Flow Field Density Research Articles

사행유로를 갖는 고분자연료전지내부에서 가스확산층을 통과하는 반응가스 우회유동에 대한 연구

A serpentine channel geometry often used in a fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from the intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with high aspect ratio of active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compressive forces. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds that of dropwise condensation in cathode channels.

Coupling mineral analysis with conceptual groundwater flow modelling: The source and fate of iron, aluminium and manganese in a back-barrier island

Mineral and aqueous geochemical data are combined with a conceptual groundwater flow model, to establish the origin and fate of iron, aluminium and manganese in the groundwater system of a small back-barrier island. The flow model domain consists of an unconfined island fresh groundwater lens overlying a semi-confined hypersaline aquifer. The two aquifers are separated by a discontinuous, clay-rich aquitard and both contain diffusion governed variable density flow fields. High concentrations of dissolved iron and manganese are associated with brackish to hypersaline groundwater, although there is no systematic relationship with salinity. Calculation of S 2−/SO 4 2− and Fe 2+/Fe 3+ redox couples and the results of thermodynamic modelling show that redox disequilibrium in the groundwater is widespread. Groundwater samples containing aqueous sulphide and ferric iron complexes are supersaturated with respect to pyrite, goethite and haematite but the prevailing state of redox disequilibrium controls mineral dissolution and precipitation. Aqueous iron in the deeper regions of both aquifers is derived from the dissolution of iron oxide–hydroxides in lateritic palaeosols controlled by seasonal fluctuations in groundwater redox state. Aqueous manganese is potentially derived from the dissolution of ilmenite and amorphous oxide–hydroxides. The oxidation of iron sulphides contributes to the aqueous iron concentration and sulphuric acid production in the shallow groundwater. The solubility of aluminium is also limited by this process, governed by acidity regulation. A significant proportion of aqueous iron is transmitted from the semi-confined to the overlying unconfined aquifer through discontinuities in the aquitard layer. Movement of metals in solution outside the island groundwater system is restricted by the presence of diffusion boundaries within variable density transition zones.

Deformation by examples: a density flow approach

AbstractIn this article, a shape transformation technique is introduced for deforming objects based on a given deformation example. The example consists of two reference shapes representing two different states of an object. The reference shapes are assumed to morph from one state to the other. The evolution between the two reference shapes determines the shape transformation function. Any given objects can then be deformed by the same transformation. A continuous 4D Radial Basis Function is used to construct a density flow field (an extension of the optical flow in computer vision) representing the shape transformation of the example in 3‐space. Objects embedded in the density flow field are deformed by moving vertices of the objects along the density flow vectors. Additional parameters are introduced to control the process of the deformation. This provides explicit control on the shape of the object obtained in the deformation process. Copyright © 2007 John Wiley & Sons, Ltd.

Numerical analysis of a proton exchange membrane fuel cell. Part 1: Model development

A full three-dimensional, non-isothermal computational fluid dynamics model of a proton exchange membrane (PEM) fuel cell with straight flow field channels has been developed. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell: convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. The feature of the algorithm developed in this work is its capability for accurate calculation of the local activation overpotentials, which in turn results in improved prediction of the local current density distribution. Fully three-dimensional results of the velocity flow field, species profiles, temperature distribution, potential distribution, and local current density distribution are presented. The model is shown to be able to understand the many interacting, complex electrochemical and transport phenomena that cannot be studied experimentally.

Algebraic iterative algorithm for deflection tomography and its application to density flow fields in a hypersonic wind tunnel

A novel algebraic iterative algorithm based on deflection tomography is presented. This algorithm is derived from the essentials of deflection tomography with a linear expansion of the local basis functions. By use of this algorithm the tomographic problem is finally reduced to the solution of a set of linear equations. The algorithm is demonstrated by mapping a three-peak Gaussian simulative temperature field. Compared with reconstruction results obtained by other traditional deflection algorithms, its reconstruction results provide a significant improvement in reconstruction accuracy, especially in cases with noisy data added. In the density diagnosis of a hypersonic wind tunnel, this algorithm is adopted to reconstruct density distributions of an axial symmetry flow field. One cross section of the reconstruction results is selected to be compared with the inverse Abel transform algorithm. Results show that the novel algorithm can achieve an accuracy equivalent to the inverse Abel transform algorithm. However, the novel algorithm is more versatile because it is applicable to arbitrary kinds of distribution.

