A short note on a 3D spectral analysis for turbulent flows on unstructured meshes

We propose two techniques for computing the energy spectra for 3D unstructured meshes that are consistent across different element types. These techniques can be particularly useful when assessing the dissipation characteristics and the suitability of several popular non-linear high-order methods for implicit large-eddy simulations (iLES). Numerical experiments demonstrate the performance of several element types for iLES of the Taylor-Green vortex, where a significantly different dissipation and dispersion mechanism for each element type is revealed. The energy spectra results are dependent on the technique selected for obtaining them, therefore an additional established technique from the literature is also included for comparison to further analyse their similarities and their differences. These techniques can be an integral tool for the tuning and calibration of non-linear high-order methods that can benefit both explicit and implicit large-eddy simulations (LES).

A new particle shifting technique for SPH methods based on Voronoi diagram and volume compensation

Particle shifting technique (PST) in smoothed particle hydrodynamics (SPH) is a useful strategy to render particle distribution isotropic for higher numerical accuracy and stability. However, the non-isotropic particle distribution near the free surfaces and volume non-conservation still often occur, which will degenerate accuracy in the long-time simulation with violent flows. In this study, the two major concerns are solved by proposing a new PST. The Voronoi cell is constructed for the particles with incomplete kernel support, and the difference between the Voronoi cell centroid and the corresponding particle location is used to determine the shifting vector, which ensures a very isotropic particle distribution and that no clustering or nonphysical gap occurs in these areas. For the free-surface particles, the shifting vector is determined according to the volume change and thus the volume conservation is ensured, where the volume is calculated conveniently with the merit of Voronoi diagram. The present method is compared with the other two advanced PST methods, and a set of challenging cases with violent flows is simulated to validate its superior properties in terms of maintaining isotropic particle distribution and volume conservation. Moreover, the efficiency of the present method is almost the same as those of other advanced PSTs. Overall, the new PST method presents a strong potential in the long-time simulation of violent flows in coastal engineering.

Early Career Award from the Research Grants Council of HK

Multi-level adaptive particle refinement method with large refinement scale ratio and new free-surface detection algorithm for complex fluid-structure interaction problems

Fluid-Structure Interaction (FSI) is a crucial problem in ocean engineering. The smoothed particle hydrodynamics (SPH) method has been employed recently for FSI problems in light of its Lagrangian nature and its advantage in handling multi-physics problems. The efficiency of SPH can be greatly improved with the Adaptive Particle Refinement (APR) method, which refines particles in the regions of interest while deploying coarse particles in the left areas. In this study, the APR method is further improved by developing several new algorithms. Firstly, a new particle refinement strategy with the refinement scale ratio of 4 is employed for multi-level resolutions, and this dramatically decreases the computational costs compared to the standard APR method. Secondly, the regularized transition sub-zone is deployed to render an isotropic particle distribution, which makes the solutions between the refinement zone and the non-refinement zone smoother and consequently results in a more accurate prediction. Thirdly, for complex FSI problems with free surface, a new free-surface detection method based on the Voronoi diagram is proposed, and the performance is validated in comparison to the conventional method. The improved APR method is then applied to a set of challenging FSI cases. Numerical simulations demonstrate that the results from the refinement with scale ratio 4 are consistent with other studies and experimental data, and also agree well with those employing the refinement scale ratio 2. A significant reduction in the computational time is observed for all the considered cases. Overall, the improved APR method with a large refinement scale ratio and the new free-surface detection strategy shows great potential in simulating complex FSI problems efficiently and accurately.