Adaptation Techniques Of Unstructured Hybrid Mesh For Complicated Configurations Of Industrial Level

The numerical schemes,turbulence models and mesh are the main ingredients to affect the simulation precision of computational fluid dynamics(CFD).The numerical schemes and turbulence models,which are widely used for engineering,have been richly calibrated.So their regularities of influence on aerodynamics are quite clear and plenty of experiences are formed.However,because there is no universal standard for mesh generation,the quality of generated mesh relies on users' experience.Moreover,the distribution of mesh elements usually cannot be adequate for flow field,which makes the numerical result strongly dependent on computing mesh.Several meshes with progressive decrease in element's size are required for traditional analysis of mesh independence,which costs large amount of manpower and computer resources.In this study,the adaptation techniques of unstructured hybrid mesh coupled with characteristic of flow field are investigated,which aims to improve the simulation precision and reduce the dependence of numerical result on mesh.Based on the principles of engineering practicability,the methods and techniques are established for the system of mesh adaptation.The main research contents and innovations are listed as follows:(1)The error estimate methods of adaptation for mesh elements are investigated based on the flow field variables.In the first place,the flow simulation is the basis for error estimate based on flow features.So both the governing equations and turbulence equations in integral form,and numerical methods are briefly introduced.Next,two discontinuous type methods for error estimate are developed,including the quasi-gradient method and the extrapolation method.Then the shear-stress ratio method is adopted to identify the core zone of vortices,and the method based on characteristic lines is presented to capture shocks.(2)The mesh optimization techniques of adaptation for hybrid mesh are investigated,which are competent for real engineering applications.Both the robustness and practicability are taken into consideration when one method is developed or adopted to establish the adaptation system.Above all,the distribution optimization of mesh elements is fulfilled adequately through refinement and coarsening for all types of mesh elements.The isotropic subdivision of element is used for mesh refinement,and its inverse procedure is used for mesh coarsening based on the relationship between elements and their descendant elements.The unrefined element adjacent refined one is converted into a polyhedron to deal with the suspending nodes.Besides,that conversion method can release direction limitation of optimization for elements in boundary layer.Based on the multi-levels data structure and the object-oriented approach,the mesh adaptation system is divided to multi-modules and then constructed.In the second place,the real geometry is approximated with local Coons patches which depends only on surface mesh.The mesh points newly inserted are move to their object location determined by parameter mapping method in order to make the refined surface mesh consistent with real geometry.Next,the dynamic mesh based on distance weight is improved by introducing a variable influence distance and a bounding-boxes acceleration method.The interior mesh elements can be effectively and correctly deformed according to the projected surface mesh,avoiding the elements near a wall overlapping or their volume turning to negative.Finally,the interpolation method from base mesh to optimized mesh is proposed to accelerate the convergence of flow simulation after mesh adaptation.(3)The parallel algorithms are developed for all modules of mesh adaptation system.First of all,the parallel algorithms for the module of error estimate are investigated,which mainly contains mesh partitioning and data communication among different parallel processes.Then the parallel algorithms for the module of elements distribution optimization are proposed,including parallel marking algorithm for refinement or coarsening attributes of elements and parallel numbering algorithm for newly inserted mesh points or elements.Thirdly,the dynamic load balance algorithms for mesh elements are developed,which consists of parallel mesh repartitioning and parallel data transferring of mesh elements and points.Then the parallel algorithm for points projecting module of surface mesh and deformation module of volume elements are proposed,respectively.Lastly,the parallel algorithms are tested using a transport configuration and a delta-wing configuration.The test cases indicate that the parallel efficiency of mesh adaptation system is quite acceptable and the total cost of time is far less than that to finish the flow simulation.(4)The applicability tests for simulation of flow with separation vortex have been carried out with three standard cases: the unsteady flow around a tri-wedge,the low Reynolds number flow around a cylinder,the flow around the 30P30 N triple airfoil and the flow around the sharp leading delta-wing at large angle of attack.The error estimate methods for vertex identification have the capability to recognize the vortex core,and the mesh optimization techniques of adaptation can iteratively improve the distribution of the elements near the zone of vortex core.The convergence of simulation has been obviously improved with mesh adaptation.The flow details,such as the distribution of velocity and pressure,the separation or reattachment lines,are compared before and after mesh adaptation,which shows the simulation precision can be improved with mesh adaptation.(5)The applicability tests for simulation of flow with shocks have also been carried out with four standard cases,including the hypersonic flow around a cylinder,the type IV shock/shock interference flow,the hypersonic flow around a sphere and the interference flow of the double half spheres.The results of these cases indicate that the shock surface can be recognized by the error estimate methods and the elements near the shock surface can be refined repeatedly with the distribution optimization techniques.After mesh adaptations,the simulation convergence can be obviously improved,the shock resolution can be distinctly increased and fine shock structure can be obtained.The flow details,such as the distribution of pressure,velocity and heating flux,are compared before and after mesh adaptation,which shows the precision of shock simulation and aerodynamic force and heating can be improved with mesh adaptation.(6)Finally,the mesh adaptation have been applied to simulate some complicated flows of real industrial aircrafts.The first case is the low-speed flow simulation for the low-aspect flywing at large angles of attack.The stall angle and the aerodynamic moments are predicted more precisely with the separation vortices adaptation.The second case is the transonic flow simulation for a typical airliner.With mesh adaptation,the shock location above the wing is predicted well and the precision of aerodynamics are improved.The last case is the aerodynamic interference simulation between a supersonic fighter and a missile loaded below the wing of the fighter.The mesh independence for the fighter and missile are analyzed with uniformly refined mesh generated by the mesh adaptation system,respectively.The precision of interference flow are improved with mesh adaptation.Overall,these cases of industrial level indicate that the mesh adaptation methodology proposed in this thesis is reliable and efficient for real engineering applications.