Abstract This paper aims to study the stability of low water head grid-connected primary frequency governing condition (PFGC) of pumped-storage unit (PSU). Firstly, a nonlinear mathematical model of the pumped-storage unit governing system (PSUGS) considering the nonlinear characteristics of water head loss and fifth-order synchronous generator is proposed, and the nonlinear state equation in power control mode is derived. On this basis, the PFGC bifurcation characteristics of the governing system coupled with the first-order and third-order synchronous generator models are explored, and the action mechanism of the excitation characteristics in the nonlinear PSUGS model is proposed in this paper on the nonlinear dynamic response of the system is revealed. Furthermore, the transient characteristics of the nonlinear PSUGS considering the fifth-order synchronous generator model under different water head conditions are studied, the multi-scale oscillation characteristics and stability of the PSUGS under low water head PFGC are analyzed, and the influence laws of hydraulic factors, mechanical factors and electrical factors on the stability of the nonlinear PSUGS are revealed. The results show that under the PFGC, the nonlinear PSUGS meets the supercritical Hopf bifurcation. The high frequency wavelet is excited by the generator excitation system in the PFGC oscillation of the governing system. At the same time, the low-frequency wavelet in the PFGC transient oscillation is mainly caused by the power grid stability and maps with the water hammer effect in the dynamic process. The results of this paper provide a theoretical basis for the stability analysis and optimal governing of low water head PFGC of PSU.
To perform a dosimetric evaluation of four different simultaneous integrated boost whole brain radiotherapy modalities with hippocampus and inner ear avoidance in the treatment of limited brain metastases. Computed tomography/magnetic resonance imaging data of 10 patients with limited (1–5) brain metastases were used to replan step-and-shoot intensity-modulated radiotherapy (sIMRT), dynamic intensity-modulated radiation therapy (dIMRT), volumetric-modulated arc therapy (VMAT), and helical tomotherapy (Tomo). The prescribed doses of 40–50 Gy in 10 fractions and 30 Gy in 10 fractions were simultaneously delivered to the metastatic lesions and the whole-brain volume, respectively. The hippocampal dose met the RTOG 0933 criteria for hippocampal avoidance (Dmax ≤17 Gy, D100% ≤10 Gy). The inner ear dose was restrained to Dmean ≤15 Gy. Target coverage (TC), homogeneity index (HI), conformity index (CI), maximum dose (Dmax), minimum dose (Dmin) and dose to organs at risk (OARs) were compared. All plans met the indicated dose restrictions. The mean percentage of planning target volume of metastases (PTVmets) coverage ranged from 97.1 to 99.4%. For planning target volume of brain (PTVbrain), Tomo provided the lowest average D2% (37.5 ± 2.8 Gy), the highest average D98% (25.2 ± 2.0 Gy), and the best TC (92.6% ± 2.1%) and CI (0.79 ± 0.06). The two fixed gantry IMRT modalities (step and shot, dynamic) provided similar PTVbrain dose homogeneity (both 0.76). Significant differences across the four approaches were observed for the maximum and minimum doses to the hippocampus and the maximum doses to the eyes, lens and optic nerves. All four radiotherapy modalities produced acceptable treatment plans with good avoidance of the hippocampus and inner ear. Tomo obtained satisfactory PTVbrain coverage and the best homogeneity index. Clinicaltrials.gov, NCT03414944 . Registered 29 January 2018
Abstract By introducing linear and constant terms with an undetermined parameter and subsequently using certain rules to determine the optimal value of the parameter, we establish analytical approximate frequencies and the corresponding periodic solutions for strongly mixed-parity nonlinear oscillators. A quadratic–cubic nonlinear oscillator is used to verify and illustrate the usefulness and effectiveness of the proposed method.
Abstract Galloping-based energy harvester was developed for scavenging small-scale wind energy. An electromechanical coupled distributed parameter model is proposed based on the Euler-Bernoulli beam theory, quasi-steady approximation and Gauss law for electromagnetic energy harvesting from the galloping. The analytical solutions are derived using the equivalent structure method and confirmed by the numerical solutions of the full coupled model. In addition to the aerodynamic properties, modal shape and structural damping, the onset speed to galloping, tip displacement and harvested power are also functions of the electric damping and modified natural frequency. Such two critical variables depend on the natural frequency, coil inductance, coupling factor, load resistance and coil resistance. The bifurcation and performance analyses are conducted. Two optimal load resistances are found for the maximal power at fixed resistance ratio. When the resistance ratio changes, the optimal load resistance is unique. Larger resistance ratio, larger electromagnetic coupling factor and higher wind speed lead to higher maximal power. For small resistance ratio, there is an optimal magnet position to maximize the power. For large resistance ratio, the power increases monotonously with the magnet position. With small electromagnetic coupling factor, the power is higher with lower tip displacement at smaller coil inductance. The maximal power occurs at the medium coil inductance with large electromagnetic coupling factor.
Osteogenesis and angiogenesis acts as an essential role in repairing large tibial defects (LTDs). Total flavonoids of rhizoma drynariae (TFRD), a traditional Chinese medicinal herb, is reported to show anabolic effects on fracture healing. However, whether TFRD could improve the bone formation and angiogenesis in LTDs remains unknown. The purpose of this study was to evaluate the effect of TFRD on bone formation and angiogenesis in LTDs in distraction osteogenesis (DO). Using a previously established fracture model, LTD rats was established with circular external fixator (CEF). All rats then randomly divided into TFRD low dosage group (with DO), TFRD medium dosage group (with DO), TFRD high dosage group (with DO), model group (with DO) and blank group (without DO). Twelve weeks after treatment, according to X-ray and Micro-CT, TFRD groups (especially in medium dosage group) can significantly promote the formation of a large number of epiphyses and improve new bone mineralization compared with model group, and the results of HE and Masson staining and in vitro ALP level of BMSC also demonstrated the formation of bone matrix and mineralization in the TFRD groups. Also, angiographic imaging suggested that total flavonoids of TFRD was able to promote angiogenesis in the defect area. Consistently, TFRD significantly increased the levels of BMP-2, SMAD1, SMAD4, RUNX-2, OSX and VEGF in LTD rats based on ELISA and Real-Time PCR. In addition, we found that ALP activity of TFRD medium dosage group reached a peak after 10 days of induction through BMSC cell culture in vitro experiment. TFRD promoted bone formation in LTD through activation of BMP-Smad signaling pathway, which provides a promising new strategy for repairing bone defects in DO surgeries.
Semi-open centrifugal pumps are widely used in various fields. However, the tip leakage vortex (TLV) has a malign effect on the impeller flow field. The structure and trajectory of a TLV under different discharge conditions were simulated and are discussed herein. Then, the characteristics of the TLV were analyzed using a new omega vortex identification method. The external characteristic and pressure fluctuation of the simulation and experiment were consistent. A secondary leakage vortex near the blade outlet was formed under the high-discharge condition. A leading-edge overflow phenomenon under the low-discharge condition and led to the formation of a reverse-flow vortex. The interface between the main flow and tip leakage flow moved toward the impeller upstream with decreased discharge. As a result, the peak of the entropy production curve moved upstream, and leading-edge overflow and reverse flow occurred. The tip leakage flow changed the blade pressure distribution, resulting in a decrease in the blade load.
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