《1. Engineering research hotspots and engineering research focus》

1. Engineering research hotspots and engineering research focus

《1.1 Development trends of engineering research hotspots》

1.1 Development trends of engineering research hotspots

The top 10 engineering research hotspots in mechanical and vehicle engineering (hereinafter referred to as the mechanical field) include mechanical engineering, naval architecture and ocean engineering, aerospace science and technology, armament science and technology, power and electrical equipment engineering and technology, transportation engineering, and other subjects (Table 1.1.1). Traditional research in mechanical engineering has focused on “Large-scale robot tactile sensors and implementations,” “Neural network model and fuzzy logic,” “Vibration isolation system,” “Piezoelectric energy harvesting,” “Fluid–solid coupling simulation,” “Instability and dynamics of a gas turbine combustor,” “Shape memory alloy,” and “Nonlinear vibration modes in mechanical engineering.” “Process design and self-adaptive control of additive manufacturing” and “Supersonic flow and aerodynamics” are emerging hotspots. Table 1.1.2 shows the annual publication of core papers involved in all hotspots in 2011–2016. The number of studies on “Process design and self-adaptive control for additive manufacturing” has significantly increased.

(1) Large-scale robot tactile sensors and implementations

A robot obtains external information through tactile sensors to perceive the external environment. Currently, tactile sensors are divided into two categories: the first category uses pressure sensing unit to detect the force, detect the vector of contact force at a single point, and form a multi-point array (e.g., an array sensor composed of many strain foils); and the second category uses an elastomer as contacting part of the sensor, and deformation occurs when the elastomer is subjected to force. Large robots have high stiffness and output power; as such, the output force in their interaction with the environment is large. To achieve accurate operation, robots should be equipped with tactile sensors with precise sensing capability. Currently, research on single-dimensional force sensor has

《Table 1.1.1》

Table 1.1.1 Top 10 engineering research hotspots in mechanical and vehicle engineering

《Table 1.1.2》

Table 1.1.2 Annual number of core papers belonging to each of the top 10 engineering research hotspots in mechanical and vehicle engineering

matured. However, detecting forces in three-dimensional directions and even multidimensional directions is necessary to perceive the interaction between the robot and the environment. The main research focuses are the optimization of flexible structures and the design of base materials. Research and development of flexible multidimensional tactile sensors are important but difficult to achieve.

(2) Process design and self-adaptive control of additive manufacturing

Metal additive manufacturing techniques are categorized according to heat source into laser, electron beam, and arc methods. Laser engineering net shaping based on powdered materials, selective laser melting, and selective electron beam melting are used in the field of aerospace, biological medicine, and molding. These processes can solve complex, integrated, and personalized manufacturing difficulties that come with traditional manufacturing techniques. However, these processes exhibit limitations in forming large parts at a rapid speed and low cost because of their complex laser and electron beam control, high cost, difficult powder preparation, easy contamination, and low formation efficiency. Wire arc additive manufacture (WAAM) uses arc as energy beam; moreover, parts are accumulated by multiple welds, with uniform chemical composition and high density. The local atmosphere does not limit the forming dimension. The efficiency of WAAM can reach kilograms per hour or even higher. The wire material exhibits less pollution and lower cost than the powder material and thus can be used in low-cost high-efficiency additive manufacturing of large aerospace components. In addition, arc is easier to achieve than laser and electron beam. The wire materials can be more easily transported and collected than the powdered materials and have significant potential in the space metal additive manufacturing field. Compared with laser and electron beam techniques, the part used in arc additive manufacturing technique possesses a rough surface, low dimensional accuracy, and coarse and big interior tissues. In-situ integration with cutting, forging, and other traditional processes is the development direction of this technology and one of the research hotspots in this field to improve the accuracy and performance of structures.

