检索结果-南通市图书馆

Hypersonic starting flow at high angle of attack

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2016年第2期29卷 297-304页

作者： Bai Chenyuan wu ziniu Department of Engineering Mechanics Tsinghua University

Compressible starting flow at small angle of attack （AoA） involves small amplitude waves and time-dependent lift coefficient and has been extensively studied before. In this paper we consider hypersonic starting flow of a two-dimensional flat wing or airfoil at large angle of attack involving strong shock waves. The flow field in some typical regions near the wing is solved analytically. Simple expressions of time-dependent lift evolutions at the initial and final stages are given. Numerical simulations by compuational fluid dynamics are used to verify and complement the theoretical results. It is shown that below the wing there is a straight oblique shock （OSW） wave, a curved shock wave （CSW） and an unsteady horizontal shock wave （USW）, and the latter moves perpendicularlly to the wing. The length of these three parts of waves changes with time. The pres- sure above OSW is larger than that above USW, while across CSW there is a significant drop of the pressure, making the force nearly constant during the initial period of time. When, however, the Mach number is very large, the force coefficient tends to a time-independent constant, proportional to the square of the sine of the angle of attack.

关键词： Hypersonic Normal force Shock waves Starting flow Unsteady flow

Generalized Kutta–Joukowski theorem for multi-vortex and multi-airfoil flow with vortex production——A general model

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2014年第1期27卷 1037-1050页

作者： Bai Chenyuan Li Juan wu ziniu School of Aerospace Tsinghua University

By using a special momentum approach and with the help of interchange between singularity velocity and induced flow velocity, we derive in a physical way explicit force formulas for twodimensional inviscid flow involving multiple bound and free vortices, multiple airfoils, and vortex production. These force formulas hold individually for each airfoil thus allowing for force decomposition, and the contributions to forces from singularities（such as bound and image vortices,sources, and doublets） and bodies out of an airfoil are related to their induced velocities at the locations of singularities inside this airfoil. The force contribution due to vortex production is related to the vortex production rate and the distance between each pair of vortices in production, thus frameindependent. The formulas are validated against a number of standard problems. These force formulas, which generalize the classic Kutta–Joukowski theorem（for a single bound vortex） and the recent generalized Lagally theorem（for problems without a bound vortex and vortex production） to more general cases, can be used to identify or understand the roles of outside vortices and bodies on the forces of the actual body, optimize arrangement of outside vortices and bodies for force enhancement or reduction, and derive analytical force formulas once the flow field is given or known.

关键词： Drag force Lift force Multibody Multiple vortices Vortex production

Modelling the spreading rate of controlled communicable epidemics through an entropy-based thermodynamic model

在线全文

学校读者我要写书评

暂无评论

Science China(Physics,Mechanics & Astronomy) 2013年第11期56卷 2143-2150页

作者： WANG WenBin wu ziniu WANG ChunFeng HU RuiFeng Department of Engineering Mechanics Tsinghua University School of Electro-Mechanical Engineering Xidian University

A model based on a thermodynamic approach is proposed for predicting the dynamics of communicable epidemics assumed to be governed by controlling eforts of multiple scales so that an entropy is associated with the system.All the epidemic details are factored into a single and time-dependent coefcient,the functional form of this coefcient is found through four constraints,including notably the existence of an inflexion point and a maximum.The model is solved to give a log-normal distribution for the spread rate,for which a Shannon entropy can be defined.The only parameter,that characterizes the width of the distribution function,is uniquely determined through maximizing the rate of entropy production.This entropy-based thermodynamic(EBT)model predicts the number of hospitalized cases with a reasonable accuracy for SARS in the year 2003.This EBT model can be of use for potential epidemics such as avian influenza and H7N9 in China.

