This paper investigates the evolution properties of the flow field structure for axial descent rotor based on the unsteady momentum source method combined with the Computational Fluid Dynamics(CFD).The study focuses o...This paper investigates the evolution properties of the flow field structure for axial descent rotor based on the unsteady momentum source method combined with the Computational Fluid Dynamics(CFD).The study focuses on analyzing the speed critical value during the onset of or exit from the Vortex Ring State(VRS),and its changing characteristics of the flow field structure.Results from the Lagrangian Coherent Structure(LCS)analysis reveal the switch particularity between the tubular slipstream and the annular vortex,suggesting the criticality change character on flow field structure which occurs in the evolution process of VRS.These criticality features,that is the VRS boundaries described by the flow structure change,as determined by this method,are consistent with the existing theoretical and experimental results,verifying the feasibility of the established analysis method in the study of such boundaries.This work provides a robust framework for studying VRS boundaries and offers insights for further research on the underlying flow mechanism.展开更多
Single-rotor helicopters under certain conditions can enter the unanticipated yaw rotation,which often leads to the aviation accident.However,the causes of this flight mode have not been definitely determined yet.Typi...Single-rotor helicopters under certain conditions can enter the unanticipated yaw rotation,which often leads to the aviation accident.However,the causes of this flight mode have not been definitely determined yet.Typically,the main researcher’s proposal is that unanticipated yaw occurs,because helicopter tail rotor enters the vortex ring state.Authors of this paper decided to study the vortex ring state from the point of propeller thrust variation while being airflow blown.Experiments were carried out with a propeller in a wind tunnel,in which they varied the airflow velocity and angle between propeller axis of rotation and airflow velocity vector to imitate tail rotor side-blowing,while varying helicopter sideslip angle.Experiments results showed that at the vortex ring state there was a tail rotor thrust drop but it was not significant and thrust restored when the angle between propeller axis of rotation and airflow velocity vector exceeded 40°.But when propeller was axial blown in the direction against propeller thrust vector with airflow velocity 10 m/s the propeller thrust dropped by 80%relative to the thrust without blowing,which may lead to the occurrence of unanticipated yaw rotation.This suggests that helicopter wind blowing against tail rotor thrust vector is more dangerous than blowing in the opposite direction which leads to the vortex ring state.展开更多
To understand the vortex-ring state and to develop an approach for predicting its boundary, a series of model rotor tests of vertical descent and oblique descent have been conducted on a newly-built test apparatus - t...To understand the vortex-ring state and to develop an approach for predicting its boundary, a series of model rotor tests of vertical descent and oblique descent have been conducted on a newly-built test apparatus - the Whirling Beam. The test results showed some unsteady aerodynamic behavior of the model rotor operating in the vortex-ring state. A very irregular variation of the rotortorque at low rate-of-descent was observed here for the first time. We considered it to be the start of the 'power settling' and determined the critical descent velocity according to this observation. A previous criterion for the vortex-ring state was modified to give a semi-empirical method for predicting the entire vortex-ring state boundary. The computed boundary shows a good correlation with the model test results and the flight experiences.展开更多
A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of...A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of bump height on the vortical flow phenomena and the underlying physical mechanisms are inves- tigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The circu- lation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Furthermore, the mechanism of flow transition from laminar to turbulent state has been revealed based on analysis of turbulent kinetic energy.展开更多
A high-resolution simulation tool for rotorcraft aerodynamics is developed by coupling CFD with a Vorticity Transport Model（VTM）. An Eulerian-based CFD module is used to model the blade near body flowfield, and a La...A high-resolution simulation tool for rotorcraft aerodynamics is developed by coupling CFD with a Vorticity Transport Model（VTM）. An Eulerian-based CFD module is used to model the blade near body flowfield, and a Lagrangian-based VTM module is employed for vortex tracking in the far wake. The coupling procedure is implemented by transmitting vortex sources to the VTM module and feeding boundary conditions back to the CFD module. The presented CFD/VTM hybrid solver is firstly validated by hover cases of three different rotor configurations. Simulation results, including the blade surface pressure distribution, rotor downwash, and hover figure of merit, exhibit favorable correlations with available experimental data. Then, a rotor operated in vertical descending flight with a fixed collective pitch is investigated. It is shown that the CFD/VTM coupling method is suitable for rotor wake simulation. Wake instabilities（far wake breakdown in hover and toroidal wake pattern in the vortex ring state） are successfully demonstrated with a moderate computational cost.展开更多
