Search

Reducing drag under high turbulence is a critical but challenging issue that has engendered great concern. This study utilizes hydrophilic tips in superhydrophobic (SHP) grooves to enhance the stability of plastron, which results in a considerable drag reduction ($DR$) up to 62 %, at Reynolds number ($Re$) reaching $2.79 \times 10^{4}$. The effect of the spacing width $w$ of the microgrooves on both $DR$ and flow structures is investigated. Experimental results demonstrate that $DR$ increases as either microgroove spacing $w$ or $Re$ increases. The velocity fields obtained using particle image velocimetry indicate that the air-filled SHP grooves induce a considerable wall slip. This slip significantly weakens the intensity of Taylor rolls, reduces local momentum transport, and consequently lowers drag. This phenomenon becomes more pronounced with increasing $w$. Furthermore, to quantify the multiscale relationship between global response and geometrical as well as driving parameters, $DR\sim (w, \phi _s, Re)$ , a theoretical model is established based on angular momentum defect theory and magnitude estimate. It is demonstrated that a decrease in the surface solid fraction can reduce wall shear, and an increase in the groove width can weaken turbulence kinetic energy production, rendering enhanced slip and drag reduction. This research has implications for designing and optimizing turbulent-drag-reducing surfaces in various engineering applications, such as transportation and marine engineering. [ABSTRACT FROM AUTHOR]

The tensile stress generated by the superposition of two reflection waves in the target plays a critical role in explaining plate-impact-induced spalling. A method of images is proposed to simulate the physical process of wave superposition and this method is applied in order to study the cavitation mechanism in single-crystal Al through molecular dynamics simulation. The critical impact-load velocity for the cavitation obtained by this method is as small as 400 m/s, which is much lower than the result (650 m/s) obtained by the conventional piston-load method. The new cavitation mechanism found is distinctively different from the conventional dislocation-entanglement-induced cavitation under high-velocity impact. The new mechanism involves two key events: firstly, a crack-like defect is formed and its relevant atomic bonds are broken under high tensile stress, resulting in a great momentum of related atoms; and secondly, previous high-momentum atoms collide with the atoms in their running way, resulting in the destruction of the original FCC structure locally and nanovoids or penny-shaped voids being formed. Additionally, the cavitation region, the number of voids, and delamination surfaces increases with the impact-load rate. [ABSTRACT FROM AUTHOR]

To theoretically analyse how void swelling and spatial heterogeneity impact the fracture behaviour of metallic materials, a coupled porous crystal plasticity framework has been developed. The proposed framework is then applied to analyse the fracture behaviour of metallic materials under different swelling conditions and types of spatial heterogeneity. It is revealed that at low porosity, the spatial heterogeneity of voids will have limited influence on the fracture behaviour of the material. However, at high porosity, owing to void swelling, a concentrated distribution of voids near grain boundaries can lead to a strong deformation localization, which can be seen as an indicator of possible quasi-brittle fracture. It is further discovered that the inner pressure of the voids tends to reduce the yield stress of the material and promote localized plastic deformation. As a result, such pressure will enhance the impact of concentrated distribution of voids, which can lead to further embrittlement of the material. The present study reveals the key role of void swelling and spatial heterogeneity in regulating the fracture behaviour of metallic materials, a step towards a better understanding over the failure mechanisms of metallic materials under extreme conditions. [ABSTRACT FROM AUTHOR]