力学进展

Study on bifurcation to chaos of surface tension gradient driven flow

GUO Ziyi, LI Kai, KANG Qi, DUAN Li, HU Wenrui

2021, 51(1): 1-28. doi: 10.6052/1000-0992-20-022

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As the primary heat and mass transfer mechanism in space through natural convection, surface tension gradient-driven convection is a complex nonlinear process concerning strong coupling between fluid flow and heat transfer. It is also a multiple parameter coupling process that exhibits complex spatial-temporal characteristics. Therefore, the mechanism of the surface tension gradient-driven convection becomes a hotspot in microgravity fluid physics. It also has many important applications, such as in space fluid and energy management. In this paper, recent experimental and numerical results on the transition of surface tension gradient-driven convection are reviewed, especially the nonlinear analysis on the flow bifurcations to chaos. There are several numerical methods to obtain the corresponding bifurcation diagrams. One is to integrate the model forward in time starting from different parameters and initial values, and others are to calculate the asymptotic flow states and bifurcation points directly. The direct numerical simulation method and time series analysis are widely used, but searching for bifurcation points from a large number of data is burdensome. Bifurcation points can be computed directly with the numerical bifurcation method, but such calculations are more difficult to implement than the direct numerical method.

Progresses and challenges of high-order-moment turbulence closure

WANG Shengye, FU Xiang, YANG Xiaoliang, ZHENG Haobang, DENG Xiaogang

2021, 51(1): 29-61. doi: 10.6052/1000-0992-20-029

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High-order-moment model is one of the focuses and frontier topics in the research of turbulence closure theory. Since Mr. Chou first established the Reynolds-stress transport equation in general turbulence, scholars have never stopped to study the high-order-moment model in order to predict the complex flow more accurately. Especially in the new century, thanks to the rapid development of computer hardware and the breakthrough of high-order numerical methods, turbulence simulation is in the course of turning from RANS into LES. In either RANS framework, LES framework, or the hybrid one, high-order-moment models are representative of advanced closure models. According to this, the paper gives a general review of the high-order-moment models. The focus includes the modeling approach of each source item, the evolution process of scale providing equations, and the research demand of numerical solution technology. Through several typical turbulence problems, their advantages over the traditional eddy-viscosity models are shown. Moreover, the integration of high-order-moment models in some CFD software is given. Finally, the future challenges and development direction of the high-order-moment turbulence model are prospected.

Progress in macro scale mechanical effects investigation of solid oxide fuel cells

XU Xinhai, WU Yuhua, YAN Zilin, ZHONG Zheng

2021, 51(1): 62-81. doi: 10.6052/1000-0992-20-023

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Solid oxide fuel cell (SOFC) is a clean and high-efficiency electric power generation device with promising application prospects. The SOFC uses the ceramic material as electrodes and electrolytes. Due to its high working temperature, mechanical failure is one of the most significant factors leading to the decrease of SOFC performance and lifetime. In-situ experimental measurements are quite limited in SOFCs. Therefore, a numerical simulation based on macro-scale mechanical models is generally employed for structure optimization of a single cell and stack. Progress in macro-scale mechanical effects of SOFCs is reviewed in this study. The principles and mathematical models of mechanical effects include residual strain, oxidation strain, chemical expansion, thermal ain, and creep strain in different SOFC manufacturing and operating stages are introduced. The latest development in the theoretical study and numerical simulation on different mechanical effects, as well as the electro-chemo-mechanical coupling effects, are summarized. In the last, the prospects of macro-scale mechanical effects investigation of SOFCs are discussed.

Advanced structural material design based on simulation and data-driven method

LI Xiang, YAN Ziming, LIU Zhanli, ZHUANG Zhuo

2021, 51(1): 82-105. doi: 10.6052/1000-0992-20-012

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Advanced structural materials have received extensive attention in the field of materials and structural design in recent years. These materials generally achieve excellent performances via structural design at multiple length-scales. In the early material design, some researchers created reasonable mathematical and mechanical models from the natural topologies; some researchers established bionic mechanical models based on the structural and functional characteristics of biological systems. Nevertheless, it isn't easy to obtain the optimal designs only based on ingenious design. To traverse the design space to search for the optimal design by trial and error is also not practical. For these reasons, the topology optimization method has been successfully applied to the design of advanced structural materials such as phononic crystals, cellular materials, etc. However, the existing topology optimization methods still have challenges in achieving accurate reverse designs. Data-driven methods can establish complex relationships of multi-dimensional variables, and they can reveal mechanical mechanisms and laws that are difficult to be discovered by traditional methods. Hence, this paper systematically reviews the development of advanced structural material design methods. Various mainstream design methods are compared and illustrated. The status of intelligent design of advanced structural materials based on data-driven methods is introduced. The prospect of this research area is discussed.

