bluuebunny/crossref_metadata_2025_split huggingface dataset:
ARX-based EEG data balancing for error potential BCI
Authors: Farabbi Andrea , Aloia Vanessa , Mainardi Luca 
Published: May 2022

Abstract

Objective. Deep learning algorithms employed in brain computer interfaces (BCIs) need large electroencephalographic (EEG) datasets to be trained. These datasets are usually unbalanced, particularly when error potential (ErrP) experiment are considered, being ErrP’s epochs much rarer than non-ErrP ones. To face the issue of unbalance of rare epochs, this paper presents a novel, data balancing methods based on ARX–modelling. Approach. AutoRegressive with eXogenous input (ARX)-models are identified on the EEG data of the ‘Monitoring error-related potentials’ dataset of the BNCI Horizon 2020 and then employed to generate new synthetic data of the minority class of ErrP epochs. The balanced dataset is used to train a classifier of non-ErrP vs. ErrP epochs based on EEGNet. Main results. Compared to classical techniques (e.g. class weights, CW) for data balancing, the new method outperforms the others in terms of resulting accuracy (i.e. ARX

91.5
%

vs CW

88.3
%

), F1-score (i.e. ARX

78.3
%

vs CW

73.7
%

) and balanced accuracy (i.e. ARX

87.0
%

vs CW

81.1
%

) and also reduces the number of false positive detection (i.e. ARX 51 vs CW 104). Moreover, the ARX-based method shows a better generalization capability of the whole model to classify and predict new data. Significance. The results obtained suggest that the proposed method can be used in BCI application for tackling the issue of data unbalance and obtain more reliable and robust performances.
Approximation of hub leakage flow in a high speed axial flow compressor rotor with adjoint method
Authors: Ito Sasuga , Furukawa Masato , Yamada Kazutoyo 
Published: May 2022

Abstract
This paper focuses the effect of hub leakage flow on the performance of a high speed transonic rotor. The rotor considered here is NASA rotor 37. General computational fluid dynamics of this rotor have been unable to predict a total pressure deficit from the hub wall to the mid span height observed in the experiments. In this study, computations have been performed on the rotor with the boundary condition assumed the leakage flow from the tip gap at the upstream of the rotor. In the computations, the velocity distribution is determined on the computational domain corresponding to the location of the tip gap. The swirl velocity at the boundary is calculated with the rotational speed and the radial length of the hub wall. The radial velocity is estimated based on the leakage mass flow. However, the mass flow is an uncertain quantity, and it has effect to the prediction of the total pressure deficit. Therefore, an adjoint method has been applied for the approximation to the uncertain quantity. The simulation results using the leakage flow boundary and the result without the boundary condition have been compared with the experiment. The comparisons have shown that the boundary condition and the approximation with the adjoint method have enabled to predict the total pressure deficit.
Lattice Boltzmann simulation of flow and sound fields around an axial flow fan
Authors: Okuyama K , Yamada K 
Published: May 2022

Abstract
In this study, not only the flow field but the aerodynamic sound field around the axial flow fan were directly simulated using the lattice Boltzmann method (LBM). To validate the LBM results obtained in this study, they were compared with the experimental results in terms of the pressure rise performance characteristics. In order to investigate the change in the flow and sound fields due to different operating points, calculations were conducted at two operating points; one is the low flow rate point where the fan is stalled and the other is the high flow rate point where the flow field in the fan is sound. In the comparison of the performance characteristics, the calculation results showed good agreement with the experimental results, and the flow fields were well reproduced with high accuracy. The analysis in terms of the vortical flow structures and the relative velocity distributions showed that the tip vortex was generated by the pressure difference between both sides of the fan blade at the high flow rate point while at the low flow rate point the flow field near the fan tip was covered with the low velocity region and the leading edge separation occurred on the tip side of the fan blade. Therefore, it was confirmed from the flow field as well as the performance that the fan blade was stalled with the blade loading lost at the low flow rate point. Large pressure fluctuations on the fan blade surface were caused by the interaction with the tip vortex from the adjacent blade at the high flow rate point and the occurrence of leading edge separation at the low flow rate point. With regard to the comparison of the far field sound spectrum, the calculation results for both two flow rate points were in good agreement with the experimental results in a low frequency range, and the aerodynamic sounds generated from the fan were predicted well with the LBM simulation.
Numerical simulation of cavitating flow around a twist hydrofoil focusing on the erosion behaviour
Authors: Wang Zhengdong , Li Linmin , Li Xiaojun , Zhu Zuchao 
Published: May 2022