Laser interferometric CT measurement of the unsteady supersonic shock-vortex flow field discharging from two parallel and cylindrical nozzles

Three-dimensional flow phenomena with unsteady shock waves and vortices have been observed in a shock tube experiment by using an interferometric CT (computed tomography) technique. A model with small ducts, which has a pair of cylindrical nozzles, is introduced in the test section of the shock tube. The model can be rotated around its central axis to change the observation angle. The projection image of density distribution for each observation angle is obtained by using a fixed Mach–Zehnder interferometer. The three-dimensional density distribution is reconstructed from these projection images. The shock Mach numbers are fixed to 2.3 at the exits of the nozzles in nitrogen gas of 19.4 kPa initial pressure. The resultant 3D density flow fields are illustrated by several visualizing methods to clarify the 3D features of shock waves, vortices and their mutual interactions.

Fixation as a Mechanism for Stabilization of Short Image Sequences

A novel method is introduced for the stabilization of short image sequences. Stabilization is achieved by means of fixation of the central image region using a variable window size block matching method. When applied to a sliding temporal window, the stabilization improves the performance of standard optic flow techniques. Due to the unique choice of fixation as the main stabilization mechanism, the proposed method not only increases the flow field density but renders certain global structural properties of the flow fields more predictable as well. This in turn is advantageous for egomotion computation.

A two-phase flow model for hydrogen evolution in an electrochemical cell

Hydrogen evolution, flow field and current density distribution in an electrochemical cell are investigated with a two-phase flow model. The mathematical model involves solutions of transport equations for the variables of each phase with allowance for inter-phase transfer of mass and momentum. The buoyancy force generated due to density difference between two phases modifies flow profile and increases fluid velocity at the vicinity of the electrode. The current density decreases over the electrode mainly because of the decrease in effective conductivity of electrolyte. It is found that the hydrogen generation significantly increases at higher electrolyte flow by reducing the residence time of bubbles over the electrode. The predicted results satisfactorily agree with data available in the literature.

Experimental and numerical investigation of an O $_{2}$ NO supersonic free jet expansion

Cold O main flow. NO two-dimensional rotational temperature and density flow field patterns in the jet behind the nozzle have been measured by laser-induced fluorescence (LIF). The spectroscopic investigations are complemented by static and impact pressure measurements along the jet centreline. Two nozzles with the same inner profile but with a short and long divergent section and small and large lip have been examined. Three experiments performed using these nozzles cover a wide range of regimes of underexpanded free jet flow, from highly oscillating multi-cycle structure to the regime with smooth deceleration of the issuing flow by the background gas. The numerical simulation of the whole flow field from the conditions in the stagnation chamber to those in the flooded space is performed in the framework of the full set of Navier–Stokes equations by employing a recently developed algorithm based on a staggered grid. The flow inside a long nozzle is shown to be strongly affected by the boundary layer, which leads to the degeneration of the isentropic core at the nozzle exit and transforms the jet regime from overexpanded for inviscid flow to underexpanded for a real flow. The model reproduces the experimental structures of the low-density free jet flow fairly well.

Observation of coherent sheared turbulence flows in the DIII-D Tokamak.

Time-resolved measurements of the turbulent density flow field in a tokamak plasma reveal low-frequency ( approximately 15 KHz), coherent oscillations in the poloidal flow, v(theta). These flow oscillations have a long poloidal wavelength (m<3) and narrow radial extent (k(r)rho(i) approximately 0.2). The estimated flow-shearing rate is of the same order of magnitude as the turbulence decorrelation rate and may thus regulate the turbulence amplitude. These features are consistent with theoretically predicted axisymmetric, self-regulating, sheared flows recognized as geodesic acoustic modes.