(3) Supersonic flow and aerodynamics

Supersonic flow plays an important role in the design of supersonic civil and military airplanes and is affected by a number of key technical issues related to supersonic aerodynamics. An advanced aerodynamic layout design of supersonic jet can be obtained by exploring the type of boundary transition and control measures under supersonic flow; such design can be used to determine the effect of shock wave/boundary layer interference on aerodynamic layout design, clarify the perturbation modes of supersonic flow and the evolutionary characteristics of the interaction of shock waves, and understand the structure characteristics of coherent vortex in supersonic turbulent boundary layer and the generation and propagation mechanism of aerodynamic noise. Research on the characteristics of the translot jet field can optimize the mixing process of fuel, reveal the physical mechanism of the combustion stability of scramjet engine, and improve theapplications of scramjet technology and scramjet engine in engineering practices. Transverse slot jet flow plays an important role in the overall thermal protection of aircraft and the internal cooling of scramjet engine. However, unsteady aerodynamic issues (e.g., flow separation and reattachment) are prone to occur under supersonic flow due to the disturbance of shock wave and the effects of supersonic shear layer and turbulence. These issues can be addressed through the following: explore new theory and mechanism; conduct advanced experimental technology research; develop efficient numerical simulation methods; emphasize the interaction mechanism between the jet field and the main flow field; systematically understand the effect of the comprehensive coupling of shock wave, expansion wave, shear layer, and boundary layer on the mixed characteristics of jet media and mainstream moving media; and finally grasp shock wave impinging and the relationship between the resulting flow separation and peak heat flow.

(4) Neural network model and fuzzy logic

Fuzzy neural network control is an intelligent control technology that combines fuzzy control and neural network. Fuzzy logic is similar to the thinking mode of humans and is used for systematic qualitative analysis and reasoning with fuzzy rules; this tool requires strong understanding and processing ability of natural language. Fuzzy logic can solve the fuzzy information problem, which is difficult to accurately quantify in a control system. Neural network is similar to the human brain, which is made up of neurons. Neural network, which exhibits self-learning ability, can adjust the weight matrix to complete the nonlinear mapping between input and output. With the rapid development of computer science, neural network and fuzzy logic can be combined while maintaining their respective advantages. However, a mathematical model is difficult to establish when many variables and strong coupling factors exist in a nonlinear system. Compared with traditional control methods, this intelligent control method can effectively improve the intelligence level of a system and improve the quality of control. The rapidly rising deep learning technology has introduced new vitality into the artificial neural network because of the development of hardware technology. Fuzzy neural network has become the frontier and significant research direction for intelligent control technology.

Future research will focus on designing a network structure, fuzzy rules, and optimized learning algorithm on a fuzzy neural network. The intersection of fuzzy theory and neural network enables theoretical and technological breakthroughs for completing each system by combining with other advanced machine learning methods. Hence, fuzzy neutral network technology is further integrated with advanced control and large data analysis methods. This technology will play an increasingly important role in the reform of intelligent manufacturing, smart grids, and other new industrial fields.

(5) Vibration isolation system

The main method for vibration isolation is to install a vibration isolation device or adopt vibration isolation technology between vibration source and transmission path. This kind of device plays a role in vibration isolation and is referred to as a vibration isolation system. A research hotspot and challenge both at home and abroad is to satisfy the strict requirements for the working environment of the processing of ultra-precision instruments and the performance indices of remote sensing equipment and the high-precision optical equipment on the satellite as the high resolution satellites in orbit (especially the strict requirements for micro vibration and ultra-low frequency vibration, ultra-low frequency, and high-performance isolation vibration system). With the traditional linear passive vibration isolation system, research of high-performance vibration isolation system is difficult because of two major contradictions, namely, difficulty in considering the system carrying capacity and the ultra-low isolation frequency and inability to consider the suppression of the peak of resonance frequency domain and the improvement of vibration isolation in the high frequency domain. The nonlinear vibration isolation technology and the active and semi-active vibration isolation technology are the main approaches used to fabricate vibration high-performance isolation systems, such as vibration isolation device with high static and low dynamic rigidity, nonlinear vibration isolation system, and nonlinear magnetic vibration isolator based on damper, and so on. A device with negative stiffness is introduced in linear vibration isolation system to design a low-frequency vibration isolation system with high static and low dynamic stiffness and ensure the bearing capacity of the system while maintaining ultra-low frequency vibration isolation. Nonlinear vibration isolation systems based on non-viscous damper can effectively inhibit vibration amplitude in the resonance frequency domain and the occurrence of jumping. Moreover, this system does not influence the vibration isolation characteristics in the high-frequency domain. A wide spectrum of effective vibration isolation systems can be achieved using intelligent materials and active control technology.