Abstract
The erosion risk in cavitating flow is a challenging issue due to the instantaneity and complexity. In this paper, the energy balance approach (EBA)-based cavitation erosion model is employed to investigate the erosion behavior around a Delft Twist-11 hydrofoil utilizing large eddy simulation (LES) approach. The results show that the predicted erosion region is qualitative agreement with the experimental paint tests. The adaptive mesh refinement (AMR) method is conducted on a coarse mesh to verify its preponderance in save computing resources and ensuring accuracy. It is found that the case using AMR obtains more accurate cavitation features within less computational cells than that of the fine mesh, including the predicted shedding frequency and U-type structure of shedding cloud.
Prediction of incipient cavitation in micro tubular propeller turbine
Authors: Jang Seungsoo , Song Ji-Hun , Kim Youn-Jea 
Published: May 2022

Abstract
Cavitation is a phenomenon that cavities or bubbles form in a liquid when static pressure falls below the vapor pressure. It can damage the machine, hence it is recommended to be avoided in the design and operation of the machine. In this study, the cavitation characteristics were numerically investigated with a change in the flow rate of a micro tubular propeller turbine. Here, the rotational speed was set to the value corresponding to the best efficiency point for each flow rate. A multiphase flow analysis incorporating water and water vapor was conducted, using the Zwart-Gerber-Belamri (ZGB) model as the cavitation model. Results show that the tip clearance is the first region where the cavitation starts to occur as the flow rate increases. At this time, the Thoma number was found to be 1.178. The second region where the cavitation occurs with further increases in the flow rate is in the vicinity of the leading edge. It turned out that the leading edge cavitation occurs when the Thoma number is below 1.111. As the tip clearance increases, the range of the flow rate that cavitation does not occur increases, but it reduces turbine performance. Therefore, the selection of appropriate tip clearance given the operating flow rate range and performance needs to be considered.
A Venturi tube design for studying travelling bubble cavitation
Authors: Li Zehao , Zuo Zhigang , Qian Zhongdong 
Published: May 2022

Abstract
Travelling bubble cavitation always appears in forms of nearly spherical travelling bubbles. These bubbles grow from the freestream/surface nuclei in the low-pressure region in the flow channel. As the inception of travelling bubble cavitation is very sensitive to the boundary layer flow, attached cavitation would be likely to appear simultaneously if the flow is not well-controlled. In this study, based on the interaction between cavitation inception and boundary layer, a Venturi tube for studying travelling bubble cavitation is designed with the aid of a computational fluid dynamics (CFD) approach. The flow channel of this Venturi tube is composed of an inlet straight pipe (10 mm × 10 mm square), a contraction section, a throat section (5 mm × 5 mm square) and two diffuser sections (diffuser 1 and diffuser 2). For visualization convenience, each cross section of the Venturi tube is square. From CFD results, no flow separation occurs near the throat section and the adverse pressure gradient is relatively small, which indicates attached cavitation may not occur. The manufactured Venturi tube is installed in a blow-down type tunnel and tested with Reynolds number at roughly 3 - 3.75 × 104 and the pressure recovery number at 2.95 - 4.41. The velocity of the Venturi inlet varies from about 6 m/s to 7.5 m/s. Experiment results shows travelling bubble cavitation can be generated successfully and pure travelling bubble cavitation inception is achieved in this Venturi tube. The formation of travelling bubbles is recorded by a Phantom VEO 710L high-speed camera with framing rate of 24000 fps.
Internal flow characteristics in the draft tube of a Francis turbine model by air injection under low flow rate condition
Authors: Kim Seung-Jun , Cho Yong , Kim Jin-Hyuk 
Published: May 2022

Abstract
Undesirable internal flow features under low flow rates in the draft tube of a Francis turbine are caused by the vortex ropes with precession. These vortex ropes cause performance degradation and increase the instability of turbine systems. The effects of anti-swirl fins on the wall of the draft tube were observed in the previous study to suppress the generation of the vortex rope. However, vortex ropes still remained near the cone of the draft tube. Thus, extra methods to suppress the vortex rope are needed. In this study, the influences of air injection were investigated from the cone of the runner to suppress the vortex rope characteristics in the draft tube of a Francis turbine model. For investigating the unsteady flow and pressure phenomena by injecting the air in the draft tube, unsteady-state Reynolds-averaged Navier–Stokes calculations were carried out with a turbulence model of a scale-adaptive simulation shear stress transport. By injecting the air into the draft tube, the hydraulic efficiency was reduced, however, the visible vortex ropes in the draft tube were suppressed. Additionally, the unsteady pressure phenomena also were reduced by injecting the air.
The development of a high-speed miniature pump with dynamic bearing
Authors: Song Xueyi , Li Hao Yuan , Li Ying , Luo Xianwu 
Published: May 2022

