Coarse Region Research Articles

Unconditionally Stable MFLTD Method for the Full Wave Electromagnetic Simulation

A new unconditionally stable numerical method for the time domain simulation of electromagnetic problems using a combination of the Laguerre polynomials in the time domain and radial basis functions (RBF) in the spatial domain named mesh-Free Laguerre time domain (MFLTD) method is described. Unconditionally stable Laguerre based method may be computationally much more efficient than the FDTD methods and using RBFs allows computing the problems with complex-shapes in a non-uniform point's distribution. A numerical example at the final part of the communication shows the efficiency of the presented method in problems with sparse and coarse regions. Using the MFLTD method for this example, we can achieve a 96% reduction in the computation time and get an acceptable degree of accuracy in comparison to the conventional mesh-free time-domain (MFTD) method.

The microstructural evolution and superplastic behavior at low temperatures of Mg–5.00Zn–0.92Y–0.16Zr (wt.%) alloys after hot extrusion and ECAP process

Abstract In this study, equal channel angular pressing (ECAP) was applied to a hot-extruded Mg–5.00Zn–0.92Y–0.16Zr (wt.%) alloy to produce an ultrafine-grained a -Mg structure of 0.6 μm with uniformly distributed fine quasicrystal Mg 3 YZn 6 particles. The basal planes in the as-ECAPed alloy were inclined approximately 45° to the ECAP direction (EPD); thus, a high Schmid factor of 0.36 for ( 0 0 0 1 ) 〈 1 1 2 ¯ 0 〉 basal slip along EPD was obtained. Then, the superplastic behavior at low temperatures of 150–250 °C and initial strain rates of 1.67 × 10 −3 s −1 –1.67 × 10 −1 s −1 of the as-ECAPed alloy was investigated and compared with that of the as-extruded alloy. The microstructural development, texture evolution and cracking behavior during tensile tests were systemically investigated by electron backscattered diffraction (EBSD) analysis. During the tensile test along the EPD, the basal planes in the as-ECAPed alloy specimen were tilted approximately 15° away from the original position; thus, the basal planes were still in a position favorable for the basal slip along the EPD. As a result, a maximum elongation of 865% was obtained at 200 °C and a strain rate of 1.67 × 10 −3 s −1 , as a result of this favored basal texture and the excellent thermal stability of the ultrafine-grained structure. This optimum superplastic temperature of 200 °C is much lower than that obtained for other Mg–Zn–Y–(Zr) alloys. In contrast, for the as-extruded alloy specimen, superplastic behavior did not occur until the critical condition of 250 °C and a strain rate of 1.67 × 10 −3 s −1 . Below this temperature and strain rate, the strong extrusion basal texture was maintained, and the cracks in the hot-worked coarse region (or un-DRXed region) were observed nucleating mainly associated with the formation of a 38° { 1 0 1 ¯ 1 } – { 1 0 1 ¯ 2 } double twin. At this temperature and strain rate, the hot-worked coarse regions were significantly refined by the dynamic recovery and the following recrystallization process. In the case, the formation of a double twin was prevented and the facture was delayed.

Effect of Tensile Strength and Microstructure on Notch-fatigue Properties of Ultrafine-grained Steels

Fatigue strength is one of the key properties in the practical use of ultrafine grained steels. Fatigue tests were conducted on notched specimens by conventional electromagnetic resonance fatigue testing machines. The electromagnetic resonance fatigue testing was carried out at 150 Hz up to 107 cycles. The investigated steels had different levels of carbon, 0.15 wt%, 0.30 wt% and 0.60 wt% with tensile strengths of 850 MPa, 950 MPa and 1105 MPa, respectively. With increasing in carbon content, the tensile strength increased and the total elongation decreased. The notched specimens never showed internal fracture and showed a clear fatigue limit. The notch fatigue limit increased with an increase in tensile strength from 850 MPa to 970 MPa, when the carbon content was 0.15 and 0.30 wt% with microstructures consisting of ultrafine ferrite grains and cementite particles. On the other hand, when the carbon content was 0.60 wt%, the notch fatigue limit decreased, though tensile strength increased to 1105 MPa. Retained pearlite grains were observed in 0.60 wt%C steel in addition to ultrafine ferrite grains and cementite particles. These retained pearlite grains which were coarse and high angle grain boundary poor regions were attributed to the lower notch fatigue limit.

