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Coupling techniques that take advantage of the mass conservation property of the volume-of-fluid (VOF) method and the sharpened interface computation of the Level Set (LS) approach are widely used for computations with high accura...
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Coupling techniques that take advantage of the mass conservation property of the volume-of-fluid (VOF) method and the sharpened interface computation of the Level Set (LS) approach are widely used for computations with high accuracy order demands. In this paper, an enhanced coupling method for interface computations in incompressible two-phase flows is presented. In the proposed method, the solution of the re-initialization LS function is reformulated in a conservative form before it is applied to estimate the interface curvature and interface normal vector. The estimated interface exhibits significant improvements in smoothness and accuracy compared to those obtained with the original VOF method and a previous coupling method. The proposed method is then implemented in an incompressible Navier-Stokes solver (interFoam) in the OpenFOAM platform to solve several benchmark tests. Good agreement between the simulated results and the analytical/benchmark solutions with well-preserved mass conservation is obtained for standard tests, including a reversed single vortex, static droplet, and rising bubble, thus demonstrating the potential of the proposed technique for both academic research and practical applications. (C) 2020 Elsevier Ltd. All rights reserved.
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Compared to a density based procedure, the level set method can acquire a clear boundary without any intermediate density elements, and has become a topical issue in topology optimization studies. In this method, the material prop...
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Compared to a density based procedure, the level set method can acquire a clear boundary without any intermediate density elements, and has become a topical issue in topology optimization studies. In this method, the material properties of the discretized elements are usually defined as the design variables, giving rise to the so called discrete or sawtooth topologies. In this paper, a topology optimization methodology based on level-set method (LSM) and radial basis function (RBF) is proposed. To enhance the performance of the LSM, an improved material interpolation model is proposed. The RBF based post-processor is proposed to smooth the preliminary optimized topology. The proposed method is applied to the topology design of a piezoelectric energy harvester. The effect of different penalization factors is compared and analyzed. The numerical results validate the feasibility and effectiveness of the proposed method.
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A coupling technique for interface simulations applied to the fields of ship hydrodynamics and multiphase flow computations is presented. The technique takes advantages of mass conservation property of the volume-of-fluid method a...
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A coupling technique for interface simulations applied to the fields of ship hydrodynamics and multiphase flow computations is presented. The technique takes advantages of mass conservation property of the volume-of-fluid method and sharpened interface computation of the Level-Set approach. An improved scheme for the gradient computation of the Level-Set function during re-initialisation is also introduced. A solver with the present coupling is built into the interFoam solver of the OpenFOAM platform. Three test cases, i.e. a free rising air bubble in still water, dam-break simulation with a hump in dry bed channel and free surface flow around a container ship are conducted to demonstrate the capability of the new solver. A good agreement between the simulated results and the benchmark and/or measured data shows the significant potential of the present technique in both academic research and practical applications.
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This paper presents a parallel Eulerian/Lagrangian multi-scale coupling procedure for two-phase flows. At the fully resolved scale, the dynamically evolving phase interface is tracked using a Eulerian approach. In regions of the f...
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This paper presents a parallel Eulerian/Lagrangian multi-scale coupling procedure for two-phase flows. At the fully resolved scale, the dynamically evolving phase interface is tracked using a Eulerian approach. In regions of the flow, where the phase interface geometry can no longer be resolved adequately, separated, small scale liquid structures are described by a Lagrangian point particle approach. The coupling procedure of these two descriptions consists of an efficient parallel algorithm that identifies tracked liquid candidate structures, removes them from the resolved Eulerian description, and inserts them into the Lagrangian description preserving their position, mass, momentum, and lower order shape. While in principle applicable to level set, Volume of Fluid, and marker particle interface tracking methods for the fully resolved scale, this paper focuses on examples from atomization simulations using the refined level set grid method.
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In the recent years, isogeometric analysis (IGA) has found wide application in modeling different types of discontinuities produced by cracks, contact surfaces and bi-material interfaces. This technique eliminates the geometry dis...
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In the recent years, isogeometric analysis (IGA) has found wide application in modeling different types of discontinuities produced by cracks, contact surfaces and bi-material interfaces. This technique eliminates the geometry discretization errors associated with the representation of complex geometries. The present paper employs the extended isogeometric analysis (XIGA) and the coupled finite element-isogeometric analysis (FE-IGA) to model large elasto-plastic deformations in bi-material engineering components. XIGA models all types of discontinuities independent of the grid chosen for analysis. Instead, the standard displacement-based approximation is enriched with additional enrichment functions to include the effects of these discontinuities in the formulation. In the coupled FE-IGA technique, IGA is used in the weak portion of the domain to eliminate the problems of mesh distortion and conventional finite element method is used in the stronger portion where mesh distortions do not occur. The transition elements are employed to couple the finite element and isogeometric portions of the domain. Finally, several numerical problems are solved by XIGA and coupled FE-IGA techniques to illustrate the applicability, efficiency and accuracy of the proposed techniques in modeling large elasto-plastic deformations in bi-material specimens. The results obtained in the present study are compared with finite element solutions which have been taken as the reference solution for the given problems.
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The physics of atomization process involve many spatial scales, generating a wide variety of liquid inclusions of different sizes with large density and viscosity ratios between liquid and gas phases. In order to correctly capture...
