The pressure pulsations in the vaneless space of pump-turbines are extremely intense and always experience rapid time variations during transient scenarios, causing structural vibrations and even more serious accidents. In this study, the mechanism behind the rapid time variations of the vaneless space pressure pulsations in a model pump-turbine during runaway was analyzed through three-dimensional (3D) numerical simulations. These results show that the high-frequency pressure pulsation components originating from rotor–stator interactions (RSI) are dominant during the whole process. These components fluctuate significantly in frequency when the working point goes through the S-shaped region of the characteristic curve, with the amplitudes increasing. Meanwhile, some low-frequency pulsations are also enhanced and become obvious. These features can be attributed to the transitions of the inter blade vortex structures (IBVSs) to the forward flow vortex structures (FFVSs) and the back flow vortex structures (BFVSs) at the impeller entrance, when the pump-turbine operates in the region with S-shaped characteristics. The FFVSs mainly cause decreases in frequency and introduce low-frequency pulsations, while the BFVSs are responsible for the unstable fluctuations. These findings contribute to the understanding of how transient flow patterns evolve and may provide new ideas about avoiding severe pressure pulsations caused by rotating stalls in the pump-turbine during transient scenarios.

Wind resistance performance research on the influence of the exterior features of high-rise buildings is an essential part of structure design. In this paper, the wind resistance tests of three high-rise building models with different concavity shapes on the horizontal plane were investigated using the wind tunnel experiment. The CAARC (Commonwealth Advisory Aeronautical Research Council) standard high-rise model was adopted as the background model (BG model for short), and the other two comparison groups with different planar configurations were designed, one with concave corners around the building (T1 model) and the other with concavity in the middle of the long side of the building (T2 model). The distribution laws of the average wind pressure coefficients of the building facades and roofs under the whole wind azimuths were analyzed comprehensively. The results show that the external shape plays a certain role in wind pressure distribution. Among the three high-rise building models, the most unfavorable positive wind pressure coefficient on the facades occurs in the T2 model (a 4.7% increment compared with the BG model), while the most unfavorable negative wind pressure coefficient appears in the T1 model (a 25% increment compared with BG model). Furthermore, it is noted that the architectural appearance of the T1 model makes the flow field around the building more streamlined, reflecting more favorable wind-resistance performance, including a maximum reduction of 47.2% in the least stresses per unit area on the roof. The research in this paper can provide some references for the design of different exterior features of high-rise buildings.

Wind resistance is one of the most important safety targets for high-rise buildings, especially slab-shaped ones with relatively large length–width ratios. In this study, the characteristics of wind pressure on a reduced-scale model of a slab-shaped high-rise building were analyzed experimentally. The experiment was conducted using the DTC Initium electronic scanning pressure measurement system in the wind tunnel at the Xiamen University of Technology, China. The spatial distribution and time-frequency characteristics of the wind pressure signals were analyzed with various wind flow directions under uniform and boundary-layer inflow conditions. The results show that both of the inflow conditions and the wind directions have significant influences on the magnitude and distribution characteristics of the wind pressure on the building surfaces. The wavelet transform-based analysis shows that the wind pressure on the building surfaces presents obvious intermittent characteristics, with the instantaneous energies pulsating intensively in the time-frequency domain, illustrating the unsteady nature of the wind pressure loads on the building. The influence and risk of the unsteady pulsating pressure loads should be considered when evaluating the wind-resistant performances of this type of building.

Rotating stall, the phenomenon of unevenly distributing stall cells rotating in the runner, is one of the most unfavorable flow patterns in the pump-turbine. It occurs at off-design operating conditions and always induces intensive pressure pulsations and hydraulic vibrations, and may cause structural failures of the pump-turbine. Therefore, the characteristics of the rotating stall should be investigated thoroughly at all possible operating conditions. In this study, the rotating stall phenomenon in a model pump-turbine during the runaway transient scenario was simulated by the three-dimensional numerical model. The entire process of the formation of the rotating stall in this scenario was analyzed by the spatial distributions of channel discharges in the runner and their time-varying features. The results show that the rotating stall evolves dynamically and demonstrates irregular local variations. The runner channel discharges can recognize the rotating stall effectively and describe their characteristics semi-quantitatively. This study presents some understandings about the rotating stall in transient scenarios and may contribute to the reveal of the reasons for the hydraulic instabilities of pump-turbines during these scenarios.

In this paper, a simple approach for modelling the reproduction of bacteria in water flow is presented. Firstly, the basic mathematical model for simulating the advection-diffusion of pollutions in water flow is reviewed and validated. Then, a solution strategy of doubling the concentration at regular intervals by the basic model is proposed. This approach is easy to be implemented in the existing flow field solver and is proved to be feasible by simulating a standard example. Finally, the spread of the bacterial pollution in a pool is simulated. The result shows that the water quality variation in the pool can be attributed to both the flow patterns of the water flow and the reproduction characteristics of the bacteria. The recirculation region limits the spread of the bacteria, which induces the continuous deterioration of water quality when considering the reproduction of the bacteria.