Exploring a Multiresolution Modeling Approach within the Shallow-Water Equations

Abstract The ability to solve the global shallow-water equations with a conforming, variable-resolution mesh is evaluated using standard shallow-water test cases. While the long-term motivation for this study is the creation of a global climate modeling framework capable of resolving different spatial and temporal scales in different regions, the process begins with an analysis of the shallow-water system in order to better understand the strengths and weaknesses of the approach developed herein. The multiresolution meshes are spherical centroidal Voronoi tessellations where a single, user-supplied density function determines the region(s) of fine- and coarse-mesh resolution. The shallow-water system is explored with a suite of meshes ranging from quasi-uniform resolution meshes, where the grid spacing is globally uniform, to highly variable resolution meshes, where the grid spacing varies by a factor of 16 between the fine and coarse regions. The potential vorticity is found to be conserved to within machine precision and the total available energy is conserved to within a time-truncation error. This result holds for the full suite of meshes, ranging from quasi-uniform resolution and highly variable resolution meshes. Based on shallow-water test cases 2 and 5, the primary conclusion of this study is that solution error is controlled primarily by the grid resolution in the coarsest part of the model domain. This conclusion is consistent with results obtained by others. When these variable-resolution meshes are used for the simulation of an unstable zonal jet, the core features of the growing instability are found to be largely unchanged as the variation in the mesh resolution increases. The main differences between the simulations occur outside the region of mesh refinement and these differences are attributed to the additional truncation error that accompanies increases in grid spacing. Overall, the results demonstrate support for this approach as a path toward multiresolution climate system modeling.

A field test study of airborne wear particles from a running regional train

Inhalable airborne particles have inverse health effects. In railways, mechanical brakes, the wheel–rail contact, current collectors, ballast, sleepers, and masonry structures yield particulate matter. Field tests examined a Swedish track using a train instrumented with particle measurement devices, brake pad temperature sensors, and speed and brake sensors. The main objective of this field test was to study the characteristics of particles generated from disc brakes on a running train with an on-board measuring set-up. Two airborne particle sampling points were designated, one near a pad–rotor disc brake contact and a second under the frame, not near a mechanical brake or the wheel–rail contact; the numbers and size distributions of the particles detected were registered and evaluated under various conditions (e.g. activating/deactivating electrical brakes or negotiating curves). During braking, three speed/temperature-dependent particle peaks were identified in the fine region, representing particles 280, 350, and 600 nm in diameter. In the coarse region, a peak was discerned for particles 3–6 µm in diameter. Effects of brake pad temperature on particle size distribution were also investigated. Results indicate that the 280-nm peak increased with increasing temperature, and that electrical braking significantly reduced airborne particle numbers. Field emission scanning electron microscope images captured particles sizing down to 50 nm. The inductively coupled plasma mass spectrometry results indicated that Fe, Cu, Zn, Al, Ca, and Mg were the main elements constituting the particles.

