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Development of CFD technology and its application in the field of aviation
2018/9/27
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Luo Lei, Jiao Zhenxun, Jiang Chongwen
(School of Aeronautics and Sciences and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191)
[Abstract] The recent development of computational fluid mechanics (CFD) technology and its application status in the aviation field are reviewed. In terms of the development of CFD technology, the three aspects of calculation format, grid method, and turbulence simulation were reviewed, and the future development direction of CFD technology was prospected. In terms of the application of CFD technology, the current situation of key application areas such as aircraft appearance optimization, rotor/helicopter, non-constant flow, multi-body separation and air intake duct are introduced.
Computational Fluid Mechanics
Fluid Dynamics, CFD) has developed rapidly since the 1960s with the continuous advancement of computer technology. Now it has penetrated into all aspects of engineering fields including aviation, aerospace, ships, water conservancy, metallurgy, construction, chemicals and other engineering fields, and has made great achievements. The aviation field is the field of early application and development of CFD technology. In more than half a century, the aviation engineering community has formed a set of effective cFD technology application methods, fully and rationally utilized the advantages of cFD technology, and effectively shortened the cycle of technology research and development and model development. Under the rapid development of the aviation field today, CFD technology has shown great application value and development potential. This article aims to review the recent development of CFD technology, look forward to the future development direction of CFD technology, and introduce the current status of CFD technology application in the aviation field.
1
CFDTechnology development
With the deepening of CFD technology, CFD faces more and more difficulties. This article introduces the new development of CFD technology from the aspects of calculation format, grid method, turbulence simulation, etc.
In the CFD field, low-order formats are widely used in engineering practical calculations due to their robustness and reliability. Although low-order formats have achieved great success in complex flow numerical simulations of complex appearances, low-order formats have large numerical dissipation and dispersion. For complex problems, such as shock waves, turbulence, nonlinear effects and multi-scale problems, a relatively order format with small dissipation and dispersion must be adopted. The efficiency of the higher-order format to achieve the same accuracy than the lower-order format is also a major advantage in engineering applications. The more developed order formats in recent years include: finite difference order format, intermittent Galerkin finite element method, ENO/wENO finite volume method, finite spectrum difference method, finite spectrum volume method, and mixed DG/Fv method. Among them, the intermittent Galerkin finite element method and the corresponding mixed method have become a hot topic in the research of higher-order formats due to their superiority. Flux Reconstruction (FR) or CPR method proposed recently
Pmcedure usingReconstmction, CPR) unifies these orderly formats under the same framework, which has attracted widespread attention from researchers.
Generating computational grids with a higher quality is a prerequisite for CFD calculations and is one of the most important factors affecting the CFD calculation structure. The larger problem with grid is the large amount of manual work, which is one of the bottlenecks of CFD work efficiency. It is the goal of grid algorithm designers to try to simplify grid generation, reduce the manual workload in grid generation, and improve the adaptability of grids to complex appearance and motion boundary problems. The main areas currently developing in grids include automated grid technology, overlapping grid indica, Cartesian grid cases and adaptive grid encryption technology. In terms of automated grid generation technology, there are still many problems that have not been completely solved, such as further improvements in various grid generation algorithms, adaptive grid encryption algorithms, and parallel grid generation algorithms. In terms of overlapping mesh, the basic mesh assembly method must consider some complex issues, such as some special issues such as thin objects and non-closed appearances; the hole digging method that adapts to multi-scale complex appearances needs further research in the basic mesh assembly method; compared with the basic mesh assembly method, automatic mesh assembly method has better grid assembly efficiency and a better degree of automation, and is more suitable for numerical calculations of non-constant flow. The recently developed Cartesian grid technology has also shown great research potential due to its superiority in CFD calculations. In particular, the cutting unit method meets the global and local conservation laws. The focus of the research will be on the numerical flux algorithm at the fast cutting interface and solving the numerical oscillation problem caused by mutations in cutting units. Adaptive grid encryption technology uses grid encryption, that is, changing the number of nodes and unit size to achieve the purpose of improving the grid solution accuracy. It is mainly divided into two basic types: global grid encryption and local grid encryption. Compared with global grid encryption, local grid encryption can improve the solution accuracy without increasing too much computing, so it is widely used in solving complex problems such as complex appearance, chemical reaction flow, and faster flow. At present, the research mainly focuses on solving two key problems in adaptive mesh: determining the location of adaptive mesh encryption and determining the grid encryption algorithm.
