505 research outputs found
Anisotropic Particles: Preparation and Study
Anisotropic particles have received significant attention in self-assembly for the large scale fabrication of hierarchical structures. Janus particles, a specific class of anisotropic particles, have two hemispheres with different materials. Due to the anisotropic nature of the particle shape and interactions, Janus particles have demonstrated interesting properties in interfacial assembly, switchable devices, cargo transport, and optical sensing. The objective of this research is to fabricate novel anisotropic Janus particles and explore their potential unique properties.;One of the driving forces arises from the previous work of bimetallic nanorods and their autonomous motion. The bimetallic nanorod systems undergo chemically powered non- Brownian motion due to the asymmetric distribution of catalytic source for a chemical fuel solution. However, the approach used to prepare the bimetallic nanorods is rather complex. The original design of bimetallic Janus particles is based on a general physical vapor deposition technique -- electron beam evaporation. The resulting bimetallic Janus particles are colloidal silica spheres coated with two differing metals on each hemisphere. This approach allows fabricating bimetallic Janus particles with various combinations of metals that are available for electron beam evaporation.;Chemical transformation of bimetallic Janus particles into other species provides an opportunity to expand the scope of anisotropic particles. The metals on the Janus particles are possible to convert to their corresponding metal oxides and metal sulfides through solid-gas heterogeneous reactions, and therefore, the chemical transformation of the parent bimetallic Janus particles produces a wide array of previously unavailable Janus particle types, including metal/metal oxide, metal/metal sulfide, metal oxide/metal oxide, metal sulfide/metal sulfide, and metal oxide/metal sulfide, which allows tuning their optical, electronic, magnetic and catalytic properties. This vast library of anisotropic particulate building blocks provides a powerful arsenal for engineering the assembly of specific targeted structures and systems.;Autonomous motion is distinctive from Brownian motion. Platinum half-coated Janus particles undergo self-propelled motion, which is induced by the catalytic decomposition of hydrogen peroxide. The average speed of the self-propelled Pt-SiO2 Janus particles increases with increasing the concentration of hydrogen peroxide. Motion direction analyses show that the probability for the Janus particles continuing to travel in nearly same direction goes higher in higher concentrations of hydrogen peroxide. Microscopic observation of the particle motion demonstrates that these Janus particles move, on average, with the platinum-coated region oriented opposite to the direction of motion. The trajectories of the autonomous motion exhibit a directed motion at short time scale but with an overall random behavior at long time scales. Huge benefit can be garnered by taking advantage of the self-propulsion component in the system. The control of the motion of the magnetic Janus particles in solutions of hydrogen peroxide is demonstrated using the external magnetic field. The magnetic Janus particles orient themselves with the equatorial plane parallel to the applied field and the motion direction is perpendicular to the field. The directed motion has a more distinct preferred direction compared to the case in the absence of magnetic field, and the applied field is verified to control the orientation, not influence the speed of the particle motion.;Anisotropic particles are unique building blocks to assemble complex structures. The surface functionalized Janus particles with alkanethiols are adsorbed at the interfaces of liquid-air and liquid-liquid, forming monolayers with metal hemispheres pointing to the same direction. By changing the liquid oil phase, the orientation of the Janus particles can be manipulated, which provides an opportunity to selectively modify the surface in either phase. The preferential orientation in the same direction at interfaces allows for direct transfer of the Janus particles while the desired faces remain in either a face-down or face-up configuration. An external intervention, magnetic field, is also sought to direct the assembly of the magnetic Janus particles. In the presence of uniform magnetic field, the magnetic Janus particles form staggered chain structures with the chain direction parallel to the direction of the applied field. These chain structures are destroyed due to the capillary force during solvent evaporation. However, these soft structures are successfully locked in place after the solution dries by the addition of ammonium carbonate to the solution, which suggests a promising way to achieve 2D or 3D super structures for the fabrication of photonic crystals and photonic devices
Experimental Study of Lean Blowout with Hydrogen Addition in a Swirl-stabilized Premixed Combustor