(6) Piezoelectric energy harvesting

In recent years, research on the conversion of vibration energy into electrical energy in the environment has attracted significant attention. When the piezoelectric material deforms under external force, the polarized charge will appear on the surface to form an electric field with high energy density. A device that uses piezoelectric effect to generate energy is called a piezoelectric energy collector; this device is characterized by simple structure, small calorific value, lack of electromagnetic interference, easy miniaturization, low carbon content, and environment friendliness.

Key technologies of piezoelectric energy harvesting mainly include study on high-performance piezoelectric materials and design of intelligent piezoelectric energy harvesting structure with low-power consumption, and simple and efficient energy harvesting circuit. The self-powered device based on piezoelectric energy harvesting has been applied for inspection of traffic infrastructures and wireless medical equipment, monitoring of indoor environment, and detection of tire pressure. Moreover, the conversion of human kinetic energy into electrical energy and the achievement of energy supply of wearable electronic devices have attracted significant attention because of their broad market prospect.

(7) Fluid–solid coupling simulation

Fluid–solid coupling refers to the interaction between solid and liquid media. Fluid–solid coupling simulation mainly investigates the coupling dynamic behavior between the solid and fluid. Fluid–solid coupling simulation is involved in the nonlinear behavior of fluid, the nonlinear constitutive equations of solid materials, and the nonlinearity of the fluid–solid coupling interface. Therefore, simulation of fluid–solid coupling is a frequent nonlinear dynamic problem. The coupling can be analyzed using numerical algorithms and simulation of the force transfer between the fluid and solid sub domains. With the development of fluid–solid coupling (e.g., arterial vascular simulation and parachute aerodynamics simulation), accurately solving large deformation, strong nonlinearity, multi-scale, spatiotemporal coupling, and multi-phase fluid-solid structure coupling is necessary. Highly precise and efficient fluid– solid coupling model and numerical algorithms have become research hotspots at home and abroad. For the fluid–solid coupling problem, uniform and non-uniform mesh generation techniques and space–time simulation techniques have good prospects for development. Fluid– solid coupling method considering time and space has a significant application prospect for improving the accuracy and efficiency of the solution.

(8) Instability and dynamics of a gas turbine combustor

Combustion instability is one of the most important research topics in the field of combustion and is widely investigated in internal combustion engines, aircraft engines, gas turbines, and other combustion phenomena. Instability induces flame discontinuity, local extinction, propagation velocity, and shock of flame surface to show cycle changes, decreased combustion efficiency, deteriorated pollutant emission and mechanical damage, and other hazards. Research on the combustion instability mechanism and its inhibition approaches must be improved to avoid hazards caused by combustion instability, expand the scope of work of advanced combustion mode, improve efficiency, and reduce emissions. Between China and other countries, the main gaps in designing and manufacturing gas turbines and its combustion chamber include the design, processing, and high-temperature resistant composite technology.

(9) Shape memory alloy

Shape memory alloy exhibits shape memory effect and pseudo elasticity (i.e., hyperelasticity) due to its thermoelastic martensitic transformation. Since their discovery in the 1960s, shape memory alloys, such as nickel titanium alloys, have been widely used in aviation, aerospace, biomedical, and other fields. The major unsolved problems in research on shape memory alloys include the following: ① thermomechanical coupling–cyclic deformation characteristics, including temperature dependence in the deformation of shape memory alloys, speed (frequency) dependence in the deformation process of pseudo elastic shape memory alloys, evolution law of thermal mechanical behavior under cyclic loading, and local effects of stress-induced martensitic transformation; ② numerical simulation of the mechanical behavior of shape memory alloys with complex structure and complex load (non-proportional load); and ③ fatigue and fracture characteristics, including crack formation and expansion under cyclic phase transformation, effect of thermal mechanical coupling on fatigue fracture behavior, and stability and macroscopic fatigue criteria under high cycle fatigue loading. In this regard, experimental and theoretical studies on shape memory alloys at micro, meso, and macro scales must be conducted to establish a multi-physical field coupling model, reveal the underlying mechanism, and simulate and predict the mechanical behavior of shape memory alloys.

