Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics

Cao, Shuyang,Wang, Mengen,Zhu, Jinwei,Cao, Jinxin,Tamura, Tetsuro,Yang, Qingshan Techno-Press 2018 Wind and Structures, An International Journal (WAS Vol.26 No.3

Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-induced wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time-averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.

Numerical study of wind profiles over simplified water waves

Cao, Shuyang,Zhang, Enzhen,Sun, Liming,Cao, Jinxin Techno-Press 2015 Wind and Structures, An International Journal (WAS Vol.21 No.3

Vertical profiles of mean and fluctuating wind velocities over water waves were studied, by performing Large-Eddy Simulations (LES) on a fully developed turbulent boundary layer over simplified water waves. The water waves were simplified to two-dimensional, periodic and non-evolving. Different wave steepness defined by $a/{\lambda}$ (a : wave amplitude; ${\lambda}$ : wavelength) and wave age defined by $c/U_b$ (c: phase velocity of the wave; $U_b$ : bulk velocity of the air) were considered, in order to elaborate the characteristics of mean and fluctuating wind profiles. Results shows that, compared to a static wave, a moving wave plays a lesser aerodynamic role as roughness as it moves downstream slower or a little faster than air, and plays more aerodynamic roles when it moves downstream much faster than air or moves in the opposite direction to air. The changes of gradient height, power law index, roughness length and friction velocity with wave age and wave amplitude are presented, which shed light on the wind characteristics over real sea surfaces for wind engineering applications.

Numerical study of wind profiles over simplified water waves

Shuyang Cao,Enzhen Zhang,Liming Sun,Jinxin Cao 한국풍공학회 2015 Wind and Structures, An International Journal (WAS Vol.21 No.3

Vertical profiles of mean and fluctuating wind velocities over water waves were studied, by performing Large-Eddy Simulations (LES) on a fully developed turbulent boundary layer over simplified water waves. The water waves were simplified to two-dimensional, periodic and non-evolving. Different wave steepness defined by a /λ ( a : wave amplitude; λ : wavelength) and wave age defined by c/Ub (c: phase velocity of the wave; Ub : bulk velocity of the air) were considered, in order to elaborate the characteristics of mean and fluctuating wind profiles. Results shows that, compared to a static wave, a moving wave plays a lesser aerodynamic role as roughness as it moves downstream slower or a little faster than air, and plays more aerodynamic roles when it moves downstream much faster than air or moves in the opposite direction to air. The changes of gradient height, power law index, roughness length and friction velocity with wave age and wave amplitude are presented, which shed light on the wind characteristics over real sea surfaces for wind engineering applications.

Numerical investigation of effects of rotating downdraft on tornado-like-vortex characteristics

Shuyang Cao,Mengen Wang,Jinwei Zhu,Jinxin Cao,Tetsuro Tamura,Qingshan Yang 한국풍공학회 2018 Wind and Structures, An International Journal (WAS Vol.26 No.3

Appropriate modeling of a tornado-like vortex is a prerequisite when studying the near-ground wind characteristics of a tornado and tornado-induced wind loads on structures. Both Ward- and ISU-type tornado simulators employ guide vanes to induce angular momentum to converge flow in order to generate tornado-like vortices. But in the Ward-type simulator, the guide vanes are mounted near the ground while in the ISU-type they are located at a high position to allow vertical circulation of flow that creates a rotating downdraft to generate a tornado-like vortex. In this study, numerical simulations were performed to reproduce tornado-like vortices using both Ward-type and ISU-type tornado simulators, from which the effects of rotating downdraft on the vortex characteristics were clarified. Particular attention was devoted to the wander of tornado-like vortices, and their dependences on swirl ratio and fetch length were investigated. The present study showed that the dynamic vortex structure depends significantly on the vortex-generating mechanism, although the time- averaged structure remains similar. This feature should be taken into consideration when tornado-like-vortex simulators are utilized to investigate tornado-induced wind forces on structures.

