Summary
For discrete element methods (DEMs), integrating the equation of motion based on Newton's second law is an integral part of the computation. Accelerations and velocities are involved even for ...modeling static mechanics problems. As a consequence, the accuracy can be ruined and numerous calculation steps are required to converge. In this study, we propose a static DEM based on discontinuous deformation analysis (DDA). The force of inertia is removed to develop a set of static equilibrium equations for distinct blocks. It inherits the advantages of DDA in dealing with distinct block system such as jointed rock structures. Furthermore, the critical numerical artifact in DDA, ie, artificial springs between contact blocks, is avoided. Accurate numerical solution can be achieved in mere one calculation step. Last but not the least, since the method is formulated in the framework of mathematical programming, the implementation can be easily conducted with standard and readily available solvers. Its accuracy and efficiency are verified against a series of benchmarks found in the literature.
This paper develops an improved disk discontinuous deformation analysis (DDDA) model to simulate mixing particles in rotary drums. The original DDDA is improved by developing a disk-movable line ...segment contact model to motivate the constant movements of the disks during the mixing operation. The analytical solution of the movement of a single disk in a rotary drum is achieved by building a force analysis model, and the improved DDDA is validated by comparing this analytical solution with the proposed numerical simulation. Then, the improved DDDA is utilized to simulate mixing 8805 disks in a rotary drum, during which the influences of rotational speed and layouts of stirring blades are studies. Model performance shows that, the improved DDDA is a feasible tool to simulate the particle mixing process in rotary drums, which may optimize schemes for mixing particles and designing mixing equipment in industry.
Display omitted
•An improved DDDA model for simulating mixing particles in rotary drums is developed.•A disk-movable line segment contact model is developed.•The analytical solution of the movement of a single disk in rotary drum is achieved.•The validity of the improved DDDA is demonstrated.•The ability of the improved DDDA in simulating mixing disks in rotary drums is verified.
The analytic continuation of the GW self-energy from the imaginary to the real-energy axis is a central difficulty for approaches exploiting the favorable properties of response functions at ...imaginary frequencies. Within a scheme merging contour-deformation and analytic-continuation techniques, we show on the basis of extensive calculations for large molecular sets that it is preferable to perform an analytic continuation of the dynamically screened-Coulomb potential W rather than the much more structured self-energy operator. The case of states lying far away from the gap, including core states, is addressed by generalizing the analytic-continuation scheme, accounting further for quasiparticle lifetimes.
AbstractThe seismic performance of Balboa Boulevard during the 1994 MW 6.7 Northridge earthquake was examined through nonlinear deformation analyses (NDAs) using advanced tools to (1) investigate the ...failure mechanism leading to ground deformations at this site; (2) evaluate the accuracy of the adopted analysis methods, engineering procedures, and state-of-the-art tools to reasonably estimate horizontal ground displacements; and (3) identify key factors and parameters contributing to earthquake-induced ground deformations at this site. One-dimensional (1D) liquefaction vulnerability indexes (LVIs) and permanent displacements using Newmark sliding block analyses were also estimated and compared against ground deformations observed after the earthquake. The geotechnical characterization of Balboa Boulevard was assessed based on field and laboratory data obtained from two investigation campaigns. Transitional probability geostatistics were used to develop stratigraphic models that capture the heterogeneity and the spatial variability patterns of sand-like and clay-like soils present at this site. The stratigraphic models were implemented in the finite difference software FLAC and the behavior of sand-like and clay-like soils simulated using the PM4Sand and PM4Silt constitutive models, respectively. Sensitivity analyses were performed to address uncertainties associated with the spatial variability of soils, input ground motions, the proportion of sand-like and clay-like soils within the soil deposit, and the strength properties of these materials. Results from NDAs suggest that a compounded effect of both liquefaction of sand-like soils and cyclic softening of clay-like soils led to the excessive ground deformations at Balboa Boulevard. This study sheds light on the importance of using appropriate engineering procedures and numerical modeling protocols in the prediction of deformation patterns, the selection of key input parameters, as well as the applicability of LVIs in complex sites.
To analyze the movement of discrete rock block systems, a nodal-based three-dimensional discontinuous deformation analysis method with contact potential (3D-NDDACP) is proposed. In the proposed ...3D-NDDACP method, tetrahedral FE meshes which can be effectively generated with existing mesh generator are adopted to discretize rock blocks to better capture their deformation. Additionally, the contact potential is incorporated to treat the contact between two adjacent blocks. The introduction of contact potential significantly simplifies the implementation of the proposed 3D-NDDACP method, since the contact force can be directly computed without distinguishing contact types between any two adjacent blocks. However, in the traditional 3D-DDA method it is essential to conduct contact type judgment before computing contact forces. Note that contact type judgment is not a trivial task for 3D problems, since many different contact types including point-to-point contact, point-to-edge contact, point-to-face contact and edge-to-edge contact are involved. With the proposed 3D-NDDACP method, three benchmark problems about the movement of rock block systems are investigated. Numerical results obtained with the proposed 3D-NDDACP method are in good agreement with the theoretical solution, which means that the proposed 3D-NDDACP method can reliably and correctly simulate the movement of rock block systems. The proposed 3D-NDDACP method warrants further investigation.
Highlights
A nodal-based 3D DDA model is proposed for modeling the discrete rock block system.