(10) Nonlinear vibration modes in mechanical engineering

Nonlinear phenomena are ubiquitous in engineering applications and considerably change the dynamic behavior of structures. Research hotspots at home and abroad include dynamics analysis and solving method, experimental test, system nonparametric identification, modal parameter identification, model updating, uncertainty analysis, and other engineering problems on nonlinear mechanical system. Modal theory is the core of linear vibration theory. Nonlinear modal theory is proposed to determine a simple and efficient method for solving multidimensional nonlinear systems (e.g., linear modes). In addition, the concept of modal theory is extended to nonlinear systems. Specifically, nonlinear mode is the extension of linear mode and is the basis for simplifying nonlinear systems. The modes of the linear system are determined, and nonlinear modes may produce bifurcations to increase the number of modes of the nonlinear system beyond the degree of freedom of the system. Scholars have developed direct solutions to nonlinear vibration modes, subspace iteration, and model reduction methods for nonlinear systems. These methods provide an efficient solution for dynamic study on bifurcation, jumping, modal coupling, force relaxation, and flutter. However, quantitative analysis of many influencing factors is difficult due to the complexity of the dynamic characteristics of nonlinear systems. Therefore, uncertainty analysis methods (e.g., Bayesian method) must be combined to address limitations on studying model identification, parameter identification, model updating, model reduction, and skeleton curve stability identification. Thus far, a complete system has not been established yet.

《1.2 Understanding of engineering research focus》

1.2 Understanding of engineering research focus

1.2.1 Large-scale robot tactile sensors and implementations

Large robots possess high stiffness, output power, and output force in their interaction with the environment. Tactile sensors with precise sensing capability are frequently required to achieve accurate operation. Research on one-dimensional force sensors has matured. However, to perceive the interaction between the robot and its environment, robot tactile sensors should detect the vertical pressure on the surface and the shear force in the horizontal direction. For example, when a robot holds an object, the tangential force must be perceived with the positive pressure. When a robot touches an object with irregular surface, the three-dimensional or even multi-dimensional force must be determined. The development of three-dimensional force tactile sensors has become an important research field on intelligent robot technology. To obtain three-dimensional force tactile sensations, scholars have used pressure-sensitive film resistors, platinum/titanium films, anisotropic carbon nanotubes, flexible capacitors, carbon black/silicone rubber, and other base materials for covering the cross structure. The main research work focuses on the optimization of flexible structures and the design of base materials.

(1) 3 D shape tactile sensor with high sensitivity

A robot obtains external information through tactile sensors to perceive the external environment. Tactile sensors are categorized into two groups. The first group uses a pressure sensing unit to detect the force and the vector of contact force at a single point and form a multi-dot array (e.g., an array sensor composed of many strain foils). The second group uses an elastomer as the contacting part of the sensor, and deformation occurs when the elastomer is subjected to force. Therefore, the size and distribution of contact force are calculated by detecting the shape and deformation of the elastomer. The generalized tactile is a general term for contact, sliding, pressure perception, and so on. In addition, tactile in the narrow sense refers to the force perception on the contact surface. Three-dimensional tactile shape refers to the tactile sensation that can perceive the shape of the object being touched. Indirect perception is mainly achieved based on changes in the amount of correlated sensing in the contact sliding process. In the current research, changes in light reflection after subjecting the fiber material to stress are used as basis to achieve a 3D tactile shape.

(2) Flexible tactile sensor

Research on flexible tactile sensor has become a hotspot in the field of robot tactile sensor, with emphases on the development and research of multi-dimensional flexible tactile sensors. Multi-dimensional flexible tactile sensors are flexible, similar to the human skin, and can attach to the surface of objects to sense and measure multidimensional information. These flexible sensors can attach to carriers with different surface shapes and roughness to rapidly and accurately sense and obtain outside information. Flexible tactile sensors fabricated with new sensitive materials and manufacturing processes exhibit flexibility and malleability, which will be increasingly similar to those of the human skin. Future research must focus on providing flexible tactile sensors with multidimensional force measurement ability.