Abstract
A high-speed miniature pump is proposed for the application in aerospace area, where high efficiency and reliability are expected for the pump under the microgravity operation condition. In this paper, a miniature centrifugal pump with dynamic bearing is selected as the objective for the performance development. The prototype of the miniature pump is manufactured and tested by experiments at the rotational speed of 10,000r/min. The comparison of hydraulic performance near the design operation shows good agreement between the numerical results and experimental data. In order to improve hydraulic performance of the pump, the effects of splitter blades, number of blades, and wrap angle of blades are analyzed numerically. For each operating condition, in order to compare the influence of splitter blades on the performance and the internal flow, the impellers with and without splitter blades are designed respectively. In addition, for the splitter blades, the influence of the splitter blade position and the splitter blade thickness are also investigated. The numerical result shows that the highest efficiency among all the impellers is achieved with the blade number of 5, wrap angle of 160° and with no splitter blades. Moreover, for the impellers with the splitter blades, hydraulic performance of the pump is mainly decided by the main blades, while the splitter blades hardly help to improve the efficiency of the pump. The study is helpful for the further application of the high-speed miniature pump in various societies.
Optimization design of the groove flow control technique to suppress flow instability in axial-flow pump
Authors: Rui Zhang 
Published: May 2022

Abstract
As an effective fluid control technology, the groove flow control technique is used to improve axial-flow pump energy performance. Determining the values of groove parameters reasonably is essential to maximize the energy performance of axial-flow pumps. In order to solve the limitation of traditional groove design in groove flow control technology, this paper presents a multi-objective groove optimization method developed by integrating OD (orthogonal design), RSM (response surface methodology), and PSO (particle swarm optimization). A case study of an axial-flow pump is conducted. By using the OD method, the sample space has been designed. The performance prediction model of axial flow pump is established by RSM. In addition, the linear weighted method is employed to construct a comprehensive evaluation function as the fitness value of PSO. Finally, an optimal combination of the groove flow control technique parameters is determined using the PSO algorithm. The result shows that this method proposed in this paper is helpful for solving multi-objective optimization of the groove flow control technique and could effectively improve the hydraulic performance of the axial-flow pump under three typical stall conditions. The optimization effectively suppresses the formation of a saddle zone. In addition, the optimization increases the pump head by 0.30, 4.41 and 9.70 m under the critical stall condition, moderate stall condition and deep stall condition, respectively. Moreover, the optimization leads to better inflow conditions for the impeller and enhances its performance under the stall conditions. This study provides a basis for the early-stage design of the groove flow control technique for axial-flow pump and provides a valuable reference for the optimal design of groove for other fluid machineries.
Influence of runner leading edge radius on the pump-turbine performance
Authors: Ono Kodai , Tsuru Wakana , Yokota Kazuhiko 
Published: May 2022

Abstract
In pump-turbines, serious fluid oscillations were reported. It is known that the performance curves of the pump-turbines have an S-shape. In order to reduce the flow oscillation, we investigated the relation between the leading edge radius and the S-shaped characteristics. The effects of the radius on the pump-turbine performance is investigated by the experiments, the numerical calculations and theoretical analyses. The numerical and experimental results agree well. The theoretical loss coefficients were calculated based on the numerical results. The leading edge vorticity causes the inner blade loss. Vortices expand with an increase of the radius.
Characteristics and suppression of vibration in cross-flow turbine with a cavity
Authors: Otsuka K , Ogawa N , Iio S , Kitahora T , Choi Y D , Inagaki M 
Published: May 2022

Abstract
Cross-flow turbine is widely used for small hydropower in run-of-river type stations because of its excellent partial load characteristics, easy manufacturing, and low cost. The Banki-Michell turbine is the origin, and various studies have focused on improving the turbine performance. The authors have developed a new cross-flow turbine with a cavity and a guide wall from the outer nozzle wall tip to suppress flow separation on the guide wall and make the casing smaller. However, little researches are focusing on vibration, and it is also essential to get knowledge for suppressing vibration and noise from the turbine aiming to install at higher head sites. Our previous studies showed that high-pressure fluctuation occurs near the outer nozzle tip, and the cavity suppresses the pressure fluctuations, but the suppression mechanism is not fully understood. Therefore, this study experimentally investigates the pressure fluctuation generation mechanism and the cavity’s effect on suppressing the vibration. Furthermore, it aims to suppress the vibration by modifying the nozzle tip shape. The vibration level was measured on a lab-scale turbine and CFD analysis to reveal the generation of pressure fluctuation. As a result, the predominant frequency of the vibration corresponds to the blade passing frequency and the pressure fluctuation at the nozzle tip. The cavity and nozzle tip shape show a significant influence on pressure and vibration characteristics and turbine performance.
Effects of air chamber and curtain wall geometries on energy conversion performance in a fixed oscillating water column-type wave energy converter
Authors: Murakami T , Imai Y , Nagata S , Takao M 
Published: May 2022