Contact forces are computed effectively using contact volumes, without the cumbersome contact type determination.
Simplest and most versatile tetrahedral meshes are always available for the proposed model.
Display omitted
A novel multi-cover searching (MCS) algorithm is presented to promote the efficiency of contact detection. As simple geometrical entities, covers are built on basic geometrical ...elements of the boundary of polygonal blocks, promoting the efficiency of rough search. Firstly, blocks possible to contact each other are identified more precisely through intersections between covers. Secondly, when performing contact type identification between two blocks, a full range search between geometrical elements is reduced into several local delicate checks. Moreover, we could embed MCS into many other contact detection methods. Several examples validate its accuracy and improved efficiency over traditional methods.
The construction of large earth/rock fill dams, albeit its remarkable progress, still relies largely on past experiences. Therefore, a comprehensive yet dependable monitoring program is particularly ...beneficial for guiding the practice. However, conventional measurements can only produce limited discrete data. This paper exploits the potential of the terrestrial laser scanning (TLS) for an accurate inventory of as-built states of a concrete-faced rockfill dam under construction and for a full-field analysis of the 3D deformation pattern over its upstream face. For the former, a well-designed 3D geodetic system, with a particular consideration of the topography, promises a regulated acquisition of high-quality and blind-zone-free point cloud at field and also eases the cumbersome data registration process while maintaining its precision in house. For the latter, a problem-tailored processing pipeline is proposed for deformation extraction. Its core idea is to achieve a highly precise alignment of the point clouds with Iterative Closed Point algorithms from different epochs in datum areas that displays a featured, undeformed geometry at stable positions across epochs. Then, the alignment transformation matrix is applied to the point clouds of respective upstream face for each epoch, followed by pairwise comparisons of multiple adjusted point clouds for deformation evaluation. A processing pipeline is used to exploit the peal scene data redundancy of the GLQ dam acquired at six different epochs. Statistical analysis shows that satisfactory accuracy for deformation detection can be repeatably achieved, regardless of the scanner's positioning uncertainties. The obtained 3D deformation patterns are characterised by three different zones: practically undeformed, outward and inward deformed zones. Their evolutions comply well with real construction stages and unique 3D valley topography. Abundant deformation results highlight the potential of TLS combined with the proposed data processing pipeline for cost-efficient monitoring of huge infrastructures compared to conventional labor-intense measurements.
Very preterm human neonates are exposed to numerous invasive procedures as part of life-saving care. Evidence suggests that repetitive neonatal procedural pain precedes long-term alterations in brain ...development. However, to date the link between pain and brain development has limited temporal and anatomic specificity. We hypothesized that early exposure to painful stimuli during a period of rapid brain development, before pain modulatory systems reach maturity, will predict pronounced changes in thalamic development, and thereby cognitive and motor function. In a prospective cohort study, 155 very preterm neonates (82 males, 73 females) born 24-32 weeks' gestation underwent two MRIs at median postmenstrual ages 32 and 40 weeks that included structural, metabolic, and diffusion imaging. Detailed day-by-day clinical data were collected. Cognitive and motor abilities were assessed at 3 years, corrected age. The association of early (skin breaks, birth-Scan 1) and late pain (skin breaks, Scans 1-2) with thalamic volumes and
-acetylaspartate (NAA)/choline (Cho), and fractional anisotropy of white-matter pathways was assessed. Early pain was associated with slower thalamic macrostructural growth, most pronounced in extremely premature neonates. Deformation-based morphometry analyses confirmed early pain-related volume losses were localized to somatosensory regions. In extremely preterm neonates early pain was associated with decreased thalamic NAA/Cho and microstructural alterations in thalamocortical pathways. Thalamic growth was in turn related to cognitive and motor outcomes. We observed regionally-specific alterations in the lateral thalamus and thalamocortical pathways in extremely preterm neonates exposed to more procedural pain. Findings suggest a sensitive period leading to lasting alterations in somatosensory-system development.
Early exposure to repetitive procedural pain in very preterm neonates may disrupt the development of regions involved in somatosensory processing, leading to poor functional outcomes. We demonstrate that early pain is associated with thalamic volume loss in the territory of the somatosensory thalamus and is accompanied by disruptions thalamic metabolic growth and thalamocortical pathway maturation, particularly in extremely preterm neonates. Thalamic growth was associated with cognitive and motor outcome at 3 years corrected age. Findings provide evidence for a developmentally sensitive period whereby subcortical structures in young neonates may be most vulnerable to procedural pain. Furthermore, results suggest that the thalamus may play a key role underlying the association between neonatal pain and poor neurodevelopmental outcomes in these high-risk neonates.
A chemical weathering model based on Discontinuous Deformation Analysis(DDA) was proposed in this study to shed light on the microscopic origin of the mechanical behaviors of cemented sandstone with ...different chemical weathering degrees. The main deterioration of the microstructure induced by chemical weathering was represented by the loss of cement between polygonous grains skeleton. By conducting uniaxial compression tests, the result revealed that the proposed model that considered the geometric and contact heterogeneity was feasible to characterize the degradation of strength and deformation properties with the enhancement of the weathering degree. In addition, the discrepant failure patterns along the degree of chemical weathering were elaborated from the initiation of fracture at the grain scale, showing a correlation to the cement and porosity.
PDF
