1,720,982 research outputs found
Physical interpretation of neural network-based nonlinear eddy viscosity models
No full textNeural network-based turbulence modeling has gained significant success in improving turbulence predictions by incorporating high fidelity data. However, the interpretability of the learned model is often not fully analyzed, which has been one of the main criticisms of neural network-based turbulence modeling. Therefore, it is increasingly demanding to provide physical interpretation of the trained model, which is of significant interest for guiding the development of interpretable and unified turbulence models. The present work aims to interpret the predictive improvement of turbulence flows based on the behavior of the learned model, represented with tensor basis neural networks. The ensemble Kalman method is used for model learning from sparse observation data due to its ease of implementation and high training efficiency. Two cases, i.e., flow over the S809 airfoil and flow in a square duct, are used to demonstrate the physical interpretation of the ensemble-based turbulence modeling. For the flow over the S809 airfoil, our results show that the ensemble Kalman method learns an optimal linear eddy viscosity model, which improves the prediction of the aerodynamic lift by reducing the eddy viscosity in the upstream boundary layer and promoting the early onset of flow separation. For the square duct case, the method provides a nonlinear eddy viscosity model, which predicts well secondary flows by capturing the imbalance of the Reynolds normal stresses. The flexibility of the ensemble-based method is highlighted to capture characteristics of the flow separation and secondary flow by adjusting the nonlinearity of the turbulence model.(c) 2023 Elsevier Masson SAS. All rights reserved
Numerical Study of Steady-Blowing Jet for a Stalled Airfoil
No full textNumerical study on flow separation control is conducted for a stalled airfoil with steady-blowing jet. It is expected that steady-blowing jet, if it is applied properly, provides additional momentum in the boundary layer flow which is susceptible to flow separation at high angles of attack. Various methods have been tested to control the stall with fluidic actuation. Major parameters in fluidic actuation are the jet location, the jet momentum, the jet angle and so on. In this study, of interest is the impact of the jet angle () on the flow control. The jet angles is the angle between the jet direction and the airfoil surface. The current study is conducted with two jet angles: 22 degrees (tangential jet) and 80 degrees (chord-normal jet). A rotorcraft-relevant flow condition is simulated with a compressible Navier-Stokes solver. The governing equations of unsteady Reynolds-averaged Navier-Stokes are numerically solved. Both static and pitching conditions of a rotor airfoil are simulated to investigate the flow control in various situations. Current 2D simulations indicate that the small jet angle 22 degrees is effective to control the stall. This tangential jet can provide additional momentum into the separating boundary layer directly. In contrast, the chord-normal jet of 80 degrees cuts through the boundary layer and blocks the external flow, resulting in a significant degrade in the aerodynamic lift. Current 3D simulation indicates that a chord-normal jet from a discrete hole in the span can induce secondary vortex which helps to mitigate the boundary layer separation on the airfoil.Maste
Numerical Modeling and Analysis of Operating Characteristic on Hydrogen Production System with PEM Water Electrolysis
No full text세계 각국이 탄소 배출량 저감을 위해 수소를 주목하면서, 친환경 수소 생산에 대한 관심이 지속적으로 증가하고 있다. 양성자 교환막 수전해(proton exchange membrane water electrolysis, PEMWE)는 수소 생산과정에서 탄소를 전혀 배출하지 않는 그린 수소의 생산방법이다. PEMWE는 100도 미만의 액체 상태의 물에 전류를 흘려서 물을 전기분해하는 방법으로 수소를 생산하고, 음극과 양극 사이에 고분자로 이루어진 proton exchange membrane이 있어 고순도의 수소를 생산할 수 있다. 또한 변동성이 큰 신재생 에너지과 연계해서 수전해를 하는 경우, PEMWE의 빠른 동적 응답성은 효율적으로 수소를 생산할 수 있게 한다. 기존에 이루어진 PEMWE에 관한 연구는 PEMWE 스택 위주로 심층적인 연구가 이루어졌다. 그러나 안정적인 PEMWE 스택 운영을 위한 balence of plants가 포함된 PEMWE 시스템의 연구는 부족하였고 특히 수소의 생산부터 생산된 수소의 후처리 단계가 포함된 수소 생산 시스템에 관한 연구는 극히 드물었다.
본 연구는 PEMWE 스택 내부의 물질 전달 현상을 반영하는 스택의 전압 모델과 수소 생산 단계부터 압축수소를 만드는 단계까지의 PEMWE 시스템을 제안하고 다양한 온도와 압력 조건하에서 스택과 시스템의 운전 특성을 조사한다. 수소의 저장형태는 다양하지만 본 연구에서는 최종적으로 고순도의 가압수소를 만드는 것을 목표로 두었다. PEMWE 스택에서 생산된 수소는 수분과 소량의 산소가 포함되어 있다. 수소의 순도를 높이기 위해서, 물리적 수분 제거단계와 화학적 흡착 단계를 통해 수소의 순도를 높였다. 가압단계에서 압축기의 기계적 부하와 수소의 연소특성을 고려하여 이단압축으로 수소를 압축하였다. 해당 시스템에서, 압력이 증가할수록 PEMWE 스택 효율은 감소하고 시스템 효율은 증가하다가 스택 효율과 비슷해졌다. 그리고 온도가 증가함에 따라 스택 효율은 낮아지고 시스템 효율은 반대의 경향을 보였다. 해당 연구의 결과는 수소의 가압단계까지 고려한 PEMWE 시스템의 작동 특성을 제공하여 목적에 맞는 최적의 운전 전략을 도출하는데 도움을 줄 수 있다.|As countries around the world pay attention to hydrogen to reduce carbon emissions, interest in eco-friendl hydrogen production has increased. Proton exchange membrane water electrolysis (PEMWE) is a green hydrogen production method that does not emit carbon at all in the hydrogen production process. PEMWE produces hydrogen by electrolyzing water by flowing an electric current through liquid water below 100 degrees Celsius and there is a proton exchange membrane made of polymer between the cathode and the anode, so that high-purity hydrogen can be produced. In addition, when water electrolysis is performed in conjunction with renewable energy with high volatility, the fast dynamic response of PEMWE enables efficient hydrogen production. Existing studies on PEMWE have been conducted in depth centering on the PEMWE stack. However, research on the PEMWE system including the balance of plants for stable PEMWE stack operation has been lacking, and in particular, research on the hydrogen production system including the post-processing of hydrogen produced from the production of hydrogen has been extremely rare.