Lean premixed combustion is widely used to achieve a better compromise between nitric oxides emissions and combustion efficiency. However, combustor operation near the lean blowout limit can render the flame unstable and lead to oscillations, flashback, or extinction, thereby limiting the potential range of lean combustion. Recent interest in integrated gasification combined cycle plants and syngas combustion requires an improved understanding of the role of hydrogen on the combustion process. Therefore, in present study, combustion of pure methane and blended methane-hydrogen has been conducted in a swirl stabilized premixed combustor. The measurement techniques implemented mainly include particle image velocimetry, CH*/OH* chemiluminescence imaging, planar laser-induced fluorescence imaging of OH radical. By investigating the flow field, heat release, flow-flame interaction, and flame structure properties, the fundamental controlling processes that limit lean and hydrogen-enriched premixed combustion with and without confinement have been analyzed and discussed. As equivalence ratio decreases, for unconfined flames, the reduced flame speed leads flame shrinking toward internal recirculation zone (IRZ) and getting more interacted with inner shear layer, where turbulence level and vorticity are higher. The flame fronts therefore experience higher hydrodynamic stretch rate, resulting in local extinction, and breaks along the flame fronts. Those breaks, in turn, entrain the unburnt fuel air mixture into IRZ passing through the shear layer with the local vortex effect, further leading to reaction within IRZ. In methane-only flames, the width of IRZ decreases, causing flames to straddle the boundary of the IRZ and to be unstable. High speed imaging shows that periodic flame rotating with local extinction and re-light events are evident, resulting in high RMS of heat release rate, and therefore a shorter extinction time scale. With hydrogen addition, flames remain in relatively axisymmetric burning structure and stable with the aid of low minimum ignition energy and high molecular diffusivity associated with hydrogen, leading to lower heat release fluctuation and a longer extinction time scale. For confined flames, however, the hydrogen effect on the extinction transient is completely opposite due to spiraling columnar burning structure, in comparison of a relatively stable conical shape in methane flames
A new discretely divergence-free positivity-preserving high-order finite volume method for ideal MHD equations
This paper proposes and analyzes a novel efficient high-order finite volume
method for the ideal magnetohydrodynamics (MHD). As a distinctive feature, the
method simultaneously preserves a discretely divergence-free (DDF) constraint
on the magnetic field and the positivity-preserving (PP) property, which
ensures the positivity of density, pressure, and internal energy. To enforce
the DDF condition, we design a new discrete projection approach that projects
the reconstructed point values at the cell interface into a DDF space, without
using any approximation polynomials. This projection method is highly
efficient, easy to implement, and particularly suitable for standard high-order
finite volume WENO methods, which typically return only the point values in the
reconstruction. Moreover, we also develop a new finite volume framework for
constructing provably PP schemes for the ideal MHD system. The framework
comprises the discrete projection technique, a suitable approximation to the
Godunov--Powell source terms, and a simple PP limiter. We provide rigorous
analysis of the PP property of the proposed finite volume method, demonstrating
that the DDF condition and the proper approximation to the source terms
eliminate the impact of magnetic divergence terms on the PP property. The
analysis is challenging due to the internal energy function's nonlinearity and
the intricate relationship between the DDF and PP properties. To address these
challenges, the recently developed geometric quasilinearization approach is
adopted, which transforms a nonlinear constraint into a family of linear
constraints. Finally, we validate the effectiveness of the proposed method
through several benchmark and demanding numerical examples. The results
demonstrate that the proposed method is robust, accurate, and highly effective,
confirming the significance of the proposed DDF projection and PP techniques.Comment: 26 page
Computing resource allocation in three-tier IoT fog networks: a joint optimization approach combining Stackelberg game and matching
Fog computing is a promising architecture to
provide economical and low latency data services for future
Internet of Things (IoT)-based network systems. Fog computing
relies on a set of low-power fog nodes (FNs) that are located
close to the end users to offload the services originally targeting
at cloud data centers. In this paper, we consider a specific
fog computing network consisting of a set of data service operators
(DSOs) each of which controls a set of FNs to provide the
required data service to a set of data service subscribers (DSSs).
