Simulation of primary atomization with an octree adaptive mesh refinement and VOF method

Summary An adaptive three-dimensional mesh refinement method based on the law of mass conservation has been introduced and tested using some analytical velocity fields as accurate in identifying singular point, asymptotic plane and drawing closed streamlines. This paper further investigates the adaptive mesh refinement method using a velocity field that is due to a uniform strain and a point vortex. Similar results have been obtained.

Appropriate mesh refinement plays a vital role in the accuracy and convergence of computational fluid dynamics solvers. This work is an extension of the previous work that further demonstrates the accuracy of the 3D adaptive mesh refinement method by comparing the accuracy measures between the ones derived from the analytical fields and those identified by the refined meshes. The adaptive mesh refinement method presented in this study is based on the law of mass conservation for three-dimensional incompressible or compressible steady fluid flows. The assessment of the performance of the adaptive mesh refinement method considers its key features such as drawing closed streamline and identification of singular points, asymptotic planes, and vortex axis. Several illustrative examples of the applications of the 3D mesh refinement method with a multi-level refinement confirm the accuracy and efficiency of the proposed method. Furthermore, the results demonstrate that the adaptive mesh refinement method can provide accurate and reliable qualitative measures of 3D computational fluid dynamics problems.

Non-convex analytical and geometrical tools for volume truncation, initialization and conservation enforcement in VOF methods

The lid-driven cavity flow problem stands as a widely recognized benchmark in fluid dynamics, serving to validate CFD algorithms. Despite its geometric simplicity, the lid-driven cavity flow problem exhibits a complex flow regime primarily characterized by the formation of vortices at the centre and corners of the square domain. This study evaluates the accuracy of the 2D velocity-driven adaptive mesh refinement (2D VDAMR) method in estimating vortex centres in a steady incompressible flow within a 2D square cavity. The VDAMR algorithm allows for an arbitrary number of finite mesh refinements. Increasing the number of successive mesh refinements results in more accurate outcomes. In this paper, the initial coarse uniform grid mesh was refined ten times for Reynolds numbers 100≤Re≤7500. Results show that VDAMR accurately identifies vortex centres, with its findings closely aligning with benchmark data from six literature sources.

High computational cost associated with numerical modeling of multiscale global atmospheric chemical transport (ACT) imposes severe limitations on the spatial resolution of fixed nonadaptive grids. Recently it has been shown that the interaction of numerical diffusion caused by the crude resolution with complex velocity field of atmospheric flows leads to large numerical errors. To address the described difficulties, the authors have developed a wavelet-based adaptive mesh refinement (WAMR) method for numerical simulation of two-dimensional multiscale ACT problems. The WAMR is an adaptive method that minimizes the number of grid points by introducing a fine grid only in the locations where fine spatial scales occur and uses high-order spatial discretization throughout the computational domain. The algorithm has been tested for several challenging ACT problems. Particularly, it is shown that the method correctly simulates dynamics of a pollution plume traveling on a global scale, producing less than 1% error with a relatively low number (~105) of grid points. To achieve such accuracy, conventional nonadaptive techniques would require more than three orders of magnitude more computational resources. The method possesses good mass conservation properties; it is shown that an error in the total pollutant mass does not exceed 0.02% for this number of points. The obtained results demonstrate the WAMR’s ability to achieve high numerical accuracy for challenging ACT problems at a relatively low computational cost.

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Three-dimensional deformation of a spherical droplet in a square duct flow at moderate reynolds numbers

In this paper, the unsteady flow of two immiscible fluids with heat transfer is studied numerically with a modified cubic B‐spine Differential Quadrature Method. Generalized Couette flow of two immiscible dusty (fluid–particle suspension) and pure (Newtonian) fluids are considered through rigid horizontal channelsfor three separate scenarios: first fornonporous plates with heat transfer, second forporous plates with uniform suction and injection and heat transfer, and third fornonporous plates with interface evolution. The stable liquid–liquid interface is considered for the two immiscible fluids in the first two cases. In the third case, it is assumed that the interface travels from one position to another and may undergo serious deformation; hence the single momentum equation based on the volume of fluid method is combined with the continuum surface approach model, and an interface tracking is proposed. The flow cases are considered to be subjected to three different pressure gradients, of relevance to energy systems—namely,applied constant, decaying, and periodic pressure gradients. For each case, the coupled partial differential equations are formulated and solved numerically using MCB‐DQM to compute the fluids velocities, fluid temperatures, interface evolution. The effects of emerging thermo‐fluid parameters, that is, Eckert (dissipation), Reynolds, Prandtl, and Froude numbers, particle concentration parameter, volume fraction parameter, pressure gradient, time, and the ratio of viscosities, densities, thermal conductivities, and specific heats on velocity and temperature characteristics are illustrated through graphs.

