling of edge tool influence functions for computer controlled optical surfacing process

Computer-controlled optical surfacing provides superior optical fabrication performance with low-cost mass production over conventional method relying heavily on the skills of optician. However, there are still lots of technical issues to be resolved in computer-controlled optical surfacing, and edge effect has been regarded as one of the most challenging tasks for years due to the unpredictable behaviors of a polishing tool. As a polishing tool approaches the edge of the workpiece, the tool-workpiece contact area decreases and this in turn accompanies the tool-workpiece misfit and non-uniform pressure distribution. Thus, the edge effects should be taken into account in deterministic polishing technique. In this paper, we suggest new edge tool influence functions by modeling the velocity and pressure distribution of a polishing tool with eccentric rotation motion. Here, the finite element analysis was used to accurately predict the non-linear behaviors of a polishing tool at the edge. We verified our proposed method by comparisons with experimental results, and it shows considerable resemblance between them.

The stress state around lined non-circular hydraulic tunnels below the water table using complex variable method

An elastic plane-strain solution for lined non-circular hydraulic tunnels, below the water table, is presented. Tunnels are assumed to be subjected to uniform ground load, and excavated in a semi-finite aquifer with fully saturated and drained geomaterial. Rock mass and concrete shows a homogenous, isotropic linear elastic behavior, and the non-seepage condition dominates the model. Muskhelishvili complex potential functions combined with conformal mapping method were used to investigate stress field around the tunnels. The solution was compared with ABAQUS finite element software through an example that showed a good agreement. Moreover, the proposed solution predicted the boundary values of the problem along the internal lining periphery much more accurately than did the ABAQUS solution. In the next part, an attempt was made to reduce the solution to be more practical and simply applicable for pressure tunnels when a large water pressure is exerted on the linings. The significance of the work is in the fact that it incorporates non-uniform distribution of water pressure for groundwater and water inside tunnels, and predicts stress components in a highly accurate and fast process. The method could also be used for the openings in half-plane medium under non-uniform ground load.

Non-axisymmetric flow characteristics in centrifugal compressor

The flow field distribution in centrifugal compressor is significantly affected by the non-axisymmetric geometry structure of the volute. The experimental and numerical simulation methods were adopted in this work to study the compressor flow field distribution with different flow conditions. The results show that the pressure distribution in volute is characterized by the circumferential non-uniform phenomenon and the pressure fluctuation on the high static pressure zone propagates reversely to upstream, which results in the non-axisymmetric flow inside the compressor. The non-uniform level of pressure distribution in large flow condition is higher than that in small flow condition, its effect on the upstream flow field is also stronger. Additionally, the non-uniform circumferential pressure distribution in volute brings the non-axisymmetric flow at impeller outlet. In different flow conditions, the circumferential variation of the absolute flow angle at impeller outlet is also different. Meanwhile, the non-axisymmetric flow characteristics in internal impeller can be also reflected by the distribution of the mass flow. The high static pressure region of the volute corresponds to the decrease of mass flow in upstream blade channel, while the low static pressure zone of the volute corresponds to the increase of the mass flow. In small flow condition, the mass flow difference in the blade channel is bigger than that in the large flow condition.

Investigations of the Armature–Rail Contact Pressure Distribution in a Railgun

In the process of electromagnetic launch, the nonuniform contact pressure distribution on the armature-rail interface kept changing with the armature moving forward. At first, the contact pressure was mainly provided by the deformation of the trailing arm, from which it could keep a good metal-metal contact at the armature-rail interface. After the pulse current was constructed, the magnetic pressure would dominate the contact pressure because it was much larger than the mechanical interference pressure. In this paper, the 3-D modeling of contact pressure distribution was carried out with the finite-element method. In the simulation of 3-D contact calculations, the current entered the armature from the trailing edge. The electromagnetic force perpendicular to the armature trailing arm was equivalent to the uniform load on the inner surface of the trailing arm. A typical contour pattern of nonuniform contact pressure distribution was acquired. Some nonuniform coefficients of contact pressure distribution were proposed and analyzed to evaluate the contact performances between the armature and rails. A series of simulations was conducted at different magnitudes of magnetic pressure, and with different armature structure parameters. The characteristics of the nonuniform coefficients were investigated and discussed. The dependences of the nonuniform coefficients on the equivalent uniform load and armature structure parameters were obtained.

Aggregate Constraints for Virtual Manipulation with Soft Fingers.

Interactive dexterous manipulation of virtual objects remains a complex challenge that requires both appropriate hand models and accurate physically-based simulation of interactions. In this paper, we propose an approach based on novel aggregate constraints for simulating dexterous grasping using soft fingers. Our approach aims at improving the computation of contact mechanics when many contact points are involved, by aggregating the multiple contact constraints into a minimal set of constraints. We also introduce a method for non-uniform pressure distribution over the contact surface, to adapt the response when touching sharp edges. We use the Coulomb-Contensou friction model to efficiently simulate tangential and torsional friction. We show through different use cases that our aggregate constraint formulation is well-suited for simulating interactively dexterous manipulation of virtual objects through soft fingers, and efficiently reduces the computation time of constraint solving.

Numerical and experimental investigations of hydro-mechanical deep drawing process of laminated aluminum/steel sheets

The application of hydro-mechanical deep drawing (HMDD) process on laminated sheets combines advantages of both process and material to improve the forming condition of poor formable light-weight metals such as aluminum alloys. In this research, the HMDD process of anisotropic laminated bimetallic sheets has been analyzed using a 3D finite element simulation with implementing Fortran based code for accurate modeling of non-uniform oil pressure distribution. To verify FE results, experimental works were conducted on the widely used laminated aluminum/steel sheets. Based on the developed FE model, parametric studies were performed to investigate the effect of material parameters such as layers thickness and lay-up arrangement on the forming condition, process window and formability of aluminum sheet as key process parameters. Results demonstrated that suitable process condition to attain a successful forming of bimetallic aluminum/steel sheets can be predicted by developed FE model reasonably. Also, It shown that wider working zone was achievable with decrease in drawing ratio, reduction in thickness of sheet with lower strength layer and also when aluminum sheet is in contact with punch (A/S lay-up). In addition, the higher limiting drawing ratio (LDR) and lower thinning in low formable aluminum sheet would be achievable in A/S lay-up than S/A lay-up.