Progress in Hypersonic Studies Using Electron-Beam-Excited X-Ray Detection

Experimental and numerical studies of the laminar shock-wave/boundary-layer interaction occurring past a hollow cylinder flare model in a Mach 10 air flow at zero incidence were conducted. Flowfield density measurements were performed by detecting x-ray emissions from the gas produced by electron-beam impact. A solution was found to make these measurements possible near the model surface. Wall pressure and heat flux measurements have been performed. All of the experimental results are compared with numerical results obtained by using Navier-Stokes and direct simulation Monte Carlo solvers

CFD based two-phase modelling of proton exchange membrane fuel cell with interdigitated flow field

Abstract The flow field structure is an important factor determining proton exchange membrane fuel cell performance. A steady three-dimensional, two-phase and non-isotherm mathematical model based on computational fluid dynamics is proposed in this paper, and the model is applied to conduct numerical study on a single proton exchange membrane fuel cell with an interdigitated flow field and an electrode area of 6·4×6·5 cm2. The distribution of gas flow field, temperature and local current density is numerically obtained, which also support some advantages of interdigitated flow fields such as fast water removal, forced convection, etc. In addition, the effects of anode and cathode relative humidity on the cell characteristics such as anode and cathode pressure loss, membrane electrical conductivity and cell performance are numerically analysed. The results indicate that keeping the relative humidity 100% of anode in-stream reactants constant and decreasing the relative humidity of cathode in-stream ...

Bioengineering. Fundamental Investigation for Developing Drug Delivery Systems and Bioprocess with Shock Waves and Bubbles. Numerical Analysis of Deformation of Cell Model and Observation of Bubble Behavior near the Cell-Membrane Model.

This paper describes the fundamental investigations for developing new technology using shock waves and bubbles, such as drug delivery systems (DDS) and bioprocess for environmental protection. To understand the fundamental phenomena, the coupling oscillation problems between the deformations of the cell including a bubble and the flow when underwater shock waves propagating are analyzed using computational fluid dynamics (CFD) and optical visualization. For this analysis, the cell is modeled as a liquid droplet including a gas bubble. The effects of (1) the differences of acoustic impedance between internal and surrounding fluid and (2) the boundary motion of the cell on the flow field (density) are discussed using ALE (Arbitrary Lagragian-Eulerian) computational method. The result shows that the effects of the differences of the acoustic impedance and the existence of a bubble in a cell are large for the deformation process of the cell itself. For the optical visualization, deformation process of a bubble near the elastic wall is observed as a model of cell membrane by shock waves to analyze the behavior of the bubble in the cell or microcapsule. From this result, it is suggested that there should be optimized or critical point to have large deformation of the bubble while changing initial position and initial diameter of the bubble in the cell and relative curvature of the wall (cell diameter), which may be enough to have large deformation to cracking the cell membrane. From these two results, it is found that if the proper parameters are selected, there are possibilities to make the large deformation enough to crack the cell membrane efficiently comparing with that has no bubbles when the shock waves and bubbles are applied to the fields of DDS or bioprocess.

A Three-Dimensional Baroclinic Model of the Irish Sea: Formation of the Thermal Fronts and Associated Circulation

A high-resolution, three-dimensional baroclinic shelf sea model is developed and applied to the determination of the various processes influencing the generation and position of thermal fronts and the associated circulation in the Irish Sea. The model has a horizontal grid resolution of approximately 3.6 km by 3.0 km and can resolve the thermal fronts in the region, in particular in the western Irish Sea, although the grid is not sufficiently fine to resolve small-scale features along these fronts produced by baroclinic instabilities. Using meteorological forcing from Dublin Airport (located close to the center of the region) and tidal forcing, a series of numerical experiments were performed to examine the processes influencing the location and dynamics of thermal fronts and associated circulation in the Irish Sea. The results show a reasonable qualitative agreement with observations of the thermal stratification and the associated circulation in the western Irish Sea. The ability of the model to reproduce the main features of the frontal structures in the region having been established, it is used to examine the interannual variability of the density flow field. Calculations using meteorological forcing from different years show that the western Irish Sea fronts and the associated cyclonic circulation are persistent features, although exact details of the fronts and their times of formation and breakdown show a large interannual variability. Model results also reveal a patch of thermal stratification in the northern part of the eastern Irish Sea, with northward thermal-density-driven currents in this region.

Image velocity, not tau, explains arrival-time judgments from global optical flow.