Investigation of Surface Integrity Up-Milling Magnesium Oxide Particle Reinforced Wood-Based Composite

Jinxin Wang,Rongsheng Jiang,Zhanwen Wu,Zhaolong Zhu,Longzhi Yang,Pingxiang Cao 한국정밀공학회 2023 International Journal of Precision Engineering and Vol.24 No.3

Metal oxide reinforced composite, as a new kind of engineering material, was an essential research topic to industry. Magnesium oxide as particle-reinforced phase, were mixed with wood fiber-matrix to enhance the mechanical properties of composite. However, the research on the surface quality of this composite is still desirable for expanding its application. In order to investigate the surface integrity of this kind particle-reinforced wood-based composite, spiral up-milling experiments were performed with different cutting depth and cutting speed. The effect of cutting speed and cutting depth on surface integrity was investigated. According to the calculation results of black pixels proportion of binary image of machined surface, surface defects were greatly affected by cutting depth rather than cutting speed. Defects, such as pile-up and debonding of particles, were usually observed under 0.5 mm cutting depth, meanwhile, extensive damage of flacking and fracture of wood fiber-matrix were usually observed under 1.5 mm cutting depth. In all, the machined surface formation mechanism of this composite can be different by changing cutting depth.

Optimal design for torsional vibration suppression of non-smooth NES

Yanbo Cao,Hongliang Yao,Jinxin Dou,Shengdong Han 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.11

Nonlinear energy sinks (NESs) have received increasing attention for their ability to passively inhibit a large amount of vibration energy over a wide range of frequencies. However, although many studies have focused on dynamics of NES, few have addressed the optimal design method of NES, especially for the non-smooth NES (NSNES). Therefore, the parameter optimization method for an NSNES which can be applied to supress torsional vibration of rotor system was developed. First, the design variables were reduced by using piecewise linear torsional stiffness to equivalently fit cubic torsional stiffness; then, taking the torsional vibration of single-disk rotor system as an example, the genetic algorithm (GA) was used to solve the optimization problem of the NSNES in torsional vibration suppression of rotor system. Finally, the optimized NSNES was verified by numerical simulation and experiment to suppress the torsional vibration of a rotor system. The results show that the optimally designed NSNES can effectively suppress torsional vibration of a rotor system. In transient vibration suppression, the optimal percentage of accumulated energy dissipation of NSNES can reach 95.3 %. For steady-state vibration suppression, the peak vibration suppression of NSNES can reach 82.2 % in the simulation and 81.9 % in the test.

POD-based analysis of time-resolved tornado-like vortices

Mengen Wang,Shuyang Cao,Jinxin Cao 한국풍공학회 2021 Wind and Structures, An International Journal (WAS Vol.33 No.1

In this study, three representative configurations of tornado-like vortices, i.e., single vortex, vortex breakdown and multi-vortex, are numerically simulated using large-eddy simulation (LES). Proper orthogonal decomposition (POD) is firstly employed to decompose flow-field snapshots into a series of orthogonal flow patterns (POD modes) and time-dependent coefficients. Then, a conditional-average analysis is conducted to obtain the four kinds of conditionally-averaged flow fields, which are then compared with instantaneous and ensemble-averaged flow fields. Next, a quadruple POD analysis is performed to decompose the instantaneous flow field into mean, coherent, transition and noise components. Finally, a qualitative analysis is implemented for unsteady vortex motions in horizontal and vertical planes. Results show that the conditional average shows larger-scale coherent structures than the classical ensemble average, while it loses the small-scale turbulent fluctuations present in instantaneous flow. The tornado vortex structure is controlled by the mean component in the single-vortex stage. With increase in swirl ratio, the tornado vortex evolves from single-vortex, to vortex-breakdown to multi-vortex, companied by kinetic energy transference to coherent and transition components. The horizontal and vertical vortex motions are essentially the results of horizontal and vertical velocity perturbations.