(3) Tactile array sensor

A large amount of perceptual information, including spatial pressure and temperature distribution, can be obtained by integrating tactile sensors in large arrays. In practical applications, most tactile array sensors possess flexible structures. For making artificial skin, tactile array sensors should not only be flexible but must also be capable of measuring contact pressure on any surface of a carrier. Accordingly, tactile array sensors must partially or fully cover the surface of an actuator. However, tactile array sensors exhibit certain limitations. First, some sensing elements of tactile sensors are rigid. A rigid sensor is implanted in the flexible material used as medium to transmit tactile information. The tactile sensor itself is inflexible, and the degree of curvature is limited. Thus, obtaining other accurate continuous distribution information is difficult. Second, the sensing element of another kind of tactile array sensor is flexible. However, this sensor can only measure one-dimensional tactile information. As such, scholars aim to research and develop tactile array sensors by using capacitance, PVDF, optical wave guidance, and other technologies. The present study mainly focuses on the flexibility, multidimensional aspects, and large-scale design of array sensors.

(4) Evaluation of grasping quality based on physics

Robust grasp is the most desirable attribute of robots. Stable grasping is the basis of the physical interaction between a robot gripper and the environment and is a foundation to achieve high-order grasping motion. The tactile information in the robot grasping process can be used to evaluate the quality of robot grip and provide regulatory information for real-time grasp of a robot gripper. Abundant tactile information is the basis of the robot gripper to enable grasping and flexible operation. Tactile theory and technology research can provide impetus for eventual reappearance of hand operations and perception through robotic systems. Robot grasping quality is mainly evaluated using closeness, operability, and stability of grasping.

The top three countries or regions with the highest number of core papers in engineering research on “Largescale robot tactile sensors and implementations” are the USA (17), Germany (11), and Switzerland (6). Countries or regions with top three average citation frequency include Switzerland (48.83), Spain (45.00), and Italy (42.25) (Table 1.2.1). Among the top 10 countries or regions producing core engineering papers, Germany–Spain and England–Italy exhibit high cooperation (Figure 1.2.1). The institutions with top two publication numbers of core engineering papers are Tech Univ Munich (4), Czech Tech Univ (4), Willow Garage Inc (3), RoyalInst Technol (3), and Columbia Univ (3). Moreover, the institutions with top three average citation frequency are ETH (97.50), Univ Zaragoza (65.00), and Univ Genoa (60.00) (Table 1.2.2). Among the top 10 institutions with the highest number of publications, the Italian Inst Technol and Univ Sheffield exhibits high cooperation (Figure 1.2.2). Countries or regions with top three publication number of core papers are Spain (3), the USA (3), and Sweden (2) (Table 1.2.3). The main institutions producing core papers are the Royal Inst Technol (2), Univ Bristol (2), and Carnegie Mellon Univ (1) (Table 1.2.4).

1.2.2 Vibration isolation system

The performance indices of high-resolution satellites, high-precision optical equipment, and so on; and the isolation and elimination of micro vibration are challenging and hot issues that should be addressed to satisfy the strict requirements for the working environment of processing and testing of ultra-precision equipment. Scholars, both at home and abroad, have focused on the previously neglected micro vibration problem of spacecraft caused by moving parts (e.g., reaction wheel, thrustor, solar panels, momentum wheel, refrigerator, antenna drive mecha-

《Table 1.2.1》

Table 1.2.1 Major producing countries/regions of core papers on the engineering research focus “Large-scale robot tactile sensors and implementations”

《Table 1.2.2》

Table 1.2.2 Major producing institutions of core papers on the engineering research focus “Large-scale robot tactile sensors and implementations”

《Figure 1.2.1》

Figure 1.2.1 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Large-scale robot tactile sensors and implementations”¹

1 In the figure, the nodes refer to the countries or regions, the size of the nodes refers to number of papers, the connecting line between nodes refers to papers published based on research cooperation, and the thickness of the connecting line indicates the number of papers based on research cooperation. These are the same in full text.

《Figure 1.2.2》

Figure 1.2.2 Collaboration network of the major producing institutions of core papers on the engineering research focus “Large-scale robot tactile sensors and implementations”

《Table 1.2.3》

Table 1.2.3 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Largescale robot tactile sensors and implementations”

《Table 1.2.4》

Table 1.2.4 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Large-scale robot tactile sensors and implementations”

nism, and other components) to progressively increase the point accuracy and point stability of spacecraft. Normal work will be affected once micro vibration resulting from the moving parts is converted into sensing elements (e.g., MW infrared sensor, laser communication equipment and space telescope, and high-precision equipment). As the core equipment of semiconductor science and technology industry, lithography is a typical representative of ultra-precision equipment. The machining accuracy of this equipment can reach the nanometer level.