In turbulence simulation, large eddy simulation (LES) has received a lot of research by industry scholars for its advantages in turbulence simulation in taking into account both computational volume and accuracy, and its technology is gradually maturing. Many recent research on LES has focused on improving the application performance of LEs in specific engineering fields, such as: simulating ultrasonic combustion, aerodynamic noise, and considering thermal radiation. In addition, although LES can significantly reduce the calculation amount compared to direct numerical modulus (DNS), its calculation amount is still relatively large for large-scale engineering applications, especially considering the flow of near-wall surfaces, so the calculation amount is still one of the difficulties restricting the further development and application of LES technology. The hybrid RANs/LES method developed to reduce the computational volume of LEs has recently become a hot topic in LES research. According to the literature, the specific methods of combining RANs and LEs are mainly distributed RANs/LEs method and coupled RANs/LES method.
Method). Distributed RANS, LES methods are divided into two methods. One is the integrated RANS-LES method (Mixed RANS-LES method).
Method), another is the non-fusion RANS-LEs method (Non-mixed RANS-LES method). Compared with the fusion RANS/LES method, the coupled RANS/LES method is more popular at present. Under this method, there are mainly two specific methods: the interface method and the region method (Segregated)
Method/zonal Method). The former uses the same velocity equation and couples stresses on the RANS-LES interface, while the latter uses different velocity equations in the RANS region and the LES region, and couples all flow parameters in the transition region.
CFD technology still has shortcomings in its ability to deal with and solve engineering problems. These shortcomings require that future CFD technologies have better computing efficiency, speed and accuracy. Therefore, this paper believes that the future development directions of CFD technology are mainly: (1) developing a more order format and improving the accuracy of calculation while making efficiency; (2) developing grid technology to simplify grid generation, reduce the manual workload in grid generation, and improve the adaptability of grids to complex appearance and motion boundary problems; (3) in turbulence simulation, propose faster computing models and physical models; (4) developing fast large-scale parallel computing methods, such as parallel computing methods based on graphics processors (GPUs).
2
CFDCurrent status of technology application in aviation
CFD technology has been widely and deeply applied to all aspects of the aviation field, greatly promoting the development of aviation science. The following will introduce the current application status of CFD technology in the aviation field in terms of aircraft appearance optimization, rotor/helicopter, non-constant flow, multi-body separation and air intake duct.
2.1Optimization of the shape of the walker
With the continuous development of CFD technology and computer technology, the design method of using CFD for aircraft appearance optimization has gradually evolved from simplicity to maturity, which is mainly reflected in two aspects: (1) After long-term research and development, the rapidity and accuracy of numerical simulation have been gradually improved. Not only has the flow control equations adopted by the CFD method experienced the development from non-stick to sticky, from linear to nonlinear, and the scope of application has gradually expanded, but the CFD code is becoming increasingly perfect in continuous engineering practice, and a large number of experiments have verified its trustworthiness; (2) The optimization design method has gone from early trial-compilation methods and orthogonal test methods to now the development of optimization algorithms based on cybernetics and search algorithms. The artificial intervention in the optimization design process has decreased, and the possibility of obtaining better results has been improved.
The use of CFD to optimize the appearance design of aircraft generally follows the following steps: (1) Parameterize the appearance of the aircraft to select appropriate design variables; (2) Determine the goal that needs to be optimized; (3) Use certain optimization methods to obtain the modification direction of the appearance and make corresponding improvements to the appearance; (4) Iterate the previous step until the design requirements are met. CFD provides the pneumatic parameters required for the optimization method in this process.
