Bottom Ring Research Articles

Resonant cavity structural design of carbon-based intrinsic metamaterial absorbers for broadening the effective absorption bandwidth

The effective absorption bandwidth (EAB) of absorptive materials has always been a bottleneck limiting their applications. Particularly, the combination of macroscopic structural design and the loss functions at the microscopic level of materials provides an efficient solution to this issue. Herein, a novel approach was proposed, integrating macroscopic structural design with microscopic loss functions to broaden the effective absorption bandwidth (EAB) through designing an intrinsic metamaterial absorber composed of a uniform blend of carbon-based absorbing materials and polydimethylsiloxane. A key innovation lies in the dual cubic resonant cavity design, where the multiple dimensions of the resonant cavities enable the absorber to achieve effective impedance matching across a wide frequency range. Additionally, the generation of a bottom ring current induces a magnetic field, which enhances the metamaterial absorber’s capability for further attenuation of electromagnetic wave energy. These innovative designs enable traditional carbon-based absorbing materials to achieve exceptional absorption performance, with the EAB being 2.37 times that of the skin structure, addressing the narrow EAB issue of traditional carbon-based absorptive materials. Experimental results demonstrate that the prepared samples achieve an absorption rate of 90 % for electromagnetic waves in the frequency range of 5.74 to 18 GHz under normal incidence, consistent with the simulation results. This absorber boasts advantages of broadband, with an equivalent thickness of only 2.94 mm. Its structure is easy to manufacture, demonstrating certain potential applications, and providing valuable insights for the work of other researchers.

Experimental and numerical studies on the effect of the reinforced tubes on the drag forces of a gravity cage system

Vertical tubes are proposed to connect the collar and bottom ring to address the significant deformation of gravity cage nets caused by currents and waves. The goal is to minimize the volume loss of the cages, which reduces the habitat available for fish. This study analyses the effect of vertical tubes on drag forces using numerical and experimental methods. Tests are conducted in a towing tank to measure drag forces and validate a numerical model that employs beam and link elements to simulate the gravity cage behaviour. The differences between numerical simulations and experiments are discussed. Cages with thin tubes show larger differences than those with thick tubes due to initial bending. Comparing the results with those of gravity cages without reinforcement, it is found that the vertical tubes increase the drag force and thicker tubes exacerbate this effect. At 0.5 m/s, the cages with reinforcement tilt, leading to a different trend in drag force with wavelength compared to the cages without reinforcement.

Strength Evaluation of the Flow Passage Stationary Structures of a Large Francis Turbine Unit

Abstract The multifield design and multifield coupling performance of the Francis turbine unit is a topic that needs to be studied deeply. In this paper, the strength evaluation of the stationary structures of the flow passage of a large Francis turbine unit under extreme conditions has been performed based on the fluid-structure coupling theory. The calculated stationary structures include the spiral casing, bottom ring, and head cover. The pressure load applied on the structures is 1.5 times the maximum head of the unit. The results show that there is stress concentration in the spiral casing, and the weld length with equivalent stress greater than 345 MPa reaches 800 mm; the maximum stress at the rib plate of the head cover reaches 357 MPa, which is greater than the yield limit 345MPa. Based on the evaluation results, the local and overall defects of the unit design can be found. The suggested strength evaluation methodology can provide references for the optimization of the unit and a theoretical basis for the operation of the unit.

Bi-directional extraordinary absorption, transmission multifunctional metamaterial based on circular metal-VO2 composites

Multifunctional metamaterials play a crucial role in advancing terahertz technology. In this paper, a multifunctional metamaterial composed solely of circular metal-vanadium dioxide (VO2) is designed and numerically demonstrated. The device comprises a top layer of circular VO2 ring, two polyimide dielectric layers, with two metal layers etched and filled with VO2, and a bottom embedded silver (Ag) circular ring dielectric layer. The metamaterial serves as a broadband and narrowband absorptive or transmissive substrate, utilizing the phase transition of VO2 between its metal and dielectric states. When VO2 is in its metal phase, the top circular VO2 ring periodic array functions as a broadband absorber, exhibiting a relative absorption bandwidth of approximately 93%. Meanwhile, the bottom circular Ag ring periodic array functions as a narrowband absorber and exhibits high sensitivity. In contrast, when VO2 is in the insulating phase, the metamaterial acts as a broadband transmittance substrate, exhibiting a maximum transmittance greater than 85%. This proposed metamaterial holds tremendous potential for utilization across various applications within the terahertz band, spanning from terahertz stealth radar to tunable biosensors and photodetectors.

