[en] Highlights: • The sub-5-μm microstructures on commercial pure titanium are creatively obtained based on cracks growth under 10 ns laser irradiation. • The distribution modification of laser energy induced by cambered microstructures was theoretically analyzed to produce nanostructures. • The sharp micro-nano structures under combined action of crack growth and hot-melt are obtained. - Abstract: This study reported on the formation of sub-5-μm microstructures covered on titanium by cracks growth under 10-ns laser radiation at the wavelength of 532 nm and its induced light modification for production of nanostructures. The electric field intensity and laser power density absorbed by commercial pure titanium were computed to investigate the self-trapping introduced by cracks and the effect of surface morphology on laser propagation characteristics. It is found that nanostructures can form at the surface with the curvature radius below 20 μm. Meanwhile, variable laser fluences were applied to explore the evolution of cracks on commercial pure titanium with or without melt as spot overlap number increased. Experimental study was first performed at the peak laser fluence of 1.063 J/cm"2 to investigate the microstructures induced only by cracks growth. The results demonstrated that angular microstructures with size between 1.68 μm and 4.74 μm was obtained and no nanostructure covered. Then, at the peak laser fluence of 2.126 J/cm"2, there were some nanostructures covered on the melt-induced curved microstructured surface. However, surface molten material submerged in the most of cracks at the spot overlap number of 744, where the old cracks disappeared. The results indicated that there was too much molten material and melting time at the peak laser fluence of 2.126 J/cm"2, which was not suitable for obtainment of perfect micro-nano structures. On this basis, peak laser fluence was reduced down to 1.595 J/cm"2 and the sharp sub–5 μm microstructures with nanostructures covered was obtained at spot overlap number of 3720.

[en] In this paper, a new multiphase SPH model with lower diffusion is developed for simulation of multiphase flows with large density ratios, complex interfaces, and strong impact. The zeroth-order density smoothing algorithm in previous models is replaced by a first-order density correction term which could effectively suppress density/pressure oscillation while lowering numerical diffusion. A switch-function-based artificial viscosity term is proposed to change the global implementation of numerical diffusion in the momentum equation into the local effectiveness depending on the strong impact. Such strategy aims to offer a good balance between the necessary numerical diffusion and the true physical viscosity of the fluid. In order to prove the robustness of the algorithm in various flow problems, five representative numerical examples, including Rayleigh–Taylor instability, internal solitary waves on the pycnocline, air bubble rising in water, oscillation of elliptic droplet and dam breaking, are presented. The expected advantages of the proposed method are demonstrated through comparison with the experimental data and previous numerical results.

[en] Highlights: • A novel interface method is developed for multiphase SPH. • An algebraic indicator is proposed for accurate and efficient interface detection. • A new surface tension formulation based on local surface reconstruction is developed. • The present surface tension formulation demonstrates its accuracy and efficiency. A novel interface method is developed in this paper for two-dimensional smoothed particle hydrodynamics (SPH) modelings of multiphase flows. The present interface method aims to resolve two essential issues in the multiphase flow simulations: the interface detection and the implementation of surface tension force. Specifically, a novel and easy-to-implement algebraic indicator is proposed to detect the interface particles. And the surface tension force is locally implemented on the interface particles by reconstructing arc lines connecting three adjacent interface particles. Compared with the previous surface reconstruction method, the present method utilizes fewer particles in the local evaluation of surface tension. The novel algebraic indicator provides more accurate and efficient detection of the interface particles, especially for the sparse or aggregated particle distributions. In the meantime, the present surface tension formulation removes the tedious process of evaluating gradient of the color index and thus significantly improves the numerical efficiency. The accuracy, flexibility and efficiency of the present interface method are comprehensively evaluated through four numerical examples, namely the sloshing in a rectangular tank, oscillation of elliptic droplet, Laplace law for a stationary droplet, and air bubble rising in water. The good agreement of the present numerical results with the published results demonstrates that the novel interface method proposed in this paper can precisely detect the interface particles with appealing sharpness, and recover the surface tension force accurately and efficiently.