A bridging scale method for granular materials with discrete particle assembly – Cosserat continuum modeling

Based on the bridging scale method (BSM) initially presented in nano mechanics [1,2], a new version of the BSM that couples discrete particle assembly modeling using the discrete element method (DEM) and Cosserat continuum modeling using the finite element method (FEM) at both micro–macro scale levels respectively is proposed for multiscale analysis of granular materials. The DEM is only applied to the limited localized regions for accurately simulating discontinuous failure phenomena in microscopic scale, meantime, the FEM that costs much less computational time and storage space covers the whole domain, with permitting to adopt different time step sizes for the time integration schemes in coarse and fine scales respectively. As a consequence, both computational accuracy and efficiency of the proposed BSM are greatly enhanced. With a bridging scale displacement (including translations and rotations) decomposition and based on the virtual work principle applied to the FEM nodes of the Cosserat continuum and the particle centers of the discrete particle assembly respectively, two decoupling sets of equations of motion of the combined coarse–fine scale system are formulated. The interfacial condition between the coarse and fine scale regions in the quasi-static and dynamic loading cases are presented and discussed. The non-reflecting boundary condition and its implementation, which is capable of effectively eliminating spurious reflected waves at the interfaces between the coarse and fine scale regions, are described in detail. The numerical results for 2D example problems demonstrate the applicability and advantages of the present BSM, and the performances of the non-reflecting boundary condition implemented to simulate dynamical responses in geo-structures composed of granular materials.

Replica time integrators

This paper is concerned with the classical problem of wave propagation in discrete models of nonuniform spatial resolution. We develop a new class of Replica Time Integrators (RTIs) that permit the two-way transmission of thermal phonons across mesh interfaces. This two-way transmissibility is accomplished by representing the state of the coarse regions by means of replica ensembles, consisting of collections of identical copies of the coarse regions. In dimension d, RTIs afford an O(n^d) speed-up factor in sequential mode, and O(n^(d + 1)) in parallel, over regions that are coarsened n-fold. In this work, we restrict ourselves to the solution of the 3d continuous wave equation, for both linear and non-linear materials. By a combination of phase-error analysis and numerical testing, we show that RTIs are convergent and result in exact two-way transmissibility at the Courant–Friedrichs–Lewy limit for any angle of incidence. In this limit, RTIs allow step waves and high-frequency harmonics to cross mesh interfaces in both directions without internal reflections or appreciable loss or addition of energy. The possible connections of RTIs with discrete-to-continuum approaches and, in particular, with the transition between molecular dynamics and continuum thermodynamics are also pointed to by way of future outlook.

Microstructures and Mechanical Properties of Heat-Resistant High-Pressure Die-Cast Mg-4Al-6RE-0.3Mn (RE= La, Ce, Pr, Nd, Wt.%) Alloys

Mg-4Al-6RE-0.3Mn (RE=La, Ce, Pr, Nd, wt.%) magnesium alloys were prepared successfully by the high-pressure die-casting technique. The microstructures and tensile properties in the temperature range from room temperature to 200 °C were investigated. It was confirmed that the Mg-Al-RE-based alloys exhibited an outstanding die-cast character. The cross-section of test bar could be divided into the fine skin region and the relatively coarse interior region by a narrow band with fine structure. Two binary Al-RE (RE=La, Ce, Pr, Nd) phases with the former being the dominant one, Al11RE3 and Al2RE, occupied large grain boundary area. All the four alloys exhibited excellent tensile properties until 200 °C, which were mainly attributed to the fine grain size and the strengthening phase Al11RE3 with high volume fraction, fine acicular/lamellar morphology, "orderly stack” distribution and good thermal stability. The results of this work provided a basis for further investigation of the new heat-resistant high-pressure die-cast magnesium alloys designed to serve at temperature up to 200 °C.

Linear Feature Detection in Polarimetric SAR Images

Recently, the use of linear features for processing remote-sensing images has shown its importance in applications. Unfortunately, traditional linear feature detection methods rely heavily on the image's local information which makes them vulnerable to the presence of noise in the image. This problem becomes particularly difficult for synthetic aperture radar (SAR) image applications where SAR images are corrupted by speckle noise. In order to overcome this problem, we propose a novel method that processes the polarimetric synthetic aperture radar (Pol-SAR) images by combining the multiscale image analysis with polarimetric information in a new fashion. A two-scale approach is adopted here. On a coarse level, the coarse regions of the linear features are extracted by a curvelet transform from a speckle noise reduced image obtained by the polarimetric whitening filter. On a fine level, we develop a fuzzy polarimetric detector to accurately locate the linear features inside the regions. The effectiveness of the proposed method is demonstrated using simulated Pol-SAR data acquired from both EMISAR and Convair-580 systems.

