Journal of Zhejiang University-SCIENCE A

Xie, Lei; Du, Qiang; Liu, Guang; Lian, Zengyan; Xie, Yaguang; Luo, Yifu

doi: 10.1631/jzus.a2200565pmid: N/A

Rotor-stator cavities are frequently encountered in engineering applications such as gas turbine engines. They are usually subject to an external hot mainstream crossflow which in general is highly swirled under the effect of the nozzle guide vanes. To avoid hot mainstream gas ingress, the cavity is usually purged by a stream of sealing flow. The interactions between the external crossflow, cavity flow, and sealing flow are complicated and involve all scales of turbulent unsteadiness and flow instability which are beyond the resolution of the Reynolds-average approach. To cope with such a complex issue, a wall-modeled large-eddy simulation (WMLES) approach is adopted in this study. In the simulation, a 20° sector model is used and subjected to a uniform pre-swirled external crossflow and a stream of radial sealing flow. It is triggered by a convergent Reynolds-averaged Navier-Stokes (RANS) result in which the shear stress transport (SST) turbulent model is used. In the WMLES simulation, the Smagoringsky sub-grid scale (SGS) model is applied. A scalar transportation equation is solved to simulate the blending and transportation process in the cavity. The overall flow field characteristics and deviation between RANS and WMLES results are discussed first. Both RANS and WMLES results show a Batchelor flow mode, while distinct deviation is also observed. Deviations in the small-radius region are caused by the insufficiency of the RANS approach in capturing the small-scale vortex structures in the boundary layer while deviations in the large-radius region are caused by the insufficiency of the RANS approach in predicting the external crossflow ingestion. The boundary layer vortex and external ingestion are then discussed in detail, highlighting the related flow instabilities. Finally, the large-flow structures induced by external flow ingress are analyzed using unsteady pressure oscillation signals.

Du, Wei; Ren, Juanjuan; Zhang, Kaiyao; Deng, Shijie; Zhang, Shuyi

doi: 10.1631/jzus.a2300010pmid: N/A

To accurately identify the potential contact loss of the China railway track system (CRTS) III prefabricated slab track, a finite element model with contact loss of self-compacting concrete (SCC) under transient impact was established. Then the vertical accelerations near impact points on the track slab surface were extracted to obtain damage-sensitive indices in the time and frequency domains. The indices were initially normalized to obtain independent items of evidence before the Dempster-Shafer (D-S) evidence theory was used to fuse these into one. Finally, a two-stage identification was performed to identify the damaged SCC area, comprising a rough identification (Stage I) and a precise identification (Stage II). The research results show that the damage indices extracted based on the transient impact response change abruptly at the damage location, and that can be used for damage identification. However, the use of a single index to determine the damage of the impact point may be misjudged. In Stage I, five damage indices of acceleration were fused to magnify the difference between the damaged point and undamaged point, thereby improving the accuracy of finding damage. In Stage II, in the area where more impact points were added, a fusion of three indices of acceleration response, that is, the absolute mean of the time domain, the maximum amplitude of the frequency domain, and the power density ratio, further narrowed down the area where damage exists. As a result, when the contact loss of SCC is greater than 50% along the thickness direction, the identification accuracy can be as high as 70% to 80%. The two-stage identification method proposed in this study can greatly improve the efficiency of interlayer damage detection of slab tracks and is expected to provide effective technical support for damage identification of track structures in the future.

Han, Yangyang; Liu, Guoping; Lu, Zhenyu; Zong, Huaizhi; Zhang, Junhui; Zhong, Feifei; Gao, Liyu

doi: 10.1631/jzus.a2200310pmid: N/A

Locomotion stability is essential for controlling quadruped robots and adapting them to unstructured terrain. We propose a control strategy with center-of-mass (CoM) dynamic planning for the stable locomotion of these robots. The motion trajectories of the swing legs are synchronized with the CoM of the robot. To implement the synchronous control scheme, we adjusted the swing legs to form a support triangle. The strategy is applicable to both static walk gait and dynamic trot gait. In the motion control processes of the robot legs, the distribution of the ground reaction forces is optimized to minimize joint torque and locomotion energy consumption. We also used an improved joint-torque controller with varied controller coefficients in the stance and swing phases. The simulation and experimental results demonstrate that the robot can complete omnidirectional locomotion in both walk and trot gaits. At a given locomotion speed, the stability margins for the robot during walking and trotting were 27.25% and 37.25% higher, respectively, than in the scheme without CoM planning. The control strategy with energy consumption optimization (ECO) reduced the energy consumption of the robot in walk and trot gaits by 11.25% and 13.83%, respectively, from those of the control scheme without ECO.

Yan, Yangyang; Zhang, Jintao; She, Junao; Liu, Wenhao; Deng, Jianghong; Zhu, Jianyang

doi: 10.1631/jzus.a2200292pmid: N/A

To improve the power-extraction performance of the Savonius vertical-axis wind turbine (S-VAWT), additional cylinders, which are used to control the fluid flow around the wind turbine blade, were introduced into the blade design. In contrast to the traditional numerical method, a mathematical model in the form of a dynamical system was used in this study. A numerical calculation program that could effectively solve the equations of wind-induced rotation of S-VAWT was developed, and combined with the Taguchi experimental method to investigate the influence of additional cylinders on the power-extraction characteristics of the S-VAWT. The results showed that the additional cylinders have a significant impact on the power-extraction performance of the S-VAWT. At 4-m/s wind speed, the average power coefficient of the S-VAWT with additional cylinders is 15% higher than that of the conventional S-VAWT. After construction of the wind turbine prototype and power-extraction tests, the results showed that compared with a conventional S-VAWT, the output power was 29% higher for the S-VAWT with additional cylinders under the same particular conditions.

Li, Hedong; Li, Yabiao; Pan, Yunfeng; Ng, P. L.; Leung, Christopher K. Y.; Zhao, Xin

doi: 10.1631/jzus.a2200405pmid: N/A

Slurry-infiltrated fiber concrete (SIFCON) is a sort of strain hardening cement-based composite material, typically made with 5%–20% steel fibers. This study focused on a novel type of SIFCON in which hooked-end steel fibers were replaced by arc-shaped steel fibers. The quasi-static compressive properties of the SIFCON were first measured. Test results suggested that using arc-shaped steel fibers in lieu of hooked-end steel fibers increased the quasi-static compressive strength by 47.1% and the strain at peak stress by 56.3%. We attribute these improvements to new crack-resisting mechanisms, namely “fiber cross-lock”, “dual bridging”, and “confinement loops”, when the arc-shaped steel fibers are introduced into SIFCON. As high impact resistance is a special property of SIFCON that is of practical significance, the dynamic compressive properties of arc-shaped steel fiber SIFCON were studied by using an 80-mm-diameter split Hopkinson pressure bar (SHPB). The results showed that the dynamic compressive strength, dynamic increase factor (DIF), and dynamic toughness of SIFCON all increased with the strain rate. The SIFCON incorporating arc-shaped steel fibers proved to have significant advantages in structural applications requiring high impact resistance.