Multi-scale Grid Research Articles

Numerical study of flow pattern modulation in a vertical phase separation condenser tube

The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities.

A conservative adaptive wavelet method for the shallow‐water equations on staggered grids

AbstractThis article presents the first dynamically adaptive wavelet method for the shallow‐water equations (SWEs) on a staggered hexagonal C‐grid. Pressure is located at the centres of the primal grid (hexagons) and velocity is located at the edges of the dual grid (triangles). Distinct bi‐orthogonal second‐generation wavelet transforms are developed for the pressure and the velocity. These wavelet transforms are based on second‐order accurate interpolation and restriction operators. Together with compatible restriction operators for the mass flux and Bernoulli function, they ensure that mass is conserved and that there is no numerical generation of vorticity when solving the SWEs. Grid refinement relies on appropriate thresholding of the wavelet coefficients, allowing error control in both the quasi‐geostrophic and inertia–gravity wave regimes. The SWEs are discretized on the dynamically adapted multiscale grid using a mass and potential‐enstrophy‐conserving finite‐difference scheme. The conservation and error control properties of the method are verified by applying it to a propagating inertia–gravity wave packet and to rotating shallow‐water turbulence. Significant savings in the number of degrees of freedom are achieved even in the case of rotating shallow‐water turbulence. The numerical dissipation introduced by the grid adaptation is quantified. The method has been designed so it can be extended easily to the icosahedral subdivision of the sphere. This work provides important building blocks for the development of fully adaptive general circulation models.

Ant Colony Optimization Based on Local Optima Breaking Mechanism for Unmanned Vehicle Path Planning in Cross-Country Environment

Intelligent vehicle is a preferable way for dealing with transportation problems. In cross-country environments, no apparent roads could be found, instead boscage, bumps and hollows, scattered materials make it hard to choose suitable paths. Path planning is a combinational optimization issue to find out a short path from many candidates. A combinational optimization algorithm was adopted to solve the problem. A multi-scale grid environment modeling mechanism was designed to model the unordered and irregular environment by a standard grid map. Then an ant colony optimization based algorithm was presented for finding out the most suitable path from the map. The algorithm introduced a local optima breaking mechanism based on early convergence judging process to avoid falling into local optima. Three criteria were designed for the process, say criteria of single grid, of static colony and of dynamic convergence. Feasibility and effectiveness of the method were verified by experiments.

Vascularization for cooling a plate heated by a randomly moving source

Here, we show that a plate heated by a moving beam can be cooled effectively by fluid that flows through a vasculature of channels embedded in the plate. The vascular designs studied are radial, grid and hybrid (radial + grid). The peak temperature of the plate changes with the path and direction of the moving beam. The strength, size and speed of the beam vary. The peak temperature increases as the beam strength and size increase and as the speed of the beam decreases. The grid and hybrid designs are robust because of loops present in the flow structure. The pressure difference that drives the fluid flow varied. The channel diameter ratios that provide greatest flow access are reported. The cooling performance of the multiscale grid structures is less sensitive to the changes in beam path than the cooling performance of the other structures studied. The effect of adding a vascular structure to the design is dramatic.

An efficient algorithm for mapping imaging data to 3D unstructured grids in computational biomechanics

Geometries for organ scale and multiscale simulations of organ function are now routinely derived from imaging data. However, medical images may also contain spatially heterogeneous information other than geometry that are relevant to such simulations either as initial conditions or in the form of model parameters. In this manuscript, we present an algorithm for the efficient and robust mapping of such data to imaging-based unstructured polyhedral grids in parallel. We then illustrate the application of our mapping algorithm to three different mapping problems: (i) the mapping of MRI diffusion tensor data to an unstructured ventricular grid; (ii) the mapping of serial cyrosection histology data to an unstructured mouse brain grid; and (iii) the mapping of computed tomography-derived volumetric strain data to an unstructured multiscale lung grid. Execution times and parallel performance are reported for each case.

Multiband Image Segmentation and Object Recognition for Understanding Road Scenes

This paper presents a novel method for semantic segmentation and object recognition in a road scene using a hierarchical bag-of-textons method. Current driving-assistance systems rely on multiple vehicle-mounted cameras to perceive the road environment. The proposed method relies on integrated color and near-infrared images and uses the hierarchical bag-of-textons method to recognize the spatial configuration of objects and extract contextual information from the background. The histogram of the hierarchical bag-of-textons is concatenated to textons extracted from a multiscale grid window to automatically learn the spatial context for semantic segmentation. Experimental results show that the proposed method has better segmentation accuracy than the conventional bag-of-textons method. By integrating it with other scene interpretation systems, the proposed system can be used to understand road scenes for vehicle environment perception.

0140 マルチスケール格子乱流による高シュミット数スカラー混合層の基本特性(OS1-8 噴流,後流および剥離流れ現象の解明と制御,オーガナイズドセッション)

High-Schmidt-number scalar mixing layers in the multiscale grid generated turbulence are experimentally investigated in a water channel by means of planar laser induced fluorescence (PLIF) technique and particle image velocimetry (PIV). The Reynolds number based on the effective mesh size and mean velocity is 2,500. Fluorescent dye (Rhodamine B; Schmidt number is about 2,100) is homogeneously premixed only in the upper stream, and therefore, the scalar mixing layer with an initial step profile develops downstream of the grid. Various turbulence quantities are evaluated and discussed.