A Coupled Molecular Dynamics/Coarse-Grained-Particle Method for Dynamic Simulation of Crack Growth at Finite Temperatures

A hybrid molecular dynamics/coarse-grained-particle (MD-CGP) method is proposed for dynamic simulation of crack growth at finite temperatures. In the present method, the MD method is applied for the non-linear elastic region such as the crack tip and dislocation core, while the CGP method is applied for the surrounding linear elastic region. For coupling the atomistic and coarse regions, extra atoms and particles are placed, respectively, beyond the interface of the atomistic and coarse systems. They move according to the Langevin-type equation. The dissipation term in the Langevin-type equation contains the velocity difference between the atomistic and coarse systems. Hence, the coupling is achieved through the damped oscillation of the difference between the two systems. It is shown in a one-dimensional simulation that the present hybrid method achieves the non-reflecting boundary condition at the interface for the entire wavenumber range at finite temperatures. Two-dimensional crack growth simulations are performed using both the present hybrid MD-CGP method and full MD method for accuracy check. In the hybrid simulation, the location of the atomistic region shifts to follow the moving crack tip. In spite of such drastic operations, the results obtained by the present hybrid simulation agree very well with those by the full MD simulation. The hybrid MD-CGP method allows us to simulate very large systems without losing atomistic information at the singular point and non-linear region, with much lesser degrees of freedom than required for the full MD method.

Microstructures and mechanical properties of heat-resistant HPDC Mg–4Al-based alloys containing cheap misch metal

Mg–4Al–xCe/La–0.3Mn (Ce/La: mixture of Ce and La, x=1, 2, 4 and 6wt.%) alloys were prepared by high-pressure die-casting. The microstructures, mechanical properties and thermal stability were investigated. The cross-section of test bar could be divided into the fine skin region and the relatively coarse interior region. Two binary Al–(Ce, La) phases with the former being the dominant one, Al11(Ce, La)3 and Al2(Ce, La), are mainly distributed along the dendrite boundaries, and La prefers to exist in Al11(Ce, La)3. The alloy with 4wt.% Ce/La exhibits high tensile properties and good heat resistance until 200°C, which were mainly attributed to the fine dendritic arm spacing and the main strengthening phase Al11(Ce, La)3, which is present in high volume fraction, and possesses fine rod-like morphology, network or “orderly stack” distribution and good thermal stability. The results of this research provide a basis for further investigation of the new low cost high-pressure die-cast Mg–Al–RE alloys designed to serve at temperature up to 200°C.

Fundus Autofluorescence and Visual Acuity in Central Serous Chorioretinopathy

To investigate the fundus autofluorescence (FAF) abnormalities in central serous chorioretinopathy (CSC) and evaluate potential correlations with visual acuity. Retrospective, observational case series. Four hundred seventy-five eyes of 238 patients with CSC. Consecutive patients with CSC underwent FAF imaging, as well as routine ophthalmologic examinations. Confluent hypoautofluorescence was defined as a region of absent autofluorescence greater than one fourth of a disk diameter. Granular hypoautofluorescence was defined if there was a grainy or coarse region of decreased fluorescence as compared with normal surrounding areas greater than one fourth of a disc diameter in size. A descending tract was a downward leading swathe of decreased autofluorescence originating from the posterior pole to extend below the inferior arcade. The pattern and frequency of FAF abnormalities and their correlations with corrected visual acuity. The mean age of the subjects was 57.1 years (standard deviation, 13.3), and 181 (76.1%) were male. Confluent and granular hypoautofluorescence was detected in the macula of 54 (11.4%) and 300 (63.2%) of 475 eyes, respectively. Descending tracts from the macula were observed in 43 (9.1%) eyes and from the optic disc in 43 (9.1%) eyes. Multiple regression analysis revealed that confluent hypoautofluorescence of the macula, granular hypoautofluorescence of the macula, and increasing age all were independent predictors of decreased visual acuity. The FAF abnormalities in CSC show multiple distinct patterns and seem to provide functional information.

