Numerical Simulation of Instability Characteristic in Pump turbines

International audienceIn classical pumps, there is single positive slope part on the curve of pump performance, named head-drop phenomenon. However, a second head-drop phenomenon is found in our experiments in the pump mode of some medium pump turbines. The flow rate of this second head-drop phenomenon is 0.68-0.71 times of the design flow rate. As shown in our experimental results, it is significantly influenced by the complex vortices in the impeller inlet domain. Using SST k-ɷ turbulence model, numerical simulations are conducted to reveal its mechanism. After comparing numerical simulation results with the experimental results, it can be concluded that this second head-drop is closely related with the complex vortices in the impeller inlet. The complex vortices, due to the transmission flow between separated flow and back flow, is responsible for this second head-drop phenomenon

FRP structure design method based on equality of stiffness: case study and practice

Nowadays, Fiber reinforced polymer materials are getting widely used in civil engineering. They have some advanced engineering characters like high stiffness-to-weight ratio, high strength-to-weight ratio, anti-electromagnetic interference, electrical isolation and its long time durability. Metal and concrete, those main kinds of construction material, are prone to be failed before the structure service life because of the environmental attacks. Those harsh attacks are oxidization, acidification, high temperature and alkali corrosion. After the maintenance methods like isolating the attacks or protecting plastic skins, the interfaces between the protections are easily to be cracked and it caused the same worry on durability problems. Fiber reinforced polymer stands for a new era of civil engineering and surely the traditional material can be substituted by fiber reinforced polymer. This paper wants to unfold a case study on how fiber reinforced polymer worked in civil structures

Mapping the potential distribution of major tick species in China

Ticks are known as the vectors of various zoonotic diseases such as Lyme borreliosis and tick-borne encephalitis. Though their occurrences are increasingly reported in some parts of China, our understanding of the pattern and determinants of ticks’ potential distribution over the country remain limited. In this study, we took advantage of the recently compiled spatial dataset of distribution and diversity of ticks in China, analyzed the environmental determinants of ten frequently reported tick species and mapped the spatial distribution of these species over the country using the MaxEnt model. We found that presence of urban fabric, cropland, and forest in a place are key determents of tick occurrence, suggesting ticks were likely inhabited close to where people live. Besides, precipitation in the driest month was found to have a relatively high contribution in mapping tick distribution. The model projected that theses ticks could be widely distributed in the Northwest, Central North, Northeast, and South China. Our results added new evidence on the potential distribution of a variety of major tick species in China and pinpointed areas with a high potential risk of tick bites and tick-borne diseases for raising public health awareness and prevention response

Mild traumatic brain injury is associated with effect of inflammation on structural changes of default mode network in those developing chronic pain

BACKGROUND: Mild traumatic brain injury (mTBI) has a higher prevalence (more than 50%) of developing chronic posttraumatic headache (CPTH) compared with moderate or severe TBI. However, the underlying neural mechanism for CPTH remains unclear. This study aimed to investigate the inflammation level and cortical volume changes in patients with acute PTH (APTH) and further examine their potential in identifying patients who finally developed CPTH at follow-up. METHODS: Seventy-seven mTBI patients initially underwent neuropsychological measurements, 9-plex panel of serum cytokines and MRI scans within 7 days post-injury (T-1) and 54 (70.1%) of patients completed the same protocol at a 3-month follow-up (T-2). Forty-two matched healthy controls completed the same protocol at T-1 once. RESULTS: At baseline, mTBI patients with APTH presented significantly increased GM volume mainly in the right dorsal anterior cingulate cortex (dACC) and dorsal posterior cingulate cortex (dPCC), of which the dPCC volume can predict much worse impact of headache on patients\u27 lives by HIT-6 (β = 0.389, P = 0.007) in acute stage. Serum levels of C-C motif chemokine ligand 2 (CCL2) were also elevated in these patients, and its effect on the impact of headache on quality of life was partially mediated by the dPCC volume (mean [SE] indirect effect, 0.088 [0.0462], 95% CI, 0.01-0.164). Longitudinal analysis showed that the dACC and dPCC volumes as well as CCL2 levels had persistently increased in patients developing CPTH 3 months postinjury. CONCLUSION: The findings suggested that structural remodelling of DMN brain regions were involved in the progression from acute to chronic PTH following mTBI, which also mediated the effect of inflammation processes on pain modulation. TRIAL REGISTRATION: ClinicalTrial.gov ID: NCT02868684 ; registered 16 August 2016

Determining global mean-first-passage time of random walks on Vicsek fractals using eigenvalues of Laplacian matrices

The family of Vicsek fractals is one of the most important and frequently-studied regular fractal classes, and it is of considerable interest to understand the dynamical processes on this treelike fractal family. In this paper, we investigate discrete random walks on the Vicsek fractals, with the aim to obtain the exact solutions to the global mean first-passage time (GMFPT), defined as the average of first-passage time (FPT) between two nodes over the whole family of fractals. Based on the known connections between FPTs, effective resistance, and the eigenvalues of graph Laplacian, we determine implicitly the GMFPT of the Vicsek fractals, which is corroborated by numerical results. The obtained closed-form solution shows that the GMFPT approximately grows as a power-law function with system size (number of all nodes), with the exponent lies between 1 and 2. We then provide both the upper bound and lower bound for GMFPT of general trees, and show that leading behavior of the upper bound is the square of system size and the dominating scaling of the lower bound varies linearly with system size. We also show that the upper bound can be achieved in linear chains and the lower bound can be reached in star graphs. This study provides a comprehensive understanding of random walks on the Vicsek fractals and general treelike networks.Comment: Definitive version accepted for publication in Physical Review

Enhanced interfacial interaction, mechanical properties and thermal stability of basalt fiber/epoxy composites with multi-scale reinforcements

In this work, epoxy (EP) resin composites with multi-scale reinforcements were prepared by hand lay-up and hot-pressing. The epoxy was reinforced with basalt fibers (BF) modified with a silane coupling agent (KH560). Carboxylated multi-walled carbon nanotubes (CNTs) were also grafted onto the surface of the modified BF using an impregnation method to achieve BF/CNTs multi-scale reinforcement. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) showed that KH560 was successfully grafted onto the BF surface. Scanning electron microscopy (SEM), indicated a better resin adhesion on the BF and thus a stronger interfacial interaction in the BF/CNTs-reinforced composite. It was observed that the mass fraction of KH560 was a significant parameter in achieving desirable CNT immobilization and mechanical properties of the composites. At an optimal mass fraction of KH560 (5%), the tensile, flexural, and interlaminar shear strength (ILSS) of the modified composite (BF-5%KH560-CNT/EP) increased by 12.5%, 20.9%, and 25.5% respectively compared with the BF-washed/EP composite due to more efficient load transfer. In addition, compared with BF-washed/EP, the decomposition onset temperature of BF-5%KH560-CNT/EP increased from 387°C to 396°C, the maximum decomposition rate temperature increased from 400°C to 408°C, and the residual weight increased by 5.8%