Research on the Hydraulic Characteristics of 1000 MW Francis Runner: Influence of Radial Installation Deviations

This paper focuses on a 1000 MW Francis turbine and conducts a numerical analysis of the entire flow passage using computational fluid dynamics (CFD) under steady-state conditions. Building upon prior research, we further investigate the impact of an uneven gap distribution caused by radial co-directional installation deviations of 0.1 mm, 0.3 mm, 0.5 mm, 1.5 mm, and 2.5 mm on the pressures and radial forces at various locations of the turbine, including the upper crown chamber, the vaneless zone, and the bottom ring chamber, all under steady-state rated operating conditions. By examining the maximum and minimum pressure values under different conditions, we determined that different pressure reading methods should be applied to distinct surfaces. Additionally, we derived the patterns illustrating how different components are affected by radial installation deviations. The significance of this study lies in its comprehensive exploration of the influence of radial installation deviations on the performance of a Francis turbine, particularly under steady-state rated conditions. Through numerical analysis, we elucidated the specific effects of various deviation magnitudes on the pressures and radial forces at different turbine locations. These findings provide valuable insights for the optimization of turbine design and operation. The novelty of this work stems from its in-depth investigation of radial co-directional installation deviations and the extensive results obtained through numerical simulations. These results hold significant implications for improving turbine performance and reliability, surpassing previous efforts in the literature.

Sensitivity analysis of the cage volume and mooring forces for a gravity cage subjected to current and waves

A numerical model is developed to calculate the mooring forces and cage volumes for a gravity cage system. The collar, nets, bottom ring and mooring system are all modelled by line elements due to the slender structure. The effect of structural parameters on the cage capacity and mooring forces is evaluated in a sensitivity analysis, to provide knowledge in the design phase of a fish cage. Heaver sinkers and nets with smaller solidity are preferred to prevent the volume reduction of fish cages but they lead to a larger mooring force or an increase in the probability of fish escaping. Linear wave theory is applied. Short waves increase the mean value of the mooring force and higher waves result in a larger amplitude of the mooring force. It is also found that short waves can change the shape of the nets while the shape of the cage subjected to long waves almost keeps constant as the one under uniform flow, which obtains a good agreement as previous experiments.

The design and fabrication of a broadband meta-material absorber based on a double-layer ring structure

A double-layer meta-material absorber based on two “ring” structures is designed and fabricated in this paper. Each layer contains a different additional microstructure, which has the property of broadening the microwave absorption band. On the top layer we introduce a split on the ring structure, and on the bottom layer we bring in a short wire. According to the simulation results, the absorption band below −10 dB expands beyond 8.6 GHz compared to the original configuration. Moreover, there is a favorable coincidence between the experimental results and the simulations. The surface current and energy distribution of different meta-material structures are compared and analyzed to explain the absorption band expansion. It is shown that the varied distribution pattern of the surface current leads to multiple absorption peaks, which are connected to each other to form a broad absorption band. Moreover, the results illustrate that it is the splitting that causes the decoherence between the surface currents, which generates the frequency dispersion and hence the broadening of the absorption band. The distribution of the electric energy is analyzed and the results show it is the additional wire that forms a toroidal capacitor with the bottom ring to deepen the absorbing band. These results imply a different way of expanding the absorption band of metamaterial absorbers.

Electromagnetic and Thermal Properties of HTS Bulk Combined With Different Ferromagnetic Structures Under Pulsed Field Magnetization

Pulsed field magnetization (PFM) is considered as the most effective method for magnetizing high temperature superconducting (HTS) bulks. Applying ferromagnetic materials around HTS bulks during PFM can increase their trapped field. This work proposes four ferromagnetic structures (disk, bottom ring, side ring, and bowl types) combined with HTS bulk. The trapped field and temperature are calculated in the HTS bulk with different ferromagnetic structures. The results show that the bowl type has the most significant ability to capture the magnetic field, while the bottom ring type has the smallest one. Compared with the trapped field of single bulk, the trapped field of the bowl type is increased by about 24% under the same PFM condition, and the temperature rise is also significantly decreased. In addition, the effects of different thicknesses of the ferromagnet on the trapped field and maximum temperature of bulk are also investigated. As the thickness of the ferromagnetic structure enlarges, the maximum temperatures of the disk and bottom ring increase, while the side ring and the bowl decrease.

Remote Selective C−H Halogenation of Arylphosphine Oxides with Ferric Halides

AbstractWe developed the remote selective C−H bromination for the synthesis of C4′‐brominated biarylphosphine oxides using FeBr3 as a brominating reagent. C3′‐Brominated products were obtained in reactions with substrates bearing a 2,4,6‐triisopropyl substituent on the bottom ring. Moreover, direct access to C5‐chlorinated peri‐substituted(1‐naphthalenyl)phosphines is developed using FeCl3 as a chlorinating reagent. These halogenated compounds can be utilized for cross‐coupling reactions to construct functionalized phosphine oxides.