Multiscale cluster lens mass mapping - I. Strong lensing modelling

We propose a novel technique to refine the modelling of galaxy cluster mass distribution using gravitational lensing. The idea is to combine the strengths of both ‘parametric’ and ‘non-parametric’ methods to improve the quality of the fit. We develop a multiscale model that allows sharper contrast in regions of higher density where the number of constraints is generally higher. Our model consists of (i) a multiscale grid of radial basis functions with physically motivated profiles and (ii) a list of galaxy-scale potentials at the location of the cluster member galaxies. This arrangement of potentials of different sizes allows us to reach a high resolution for the model with a minimum number of parameters. We apply our model to the well-studied cluster Abell 1689. We estimate the quality of our mass reconstruction with a Bayesian Monte Carlo Markov Chain sampler. For a selected subset of multiple images, we manage to halve the errors between the positions of predicted and observed images compared to previous studies. This is due to the flexibility of multiscale models at intermediate scale between cluster and galaxy scale. The software developed for this paper is part of the public lenstool package which can be found at http://www.oamp.fr/cosmology/lenstool.

Applicability of the Lattice Boltzmann Method for Tsunami Modeling

The Lattice Boltzmann Method (LBM) is one of the robust CFD models to solve the fluid dynamics. In the present study, the LBM for the Shallow Water Equations is developed to simulate tsunami propagation and coastal inundation. The offshore boundary conditions and nesting technique for multi-scale grid are developed for practical use of LBM. The model is validated, for a potential tsunami scenario by the comparison with the Finite Difference Method. The results imply that LBM has a sufficient accuracy and robustness for tsunami propagation and inundation modeling.

Convergence, adaptive refinement, and scaling in 1D peridynamics

AbstractWe introduce here adaptive refinement algorithms for the non‐local method peridynamics, which was proposed in (J. Mech. Phys. Solids 2000; 48:175–209) as a reformulation of classical elasticity for discontinuities and long‐range forces. We use scaling of the micromodulus and horizon and discuss the particular features of adaptivity in peridynamics for which multiscale modeling and grid refinement are closely connected. We discuss three types of numerical convergence for peridynamics and obtain uniform convergence to the classical solutions of static and dynamic elasticity problems in 1D in the limit of the horizon going to zero. Continuous micromoduli lead to optimal rates of convergence independent of the grid used, while discontinuous micromoduli produce optimal rates of convergence only for uniform grids. Examples for static and dynamic elasticity problems in 1D are shown. The relative error for the static and dynamic solutions obtained using adaptive refinement are significantly lower than those obtained using uniform refinement, for the same number of nodes. Copyright © 2008 John Wiley & Sons, Ltd.

Computation of aeolian tones from twin-cylinders using immersed surface dipole sources

Efficient numerical method is developed for the prediction of aerodynamic noise generation and propagation in low Mach number flows such as aeolian tone noise. The proposed numerical method is based on acoustic/viscous splitting techniques of which acoustic solvers use simplified linearised Euler equations, full linearised Euler equations and nonlinear perturbation equations as acoustic governing equations. All of acoustic equations are forced with immersed surface dipole model which is developed for the efficient computation of aerodynamic noise generation and propagation in low Mach number flows in which dipole source, originating from unsteady pressure fluctuation on a solid surface, is known to be more efficient than quadrupole sources. Multi-scale overset grid technique is also utilized to resolve the complex geometries. Initially, aeolian tone from single cylinder is considered to examine the effects that the immersed surface dipole models combined with the different acoustic governing equations have on the overall accuracy of the method. Then, the current numerical method is applied to the simulation of the aeolian tones from twin cylinders aligned perpendicularly to the mean flow and separated 3 diameters between their centers. In this configuration, symmetric vortices are shed from twin cylinders, which leads to the anti-phase of the lift dipoles and the in-phase of the drag dipoles. Due to these phase differences, the directivity of the fluctuating pressure from the lift dipoles shows the comparable magnitude with that from the drag dipoles at 10 diameters apart from the origin. However, the directivity at 100 diameters shows that the lift-dipole originated noise has larger magnitude than, but still comparable to, that of the drag-dipole one. Comparison of the numerical results with and without mean flow effects on the acoustic wave emphasizes the effects of the sheared background flows around the cylinders on the propagating acoustic waves, which is not generally considered by the classic acoustic analogy methods. Through the comparison of the results using the immersed surface dipole models with those using point sources, it is demonstrated that the current methods can allow for the complex interactions between the acoustic wave and the solid wall and the effects of the mean flow on the acoustic waves.

Analysis of planar circuits using a multigrid iterative method

An iterative method based on the wave concept and multiscale grid is presented to analyse complex planar circuits with reduced calculation time. This approach consists in substituting selected pixels on the edges of metallic strips by their equivalent surface impedances. An implementation of the presented theory is shown for extraction of S-parameters of open-end microstrip lines and single-loop inductors. Comparisons with experimental published results are reported.

Equivalent impedance boundary conditions for refined meshes applied to planar circuits

The application of an iterative method based on the wave concept to model planar circuits using a multiscale grid is investigated. The novelty of this method consists in substituting selected coarse meshes by their equivalent surface impedances. This approach offers high spatial-resolution with low computing resources, and results are presented to demonstrate its efficiency.