The effective diffusivity of oxygen in a cathode catalyst layer of PEMFC depends on both the catalyst layer microstructure and the changes in oxygen pathways due to liquid water. In this paper, various aspects of the effective diffusivity are investigated using pore-scale simulation, with emphasis on the effects of Knudsen diffusion. Based on microstructures reconstructed by the spherebased simulated annealing method, both the higher-order lattice Boltzmann method (LBM) and the continuum diffusion equation with the Bosanquet approximation are used to evaluate the effective diffusivity considering Knudsen diffusion. Results show that the continuum diffusion equation reproduces the results from the higher-order LBM that simulates the effects of Knudsen diffusion based on the kinetic theory, when the mean pore size is evaluated using the erosion-dilation method or the chord length distribution. It is also shown that the change of the pore size with water saturation levels should be considered to predict the effects of Knudsen diffusion accurately. The dependence of the effective diffusivity on the agglomerate size is explained by the effects of Knudsen diffusion. Derjaguin's correction for Knudsen diffusion is found to have negligible effects on the effective diffusivity for the present cases. The empirical correlations of various parameters are presented.
This paper presents an analysis of the discretization errors in the non-equilibrium models for the subfilter variance of the mixture fraction, a key quantity to model in large eddy simulation (LES) of turbulent mixing and combustion. Two discretely distinct formulations of the non-equilibrium models that solve the transport equations to obtain the subfilter variance, i.e., the second moment transport equation (STE) and the variance transport equation (VTE), are analyzed. By deriving discrete equations for the evolution of subfilter variance by the two formulations, it is seen that the difference originates primarily from the product rule of differentiation applied to the scalar convection term, which does not hold discretely. LES of scalar mixing in a planar jet is performed to illustrate the outcome of the analysis. Results show that the discrete product rule error is significant and of the same order as the production and dissipation terms on average. A priori analysis using direct numerical simulation (DNS) data for scalar mixing in homogeneous isotropic turbulence is also performed. From the analysis, it is seen that the VTE model under-predicts the subfilter variance, whereas the STE model over-predicts it substantially with sharp oscillations.
Visual exploration of flow fields is important for studying dynamic systems. We introduce semantic flow graph (SFG), a novel graph representation and interaction framework that enables users to explore the relationships among key objects (i.e., field lines, features, and spatiotemporal regions) of both steady and unsteady flow fields. The objects and their relationships are organized as a heterogeneous graph. We assign each object a set of attributes, based on which a semantic abstraction of the heterogeneous graph is generated. This semantic abstraction is SFG. We design a suite of operations to explore the underlying flow fields based on this graph representation and abstraction mechanism. Users can flexibly reconfigure SFG to examine the relationships among groups of objects at different abstraction levels. Three linked views are developed to display SFG, its node split criteria and history, and the objects in the spatial volume. For simplicity, we introduce SFG construction and exploration for steady flow fields with critical points being the only features. Then we demonstrate that SFG can be naturally extended to deal with unsteady flow fields and multiple types of features. We experiment with multiple data sets and conduct an expert evaluation to demonstrate the effectiveness of our approach.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.