The time-to-passage (TTP; i.e., the time) until an object passes an observer is optically specified by global tau, a variable that operates on the expansion rate of the angle subtended by an object relative to the observer's heading. M. K. Kaiser and L. Mowafy (1993) provided evidence for observers' sensitivity to global tau in a 3-D cloud of point lights. This interpretation is challenged, and it is suggested that TTP judgments are based on a related but much simpler variable, the image velocity of the object. The present study reexamined several factors that are relevant for the extraction of global tau. When global tau and image velocity were brought into conflict by varying the lateral offsets of the targets, observers showed a strong tendency to rely on the latter variable. Other factors that are supposed to affect TTP judgments only if observers relied on global tau, such as flow-field density and gazemovement angle, did not affect performance.

Experimental Rarefied Density Flowfields at Hypersonic Conditions over 70-Degree Blunted Cone

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Improved Accuracy in Gradient-Based Optical Flow Estimation

Optical flow estimation by means of first derivatives can produce surprisingly accurate and dense optical flow fields. In particular, recent empirical evidence suggests that the method that is based on local optimization of first-order constancy constraints is among the most accurate and reliable methods available. Nevertheless, a systematic investigation of the effects of the various parameters for this algorithm is still lacking. This paper reports such an investigation. Performance is assessed in terms of flow-field accuracy, density, and resolution. The investigation yields new information regarding pre-filter, differentiator, least-squares neighborhood, and reliability test selection. Several changes to previously-employed parameter settings result in significant overall performance improvements, while they simultaneously reduce the computational cost of the estimator.

Optical flow by nonlinear relaxation

In this paper, we present a local approach to the computation of raw optical flow fields based on nonlinear relaxation, which is explicitly designed to be coupled with a nonlinear vector filtering technique so as to achieve an effective rejection of the outliers. We derive the approach from a standard linear algorithm—the one-dimensional least squares. This is split down into two steps. First, the linear algorithm is formulated as an iterative procedure, in which image points in a suitable geometric neighborhood are made to interact each other. Second, a nonlinearity term in the elementary interaction between neighbors is added, which depends on the Gaussian interaction of elements of a velocity neighborhood. The results of a comparison with two linear local techniques show clearly the effectiveness of the approach, both in terms of motion boundary preservation and of optical flow field density.

Application of quantitative two-line OH planar laser-induced fluorescence for temporally resolved planar thermometry in reacting flows

A temporally resolved approach for measurement of two-dimensional temperature fields in reacting flows is experimentally investigated. The method, based on planar laser-induced fluorescence of the hydroxyl (OH) radical, is applicable in many combustion environments, including variable density flow fields. As a means of examining the accuracy of the technique, temperature images, from 1300 to 3000 K and 0.4 to 3 atm, have been acquired in shock-heated H(2)-O(2)-Ar flows with a two-laser, two-image ratio scheme. A complete measurement system for producing accurate, effectively instantaneous temperature images is described; the system includes single-shot monitors for laser-sheet energy distributions and spectral profiles. Temperature images obtained with the OH A(2)Σ(+) ? X(2)II (1, 0) P(1)(7)-Q(2)(11) transition pair exhibit a systematic error of only 7% over the entire range of conditions, with the error most likely dominated by shot-to-shot fluctuations in the lasers'spectral profiles. The largest error source in the instantaneous temperature images is photon shot noise. A group of OH transition pairs that provide good temperature sensitivity and strong signals for reduced shot-noise error over a range of flow-field conditions is also presented.

Transonic shockwave/turbulent boundary layer interactions on a porous surface

AbstractTransonic shockwave/turbulent boundary layer interactions on a porous surface above a closed plenum chamber have been studied experimentally in the choked flow of a windtunnel test-section. The equivalent freestream Mach number is 0.76 and results were obtained for three shock strengths. Without the porous surface the Mach numbers ahead of the shock were 1.13, 1.18 and 1.26. The respective shock Mach numbers with the porous surface were 1.10, 1.11 and 1.19. Laser holographic interferometry results are used to measure the density flowfield and examine the nature of the interaction. These results show that the interaction on the porous surface is modified by a thin shear layer adjacent to the surface and the weakening of the Shockwave is attributed to this. The interaction was also studied by solving the two-dimensional Reynolds-averaged Navier-Stokes equations together with the two-layer algebraic eddy-viscosity model of Baldwin-Lomax modified with appropriate corrections for surface transpiration. The computed results show excellent agreement with the experimental data. The examination of these numerical results shows that the surface transpiration occurs at a low subsonic velocity and suggests that the effect of the transpiration through the porous surface on the interaction may be optimised.