The aim of this paper is to utilize a numeri- cal model to compute bubble motion in quiescent Newto- nian and viscoelastic liquids. For our numerical method, we use a coupled level set and volume-of-fluid method with a second order treatment for the jump conditions related to surface tension. We investigate axisymmetric gas-liquid systems with large density and viscosity ratios as well as buoyancy-driven flows with complex changes in topology. We present comparisons to previous compu- tational results as well as experimental results. keyword: Level set, Volume-of-fluid, Viscoelastic, Projection method, Adaptive mesh refinement

The magnetic actuation of ferrofluid droplets offers an inspiring tool in widespread engineering and biological applications. In this study, the dynamics of ferrofluid droplet generation with a Drop-on-Demand feature under a non-uniform magnetic field is investigated by multiscale numerical modeling. Langevin equation is assumed for ferrofluid magnetic susceptibility due to the strong applied magnetic field. Large and small computational domains are considered. In the larger domain, the magnetic field is obtained by solving Maxwell equations. In the smaller domain, a coupling of continuity, Navier Stokes, two-phase flow, and Maxwell equations are solved by utilizing the magnetic field achieved by the larger domain for the boundary condition. The Finite volume method and coupling of level-set and Volume of Fluid methods are used for solving equations. The droplet formation is simulated in a two-dimensional axisymmetric domain. The method of solving fluid and magnetic equations is validated using a benchmark. Then, ferrofluid droplet formation is investigated experimentally, and the numerical results showed good agreement with the experimental data. The effect of 12 dimensionless parameters, including the ratio of magnetic, gravitational, and surface tension forces, the ratio of the nozzle and magnetic coil dimensions, and ferrofluid to continuous-phase properties ratios are studied. The results showed that by increasing the magnetic Bond number, gravitational Bond number, Ohnesorge number, dimensionless saturation magnetization, initial magnetic susceptibility of ferrofluid, the generated droplet diameter reduces, whereas the formation frequency increases. The same results were observed when decreasing the ferrite core diameter to outer nozzle diameter, density, and viscosity ratios.

Breakup mode transformation of leaky dielectric droplet under direct current electric field

We study the stability of two-fluid flow through a plane channel at Reynolds numbers of 100–1000 in the linear and nonlinear regimes. The two fluids have the same density but different viscosities. The fluids, when miscible, are separated from each other by a mixed layer of small but finite thickness, across which the viscosity changes from that of one fluid to that of the other. When immiscible, the interface is sharp. Our study spans a range of Schmidt numbers, viscosity ratios and locations and thicknesses of the mixed layer. A region of instability distinct from that of the Tollmien–Schlichting mode is obtained at moderate Reynolds numbers. We show that the overlap of the layer of viscosity-stratification with the critical layer of the dominant disturbance provides a mechanism for this instability. At very low values of diffusivity, the miscible flow behaves exactly like the immiscible one in terms of stability characteristics. High levels of miscibility make the flow more stable. At intermediate levels of diffusivity however, in both linear and nonlinear regimes, miscible flow can be more unstable than the corresponding immiscible flow without surface tension. This difference is greater when the thickness of the mixed layer is decreased, since the thinner the layer of viscosity stratification, the more unstable the miscible flow. In direct numerical simulations, disturbance growth occurs at much earlier times in the miscible flow, and also the miscible flow breaks spanwise symmetry more readily to go into three-dimensionality. The following observations hold for both miscible and immiscible flows without surface tension. The stability of the flow is moderately sensitive to the location of the interface between the two fluids. The response is non-monotonic, with the least stable location of the layer being mid-way between the wall and the centreline. As expected, flow at higher Reynolds numbers is more unstable.

A multiphase MPS method for bubbly flows with complex interfaces

Breakthrough pressure of natural gas in rock is an important evaluation parameter in gas reservoir development. In this study, experimental measurements of porosity and permeability of the caprock core were carried out. A digital core model was obtained by body rendering of rock CT (Computed Tomography) slices using Avizo software. Then, the grid model was imported into Fluent software for a numerical simulation of natural gas breakthrough pressure in the core. The innovations of this paper include (1) an accurate calculation method of cap breakthrough pressure based on the VOF method; (2) the method performs a perceptual analysis of average pore radius, wettability, roughness height, and viscosity ratio.

In the phase-field simulation of dendrite growth during the solidification of an alloy, the computational cost becomes extremely high when the diffusion length is significantly larger than the curvature radius of a dendrite tip. In such cases, the adaptive mesh refinement (AMR) method is effective for improving the computational performance. In this study, we perform a three-dimensional dendrite growth phase-field simulation in which AMR is implemented via parallel computing using multiple graphics processing units (GPUs), which provide high parallel computation performance. In the parallel GPU computation, we apply dynamic load balancing to parallel computing to equalize the computational cost per GPU. The accuracy of an AMR refinement condition is confirmed through the single-GPU computations of columnar dendrite growth during the directional solidification of a binary alloy. Next, we evaluate the efficiency of dynamic load balancing by performing multiple-GPU parallel computations for three different directional solidification simulations using a moving frame algorithm. Finally, weak scaling tests are performed to confirm the parallel efficiency of the developed code.