In recent years, aircraft appearance optimization methods for drag reduction are one of the research hotspots, and many researches on numerical optimization designs of CFD models based on Euler equation/N-S equation have emerged. In terms of full aircraft optimization, Lyu and Martins used the Euler equation solver, RANS equation solver (S-A turbulence model) and discrete accompanying variable method to realize the drag reduction appearance optimization design of the wing body fusion aircraft (Figure 1); Gagnon et al. used the Euler equation solver combined with the discrete accompanying variable method to combine several non-traditional layouts: wing body combinatorial layout, c-shaped wing body fusion (BwB) layout, box-wing layout and support wing (strut-brace) layout, and concluded that the support wing layout (Figure 1) has good drag reduction potential; in terms of airfoil drag reduction optimization, Poole et al. used viscous flow (Eu) The ler) solver uses a global search algorithm (gravity search) and a gradient algorithm to optimize the drag reduction of the two airfoils, and obtains shockfree results in the optimization design of the global search algorithm; Koziel et al. used the response surface model to propose a multi-objective optimization design method combining relatively low-confidence CFD models (both solved by Euler equations, but different grid thicknesses). While improving the computing efficiency, it realizes the drag reduction and increase design of the transsonic airfoil; Chen et al. used the RANS equation solver (SA turbulence model) and the multi-objective genetic algorithm to optimize the drag reduction and increase of the airfoil, which increased the lift force and eliminated the shock wave, achieving the purpose of drag reduction.
2.2Unsteady flow
When the airplane's orbital flow is separated, the flow structure becomes very complex and the flow exhibits significant non-constant properties. The unsteady effect may lead to obvious asymmetry and instability in the aerodynamics of the wings, front fuselage and each control rudder surface, which strongly affects the aircraft's handling performance. Fully understanding of the non-constant effects of separate flow and mastering its laws will help achieve strong control and utilization of flow. With the improvement of computer computing capabilities and the development of numerical methods, especially the solution method of N-S equations, CFD has become a powerful tool for studying the problem of unsteady flow by aircraft.
In recent years, some progress has been made in the study of non-static simulation around the current using the CFD method of solving the non-static N-S equation. Luckring and others reviewed the current status of CFD prediction of aerodynamics of F-16XL fighter aircraft and briefly reviewed CAWAP (Cranked)
The research results achieved in the Arrow Wing Aerodynamics Project series plan in the past two decades. CAWAP's newer research uses the non-steady RANS (uRANS) method and the separation vortex simulation (DES) method to simulate the entire aircraft circumference flow under low-speed (Mach number 0.24) and large angle of attack (200) flight conditions, showing the flow structure of the circumference flow, and obtaining better aerodynamic prediction results than the regulating method. Clifton et al. introduced the numerical simulation study of the non-constant aerodynamics of the F-22 fighter when performing J-shaped turning maneuvers (Figure 2). The simulation adopts the URANS method, and the obtained aircraft aerodynamics and torque coefficient are well matched with the flight test and wind tunnel test data provided by LockheedMartin. Forsythe et al. introduced a numerical simulation study of sudden wing stall (Aws) patterns during transonic flight of F/A-18E fighter. The simulation adopts the DES method and studies the non-static effects of aircraft around the flow at several angles of attack (7°-12°). The calculated non-static aerodynamics are in line with the experimental data. The conclusion points out that shock wave oscillation and shock wave induced separation flow at the wing front edge sawtooth are the reasons for the sudden wing stall.
2.3Rotor value lift design
The flow field and aerodynamic performance of the rotor have an important impact on the performance, flight quality, noise and vibration characteristics of the helicopter. Using CFD method to accurately calculate the flow field and performance of the rotor is an important development direction of helicopter aerodynamics. In recent years, certain progress has been made in numerical simulation methods, optimization design and multidisciplinary comprehensive research of rotors.