This research proposes a voltage model reflecting the mass transfer phenomena inside the PEMWE stack, and a PEMWE system from the hydrogen production stage to the compressed hydrogen production stage and investigates the operating characteristics of the stack and system under various temperature and pressure conditions. Although there are various types of hydrogen storage, this study aimed to produce high-purity pressurized hydrogen finally. Hydrogen produced by PEMWE contains water and a small amount of oxygen, in order to increase the purity of hydrogen, the purity of hydrogen was increased through a physical moisture removal step and a chemical adsorption step. In the pressurization step, hydrogen is compressed through two-stage compression in consideration of the mechanical load of the compressor and the combustion characteristics of hydrogen. In this system, as the pressure increased, the PEMWE stack efficiency decreased and the system efficiency increased and then approached the stack efficiency. However, as the temperature increased, the stack efficiency decreased and the system efficiency showed the opposite trend. The results of this study will provide the operating characteristics of the PEMWE system considering even the hydrogen pressurization stage, which will help to derive the optimal operation strategy for the purpose.Maste
Turbulence Model Enhancement Using Field Inversion for an Axisymmetric Blunt Body
No full textSeparated flows are challenging to analyze numerically using Reynolds-averaged Navier-Stokes (RANS) simulation. A typical turbulence model for RANS is often unreliable for massively separated flow. Thus, this study aims to enhance a turbulence model for separated flows around blunt bodies. Numerical analysis of flows around axisymmetric blunt circular cylinders with various fineness ratios is conducted by solving the RANS equations with the Spalart-Allmaras (SA) turbulence model. The field inversion method is used to correct the SA model equation using sparse experimental data, and the proper correction is determined through an optimization process which minimizes errors associated with parameters of interest.
In the current field inversion process, the SA model is modified using the aerodynamic drag measured in experiments. Although the optimization helps improve the SA model for the target drag, the overall flow field is not appropriately modified according to the test data. Therefore, the centerline velocity in the wake region is additionally included in the optimization process to modify the SA model. Consequently, the optimization involving both the aerodynamic drag and the wake centerline velocity significantly enhances the SA model, improving the prediction capability for the overall velocity field and the aerodynamic drag at the same time.Maste
Direct-Numerical Simulation with the Stability Theory for Turbulent Transition in Hypersonic Boundary Layer
No full textLaminar-to-turbulent transition in hypersonic boundary layer is numerically investigated using the direct-numerical simulation (DNS) method combined with the linear stability theory (LST). The DNS-LST framework is validated first for 2D hypersonic boundary layer. The growth of the Mack second mode matches well to previous DNS data. A complete 3D turbulent transition at Mach 6 is computed in the current DNS simulation to demonstrate the capability of the current method for a whole 3D turbulent transition scenario. Two instability modes are assigned at the DNS inlet for the fundamental breakdown in the hypersonic boundary layer: one Mack second mode (the fundamental mode) and one pair of oblique waves of the fundamental frequency. These instability modes are obtained from the LST. The current DNS successfully resolves the 3D turbulent transition in the hypersonic boundary layer. Computational data are investigated to identify major flow features associated with the fundamental breakdown phenomena. Turbulent statistics is also analyzed to discuss the overall fidelity of the current DNS.Maste
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Numerical investigation of impulsively generated high-speed jet for dynamic stall suppression
No full textA computational study of impulsively actuated high-speed jets for dynamic stall suppression is conducted to control the aerodynamic erformance of a lifting body. Jet flows are applied to the VR-12 airfoil under dynamic stall conditions. The current study demonstrates that an energetic interaction between the high-momentum jet and dynamic stall vortex is able to suppress two critical stall events, i.e., the moment and lift stalls, in the dynamic stall phenomena. The controlling mechanism of such stall events is explored using the single-jet flow under various actuation timings in a pitch period. Based on the derived mechanism of the stall control, multiple jets are utilized to suppress the severe hysteresis of the dynamic stall, because the single actuation could control only one of the stall events and not the both. Consequently, the numerical study with multiple-jet actuation reveals that the actuation of a few (3–5) jets near the lift and moment stall events could mitigate the dynamic stall as effectively as a large number (nearly 20) of jets over the entire pitch period. Furthermore, a higher actuation frequency provides an additional control benefit for dynamic stall suppression because of concentrated pulses around the two critical stall events.FALSEsciescopu
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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