How to allocate the limited computing resources of FNs to all
the DSSs to achieve an optimal and stable performance is an
important problem. Therefore, we propose a joint optimization
framework for all FNs, DSOs, and DSSs to achieve the optimal
resource allocation schemes in a distributed fashion. In the
framework, we first formulate a Stackelberg game to analyze
the pricing problem for the DSOs as well as the resource allocation
problem for the DSSs. Under the scenarios that the DSOs
can know the expected amount of resource purchased by the
DSSs, a many-to-many matching game is applied to investigate
the pairing problem between DSOs and FNs. Finally, within the
same DSO, we apply another layer of many-to-many matching
between each of the paired FNs and serving DSSs to solve
the FN-DSS pairing problem. Simulation results show that our
proposed framework can significantly improve the performance
of the IoT-based network systems
Conservative P1 Conforming and Nonconforming Galerkin Fems: Effective Flux Evaluation via a Nonmixed Method Approach
Given a P1 conforming or nonconforming Galerkin finite element method (GFEM) solution ph, which approximates the exact solution p of the diffusion-reaction equation −∇·K∇p + αp = f with full tensor variable coefficient K, we evaluate the approximate flux uh to the exact flux u = −K∇p by a simple but physically intuitive formula over each finite element. The flux is sought in the continuous (in normal component) or the discontinuous Raviart–Thomas space. A systematic way of deriving such a formula is introduced. This direct method retains local conservation property at the element level, typical of mixed methods (finite element or finite volume type), but avoids solving an indefinite linear system. In short, the present method retains the best of the GFEM and the mixed method but without their shortcomings. Thus we view our method as a conservative GFEM and demonstrate its equivalence to a certain mixed finite volume box method. The equivalence theorems explain how the pressure can decouple basically cost free from the mixed formulation. The accuracy in the flux is of first order in the H(div;Ω) norm. Numerical results are provided to support the theory
Is the Classic Convex Decomposition Optimal for Bound-Preserving Schemes in Multiple Dimensions?
Since proposed in [X. Zhang and C.-W. Shu, J. Comput. Phys., 229: 3091--3120,
2010], the Zhang--Shu framework has attracted extensive attention and motivated
many bound-preserving (BP) high-order discontinuous Galerkin and finite volume
schemes for various hyperbolic equations. A key ingredient in the framework is
the decomposition of the cell averages of the numerical solution into a convex
combination of the solution values at certain quadrature points, which helps to
rewrite high-order schemes as convex combinations of formally first-order
schemes. The classic convex decomposition originally proposed by Zhang and Shu
has been widely used over the past decade. It was verified, only for the 1D
quadratic and cubic polynomial spaces, that the classic decomposition is
optimal in the sense of achieving the mildest BP CFL condition. Yet, it
remained unclear whether the classic decomposition is optimal in multiple
dimensions. In this paper, we find that the classic multidimensional
decomposition based on the tensor product of Gauss--Lobatto and Gauss
quadratures is generally not optimal, and we discover a novel alternative
decomposition for the 2D and 3D polynomial spaces of total degree up to 2 and
3, respectively, on Cartesian meshes. Our new decomposition allows a larger BP
time step-size than the classic one, and moreover, it is rigorously proved to
be optimal to attain the mildest BP CFL condition, yet requires much fewer
nodes. The discovery of such an optimal convex decomposition is highly
nontrivial yet meaningful, as it may lead to an improvement of high-order BP
schemes for a large class of hyperbolic or convection-dominated equations, at
the cost of only a slight and local modification to the implementation code.
Several numerical examples are provided to further validate the advantages of
using our optimal decomposition over the classic one in terms of efficiency
Resident Attitudes toward Dark Tourism, a Perspective of Place-based Identity Motives
Place-based identity theories prove to be valid in better understanding resident attitudes towards support for tourism. Yet, its effectiveness is not verified in the context of dark tourism and resident attitudes towards dark tourism remains unknown. Based on a survey of 526 local residents in China’s Yingxiu, the epicentre of the Great Wenchuan Earthquake, the authors examined the relationships between the local residents’ place-based identity motives and their attitudes towards support for dark tourism development. Results show that the motive of ‘belonging/meaning’ is one of the most important determinants; residents’ involvement in dark tourism and bereavement affect their identity motives and attitudes towards support for dark tourism. The theoretical contributions and managerial implications are discussed
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