The universal steady lift and drag theory and the physical origin of lift

WU Jiezhi, LIU Luoqin, LIU Tianshu

2021, 51(1): 106-129. doi: 10.6052/1000-0992-20-014

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Since the birth of modern aerodynamics, various theories on lift and drag have been developed and validated extensively in aeronautical applications. However, the far-field force theory had long remained at low-speed incompressible flow. Based on the analytical solutions of the linearized Navier-Stokes equations in the steady far field, the authors and their collaborators extended the classic Kutta-Joukowski lift theorem to both two- and three-dimensional viscous and compressible flows, and thus filled the long-standing gap in theoretical aerodynamics. Why can the simple formulas based on linearized approximation still be accurately valid for highly nonlinear complex flows? This issue of great interest involves the methodological characteristics and physical mechanism behind the unified force theory and is the first task of this article. Moreover, there has been an abnormal phenomenon regarding the physical origin of lift that, despite the already mature and fully verified rigorous lift theory, various different hypotheses still keep surfacing frequently in various nonscientific publications and media. This indicates that the issue is really complicated and has not been thoroughly clarified in textbooks, monographs, and classrooms around the world. Now, the universality and high conciseness of the unified theory enable one to reach a clear answer to this issue by rigorous logical arguments in the most direct way. This is the second task of this article.

On supersonic combustion and hypersonic propulsion

JIANG Zonglin

2021, 51(1): 130-140. doi: 10.6052/1000-0992-21-008

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The advanced engine has been the core technology of the aviation industry for several decades. The air-breathing hypersonic propulsion is the top problem for future aerospace flight. The engine's performance depends on its energy conversion method and combustion mode, and its relevant theory is of fundamental and revealing significance. In this paper, the supersonic combustion is discussed first since it is the theoretical basis for the research and development of scramjet engines. Then, by reviewing related research progresses, three criteria of the air-breathing hypersonic ramjet propulsion are established. The first one can be used to determine the local subsonic or supersonic flow states of combustion products in supersonic reacting gas flows, revealing the mechanism of the upstream-traveling shock wave. The second one defines the critical Mach number for hypersonic ramjet operation for all the combustion modes, and is a necessary condition that needs to be considered in the engine design under the equivalent ratio combustion. The last one gives a critical wedge angle corresponding to the CJ oblique detonation, and its physical basis is the critical initiation state of detonation. Finally, the experimental research progress on the stationary oblique detonation ramjet (Sodramjet) engine is summarized, and its feasibility as a hypersonic engine for future aerospace flight is demonstrated.

Kelvin Equation for atomic scale capillary condensation

CHANG Tienchong

2021, 51(1): 141-144. doi: 10.6052/1000-0992-20-037

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Experiments reveal that capillary condensation of water at the atomic scale follows the Kelvin Equation.

Tong Binggang: A pioneer in the practice of engineering science

Yu Yongliang, SUN Dejun, LU Xiyun

2021, 51(1): 145-154. doi: 10.6052/1000-0992-21-004

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Tong Bing-Gang (1927——2020), an academician of the Chinese Academy of Sciences, is a fluid dynamicist, educationalist, and pioneer in the practice of engineering science. After introducing the life of Prof. Tong briefly, this article highlights his achievements in the practice of engineering science from three branches of fluid mechanics, namely unsteady flow and vortex motion, biomechanics for animal locomotion, and aerothermodynamics. Finally, Prof. Tong's significant contribution to mechanics education in China is briefly summarized.

Pilot-wave hydrodynamics^*

John W. M. Bush

2021, 51(1): 155-177. doi: 10.6052/1000-0992-21-011

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Yves Couder, Emmanuel Fort, and coworkers recently discovered that a millimetric droplet sustained on the surface of a vibrating fluid bath may self-propel through a resonant interaction with its own wave field. This article reviews experimental evidence indicating that the walking droplets exhibit certain features previously thought to be exclusive to the microscopic, quantum realm. It then reviews theoretical descriptions of this hydrodynamic pilot-wave system that yield insight into the origins of its quantumlike behavior. Quantization arises from the dynamic constraint imposed on the droplet by its pilot-wave field, and multimodal statistics appear to be a feature of chaotic pilot-wave dynamics. I attempt to assess the potential and limitations of this hydrodynamic system as a quantum analog. This fluid system is compared to quantum pilot-wave theories, shown to be markedly different from Bohmian mechanics and more closely related to de Broglie's original conception of quantum dynamics, his double-solution theory, and its relatively recent extensions through researchers in stochastic electrodynamics.

2021, 51(1): 178-187.

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2021 Vol. 51, No. 1

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