The charge distribution in thin mesoscopic superconducting ring is studied by the phenomenological GinzburgLandau theory. In the giant vortex states we find that the mesoscopic rings may present three kinds of charge ...The charge distribution in thin mesoscopic superconducting ring is studied by the phenomenological GinzburgLandau theory. In the giant vortex states we find that the mesoscopic rings may present three kinds of charge distribution while the disk only owns the first two kinds. The charge near the inner radius may change its sign from negative to positive with increasing applied field. In the multivortex state we find that there exist saddle-point states and stable multivortex states. The distribution of charge and the superconducting electron density in the （0：2） saddle states and the （0：4）, and （1：5） stable multivortex states has also been studied. The contour plot of the charge distribution and the Cooper pair density distribution are given.展开更多
The original free vortex wake model was used for numerical investigation.Calculation of the aerodynamic characteristics in hover and vertical descent modes in the range of vertical descent speed of 0–30 m/s including...The original free vortex wake model was used for numerical investigation.Calculation of the aerodynamic characteristics in hover and vertical descent modes in the range of vertical descent speed of 0–30 m/s including the Vortex Ring State(VRS)area was performed.The calculations were carried out under the condition of variable blade pitch angle values providing a fixed timeaverage thrust value.Visualization data of free vortex wake shapes,flow structures,and velocity fields were obtained and analyzed.The time-dependences of the rotor’s thrust and torque coefficients were obtained and analyzed.The obtained data allows determining the boundaries of the VRS area by various criteria such as rotor thrust and torque pulsations,growth of rotor power consumption relative to the hover,growth of rotor induced velocities relative to the hover,and growth of the required rotor blade pitch angles values.The results of the study are compared with experimental and calculated data of other authors and can significantly supplement the available results of experimental and computational studies in this area.展开更多
基金funded by“the Fundamental Research Funds for the Central Universities”(Grant No.24X010202133).
文摘This paper investigates the evolution properties of the flow field structure for axial descent rotor based on the unsteady momentum source method combined with the Computational Fluid Dynamics(CFD).The study focuses on analyzing the speed critical value during the onset of or exit from the Vortex Ring State(VRS),and its changing characteristics of the flow field structure.Results from the Lagrangian Coherent Structure(LCS)analysis reveal the switch particularity between the tubular slipstream and the annular vortex,suggesting the criticality change character on flow field structure which occurs in the evolution process of VRS.These criticality features,that is the VRS boundaries described by the flow structure change,as determined by this method,are consistent with the existing theoretical and experimental results,verifying the feasibility of the established analysis method in the study of such boundaries.This work provides a robust framework for studying VRS boundaries and offers insights for further research on the underlying flow mechanism.
文摘Single-rotor helicopters under certain conditions can enter the unanticipated yaw rotation,which often leads to the aviation accident.However,the causes of this flight mode have not been definitely determined yet.Typically,the main researcher’s proposal is that unanticipated yaw occurs,because helicopter tail rotor enters the vortex ring state.Authors of this paper decided to study the vortex ring state from the point of propeller thrust variation while being airflow blown.Experiments were carried out with a propeller in a wind tunnel,in which they varied the airflow velocity and angle between propeller axis of rotation and airflow velocity vector to imitate tail rotor side-blowing,while varying helicopter sideslip angle.Experiments results showed that at the vortex ring state there was a tail rotor thrust drop but it was not significant and thrust restored when the angle between propeller axis of rotation and airflow velocity vector exceeded 40°.But when propeller was axial blown in the direction against propeller thrust vector with airflow velocity 10 m/s the propeller thrust dropped by 80%relative to the thrust without blowing,which may lead to the occurrence of unanticipated yaw rotation.This suggests that helicopter wind blowing against tail rotor thrust vector is more dangerous than blowing in the opposite direction which leads to the vortex ring state.