Effect of Ce on microstructure, mechanical properties and corrosion behavior of high-pressure die-cast Mg–4Al-based alloy

Mg–4Al– xCe–0.3Mn ( x = 0, 1, 2, 4 and 6 wt.%) alloys were prepared by high-pressure die-casting. The microstructures, mechanical properties and corrosion behavior were investigated. The cross-section of test bar is divided into the fine skin region and the relatively coarse interior region by a narrow band. The dendritic arm spacing is greatly reduced and the secondary phases Al 11Ce 3 and (Al, Mg) 2Ce with the former being the dominant one substitute the Mg 17Al 12 phase with addition of Ce. When Ce content reaches 4 wt.%, the alloy exhibits an optimal cost performance ratio. The improved mechanical properties maintained up to 200 °C are mainly related to the fine grain size and the main strengthening phase Al 11Ce 3, which is present in a high volume fraction, and possesses fine acicular morphology and relatively good thermal stability. The improved corrosion resistance is attributed to the microstructure modification of the alloys and the corrosion product films.

A domain-reduction approach to bridging-scale simulation of one-dimensional nanostructures

We present a domain-reduction approach for the simulation of one-dimensional nanocrystalline structures. In this approach, the domain of interest is partitioned into coarse and fine scale regions and the coupling between the two is implemented through a bridging-scale interfacial boundary condition. The atomistic simulation is used in the fine scale region, while the discrete Fourier transform is applied to the coarse scale region to yield a compact Green’s function formulation that represents the effects of the coarse scale domain upon the fine/coarse scale interface. This approach facilitates the simulations for the fine scale, without the requirement to simulate the entire coarse scale domain. After the illustration in a simple 1D problem and comparison with analytical solutions, the proposed method is then implemented for carbon nanotube structures. The robustness of the proposed multiscale method is demonstrated after comparison and verification of our results with benchmark results from fully atomistic simulations.

Microstructural characterization of oxide morphologies on Ni and Inconel foams exposed to concentrated solar radiation

The microstructural characterization of oxide surface morphologies formed on both high purity Ni and Inconel foams exposed to concentrated solar radiation is investigated by the use of scanning electron microscopy (SEM) and EDXS. Oxidation was achieved by concentrated solar radiation at temperatures ranging between 200 and 1100 °C for 30 min under conditions of dynamic air flow. SEM observations revealed a rapid homogeneous oxidation in the Ni-foam with three different surface oxide structures formed in relation with the process temperature. An important effect of substrate morphological and compositional features was only observed in Inconel foams, independent of the test temperature, with the existence of coarse and fine oxide regions denoting the non-uniformity of the oxidation process. The above-presented results give evidence for a potential use of the tested foam materials as volumetric solar receivers.

Crystallographic Texture of Induction-Welded and Heat-Treated Pipeline Steel

Large-diameter steel pipes are produced by induction seam-welding followed by induction-assisted heat treatment. The microstructure and distribution of crystal orientations have been studied and related to the mechanical properties of the welded regions. The welding and heat-treatment process leads to a microstructure, a simple observation of which can not explain the observed variations in toughness in the vicinity of the welding joint, because the crystallographic grain size, which represents the scale of similarly oriented adjacent grains, is much coarser than the ordinary grain size. Furthermore, heating the affected zone into the austenite phase field followed by cooling does not completely eliminate the coarse regions of similarly oriented grains. The consequences of this on mechanical properties are discussed.