关键词：热力学模型传播速率信息熵流行病传染性控制建模对数正态分布

Kutta-Joukowski force expression for viscous flow

在线全文

学校读者我要写书评

暂无评论

Science China(Physics,Mechanics & Astronomy) 2015年第2期58卷 90-94页

作者： LI Juan XU YiZhe wu ziniu Department of Engineering Mechanics Tsinghua University

The Kutta Joukowski(KJ) theorem, relating the lift of an airfoil to circulation, was widely accepted for predicting the lift of viscous high Reynolds number flow without separation. However, this theorem was only proved for inviscid flow and it is thus of academic importance to see whether there is a viscous equivalent of this theorem. For lower Reynolds number flow around objects of small size, it is difficult to measure the lift force directly and it is thus convenient to measure the velocity flow field solely and then, if possible, relate the lift to the circulation in a similar way as for the inviscid KJ theorem. The purpose of this paper is to discuss the relevant conditions under which a viscous equivalent of the KJ theorem exists that reduces to the inviscid KJ theorem for high Reynolds number viscous flow and remains correct for low Reynolds number steady flow. It has been shown that if the lift is expressed as a linear function of the circulation as in the classical KJ theorem, then the freestream velocity must be corrected by a component called mean deficit velocity resulting from the wake. This correction is small only when the Reynolds number is relatively large. Moreover, the circulation, defined along a loop containing the boundary layer and a part of the wake, is generally smaller than that based on inviscid flow assumption. For unsteady viscous flow, there is an inevitable additional correction due to unsteadiness.

关键词：粘性流动高雷诺数粘流塔直接测量自由流速度低雷诺数线性函数

Review of shock wave detection method in CFD post-processing

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2013年第3期26卷 501-513页

作者： wu ziniu Xu Yizhe Wang Wenbin Hu Ruifeng School of Aerospace Tsinghua University

In the present computational fluid dynamics （CFD） community, post-processing is regarded as a procedure to view parameter distribution, detect characteristic structure and reveal physical mechanism of fluid flow based on computational or experimental results. Field plots by contours, iso-surfaces, streamlines, vectors and others are traditional post-processing techniques. While the shock wave, as one important and critical flow structure in many aerodynamic problems, can hardly be detected or distinguished in a direct way using these traditional methods, due to possible confusions with other similar discontinuous flow structures like slip line, contact discontinuity, etc. Therefore, method for automatic detection of shock wave in post-processing is of great importance for both academic research and engineering applications. In this paper, the current status of methodologies developed for shock wave detection and implementations in post-processing platform are reviewed, as well as discussions on advantages and limitations of the existing methods and proposals for further studies of shock wave detection method. We also develop an advanced post-processing software, with improved shock detection.

关键词： Aerodynamics Automatic detection Computational fluid dynamics Shock wave Post-processing

A hypersonic lift mechanism with decoupled lift and drag surfaces

在线全文

学校读者我要写书评

暂无评论

Science China(Physics,Mechanics & Astronomy) 2013年第5期56卷 981-988页

作者： XU YiZhe XU ZhiQi LI ShaoGuang LI Juan BAI ChenYuan wu ziniu School of Aerospace Tsinghua University

In the present study, we propose a novel lift mechanism for which the lifting surface produces only lift. This is achieved by mounting a two-dimensional shock-shock interaction generator below the lifting surface. The shock-shock interaction theory in conjunction with a three dimensional correction and checked with computational fluid dynamics (CFD) is used to analyze the lift and drag forces as function of the geometrical parameters and inflow Mach number. Through this study, though limited to only inviscid flow, we conclude that it is possible to obtain a high lift to drag ratio by suitably arranging the shock interaction generator.

关键词：升力面高超声速阻力机制计算流体动力学激波干扰表面干扰发生器

Transit time difference and equal or non-equal transit time theory for airfoils with lift

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2022年第2期35卷 8-11页

作者： Chenyuan BAI ziniu wu Ministry of Education Key Laboratory of Fluid Mechanics Beihang UniversityBeijing 100083China Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China

The transit time difference of fluid particles moving along the upper and lower surfaces of a lift-producing airfoil is studied here both theoretically and numerically.We show that,under thin airfoil assumption and for potential flow,the transit time difference is equal to the circulation divided by the square of the inflow velocity and the lift coefficient is equal to half of the number of chords travelled by the airfoil during the transit time difference.An analysis of transit time difference for very thick airfoil(c.***.very large angle of attack)suggests the transit time may change sign beyond thin airfoil assumption,a conclusion supported by an example of flow with an attached vortex.Thus,fluid particles may transit the upper surface with less,equal and more time than those transiting the lower surface for lift producing airfoils,depending on the configuration of flow structure and geometry.