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The physics of atomization process involve many spatial scales, generating a wide variety of liquid inclusions of different sizes with large density and viscosity ratios between liquid and gas phases. In order to correctly capture the dynamics of these phenomena, each scale needs to be resolved with an appropriate method to ensure the conservation of physical quantities (mass, momentum) as well as the jump conditions across the liquid-gas interface. To address these problems, an original multi-scale methodology has been developed. It consists of a core coupled Level-Set/Volume-of-Fluid method (CLSVOF) for accurate capture of primary atomization, an adaptive mesh refinement technique (oct-tree AMR) to dynamically optimize the structured Cartesian mesh and a particle tracking algorithm to capture droplet dynamics. An improved Eulerian-Lagrangian coupling has been developed to ensure a smooth transition between the Eulerian and the Lagrangian modelling frameworks in the zones where they reach their respective limits of validity. The overall procedure is tested on simplified numerical tests and validated on a realistic planar liquid sheet atomization case. Results highlight the ability of the present method to reproduce the whole atomization process, from large scale instabilities to small droplet dynamics, and allow a preliminary statistical spray analysis. (C) 2017 Elsevier Ltd. All rights reserved.
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This study presents a new approach for segmentation and reconstruction of newborn's skull including bones, fontanels, and sutures from computed tomography (CT) images. The segmentation approach relies on propagation of a pair of i...
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This study presents a new approach for segmentation and reconstruction of newborn's skull including bones, fontanels, and sutures from computed tomography (CT) images. The segmentation approach relies on propagation of a pair of interacting smooth surfaces based on geodesic active regions. These surfaces evolve in opposite directions; the exterior surface moves inward while the interior one moves in outward direction. The moving surfaces are forced to stop when arriving at the outer or the inner surface of the cranial bones using edge information. Since fontanels and sutures are not directly detectable in CT images, this method imposes specific propagation constraints for coupled interfaces to prevent the moving surfaces from intersecting each other and penetrating into the opposite region. Finally, an algorithm for level set initialization is introduced which enforces the evolving surfaces to conform to the shape of the head. The proposed method was evaluated using 18 neonatal CT images. The segmentation results achieved by the suggested method have been compared with manual segmentations by two different raters, performed to establish a reliable reference. The comparison of the two segmentation results using the Dice similarity coefficient and modified Hausdorff distance shows that the proposed approach provides satisfactory results.
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Developing global, high-dimensional potential energy surfaces (PESs) is a formidable task. Beside the challenges of PES fitting and fitting set generation, one also has to choose an electronic structure method capable of deliverin...
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Developing global, high-dimensional potential energy surfaces (PESs) is a formidable task. Beside the challenges of PES fitting and fitting set generation, one also has to choose an electronic structure method capable of delivering accurate potential energy values for all geometries in the fitting set, even in regions far from equilibrium. Such regions are often plagued by Hartree-Fock (HF) convergence issues, and even if convergence is achieved, self-consistent field (SCF) procedures that are used to obtain HF solutions offer no guarantee that the solution found is the lowest-energy solution. We present a study of the reactant regions of CH3OH + OH center dot, C2H6 + F center dot, and CH3NH2 + Cl center dot, where the SCF procedure often converges to a higher-energy state or fails to converge, resulting in erratic post-HF energies and regions where no energy is obtained, both of which are major obstacles for PES development. We introduce a pragmatic method for automatically finding better HF solutions (dubbed ManyHF) and present evidence that it may extend the applicability of single-reference methods to some systems previously thought to require multireference methods. Published under an exclusive license by AIP Publishing.
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Accurate segmentation of magnetic resonance (MR) images remains challenging mainly due to the intensity inhomogeneity, which is also commonly known as bias field. Recently active contour models with geometric information constrain...
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Accurate segmentation of magnetic resonance (MR) images remains challenging mainly due to the intensity inhomogeneity, which is also commonly known as bias field. Recently active contour models with geometric information constraint have been applied, however, most of them deal with the bias field by using a necessary pre-processing step before segmentation of MR data. This paper presents a novel automatic variational method, which can segment brain MR images meanwhile correcting the bias field when segmenting images with high intensity inhomogeneities. We first define a function for clustering the image pixels in a smaller neighborhood. The cluster centers in this objective function have a multiplicative factor that estimates the bias within the neighborhood. In order to reduce the effect of the noise, the local intensity variations are described by the Gaussian distributions with different means and variances. Then, the objective functions are integrated over the entire domain. In order to obtain the global optimal and make the results independent of the initialization of the algorithm, we reconstructed the energy function to be convex and calculated it by using the Split Bregman theory. A salient advantage of our method is that its result is independent of initialization, which allows robust and fully automated application. Our method is able to estimate the bias of quite general profiles, even in 7T MR images. Moreover, our model can also distinguish regions with similar intensity distribution with different variances. The proposed method has been rigorously validated with images acquired on variety of imaging modalities with promising results.
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This paper presents a novel approach based on the coupled finite element (FE) and element free Galerkin (EFG) method to model fatigue crack growth in 2-D specimens containing different types of material discontinuities like holes ...
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This paper presents a novel approach based on the coupled finite element (FE) and element free Galerkin (EFG) method to model fatigue crack growth in 2-D specimens containing different types of material discontinuities like holes and bi-material interfaces. In this approach, EFGM is used to discretize the domain near the crack whereas the conventional FEM is employed in the rest of the domain. The shape functions of the transition elements have been obtained by using the ramp function. The level set method has been used to track different discontinuities present in the domain. Finally, several two dimensional numerical problems are presented to demonstrate the applicability and efficiency of the proposed technique in modelling fatigue crack growth in presence of material discontinuities. The effect of various material irregularities on fatigue life, critical crack length and crack growth paths has been investigated in the present study. The results show that the critical crack length and the fatigue life of the cracked component reduce due to the presence of a weak bi-material discontinuity in it. The weaker discontinuities increase the fatigue life of the cracked specimen, whereas the stronger discontinuities slightly increase the fatigue life of the cracked component. The presence of holes in a cracked specimen reduces the fatigue life and the critical crack length at final failure. It was also observed that the holes and the weaker discontinuities exert some sort of attractive effect on the crack during its propagation through the domain.
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