Numerical Simulation of Temperature Field Optimization to Enhance Nitrogen Transfer in GaN Crystal Growth by the Na-Flux Method.

Improved nitrogen transport is crucial for enhancing the growth rate of GaN crystals using the Na-flux method. This study investigates the nitrogen transport mechanism during the growth of GaN crystals by the Na-flux method using a combination of numerical simulations and experiments. The results indicate that the temperature field affects the effect of nitrogen transfer, and we propose a novel bottom ring heating approach to optimize the temperature field and enhance nitrogen transfer during the growth of GaN crystals. The simulation results demonstrate that optimizing the temperature field improves nitrogen transfer by causing convection within the melt to float up from the crucible wall and sink at the crucible center. This enhancement improves the nitrogen transfer from the gas-liquid interface to the GaN crystal growth surface, thereby accelerating the growth rate of GaN crystals. Additionally, the simulation results indicate that the optimized temperature field substantially reduces polycrystalline generation at the crucible wall. These findings are also a realistic guide to the growth of other crystals in the liquid phase method.

Ultrasound vibration energy harvesting from a rotary-type piezoelectric ultrasonic actuator

Ultrasound vibrations, which are abundant in our everyday environment, have long been widely used in industrial applications and medical therapeutics. While there have massive number of studies on harvesting the low-frequency vibration energy to power the low-powered electronic devices, there are few literatures on the ultrasound vibration energy harvesting. In order to validate the feasibility of ultrasound vibration energy harvesting, this article takes a rotary-type piezoelectric ultrasound actuator (PUA) as the research object, a new PZT ring with appropriate zoning and polarization is bonded onto its elastomer top surface to harvest the vibration energy induced by the ultrasound waves. The manufacturing process and operating mechanism of the modified PUA are firstly introduced. Then, the electromechanical coupled model is derived based on the equivalent circuit method, and the dynamic and energetic characteristics of the ultrasound vibration energy harvesting under three excitation modes (i.e., two standing waves and one traveling wave excitations) are theoretically studied. The experimental results testify the theoretical values, and the feasibility of ultrasound vibration energy harvesting is also validated by experiments and an application example. When the PUA is excited by traveling ultrasound wave with amplitude 67 V and frequency 35.1 kHz, the maximum harvested power of each sector of the new PZT ring and λ/4 sector of the bottom PZT ring reach 58 mW and 82.4 mW, respectively, which fully meet the power supplying requirements of low-powered electronic products and implantable medical systems.

Spectroelectrochemical behavior of parallel arrays of single vertically oriented Pseudomonas aeruginosa cells

Pseudomonas aeruginosa is a Gram-negative opportunistic human pathogen responsible for a number of healthcare-associated infection. It is currently difficult to assess single cell behaviors of P. aeruginosa that might contribute to acquisition of antibiotic resistance, intercellular communication, biofilm development, or virulence, because mechanistic behavior is inferred from ensemble collections of cells, thus averaging effects over a population. Here, we develop and characterize a device that can capture and trap arrays of single P. aeruginosa cells in individual micropores in order to study their behaviors using spectroelectrochemistry. Focused ion beam milling is used to fabricate an array of micropores in a Au/dielectric/Au/SiO2-containing multilayer substrate, in which individual micropores are formed with dimensions that facilitate the capture of single P. aeruginosa cells in a predominantly vertical orientation. The bottom Au ring is then used as a working electrode to explore the spectroelectrochemical behavior of parallel arrays of individual P. aeruginosa cells. Application of step-potential or swept-potential waveforms produces changes in the fluorescence emission that can be imaged and correlated with applied potential. Arrays of P. aeruginosa cells typically exhibit three characteristic fluorescence behaviors that are sensitive to nutritional stress and applied potential. The device developed here enables the study of parallel collections of single bacterial cells with well-defined orientational order and should facilitate efforts to elucidate methods of bacterial communication and multidrug resistance at the single cell level.

