Subgridding Interface Research Articles

Simulation of self-aerated flows by switching interface closures

This work presents a numerical study of a skimming flow regime in a stepped spillway structure, which is a complex two-phase flow with self-aeration phenomenon. The numerical models employed consist in a hybrid approach capable of switching between interface resolving and interface modelling closures on a local level, and a state-of-the-art volume-of-fluid (VOF) method. A new switch criterion for the hybrid approach is proposed, allowing calculations on irregular grid cells. A detached eddy simulation (DES) turbulence model is selected in order to directly resolve the turbulent flow structures triggering the onset of self-aeration. The main flow properties computed in the non-aerated and aerated region are confronted with experimental measurements. Compared to the VOF method, the hybrid approach provides an overall better prediction of time-averaged air concentration, highlighting the positive outcome of modelling sub-grid interface structures.

Sensitivity of macroscopic transport calculations to uncertain microscale relationships during metal alloy solidification

The formation of grains during metallic solidification results in a multiphase system consisting of many moving interfaces. Models have been formulated in terms of variables describing the microscopic details of interface motion during solidification, but the computational cost prohibits their extension to the length scale of industrial castings. For this reason, macroscopic transport equations are mathematically formulated using averaging methods to replace the instantaneous description of interfaces. The most common approach is the volume-averaged, Eulerian approach relying on both the proper derivation of macroscopic field equations as well as uncertain interfacial conditions used to account for the average behavior of the interphase transfer occurring on the sub-grid length scale. In this work, common models used to describe the microsegregation occurring during solidification are evaluated in terms of the uncertainty of parametric inputs invoked in each model and their effect on macroscale predictions. We show that the qualitative macroscale predictions are not significantly changed by using more sophisticated microsegregation models, however significant uncertainty in input parameters used to describe the unresolved sub-grid interface is accumulated. Therefore, the use of simplified models is preferred until the uncertainty in model inputs can be reduced.

An Improved Grid Current and DC Capacitor Voltage Balancing Method for Three-Terminal Hybrid AC/DC Microgrid

In this paper, a three-terminal ac/dc hybrid microgrid with two dc terminals and one ac terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based ac grid interface and two dual active bridge (DAB) converters based dc subgrid interface that connects two isolated dc buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB dc rails according to the system operation requirement. To overcome the imbalanced grid currents and dc rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid ac/dc microgrids show that the generalized effectiveness of the proposed three-phase ac current and dc rail voltage balancing method.

Numerical calculation of permeability of periodic porous materials: Application to periodic arrays of spheres and 3D scaffold microstructures

SummaryIn this paper, an efficient numerical method is proposed to calculate the anisotropic permeability in porous materials characterized by a periodic microstructure. This method is based on pore‐scale fluid dynamic simulations using a static volume of fluid method. Unlike standard solution procedures for this type of problem, we here solve an average constitutive equation over both fluid and solid domain by use of a subgrid model to accurately capture momentum transfer from the fluid to solid interface regions. Using numerical simulations on periodic arrays of spheres, we first demonstrate that, by using the subgrid interface model, more accurate results can be produced, for the velocity and pressure fields, than via more conventional approaches. We then apply numerical upscaling over the unit cell to calculate the full anisotropic permeability from the pore‐scale numerical results. The obtained permeability values for a variety of periodic arrays of spheres in different arrangements and packing orders are in good agreement with semianalytical results reported in literature. This validation allows for the permeability assessment of more complex structures such as isotropic gyroid structures, or anisotropic cases, here modeled in their simplest form, the ellipsoidal inclusion.

A Novel FDTD Subgridding Method With Improved Separated Temporal and Spatial Subgridding Interfaces

A novel finite-difference time-domain (FDTD) subgridding method with improved separated temporal and spatial subgridding interfaces is proposed in this letter. In the improved temporal subgridding algorithm, the Taylor expansion is used to obtain the missing magnetic fields at temporal subgridding interface. For improving the stability, the electric fields at temporal subgridding interface and the magnetic fields in subgrid nearest to the temporal subgridding interface are modified at the end of each coarse time. In the improved spatial subgridding algorithm, the loop integrations of magnetic fields in coarse grid nearest to the spatial subgridding interface are amended. With numerical simulation, the efficiency and high late time stability of the proposed method have been verified. Finally, this proposed method could well resolve the difficulty of material traversing the temporal and spatial subgridding interfaces.