关键词： Airfoil Equal transit time theory Lift Non-equal transit time theory Transit time

Symmetric Mach reflection configuration with asymmetric unsteady solution

在线全文

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2023年第4期36卷 75-78页

作者： Chenyuan BAI Miaomiao WANG ziniu wu Ministry of Education Key Laboratory of Fluid Mechanics Beihang UniversityBeijing 100083China Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China

Symmetric Mach reflection in steady supersonic flow has been usually studied by solving a half-plane problem with the symmetric line treated as reflecting surface,thus losing the opportunity to discover antisymmetric flow structures.Here in this paper we treat this problem as an entire-plane problem.Using an unsteady numerical approach,we find that the two sliplines exhibit antisymmetric unsteadiness if the Mach stem height is small while the flow remains symmetric if the Mach stem height is large.The mechanism by which disturbance,generated in the downstream of the flow duct between the two sliplines,propagates upstream is identified and it is also shown that the interaction between the transmitted expansion waves and the sliplines increases the amplitude of the unstable modes.The present study suggests a new type of compressible jet that deserves further studies.

关键词： Antisymmetric solution Mach reflection Shock reflection Supersonic flow

氮素穗肥对杂交籼稻籽粒灌浆影响及其与淀粉合成关键酶活性间关系

在线全文

学校读者我要写书评

暂无评论

中国水稻科学 2024年第1期38卷 48-56页

作者：吴子牛何丽梅熊莹陈凯瑞杨志远孙永健吕旭马均四川农业大学水稻研究所/作物生理生态及栽培四川省重点实验室成都610000

【目的】为明确氮素穗肥对杂交稻籽粒灌浆和淀粉合成关键酶活性的影响。【方法】选用两个直链淀粉含量差异较大的杂交稻品种锦优1319(高直链淀粉)和蓉优702(低直链淀粉),在不同的氮素穗肥处理下(设0、60、90和120 kg/hm^(2)4个纯氮水平... 详细信息

【目的】为明确氮素穗肥对杂交稻籽粒灌浆和淀粉合成关键酶活性的影响。【方法】选用两个直链淀粉含量差异较大的杂交稻品种锦优1319(高直链淀粉)和蓉优702(低直链淀粉),在不同的氮素穗肥处理下(设0、60、90和120 kg/hm^(2)4个纯氮水平,分别用CK、N1、N2、N3表示)研究氮素穗肥水平对籽粒灌浆和淀粉合成关键酶活性的影响。【结果】不同穗肥处理下杂交稻籽粒灌浆特性差异显著,且不同类型品种表现各异,其中氮素穗肥的施用显著降低了稻米的直链淀粉含量。锦优1319的Gmax和Gmean随氮素穗肥水平的变化均表现为N2>CK>N1>N3,而蓉优702则表现为N3>N2>CK>N1;施用氮素穗肥显著增加了花后5~25 d籽粒腺苷二磷酸焦磷酸化酶(ADPG)和淀粉分支酶(DBE)的活性,降低了花后5~15 d籽粒GBSS活性,降低了花后5~25 d籽粒淀粉合成酶(SSS)和SBE活性。【结论】高直链淀粉品种锦优1319籽粒灌浆的最佳氮素穗肥施用量低于低直链淀粉品种蓉优702:其中锦优1319的最佳氮素穗肥施用量为90 kg/hm^(2),蓉优702为120 kg/hm^(2)。施用氮素穗肥显著降低了GBSS、SSS的活性,加之SBE活性下降,DBE活性提高,最终降低稻米直链淀粉含量。本研究条件下,籽粒ADPG活性未对稻米直链淀粉含量产生显著影响。

关键词：水稻氮素穗肥籽粒灌浆淀粉合成酶