Atroposelective desymmetrization of 2-arylresorcinols via Tsuji-Trost allylation

Palladium-catalyzed asymmetric allylic alkylation has proven to be a powerful method for the preparation of a wide variety of chiral molecules. However, the catalytic and atroposelective allylic alkylation is still rare and challenging, especially for biaryl substrates. Herein, we report the palladium-catalyzed desymmetric and atroposelective allylation, in which the palladium complex with a chiral phosphoramidite ligand enables desymmetrization of nucleophilic 2-arylresorcinols in a highly enantioselective manner. With the aid of the secondary kinetic resolution effect, a wide variety of substrates containing a hydroxymethyl group at the bottom aromatic ring are able to provide O-allylated products up to 98:2 er. Computational studies show an accessible quadrant of the allylpalladium complex and provide three plausible transition states with intra- or intermolecular hydrogen bonding. The energetically favorable transition state is in good agreement with the observed enantioselectivity and suggests that the catalytic reaction would proceed with an intramolecular hydrogen bond.

Force and stiffness behavior of natural rubber based magnetorheological elastomer bushing

This research introduces a magnetorheological elastomer (MRE) bushing that has the potential to be applied to vibration control for automotive applications. An annular shape of MRE bushing is designed and fabricated by natural rubber (NR) based MRE with homogenous distribution of carbonyl iron particles (CIPs). The component consists of five parts, which are the inner and outer pipes, MRE, coil bobbin that wound by an electromagnetic coil, top and bottom ring plates, and housing. Based on a conceptual design, the electromagnetic circuit is simulated using Finite Element Method Magnetics (FEMM) software for analyzing the distribution of magnetic flux. The fabricated MRE bushing is undergone a compression test and load adhesion test for the performance evaluation. The compression test is conducted by using the Universal Testing Machine (UTM) under various applied currents to obtain the force-displacement and stiffness behavior of the device. This study demonstrated that higher forces and stiffness are achieved compared to other MRE bushings. From here, at 5.5 mm of displacement, the ranges of forces are from 7.1 kN (off-state) to 8.5 kN (on-state at 2.5 A). Furthermore, the stiffness is increased by 19% from off-state to 2.5 A. Overall, the fabricated MRE bushing shows a significant response with the presence of the magnetic field from the simulation studies and experimental results. Thus, it has the potential for vibration control due to the ability to control rigidity.

Stress Characteristic Analysis of Pump-Turbine Head Cover Bolts during Load Rejection Based on Measurement and Simulation

It is not uncommon for pump-turbine units in pumped storage power plants to experience load rejections due to the sudden disconnection of the generator from the power grid. Load rejection can suddenly increase the rotating speed of the pump-turbine and cause strong pressure fluctuations in the flow passage of the pump-turbine unit. During load rejection, the strong pressure fluctuations caused by the water hammer effect can cause strong structural vibrations, high stresses and even damage to the turbine runner, head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts. In order to study, in detail, the flow-induced stress characteristics of the prototype pump-turbine unit, and the pressure variations during load rejection in a high-head pumped storage power plant were measured first. Then the measured data were used to set up computational fluid dynamics (CFD) simulations in the entire flow passage of the prototype pump-turbine and to calibrate the simulation results. The calculated pressure distributions in the flow passage during load rejection were exported and mapped on the finite element model of the stationary structures of the pump-turbine unit so that the flow-induced stresses on the head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts can be calculated. The results of the analysis show that the maximum stresses in the head cover bolts and bottom ring bolts are located on the rounded corner of the bolt near the stay ring and that the stresses in the bolts vary with time during load rejection. The maximum stresses of the head cover bolts are higher than the maximum stresses of the bottom ring bolts, and the maximum stresses of the bolts are above two-thirds of the yield strength of the bolt material. It is recommended to use larger nominal diameter bolts to avoid damage to the connecting bolts of the pump-turbine unit.

Flow-Induced Dynamic Behavior of Head-Cover Bolts in a Prototype Pump-Turbine during Load Rejection

In order to ensure stable grid operatiFon and improve power quality, active or passive load rejection of pumped storage power stations (PSPS) inevitably occurs from time to time. The rapid closing of the guide vanes will cause drastic changes in pressure pulsations in the flow channel of the pump-turbine (PT) unit. The high-level pressure pulsations during load rejection transfer to the entire flow passage of the PT unit and generate strong vibrations on the head-cover and the connecting bolts. In this study, the 1D/3D joint simulation of the pipeline in a pumped storage power station and the turbine flow channels including the flow domains of the runner, crown chamber, band chamber, upper and lower labyrinths and pressure balance tubes is carried out first. Then, by applying the calculated pressure loads on the head-cover, stay vanes and bottom ring of the PT unit, the flow-induced dynamic behavior of the structures including the head-cover bolts is analyzed in detail. The results demonstrate that pressure loads on head-cover bolts change dramatically during the load rejection process. The flow-induced deformation of the inner head-cover during the load rejection is larger than that of other structures, and the flow-induced displacement and stress of different head-cover bolts are not uniform. The achieved conclusions in this study can be a useful reference for the design and operation of head-cover bolts for other PT units and high-head Francis turbine units.