文摘To understand the vortex-ring state and to develop an approach for predicting its boundary, a series of model rotor tests of vertical descent and oblique descent have been conducted on a newly-built test apparatus - the Whirling Beam. The test results showed some unsteady aerodynamic behavior of the model rotor operating in the vortex-ring state. A very irregular variation of the rotortorque at low rate-of-descent was observed here for the first time. We considered it to be the start of the 'power settling' and determined the critical descent velocity according to this observation. A previous criterion for the vortex-ring state was modified to give a semi-empirical method for predicting the entire vortex-ring state boundary. The computed boundary shows a good correlation with the model test results and the flight experiences.
基金supported by the National Natural Science Foundation of China(11202100)the Natural Science Fund in Jiangsu Province(BK2011723)
文摘A vortex ring impinging on a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re = 4 × 104 based on the initial translation speed and diameter of the vortex ring. The effects of bump height on the vortical flow phenomena and the underlying physical mechanisms are inves- tigated. Based on the analysis of the evolution of vortical structures, two typical kinds of vortical structures, i.e., the wrapping vortices and the hair-pin vortices, are identified and play an important role in the flow state evolution. The circu- lation of the primary vortex ring reasonably elucidates some typical phases of flow evolution. Furthermore, the mechanism of flow transition from laminar to turbulent state has been revealed based on analysis of turbulent kinetic energy.
基金co-supported by the Funding of Jiangsu Innovation Program for Graduate Education of China(No.KYLX16_0389)the Fundamental Research Funds for the Central Universities of China
文摘A high-resolution simulation tool for rotorcraft aerodynamics is developed by coupling CFD with a Vorticity Transport Model（VTM）. An Eulerian-based CFD module is used to model the blade near body flowfield, and a Lagrangian-based VTM module is employed for vortex tracking in the far wake. The coupling procedure is implemented by transmitting vortex sources to the VTM module and feeding boundary conditions back to the CFD module. The presented CFD/VTM hybrid solver is firstly validated by hover cases of three different rotor configurations. Simulation results, including the blade surface pressure distribution, rotor downwash, and hover figure of merit, exhibit favorable correlations with available experimental data. Then, a rotor operated in vertical descending flight with a fixed collective pitch is investigated. It is shown that the CFD/VTM coupling method is suitable for rotor wake simulation. Wake instabilities（far wake breakdown in hover and toroidal wake pattern in the vortex ring state） are successfully demonstrated with a moderate computational cost.
基金supported by the National Natural Science Foundation of China （Grant No.60371033）
文摘The charge distribution in thin mesoscopic superconducting ring is studied by the phenomenological GinzburgLandau theory. In the giant vortex states we find that the mesoscopic rings may present three kinds of charge distribution while the disk only owns the first two kinds. The charge near the inner radius may change its sign from negative to positive with increasing applied field. In the multivortex state we find that there exist saddle-point states and stable multivortex states. The distribution of charge and the superconducting electron density in the （0：2） saddle states and the （0：4）, and （1：5） stable multivortex states has also been studied. The contour plot of the charge distribution and the Cooper pair density distribution are given.
文摘The original free vortex wake model was used for numerical investigation.Calculation of the aerodynamic characteristics in hover and vertical descent modes in the range of vertical descent speed of 0–30 m/s including the Vortex Ring State(VRS)area was performed.The calculations were carried out under the condition of variable blade pitch angle values providing a fixed timeaverage thrust value.Visualization data of free vortex wake shapes,flow structures,and velocity fields were obtained and analyzed.The time-dependences of the rotor’s thrust and torque coefficients were obtained and analyzed.The obtained data allows determining the boundaries of the VRS area by various criteria such as rotor thrust and torque pulsations,growth of rotor power consumption relative to the hover,growth of rotor induced velocities relative to the hover,and growth of the required rotor blade pitch angles values.The results of the study are compared with experimental and calculated data of other authors and can significantly supplement the available results of experimental and computational studies in this area.