Micro vibration factors, which can be ignored in traditional processing and testing equipment, will have a fatal effect on the accuracy of lithography. Therefore, the inhibition of micro vibration has become one of the major bottlenecks in the development of high-resolution and high-precision spacecraft and ultra-precision equipment. Micro vibration is mainly characterized by small amplitude, wide frequency band, and difficult controllability. The frequency of micro vibration ranges from extremely low to thousands of Hertz. Vibration energy ranges from several Hertz to hundreds of Hertz and is difficult to decay. Low-amplitude micro vibration possesses a complex propagation mechanism of mechanical structure and is thus difficult to remove from the vibration source. Nonlinear vibration isolation technology and active and semi-active vibration isolation technology are the main techniques used to suppress micro vibration; these technologies include high-static and low-dynamic stiffness (HSDLS) vibration isolation device, nonlinear vibration isolation system, active/semi-active isolation system, and so on.

(1) High-static and low-dynamic stiffness (HSDLS) vibration isolation device

The HSDLS vibration isolation device has a wide application prospect for solving the limitations of system carrying capacity and ultra-low isolation frequency. Quasi zero stiffness isolator (or the vibration isolation system with high-static and low-dynamic stiffness) formed by the parallel connection of positive and negative stiffness elements has become a hotspot in the field of global engineering. An elastic element with positive stiffness generally refers to vertical spring, and many forms of negative stiffness regulators exist. Quasi zero stiffness isolator is composed of the positive stiffness elastic element and the negative stiffness regulating mechanism in parallel. The positive stiffness elastic element is used to bear the main load. The negative stiffness regulating mechanism is used to counteract the stiffness of the positive stiffness elastic element and increase the likelihood of leading the stiffness of the system at the static equilibrium position to zero. The quasi zero stiffness vibration isolator has high static stiffness and low dynamic stiffness. High static stiffness can improve the bearing capacity of the isolator and reduce static displacement, whereas low dynamic stiffness can reduce the natural frequency of the system and obtain a wider isolation frequency band than the linear isolator.

(2) Nonlinear vibration isolation system

Advanced nonlinear isolation methods based on nonlinear damper and nonlinear stiffness technology have excellent isolation performance and high reliability. For example, vibration isolation, which can be used in low frequency and/or wide frequency ranges, has a significant application potential in the ultra-low frequency vibration isolation field. For designing a traditional linear passive vibration isolation system, a contradiction exists between the damping ratio and the high frequency attenuation capability. This contradiction can be solved through nonlinear damping technology. The design that uses nonlinear damping model to limit the amplification of resonance and improve the attenuation ability of high frequency vibration has become the focus of current research. This nonlinear damping vibration isolation system can provide a large damp in the case of slight amplitude vibration. This system not only can effectively control the resonance amplification factor at the resonance peak but also provide improved vibration isolation effect in the high frequency range. The vibration isolation performance of nonlinear damping is superior to that of linear damping in the wide frequency range. Combining nonlinear stiffness vibration isolation system (e.g., HSDLS vibration isolation system) may play an important role in the design of ultra-low frequency vibration isolator with high performance.

(3) Active/semi-active vibration isolation system

Active/semi-active vibration isolation technology can be used to effectively inhibit the micro vibration of spatial structure and ultra-precision instruments; this technology particularly solves the vibration isolation problem in the low frequency range and is characterized by low energy consumption and cost. In active vibration isolation, actuators and sensors are commonly used to provide control force and feedback signals, respectively. Active feedback of control can be used to regulate the stiffness and damping of the system. This mechanism can isolate vibration in the low frequency domain. Compared with passive vibration isolation systems, active vibration isolation systems are more advantageous in terms of the vibration isolation in the low frequency domain and resonance region and have a wide application prospect in the vibration isolation design of ultra-precision instruments. However, if micro vibration isolation in the low frequency domain is completely achieved using active vibration isolation technology, then the reliability of the system may be reduced. Semi-active vibration isolation technology, which combines the advantages of passive vibration isolation and active control, can demonstrate improved vibration isolation performance than the passive or active vibration isolation system. The technology can be effectively used to isolate the micro vibration of orbital spacecrafts in narrow-band, wide-band, and low frequency range. This technology is an effective method for suppressing micro vibration in space engineering.