Understanding the microscopic moisture migration in pore space using DEM simulation

The deformation of soil skeleton and migration of pore fluid are the major factors relevant to the triggering of and damages by liquefaction. The influence of pore fluid migration during earthquake has been demonstrated from recent model experiments and field case studies. Most of the current liquefaction assessment models are based on testing of isotropic liquefiable materials. However the recent New Zealand earthquake shows much severer damages than those predicted by existing models. A fundamental cause has been contributed to the embedded layers of low permeability silts. The existence of these silt layers inhibits water migration under seismic loads, which accelerated liquefaction and caused a much larger settlement than that predicted by existing theories. This study intends to understand the process of moisture migration in the pore space of sand using discrete element method (DEM) simulation. Simulations were conducted on consolidated undrained triaxial testing of sand where a cylinder sample of sand was built and subjected to a constant confining pressure and axial loading. The porosity distribution was monitored during the axial loading process. The spatial distribution of porosity change was determined, which had a direct relationship with the distribution of excess pore water pressure. The non-uniform distribution of excess pore water pressure causes moisture migration. From this, the migration of pore water during the loading process can be estimated. The results of DEM simulation show a few important observations: (1) External forces are mainly carried and transmitted by the particle chains of the soil sample; (2) Porosity distribution during loading is not uniform due to non-homogeneous soil fabric (i.e. the initial particle arrangement and existence of particle chains); (3) Excess pore water pressure develops differently at different loading stages. At the early stage of loading, zones with a high initial porosity feature higher porosity changes under the influence of external loading, which leads to a larger pore pressure variation (increase or decrease) in such zones. As the axial strain increases, particle rearrangement occurs and final porosity distribution has minor relationship with the initial condition, and the pore pressure distribution becomes irregular. The differences in the pore pressure development imply that water will migrate in the pore space in order to balance the pore water pressure distribution. The results of this simulation offer an insight on the microscale water migration in the soil pore space, which is important for holistic description of the triggering of soil liquefaction in light of its microstructure.

Exact Solutions for One-Dimensional Transient Gas Flow in Porous Media with Gravity and Klinkenberg Effects

Attempts have been made to find exact solutions for the one-dimensional transient gas flow equation in porous media. By introducing a traveling wave variable, a traveling wave solution of the gas flow equation has been found. The traveling wave solution is presented in an explicit form of the space and time variables, and it takes into account both gravity and Klinkenberg effects (pressure-dependent permeability). We investigated the properties of the traveling wave solution and the effect of some parameters such as the Klinkenberg coefficient. A numerical study has been carried out, which confirms the stability of the traveling wave solution. The traveling wave solution is then used to derive two benchmark solutions defined over the semi-infinite domain. The first one assumes uniform initial gas pressure and non-uniform boundary condition, and the second assumes uniform boundary condition and non-uniform initial distribution of the gas pressure. The benchmark solutions are easy to use and are useful for validating numerical solutions. Two illustrative examples are presented in order to compare the benchmark solutions with the numerical solutions. The results show good agreements between the solutions.

A review of the role of the partial pressure of carbon dioxide in mechanically loaded tissues: the canary in the cage singing in tune with the pressure ulcer mantra.

Pressure ulcers (PUs) can occur in any situations where people are subjected to non-uniform distribution of pressure over a prolonged period. They can have devastating effects on the patients' well-being and in extreme conditions can prove fatal. In addition to traditional wisdom implicating mechanically induced ischaemia, there is strong evidence that other mechanisms play a role in the cascade of events which can initiate the PU damage process at the cellular level. Some of these refer to a metabolic imbalance with compromised delivery of nutrients and accumulation of waste products in the local environment of the cells. The approach of much research has focused on the measure of oxygen in compressed tissues as a means of predicting early damage. However, the present review adopting a hierarchical approach, using length scales ranging from cells through to human models, has revealed compelling evidence which highlights the importance of carbon dioxide levels and associated concentration of other metabolites, such as lactate and purines. The temporal profiles of these metabolites have been monitored in the various models subjected to periods of mechanical-induced loading where the localized cells have converted to anaerobic metabolism. They reveal threshold levels of carbon dioxide which might be indicative of early tissue damage during both mechanical-induced ischaemia and subsequent reperfusion and an appropriate sensor could be used in a similar manner to the long-standing "canary in a cage" method to detect toxic gasses in enclosed mines.

Fabrication of high performance thin-film transistors via pressure-induced nucleation.

We report a method to improve the performance of polycrystalline Si (poly-Si) thin-film transistors (TFTs) via pressure-induced nucleation (PIN). During the PIN process, spatial variation in the local solidification temperature occurs because of a non-uniform pressure distribution during laser irradiation of the amorphous Si layer, which is capped with an SiO2 layer. This leads to a four-fold increase in the grain size of the poly-Si thin-films formed using the PIN process, compared with those formed using conventional excimer laser annealing. We find that thin films with optimal electrical properties can be achieved with a reduction in the number of laser irradiations from 20 to 6, as well as the preservation of the interface between the poly-Si and the SiO2 gate insulator. This interface preservation becomes possible to remove the cleaning process prior to gate insulator deposition, and we report devices with a field-effect mobility greater than 160 cm2/Vs.