Towards accurate numerical fluid dynamics of pump-turbine runners in deep partial load operation

The operation of pumped storage power plants at off-design conditions requires an understanding of the flow field and the resulting damage to ensure a safe and economic operation. Within this study, CFD simulations are carried out for an existing pump-turbine at deep part load operation to enable calculating the damage in later structural analyses. Furthermore, this study includes the comparison between the SST and SBES turbulence model at different mesh sizes. The simulation results are compared with model test data regarding head, torque and pressure fluctuations at a bottom ring and draft tube cone sensor. Whereas all simulation setups are within a reasonable range to the available model test data, the pressure fluctuations on the runner blades as well as the resolved vortices within the runner blade channels show a qualitative difference between both models. The SBES turbulence model resolves more vortices and thus different pressure fluctuations. Subsequent structural analyses show higher accuracy of the SBES turbulence model compared with the SST model. A mesh refinement from 25 million nodes to 80 million nodes with the SBES model reveals a better resolution of vortices, but only local differences in pressure fluctuations.

Flow Characteristics Analysis of a High-Head Prototype Pump-Turbine During Turbine Start-Up: Effects of the Clearance

The dramatic changes in the internal flow and the corresponding structural behavior during the turbine start-up transient process of pump-turbines are extremely complex. The clearances in the upper crown chamber and bottom ring chamber affect the results of the flow field and structural field of the pump-turbine runner. Most of the previous studies ignored the effects of the clearance flow field to simplify the numerical simulations. In this study, numerical calculations were performed on the entire flow passage of a high-head prototype pump-turbine during the start-up in turbine mode, and the model with and without the clearance are analysed respectively. The causes of the flow field characteristic difference and external characteristic difference caused by the existence of intermediate clearance in the model are studied in detail. The results show that the clearance flow field has a great influence on the axial forces on the runner, which is mainly due to the higher pressure of the clearance flow field compared with the flow field in the runner; At the same time, because the existence of clearance flow field only has a small effect on the distribution of internal flow field, the hydraulic torque of runner considering clearance effects is basically the same as the one without clearance.

Unraveling the Key Role of the Benzyl Group in the Synthesis of CL-20 Precursor HBIW.

As one of the most important energetic material molecules, hexanitrohexaazaisowurtzitane (CL-20) can only be synthesized using an amine with a benzyl group. Moreover, the reaction mechanism remains unexplored and the special role of the benzyl group has not been revealed. To address these issues, we perform an extensive theoretical study to investigate the synthesis mechanism of CL-20 precursor HBIW by employing density functional theory. Our calculated results demonstrate that the benzyl group can reduce the energy of the intermediate and the energy barrier of the rate-determining step due to the π–π stack interaction between two benzene rings of the benzyl group. For the first time, we revealed that the reactions can produce 16 intermediates with different chiralities during the formation of the first two side five-membered rings and only two of which can further form the bottom six-membered ring and finally obtain the product HBIW. The steric hindrance effect of the benzyl group leads to the formation of a higher-energy intermediate first, thereby providing an opportunity to correct the wrong chirality. All of these factors make the diimine with the benzyl group the most suitable reactant for the synthesis of CL-20.

Head-Cover vibration investigation of a Prototype Reversible Pump-Turbine Unit during Start-up in Pump mode. Part I: Fluid dynamic analysis

Reversible pump-turbine units of pumped storage power stations can generate power in turbine mode and store electric energy in pump mode for load balancing of the electric power grid. During the start-up transient process in pump mode of a reversible pump-turbine, the flow pattern and the pressure distribution in the flow channel change dramatically with the increasing opening angle of guide vanes. The unsteady pressure of the flow passing channel can cause severe vibration on the head-cover, bottom ring, and other stationary structures around the impeller. In this paper, a coupled 1D/3D co-simulation approach is developed to investigate the pressure distribution characteristics of the flow field during the start-up transient process in pump mode for a large prototype reversible pump-turbine unit. The entire model of flow channel from spiral casing to draft tube is constructed and the flow characteristics of key fluid fields such as spiral casing, stay vanes, guide vanes, impeller, and draft tube are analyzed. The simulation results show that under the condition of minimum guide vane opening the flow is insufficient and vortices occur on the impeller blades. The pressure distribution results of the flow channel from 3D fluid dynamic analysis during the start-up transient process in pump mode are exported as boundary conditions for the subsequent fluid-structure coupling analysis of the head-cover vibration investigation.