Countries or regions with top three publication numbers of core papers in engineering research focusing on “Vibration isolation system” are China (26), England (9), Korea (2), Brazil (2), and the USA (2). Countries or regions with top three average citation frequency are Korea (38.50), Brazil (36.50), and India (33.00) (Table 1.2.5). Among countries/ regions with top three publication number, China and England exhibit high cooperation (Figure 1.2.3). Institutions with top two publication number of core papers are Hong Kong Polytech Univ (6), Univ Southampton (5), Natl Univ Def Technol (5), and Tongji Univ (5). Institutions with top three average citation frequency are Univ Bristol (39.50), Univ Ulsan (38.50), and UNESP Ilha Solteira (36.50) (Table 1.2.6). Among the institutions with top ten publication number, Hong Kong Polytech Univ–Tongji Univ and Beihang Univ–Sci Technol Inertial Lab exhibit high cooperation (Figure 1.2.4). In 2011–2016, 26 core papers are related to engineering research focusing on “Vibration isolation system” published in China. Institutions with many published papers in Mainland China cover the National University of Defense Technology, Tongji University, Shanghai Jiao Tong University, Beijing University of Aeronautics and Astronautics, and so on. Countries or regions with top three publication number of core papers cited by core papers are China (15), England (2), Iran (1), and the USA (1) (Table 1.2.7). The main institutions producing core papers cited by core papers are Tongji Univ (4), Hong Kong Polytech Univ(4), and Beihang Univ (3) (Table 1.2.8).

1.2.3 Supersonic flow and aerodynamics

Supersonic flow plays an important role in the design of supersonic civil and military airplanes and is affected by a number of key technical issues related to supersonic aerodynamics. This paper aims to achieve the following: explore the type of boundary transition and control measures under supersonic flow (mainly the first mode unstable wave and transverse mode wave), determine the effect of shock wave/boundary layer interference on aerodynamic layout design, clarify the perturbation modes of supersonic flow and the evolutionary characteristics of the interaction of shock waves, understand the structural characteristics of coherent vortex in supersonic turbulent boundary layer and the generation and propagation

《Table 1.2.5》

Table 1.2.5 Main producing countries or regions of core papers on the engineering research focus “Vibration isolation system”

《Table 1.2.6》

Table 1.2.6 Main producing institutions of core papers on the engineering research focus “Vibration isolation system”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network of the major producing countries or regions of core patents on the engineering development focus “Vibration isolation system”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network of the major producing institutions of core patents on the engineering development focus “Vibration isolation system”

《Table 1.2.7》

Table 1.2.7 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Vibration isolation system”

《Table 1.2.8》

Table 1.2.8 Main producing institutions of core papers that are cited by core papers on the engineering research focus “Vibration isolation system”

mechanism of aerodynamic noise, investigate the new principle and technology of temperature reduction and noise reduction, and achieve advanced aerodynamic layout design for supersonic jets.

(1) Characteristics of translot jet-flow field

Translot jet flow refers to that the jet-flow direction is parallel to the main direction of movement; a condensing agent is also used in jet flow to reduce the temperature of object surface and change the structure of the flow field. Translot jet flow plays an important role in the overall thermal protection of aerocraft and internal cooling of detached engines. Unsteady aerodynamic problems (e.g., flow separation and reattachment under supersonic flow) are prone to occur due to the effect of shock wave interference, supersonic shear layer, and turbulence. Moreover, the relative velocity between the jet flow field and the supersonic main flow leads to a complex vortex structure in the shear layer, thereby producing strong acoustic radiation. Hence, scholars must explore new theories and mechanisms, conduct advanced experimental technology research, and develop efficient numerical simulation methods. This paper emphasizes the interaction mechanism between the jet field and the main flow field; systematically understands the effect of the comprehensive coupling of shock wave, expansion wave, shear layer, and boundary layer on the mixing characteristics of jetflow medium and mainstream moving medium; and determines the relationship between shock wave effect, flow separation, and peak thermal flow.

