Computational Fluid Dynamics (CFD) simulation software is the digital expression of fluid-related mathematical physical knowledge and engineering practice experience, significantly boosting the digital transformation of industry. However, the development of large-scale industrial CFD software is extremely difficult, requiring consideration of factors such as the characteristics of diverse functions, stable systems, superior performance, and friendly interaction. Relying on the National Numerical Windtunnel (NNW) project, we develop a general flow field simulation software, NNW-FlowStar, which has been applied in aviation, aerospace and other industries. The software is developed based on the unstructured finite volume method and large-scale parallel computing technology, combining modern software engineering thinking. With advanced numerical methods, good calculation efficiency and user-friendly interface, it can conduct aerodynamic numerical simulations of various complex shapes.
In this paper, we review the recent development in our research group on the construction of high order finite volume schemes on unstructured grids with compact stencils. The second order finite volume schemes have been applied extensively in commercial and in-house Computational Fluid Dynamics (CFD) software. When one tries to further increase the order of accuracy of the finite volume schemes, the large reconstruction stencil will be an inevitably encountered problem and is considered as the main technical bottleneck of the high order finite volume schemes. In recent years, the high order reconstruction algorithms on the compact stencils have been systematically studied in our group.
With the fast development of CFD techniques in recent years, CFD numerical simulation software has been widely used in the development of aircrafts, and plays an increasingly important role in most stages of aerospace and other fields. The National Numerical Windtunnel (NNW) Project adheres to the principle of "using while building". By integrating the software of structural mechanics, flight mechanics, engineering thermodynamics, acoustics, optics, electromagnetics, multiphase fluid mechanics and other disciplines, the project is playing an active role in aerospace, transportation, energy and power, environmental protection and disaster mitigation, and other disciplines.
Compared with traditional structured and unstructured grids, the Cartesian grid has the advantages of automation and high quality, and therefore is an important development direction of grid technology in the future. Relying on the National Numerical Windtunnel (NNW) Project basic research system, this paper studies viscous adaptive Cartesian grid methodology, focusing on the grid generation technology, adaptation method, and viscous wall boundary treatment method, for the development of Cartesian mesh generation software. For the grid generation technology, optimization of the quadtree or octree data structure is conducted, starting from optimization of the Cartesian grid data structure and based on the idea of the fully thread tree data structure; a more stable K-dimensional tree method compared to the traditional one is constructed aiming at the information search of facets. For the determination of grid types
The Spatial Artificial Neural Network (SANN) model is applied to perform Large Eddy Simulations (LES) of highly compressible turbulence at high turbulent Mach numbers of 0.6, 0.8 and 1.0 under the National Numerical Windtunnel (NNW) Project. In our previous studies, we developed the SANN model for incompressible and weakly compressible turbulence based on multi-scale spatial structures of turbulence. However, generations of shock waves in highly compressible turbulence pose great challenges to LES. This paper discusses the applicability of the SANN models for LES of highly compressible turbulence. It has been demonstrated that the correlation coefficients of the SANN model can be larger than 0.995.
Prediction of the thermal environment in the cabin is essential for aircraft thermal control and heat protection design and optimization, and is also important to system redundancy reduction and thermal safety. Due to the influence of multi-scale effects, it is difficult to improve the computational efficiency and accuracy of existing prediction methods. With the support of the National Numerical Windtunnel (NNW) Project, the space-time coupling model for multi-zone cooperative advancement and the adaptive resolution recognition algorithm for the fluid/solid interface are improved.
Predictions of the heat transfer processes in composite materials are important for designs of thermal protection structures of hypersonic vehicles. The corresponding model is also an essential part of the multiphase-multicomponent sub-model of the National Numerical Windtunnel Project. In this work, a multiscale asymptotic analysis method is used to study the problem of coupled conduction-radiation heat transfer for high temperature composite materials with periodic structures. Both the conduction equation and radiative transfer equation are analyzed.
Covering more than 60% of the satellite surface, multi-layer thermal insulation is an important medium in restraining the sources of strong electromagnetic environment in space, as well as a necessary thermal control component. Compared with internal components, the satellite surface is directly impacted and acted upon by energetic particles, resulting in a serious electrostatic threat on orbit. The high energy electrons can easily penetrate the milli-meter thin film of the multi-layer insulation, deposit on the internal dielectric material of the insulation and finally form electric fields. According to the composite structure characteristics of multi-layer thermal insulation components, the reasonably
The compressive properties of Cf/Al composites with four different braided structures prepared by vacuum pressure infiltration method, i.e. 3D five direction, 3D orthogonal, laminated puncture and 2.5D shallow-straight joint woven, are tested at 350℃ and 400℃, respectively. The high-temperature compressive properties of Cf/Al composites with different braided structures and the influence of temperature on the compressive properties are discussed, the fracture morphology of laminated puncture structures is further observed by SEM and the compressive failure mechanism is analyzed. The results show different compressive properties of composites with different braided structures at high temperature.
hermal oxidation of titanium alloys can improve their overall properties by forming an oxide layer and an oxygen diffusion layer on the surface. Severe plastic deformation of titanium alloys before thermal oxidation can introduce crystal defects such as high density of grain boundaries, dislocations, twin boundaries and high distortion energy, which can promote the absorption of oxygen atom, reduce oxide nucleation temperature, accelerate the growth of oxide film, and promote the diffusion of oxygen atoms into the matrix. Thus, a denser and thicker oxide layer and a deeper oxygen diffusion zone can be formed to obtain better performance. In the study, thermal oxidation of severe plastic deformed titanium alloys is review.