Microstructures and mechanical properties of heat-resistant high-pressure die-cast Mg–4Al– xLa–0.3Mn ( x = 1, 2, 4, 6) alloys

Mg–4Al– xLa–0.3Mn ( x = 1, 2, 4 and 6 wt.%) magnesium alloys were prepared successfully by the high-pressure die-casting technique. The microstructures, tensile properties in the temperature range from room temperature to 300 °C and heat resistance were investigated. It was confirmed that the Mg–Al–La-based alloys exhibited outstanding die-cast character. The cross-section of test bar could be divided into two parts by a narrow band, that is, the fine skin region and the relatively coarse center region. When addition of La reached 4 wt.%, the die-cast alloy exhibited high tensile properties and good heat resistance up until 200 °C. It was demonstrated that the outstanding mechanical properties were mainly attributed to the fine grain size and the strengthening phase Al 11La 3, which was in high volume fraction, fine acicular/lamellar morphology, “orderly stack” distribution and excellent thermal stability. The result of this research provided a basis for further investigation of the new high-pressure die-cast Mg–Al–RE alloys designed to serve at temperature up to 200 °C.

Domain Decomposition Preconditioners for Multiscale Flows in High-Contrast Media

In this paper, we study domain decomposition preconditioners for multiscale flows in high-contrast media. We consider flow equations governed by elliptic equations in heterogeneous media with a large contrast in the coefficients. Our main goal is to develop domain decomposition preconditioners with the condition number that is independent of the contrast when there are variations within coarse regions. This is accomplished by designing coarse-scale spaces and interpolators that represent important features of the solution within each coarse region. The important features are characterized by the connectivities of high-conductivity regions. To detect these connectivities, we introduce an eigenvalue problem that automatically detects high-conductivity regions via a large gap in the spectrum. A main observation is that this eigenvalue problem has a few small, asymptotically vanishing eigenvalues. The number of these small eigenvalues is the same as the number of connected high-conductivity regions. The coars...

SAR image edge detection using curvelet transform and Duda operator

Edge detection plays a critical role in image reconstruction of remote imaging systems. To gain better quality detection, a novel method is proposed that combines the curvelet transform and Duda operator in a special fashion for processing synthetic aperture radar (SAR) images. To extract the coarse regions in the image, the curvelet transform is first applied to the image and then, after some processing, the Duda operator locates the edges precisely inside the regions. The superiority of the proposed method is demonstrated for Pi-SAR image data.

Comparison of adjoint and analytical Bayesian inversion methods for constraining Asian sources of carbon monoxide using satellite (MOPITT) measurements of CO columns

We apply the adjoint of an atmospheric chemical transport model (GEOS‐Chem CTM) to constrain Asian sources of carbon monoxide (CO) with 2° × 2.5° spatial resolution using Measurement of Pollution in the Troposphere (MOPITT) satellite observations of CO columns in February–April 2001. Results are compared to the more common analytical method for solving the same Bayesian inverse problem and applied to the same data set. The analytical method is more exact but because of computational limitations it can only constrain emissions over coarse regions. We find that the correction factors to the a priori CO emission inventory from the adjoint inversion are generally consistent with those of the analytical inversion when averaged over the large regions of the latter. The adjoint solution reveals fine‐scale variability (cities, political boundaries) that the analytical inversion cannot resolve, for example, in the Indian subcontinent or between Korea and Japan, and some of that variability is of opposite sign which points to large aggregation errors in the analytical solution. Upward correction factors to Chinese emissions from the prior inventory are largest in central and eastern China, consistent with a recent bottom‐up revision of that inventory, although the revised inventory also sees the need for upward corrections in southern China where the adjoint and analytical inversions call for downward correction. Correction factors for biomass burning emissions derived from the adjoint and analytical inversions are consistent with a recent bottom‐up inventory on the basis of MODIS satellite fire data.