(2) Characteristics of lateral jet-flow field

If Ma > 3.0, significantly increasing the propulsion thermal efficiency of gas turbine through isobaric combustion and turbofan engine is difficult. Ramjet engine is pressurized by geometry changes to meet the pressure demand in the combustion chamber. The simple structure, light weight, and improved performance of this engine under high flight Mach number have gained increased research attention at home and abroad. In scramjet engines, the residence time of supersonic flow is extremely short. Achieving a rapid mixture between fuel and air to promote highly efficient and stable combustion in the chamber while minimizing the total pressure loss as much as possible is the most fundamental technical challenge in the design of super-combustion engine. Lateral jet flow is a common fuel injection method that exhibits deep penetration and good mixing efficiency. Study on the characteristics of lateral jet-flow field not only can optimize the mixing process of fuel but also reveal the physical mechanism of combustion stability of scramjet for early application of scramjet technology and engine scramjet in the engineering field.

Countries or regions with top three publication numbers of core papers on the engineering research focus “Supersonic flow and aerodynamics” are China (25), England (8) , t h e USA ( 5) , and Iran ( 5) . Countries or regions with top three average citation frequency are England (21.38), Egypt (19) , a n d Turkey ( 16) ( Table 1.2.9). Among the countries or regions with top 10 publication number, China and England exhibit high cooperation (Figure 1.2.5). Institutions with top three publication number of core papers are Natl Univ Def Technol (21), Univ Leeds (6), and Univ Michigan (5). Institutions with top three average citation frequency are Univ Leeds (22.67), Mil Tech Col (19), and Univ Sheffield (19) (Table 1.2.10). Among top ten institutions, Natl Univ Def Technol and Univ Leeds exhibit high cooperation (Figure 1.2.6). In 2011–2016, 25 core papers related to engineering studies focusing on “Supersonic flow and aerodynamics”. The institutions with other publication number are National University of Defense Technology and Harbin Institute of Technology.

《Table 1.2.9》

Table 1.2.9 Major producing institutions of core papers on the engineering research focus “Supersonic flow and aerodynamics”

Countries or regions with top three publication number of core papers cited by core papers are China (19), Iran (4) , a n d E n g l a n d ( 3) ( Table 1.2.11). The main institutions producing core papers cited by core papers are Natl Univ Def Technol (18), Babol Univ Technol (3), and Univ Leeds (3) (Table 1.2.12).

《2 Engineering development hotspots and engineering development focus》

2 Engineering development hotspots and engineering development focus

《2.1 Development trends of engineering development hotspots》

2.1 Development trends of engineering development hotspots

The top 8 engineering development hotspots in the field of mechanical and vehicle engineering include mechanical engineering, naval architecture and ocean engineering, aerospace science and technology, armament science and technology, power and electrical equipment engineering and technology, transportation engineering, and other subjects (Table 2.1.1). Among which, “Sensor technology,” “Advanced semiconductor technology and equipment,” “Power system of electric vehicles,” “New propulsion systems,” “Imaging technology,” “Design and manufacturing of high-efficiency, and low-emission engines” are further development of traditional research; “Surgical robot and key functional components,” and “Unmanned vehicles” are emerging hotspots. All core patents involved in these hotspots in 2011–2016 are disclosed in Table 2.1.2.

《Table 1.2.10》

Table 1.2.10 Major producing institutions of core papers on the engineering research focus “Supersonic flow and aerodynamics”

《Figure 1.2.5》

Figure 1.2.5 Collaboration network of the major producing countries or regions of core papers on the engineering development focus “Supersonic flow and aerodynamics”

《Figure 1.2.6 》

Figure 1.2.6 Collaboration network of the major producing institutions of core papers on the engineering development focus “Supersonic flow and aerodynamics”

《Table 1.2.11》

Table 1.2.11 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Supersonic flow and aerodynamics”

《Table 1.2.12》

Table 1.2.12 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Supersonic flow and aerodynamics”

《Table 2.1.1 》

Table 2.1.1 Top 8 engineering development hotspots in mechanical and vehicle engineering

(1) Sensor technology

A sensor is a device that converts acquired information into another form in accordance with certain laws of specific phenomenon in physics, chemistry, or biology. Sensor technology has been increasingly used in aerospace, transportation, machinery manufacturing, petrochemical, technical supervision and testing, and other technical fields. The focuses of sensor technology development include the

《Table 2.1.2》

Table 2.1.2 Annual number of core patents belonging to each of the Top 8 engineering development hotspots in mechanical and vehicle engineering