Continuous innovations in the fundamental theories of Computational Fluid Dynamics (CFD) are of great importance for capability expansion and popularization of the numerical simulation software. In the National Numerical Windtunnel (NNW) Project, several key fundamental issues in CFD are studied. Through endeavors of the research teams during the past three years, the project has accomplished a series of meaningful achievements, and developed many original methods and models for transition and turbulence computation, multiphase and multi-medium flow computation, multi-physics field computation, and high-accuracy computation. This paper provides brief reviews on related research progress. The methods and models with high-maturity will be integrated into the software in the NNW Project and released to the whole country.
Wind-blown sand in the atmospheric surface layer is a typical two-phase flow characterized by the splashing process as sand particles impacting the erodible surface bed, multi-field coupling among turbulent wind, electrostatic field and charged sand particles, and multiscale and trans-scale problem. Therefore, accurate simulation and prediction of wind-blown sand has been very challenging. In this paper, we review the research progresses obtained in the past 30 years, including mainly simulation of multi-field coupling of wind-blown sand and scale-coupled model of aeolian geomorphology based on Reynolds-Averaged Navier-Stokes (RANS) equations, and simulation of sand movement based on Large Eddy Simulation (LES) and high Reynolds number wall turbulence. Shortcomings of existing numerical simulation methods for aeolian sand movement and the directions worthy of further research are also discussed.
With the current improvement in computing power and storage performance, the scale of flow data output is getting larger and larger, and the requirements for hardware and software algorithms for visualization applications of flow data have also increased. Supported by the National Numerical Windtunnel (NNW) Project, a high-performance flow parallel particle tracking data management system is developed to help users explore and analyze large-scale flow field data. The system provides a variety of efficient data management methods for flow data, and optimizes data prefetching and load balancing in the process of parallel particle tracing on supercomputer clusters. For the streamline (or pathline) and process work record data generated in the particle tracking process, the system supports users to conduct performance diagnosis and analysis on the local platform. Two cases using different flow field data sets verify the effectiveness of the system proposed.
To meet the need for hypersonic three-dimensional boundary layer transition prediction for the National Numerical Windtunnel (NNW) Project, we study the hypersonic cross-flow transition criterion and the transition model. The hypersonic database is expanded using the linear stability theory of the eN method, a fully-localized hypersonic cross-flow transition criterion constructed combining the cross flow strength and surface roughness, a cross-flow extension of the hypersonic modified γ-Reθ transition model conducted based on the Chant 2.0 computing platform, and a C-γ-Reθ transition model suitable for hypersonic 3D boundary layer transition prediction established. The transition model is used to predict the cross-flow transition on hypersonic sharp cones in multiple states, and the predicted results are in good accordance with the experimental results.
An infrared imaging technology is used to study the rule of fatigue heat dissipation evolution of a new Al-Li alloy AA2198 under a high-frequency fatigue test condition of 100 Hz. It is found that fatigue heat dissipation on specimens shows fluctuation characteristics under different stress conditions, with its average temperature inclined to increase with the rise of the loading stresses. However, the temperature rise on specimens during fatigue test is not obvious, and the amplitudes of temperature variation under different stress condition is generally smaller than 1℃. The beginning of fatigue test and the moment of fatigue fracture occurrence are accompanied by a rapid temperature rise. An energy conversion theory model is proposed to explain the evolution process of fatigue heat dissipation. Meanwhile, the residual compressive stress formed on specimen subsurface during the shot peening process helps to intensify the fatigue crack closure, inhibiting the temperature rise on specimens.
Stratospheric airships are the frontier research focus with strong innovativeness and huge technical challenges. Stratobus is the only on-going large long-endurance stratospheric airship project abroad. The basic scheme and overall progresses of Stratobus are first summarized, followed by generalization and analysis of the technical proposal and development status of the key technologies, including envelope materials, solar cells, regenerative fuel cells, and gondolas. Additionally, three important balance conditions determining the design feasibility of the stratospheric airships are analyzed using numerical simulation, involving balances between buoyancy and gravity; power production and power consumption; and thrust and drag. Some experiences and lessons from the Stratobus project are finally presented, providing reference for the design of and research on stratospheric airships.
Time-Limited Dispatch (TLD) can improve the reliability of aircraft dispatch and reduce operational losses caused by flight delays or cancellations. A Markov model of a typical engine control system is established for TLD analysis, a steady-state frequency formula is derived using a Continuous-Time Markov Chain (CTMC), and a system unit time cost model is constructed. Taking average safety requirements and dispatch reliability requirements as constraints, failure dispatch time as decision variables, and the system unit time cost and dispatch reliability as optimization goals, we establish the single and multiple goals of TLD analysis. An engineering application example is given to verify the effectiveness of the model by combining the simplified engine control system and the Full Authority Digital Electronic Control (FADEC) system. Examples show that the proposed method can reduce operating costs while ensuring the satisfaction of the safety and dispatch reliability requirements of the aircraft.