Study Of High Resolution Numerical Method For Helicopter Rotor In Vortex Ring State

The Vortex Ring State(VRS)is a special flight state of helicopter.When entering this state,the main/ tail rotors consume the engine power but cannot effectively generate the thrust and operating moment,which induces the so-called ‘settling with power' phenomenon that may result in helicopter crash.In order to further understand the influence of flow field characteristics on rotor aerodynamic performance in the vortex ring state,it is very important to accurately simulate the evolution process of flow features in this state.The vortex ring state flow field is mainly featured by the evolution of multi-scale vortices.Therefore,improving the spatial high resolution of the vortex flow field is the key to accurately simulating the vortex ring state flow field.In order to study the evolution mechanism of the vortex ring state flow field,aiming at the difficulties in the numerical simulation,a set of high efficiency,high resolution,dynamic overlapping grid with adaptive mesh refinement,and corresponding high fidelity complex vortex system capture and evolution analysis method is developped in this paper.Based on this method,the unsteady evolution of vortex system in hover and axial descent conditions are numerically studied.The generation and development,as well as the mechanism of asymmetrical failure evolution of vortex ring are analyzed emphatically.From the aspect of fluid physics,the fundamental mechanism of helicopter rotor disc oscillation / overturning is revealed,which verifies the accuracy and efficiency of the developed method to simulate such a complex flow field.This work provides a solid technical foundation for further study the mechanism of the VRS and thus for the helicopter flight control anysis.The contribution of this paper is mainly focused on the following aspects:1,In the rotor flow field computational method,(?)a high accuracy unsteady vortex flow field calculation method was developed.In the framework of multi-block structured grids,the fifth-order WENO-sixth-order central dissipation adaptive hybrid reconstruction scheme,Arbitary Lagrangian-Euler method,and low Mach number preconditioning technology are integrated based on the non-inertial coordinate system,which achieves high resolution DDES simulation for the rotor flow field.(?)block-structured grid technology with spatial resolution adaptive was developped.Aiming at the multi-scale problem of vortex structures in rotor flow field,Adaptive Mesh Refinement(AMR)based on octree data structure is introduced and non-dimensional Q criterion is adopted to adaptively adjust the grid spatial resolution,achieving high-resolution capture of multi-scale vortex evolution in rotor flow field.(?)In the context of parallel computing,a fully automatic overset grid assembly method based on implicit hole cutting algorithm was developed.Aiming at the dynamic overlap problem of multi-block structured body-fitted grid and AMR Cartesian background grids,geometric collision detection technology is introduced to filter the overlapping background grids to minimize the number of overset-related grid blocks and improve the assembly efficiency and operation robustness.The MPI multi-communicator technique is used for coupling the background grid solver and the near-body grid solver to achieve the overall function of the code while maintaining the independence of these two solvers.And also,an effective method to realize the load balance among the modules is presented.2,In the simulation and analysis of rotor flow field,(?)the evolution characteristics of complex vortex system in hover flow field were analyzed.The hovering flow field of the two-bladed Caradonna-Tung rigid rotor is used for verifying the compliance of the algorithm,and the Q criterion is used to show the flow details of the complex vortex evolution in the flow field.The analysis of the vortex core features validates Scully Vortex model and the Squire model with the computed results,which further confirms the validity and reliability of classical vortex core models in traditional wake computing technology(fixed,prescribed and free wake model).The dominant modes of the vortex space distribution and the evolution characteristics of the vortex system in the axial section are obtained using proper orthogonal decomposition and the lagrange coherent structure method.The rolling up of the trailing edge vortex sheet has an important influence on the vertical velocities of the tip vortex.The non-linear phenomena such as the vortex pairing,co-rotation and leapfrog of the adjacent tip vortex in the far wake region lead to the fragmentation and turbulence of the wake structure.(?)The mechanism of asymmetric breaking of the vortex ring in the axial descent flow field was studied.The numerical simulation of the rotor flow field with different axial descending velocity is carried out.The behavior of the vortex system in the flow field is analyzed using the vortex ring evolution theory.It is found that the circulation of the vortex ring has the growth limitation and accompanied by a secondary growth,which is caused by leapfrog and root vortex ring break down.The radial and axial instabilities of the vortex ring lead to two kinds of asymmetric breaking modes,which are the main causes of the danger of overturning of the rotor disk.By analyzing the effective angle-of-attack distribution in the disk plane at large descent speed,it is found that the periodic radial tensile crushing of the rotor root vortex ring causes significant oscillations in the blade load,and the rotor disk is extremely prone to rotate along the direction of vortex tube stretching to cause overturning.