In terms of numerical simulation methods, Li Chunhua and others established a tilt rotor free wake analysis method based on lift surface theory and rolled upward vortex model; Crozon and others conducted numerical simulation research on the rotor flow in the hull wake (Figure 3), adopted the uRANS method and DES method, and used the cross-sectional boundary conditions in the calculation, and the calculation results were better in line with the test; in terms of optimization design, Leusink and others adopted the RANS method (k -ω turbulence model) optimized the shape of the rotor blade to improve the aerodynamic performance of the rotor; in the comprehensive research of multidisciplinary studies, Marpu et al. proposed a fast predictor of the rotor force calculation method coupled with CFD and computational structural mechanics (CSD) to study the maneuverability characteristics of a certain type of helicopter; Wang Junyi et al. calculated the aerodynamic elastic load of the new type of paddle tip rotor based on the CFD/CsD coupling method, pointing out that the rear swept tip can effectively improve the aerodynamic performance of the rotor.
2.4Multibody separation
The more representative multi-body separation problems in the aviation field include: hood separation, built-in bomb cabin opening process, aircraft cabin cover separation, cockpit ejection, external object release, etc. The multi-body separation flow field under faster conditions exhibits relatively nonlinear and non-constant characteristics, and there are still many problems that need to be solved in multi-body separation. Some mesh technologies in CFD technology, such as overlapping mesh technology, dynamic mesh technology and nested mesh technology, are very suitable for studying multi-body separation problems. Yang et al. proposed an adaptive overlapping Cartesian/non-structured grid combined with a down-order model, and conducted numerical simulation research on multi-plug delivery; Sickles et al. proposed a CFD-based multi-body motion system that simulated the process of plug-in continuous delivery, and the calculation results were well matched with the flight test data; Berglind et al. compared the time-accurate CFD method and the accurate normal CFD method for plug-in delivery problems, and verified the effectiveness of the correct normal method; Finlev et al. used the overlapping grid method to simulate and study the separation of the secondary fuel tank, and the results were better matched with the flight test; Tao Ruyi et al. used the mixed grid generation method to carry out numerical simulation research on the interference aerodynamic characteristics of the time-accurate mother bomb; Li Peng used the dynamic grid method to simulate the flow field of the sub-fuel tank.
2.5Air inlet
The air intake duct is an important part of the aircraft propulsion system, and its aerodynamic performance has a great impact on the performance of the engine. In recent years, certain progress has been made in air intake duct design/optimization design, research on aerodynamic mechanism, and research on flow control methods. In terms of supersonic intake ducts, Zhong Yicheng and others used the CFD method to complete a design of a spacerless intake duct (DsI) based on the wave-folding principle, and studied its aerodynamic characteristics; Loth et al. studied the shock wave/boundary layer interference phenomenon of the intake duct, and obtained the Mach number range of this phenomenon; Troia et al. used numerical simulation methods to weaken the shock wave boundary layer interference with passive original flow control, achieving the goal of no overflow in the intake duct. In terms of subsonic intake channels, Trapp et al. used cFD method to perform numerical simulation of the DLR-F6 model and studied the inlet vortice effect when the aircraft was near the ground; HaU et al. used CFD method to optimize the inlet spiral of the turbofan engine to improve the total pressure recovery coefficient; Yi et al. used CFD method to optimize the vortice generator of the S-type intake channel, reducing the distortion coefficient by 80%.
3Conclusion
This paper reviews the recent developments of computational fluid mechanics (CFD) technology and its current application status in the aviation field. In terms of CFD technology development, this paper reviews three aspects: calculation format, grid method, and turbulence simulation, and believes that in the future, CFD should focus on development in relatively order formats, grid technology applicable to complex appearance and motion boundary problems, fast and fast turbulence physics models, and large-scale parallel computing methods. In terms of the application of CFD technology, the current situation in key application areas such as aircraft appearance optimization, rotor/helicopter, non-constant flow, multi-body separation and air intake duct were introduced, indicating that cFD technology has now penetrated into all aspects of aviation engineering and provided technical support and guarantee for the rapid development of the aviation field. With the rapid development of the aviation field, CFD technology will also usher in greater opportunities and challenges.