Abstract
An oscillating water column-type wave energy extractor is composed of an air chamber, an air turbine and a generator. The energy conversion process is compounded of the primary conversion in an air chamber and the secondary conversion of an air turbine. The oscillating water column-type wave energy extractor is considered to be safe even under dirty weather conditions. For societal implementation, there is a need to make a design method which can consider the water surface motion, the pressure fluctuation in an air chamber, the rotations of an air turbine and a generator. At here, the authors carried out the 2-dimensional wave tank tests using the model oscillating water column equipped with the impulse turbine and a generator to obtain the experimental data needed to make this design method. In the experiments, the effects of the air chamber and curtain wall geometries on energy conversion performance in regular wave conditions were investigated. As the results, it is noticed that the geometry of the curtain wall and the vortex shedding from the corners of curtain wall affect the primary conversion efficiency.
Numerical investigation on flow collector with guide vane for tidal energy conversion
Authors: Sakaguchi M , Hirayama K , Kinoue Y , Murakami T , Shiomi N , Imai Y , Nagata S , Takao M 
Published: May 2022

Abstract
In ocean-based renewable energies, such as wave motion, ocean current, tidal current and ocean thermal energy, many studies have been conducted on the wave energy conversion system and they have been tested in many countries. In this study, a tidal current energy is focused. Collecting a tidal flow is very important because it is generally known that a power available from a stream of water is proportional to the cube of the free stream velocity of the current. In this research, a bi-directional turbine system is investigated for a tidal flow that changes the direction periodically. Numerical investigations are conducted on a bi-directional flow collector with guide vane to collect the flow, in which the effects of vane skew angle and blade number are investigated. Calculated results showed that the collector with eight guide vanes was the highest angular moment at turbine inlet. The axial velocity at turbine inlet was larger for smaller value of the vane skew angle, whereas the tangential velocity at the turbine inlet was larger for larger value of the vane skew angle. As the results of both the profiles of axial and tangential velocity, the angular moment took the local maximum value at around the angle of 180 deg. in this study.
Influence of the blade leaning angle on the performance of a radial impulse turbine for OWC converters
Authors: Valladares Aitor Vega , Díaz Manuel Garcia , Pereiras Bruno , Pérez José Gonzalez 
Published: May 2022

Abstract
Oscillating Water Column systems (OWC) have been in the spotlight in the last 20 years since these devices are considered one of the most promising devices among wave energy technology. These systems produce electricity by means a generator driven by a turbine, which takes advantage of the bidirectional flow created by the OWC itself. Among these turbines suitable for bidirectional flows, it is possible to find radial impulse turbines, which are the focus of this work. Traditionally, the radial impulse turbines have shown lower efficiencies than their competitors. However, the radial turbines present interesting mechanical features and, recently, some research show that the difference has been reduced. Following this thread, this work deals with another modification in the radial impulse turbine looking for a further improvement. By using a validated CFD model, it has been analysed the influence of the lean angle of the blade. Until now, all the turbines present in the literature are leaned zero degrees, leading to a strong interaction between the guide vanes and the blades. This work shows results of the same turbine, equipped with blades leaning from -5deg to 25deg, in order to determine the influence such a modification on the maximum total-to-static efficiency. Results have revealed a slight improvement in the maximum efficiency for positive leaning angles, whereas negative angles drive the turbine to worse performance.
Analytical study on a turbine housing with inner insulation structure for rapid catalyst light-off
Authors: Kitamura T , Hoshi T , Ebisu M 
Published: May 2022

Abstract
The analytical study on a turbine housing with inner insulation structure has been conducted for rapid light-off of a catalyst unit, which is located at the downstream side of turbochargers. CHT (Conjugate Heat Transfer) calculations, working for simulating heat transfer with mutual dependence between solid structures and fluid, are applied to the turbocharger including the turbine section, the bearing housing and the catalyst unit to evaluate the time reduction for activation of the catalyst unit. Feasible inner-insulation structures to be installed in the turbine housing have been also designed. Eventually, the inner-insulated turbine housing has been proven highly effective for rapid catalyst light-off.