Finite-Difference Time-Domain Analysis of the Shielding Effectiveness of Metallic Enclosures With Apertures Using a Novel Subgridding Algorithm

One of the chief challenges of the finite-difference time-domain (FDTD) subgridding algorithm is developing a field-coupling scheme to avoid late-time instability resulting from spatial interface between regions with different meshes. This paper introduces the concept of the nonuniform grid method to appropriately handle the tangential electric component on the spatial interface of the two meshes. This study analyzed shielding effectiveness of metallic enclosures with apertures. A novel subgridding algorithm was employed to locally refine the region containing a thin perfect electric conducting (PEC) film with apertures through FDTD simulation. Image theory was implemented to obtain an interpolation scheme of the magnetic fields as the sub-boundary surface when a thin PEC film traverses the subgridding interface. The numerical accuracy and stability of the proposed method were verified by comparing with results from other established methods.

Huygens Subgridding for 3-D Frequency-Dependent Finite-Difference Time-Domain Method

Subgridding methods are often used to increase the efficiency of the wave propagation simulation with the Finite-Difference Time-Domain method. However, the majority of contemporary subgridding techniques have two important drawbacks: the difficulty in accommodating dispersive media and the inability for physical interfaces to cross the subgridding interface. This paper presents an extension of the frequency-dependent Huygens subgridding method from one dimension to three dimensions. Frequency dependency is implemented via the Auxiliary Differential Equation approach using the one-pole Debye relaxation model. Numerical experiments indicate that subgridding interfaces can be placed in various Debye media as well as across the physical interface .

A spectrally refined interface approach for simulating multiphase flows

This paper presents a novel approach to phase-interface transport based on pseudo-spectral sub-grid refinement of a level set function. In each flow solver grid cell, a set of quadrature points is introduced on which the value of the level set function is known. This methodology allows to define a polynomial reconstruction of the level set function in each cell. The transport is performed using a semi-Lagrangian technique, removing all constraints on the time step size. Such an approach provides sub-cell resolution of the phase-interface and leads to excellent accuracy in the transport, while a reasonable cost is obtained by pre-computing some of the metrics associated with the polynomials. To couple this approach with a flow solver, an converging curvature computation is introduced. First, a second order explicit distance to the sub-grid interface is reconstructed on the flow solver mesh. Then, a least squares approach is employed to extract the curvature from this distance function. This technique is found to combine the high accuracy and good conservation found in the particle level set method with the converging curvature usually obtained with classical high order PDE transport of the level set function. Tests are presented for both transport as well as two-phase flows, that suggest that this technique is capable of retaining the thin liquid structures that are expected in turbulent atomization of liquids.

Improved FDTD subgridding algorithms via digital filtering and domain overriding

In numerical simulations of Maxwell's equations for problems with disparate geometric scales, it is often advantageous to use grids of varying densities over different portions of the computational domain. In simulations involving structured finite-difference time-domain (FDTD) grids, this strategy is often referred as subgridding (SG). Although SG can lead to major computational savings, it is known to cause instabilities, spurious reflections, and other accuracy problems. In this paper, we introduce two strategies to combat these problems. First, we present an overlapped SG (OSG) approach combined with digital filters (in space). OSG can recover standard SG (SSG) schemes but it is based upon a more general, explicit separation between interpolation/decimation operations and the FDTD field update itself. This allows for a better classification of errors associated with the subgrid interface. More importantly, digital filters and phase matching techniques can be then employed to combat those errors. Second, we introduce SG with a domain overriding (SG-DO) strategy, consisting of overlapped (sub)grid regions that contain auxiliary (buffer) subdomains with perfectly matched layers (PML) to allow explicit control on the reflection and transmission properties at SG interfaces. We provide two-dimensional (2-D) numerical examples showing that residual errors from 2-D SG-DO FDTD simulations can be significantly reduced when compared to SSG schemes.