41,445 research outputs found
Phonon Bottleneck Identification in Disordered Nanoporous Materials
Nanoporous materials are a promising platform for thermoelectrics in that
they offer high thermal conductivity tunability while preserving good
electrical properties, a crucial requirement for high- effciency thermal energy
conversion. Understanding the impact of the pore arrangement on thermal
transport is pivotal to engineering realistic materials, where pore disorder is
unavoidable. Although there has been considerable progress in modeling thermal
size effects in nanostructures, it has remained a challenge to screen such
materials over a large phase space due to the slow simulation time required for
accurate results. We use density functional theory in connection with the
Boltzmann transport equation, to perform calculations of thermal conductivity
in disordered porous materials. By leveraging graph theory and regressive
analysis, we identify the set of pores representing the phonon bottleneck and
obtain a descriptor for thermal transport, based on the sum of the pore-pore
distances between such pores. This approach provides a simple tool to estimate
phonon suppression in realistic porous materials for thermoelectric
applications and enhance our understanding of heat transport in disordered
materials
From Tomography to Material Properties of Thermal Protection Systems
A NASA Ames Research Center (ARC) effort, under the Entry Systems Modeling (ESM) project, aims at developing micro-tomography (micro-CT) experiments and simulations for studying materials used in hypersonic entry systems. X-ray micro-tomography allows for non-destructive 3D imaging of a materials micro-structure at the sub-micron scale, providing fiber-scale representations of porous thermal protection systems (TPS) materials. The technique has also allowed for In-situ experiments that can resolve response phenomena under realistic environmental conditions such as high temperature, mechanical loads, and oxidizing atmospheres. Simulation tools have been developed at the NASA Ames Research Center to determine material properties and material response from the high-fidelity tomographic representations of the porous materials with the goal of informing macroscopic TPS response models and guiding future TPS design
Model reduction for molecular diffusion in nanoporous media
Porous materials are widely used for applications in gas storage and
separation. The diffusive properties of a variety of gases in porous media can
be modeled using molecular dynamics simulations that can be computationally
demanding depending on the pore geometry, complexity and amount of gas
adsorbed. We explore a dimensionality reduction approach for estimating the
self-diffusion coefficient of gases in simple pores using Langevin dynamics,
such that the three-dimensional (3D) atomistic interactions that determine the
diffusion properties of realistic systems can be reduced to an effective
one-dimensional (1D) diffusion problem along the pore axis. We demonstrate the
approach by modeling the transport of nitrogen molecules in single-walled
carbon nanotubes of different radii, showing that 1D Langevin models can be
parametrized with a few single-particle 3D atomistic simulations. The reduced
1D model predicts accurate diffusion coefficients over a broad range of
temperatures and gas densities. Our work paves the way for studying the
diffusion process of more general porous materials as zeolites or
metal-organics frameworks with effective models of reduced complexity.Comment: 8 pages, 6 figure
Effective elastic properties of randomly distributed void models for porous materials
This is the post-print version of the final paper published in International Journal of Mechanical Sciences. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.Many 2D analytical models are available for estimating the effective elastic properties of porous materials. Most of these models adopt circular voids of a uniform diameter in superlattice arrays, such as unit void or periodically positioned models. There are two principal issues in a realistic representation of porous materials: the random distribution of a statistically sufficiently large number of voids in the model, and the random distribution of the size and position of the voids. Numerical schemes such as the FEM or the BEM have also been presented to cater for regular patterned circular voids. However, due to the large number of elements needed to produce sufficient accuracy for the curved boundary of circular voids or modelling a statistically sufficient number of voids with a random distribution in both the void size and the position, no such model has yet been produced.
Modelling based on an FEM approach using a simplified approximation for void geometry is proposed here for the calculation of the effective elastic properties of porous solids. A plane strain model of a square geometry is adopted for a 2D array of voids. This simplified square shape allows a large number of voids to be simulated with a random distribution for both void sizes and their locations. The problem of anisotropy, which arises from the square shape, is discussed. It is verified that along the two principal directions (parallel to the sides of the square voids), the elastic properties remain the same as those predicted by using a circular void geometry. This square-shaped approximation, with its reduced requirement for FE analysis, has the potential to be extended to 3-dimensional modelling for a realistic simulation of engineering materials.University of Aberdee
Observation of anisotropic diffusion of light in compacted granular porous materials
It is known that compaction of granular matter can lead to anisotropic
mechanical properties. Recent work has confirmed the link to pore space
anisotropy, but the relation between compression, mechanical properties and
material microstructure remains poorly understood and new diagnostic tools are
needed. By studying the temporal and spatial characteristics of short optical
pulses diffusively transmitted through compacted granular materials, we show
that powder compaction can also give rise to strongly anisotropic diffusion of
light. Investigating technologically important materials such as
microcrystalline cellulose, lactose and calcium phosphate, we report increasing
optical anisotropy with compaction force and radial diffusion constants being
up to 1.7 times the longitudinal. This open new and attractive routes to
material characterization and investigation of compression-induced structural
anisotropy. In addition, by revealing inadequacy of isotropic diffusion models,
our observations also have important implications for quantitative spectroscopy
of powder compacts (e.g., pharmaceutical tablets).Comment: New version with significantly improved presentation. Data and
argumentation identical to previous versio
Model Reduction for Multiscale Lithium-Ion Battery Simulation
In this contribution we are concerned with efficient model reduction for
multiscale problems arising in lithium-ion battery modeling with spatially
resolved porous electrodes. We present new results on the application of the
reduced basis method to the resulting instationary 3D battery model that
involves strong non-linearities due to Buttler-Volmer kinetics. Empirical
operator interpolation is used to efficiently deal with this issue.
Furthermore, we present the localized reduced basis multiscale method for
parabolic problems applied to a thermal model of batteries with resolved porous
electrodes. Numerical experiments are given that demonstrate the reduction
capabilities of the presented approaches for these real world applications
Analogy electromagnetism-acoustics: Validation and application to local impedance active control for sound absorption
An analogy between electromagnetism and acoustics is presented in 2D. The
propagation of sound in presence of absorbing material is modeled using an open
boundary microwave package. Validation is performed through analytical and
experimental results. Application to local impedance active control for free
field sound absorption is finally described
A GPU-accelerated package for simulation of flow in nanoporous source rocks with many-body dissipative particle dynamics
Mesoscopic simulations of hydrocarbon flow in source shales are challenging,
in part due to the heterogeneous shale pores with sizes ranging from a few
nanometers to a few micrometers. Additionally, the sub-continuum fluid-fluid
and fluid-solid interactions in nano- to micro-scale shale pores, which are
physically and chemically sophisticated, must be captured. To address those
challenges, we present a GPU-accelerated package for simulation of flow in
nano- to micro-pore networks with a many-body dissipative particle dynamics
(mDPD) mesoscale model. Based on a fully distributed parallel paradigm, the
code offloads all intensive workloads on GPUs. Other advancements, such as
smart particle packing and no-slip boundary condition in complex pore
geometries, are also implemented for the construction and the simulation of the
realistic shale pores from 3D nanometer-resolution stack images. Our code is
validated for accuracy and compared against the CPU counterpart for speedup. In
our benchmark tests, the code delivers nearly perfect strong scaling and weak
scaling (with up to 512 million particles) on up to 512 K20X GPUs on Oak Ridge
National Laboratory's (ORNL) Titan supercomputer. Moreover, a single-GPU
benchmark on ORNL's SummitDev and IBM's AC922 suggests that the host-to-device
NVLink can boost performance over PCIe by a remarkable 40\%. Lastly, we
demonstrate, through a flow simulation in realistic shale pores, that the CPU
counterpart requires 840 Power9 cores to rival the performance delivered by our
package with four V100 GPUs on ORNL's Summit architecture. This simulation
package enables quick-turnaround and high-throughput mesoscopic numerical
simulations for investigating complex flow phenomena in nano- to micro-porous
rocks with realistic pore geometries
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