Unsteady supersonic base flows around three afterbodies, cylindrical （Cy）, boattailed （BT） and three-step （MS）, are investigated in this paper. Reynolds-averaged Navier-Stokes （RANS） and two RANS/LES （large-e...Unsteady supersonic base flows around three afterbodies, cylindrical （Cy）, boattailed （BT） and three-step （MS）, are investigated in this paper. Reynolds-averaged Navier-Stokes （RANS） and two RANS/LES （large-eddy simulation） hybrid methods, detached eddy simulation （DES） and delayed-DES （DDES）, are used to predict the base flow characteristics around the baseline Cy afterbody. All the RANS and hybrid methods are based on the two-equation SST （shear-stress transport） model with compressible corrections （CC）. According to the comparison of measurements, both DES and DDES can produce more satisfactory results than RANS. RANS can only present the ＂stable＂ flow pat- terns, while the hybrid methods can demonstrate unsteady flow structures. DDES and DES results are little different from one another although the latter exhibits better agreement with the experiment. DES is taken to investigate the 5° BT and three-step afterbodies. The mean flow data and the instantaneous turbulent coherent structures are compared against available measurements.展开更多
This research examines the vortex behaviors and aerodynamic forces in dynamic stall phenomena at a transitional Reynolds number(Re=90000)using experimental and numerical approaches.Periodic sinusoidal pitching motion ...This research examines the vortex behaviors and aerodynamic forces in dynamic stall phenomena at a transitional Reynolds number(Re=90000)using experimental and numerical approaches.Periodic sinusoidal pitching motion at two different reduced frequencies is used to achieve the dynamic stall of a NACA 0012 airfoil.Several leading edge vortices form and detach in the dynamic stall stage.The flow then quickly transitions to a full separation zone in the stall stage when the angle of attack starts to decrease.There is discrepancy between the phaseaveraged and instantaneous flow field in that the small flow structures increased with angle of attack,which is a characteristic of the flow field at the transitional Reynolds number.The interaction between the streamwise vortices in the three-dimensional numerical results and the leading edge vortex are the main contribution to the turbulent flow.In addition,the leading edge vortex that supplies vortex lift is more stable at higher reduced frequency,which decreases the lift fluctuation in the dynamic stall stage.The leading edge vortex at higher reduced frequency is strong enough to stabilize the flow,even when the airfoil is in the down-stroke phase.展开更多
This paper deals with the asymptotic behavior of multistep Runge-Kutta methods for systems of delay differential equations (DDEs). With the help of K.J.in't Hout's analytic technique for the numerical stabilit...This paper deals with the asymptotic behavior of multistep Runge-Kutta methods for systems of delay differential equations (DDEs). With the help of K.J.in't Hout's analytic technique for the numerical stability of onestep Runge-Kutta methods, we obtain that a multistep Runge-Kutta method for DDEs is stable iff the corresponding methods for ODEs is A-stable under suitable interpolation conditions.展开更多
通过采用三维瞬态DDES数值方法模拟强横风下,在路堤上运行的高速列车周围流场,对比3,6,9和12 m 4种路堤高度对高速列车瞬态气动性能的影响。研究结果表明:强横风下,随着路堤高度的增加,列车两侧压力差增大,并影响列车周围流速分布,使得...通过采用三维瞬态DDES数值方法模拟强横风下,在路堤上运行的高速列车周围流场,对比3,6,9和12 m 4种路堤高度对高速列车瞬态气动性能的影响。研究结果表明:强横风下,随着路堤高度的增加,列车两侧压力差增大,并影响列车周围流速分布,使得流场情况更为复杂。瞬态流场结构显示,在路堤高度增加之后,车体背风侧的涡结构逐渐由体积较小、脉动频率较高、能量较小的分离状逐渐转变为融合度更高、体积更大、脉动频率较低、能量较大的涡结构,将会使得车体运动的稳定性受到更大影响,更容易发生倾覆危险。从气动力来看,随着路堤高度的增加,头车受到的气动载荷增加较大。展开更多
With the increase of blade loading, the loss prediction model used in the design process needs refinement and improvement to meet the high-performance design. For the turbine design, most of existing profile loss mode...With the increase of blade loading, the loss prediction model used in the design process needs refinement and improvement to meet the high-performance design. For the turbine design, most of existing profile loss models are developed for subsonic and transonic cases and their accuracy in high Mach number flow are limited. The primary research interest of this work is to study the flow mechanism of turbine cascade with high Mach number and the related profile loss. In this work, a transonic turbine cascade with strong shock wave is numerically studied with Reynolds Averaged Navier-Stokes(RANS). Also, to overcome the limitations of RANS modeling, Delayed Detached Eddy Simulation(DDES) type high-fidelity turbulence simulation is also conducted. Based on the numerical results, the primary loss sources, including the boundary layer loss, the trailing loss and the shock loss are analyzed and results from existing loss models are assessed. The results from current work may help to develop refinement profile loss model for the design of turbine cascade working in the high Mach number regime.展开更多
Presents information on a study which dealt with the error behavior and the stability analysis of a class of linear multistep methods with the Lagrangian interpolation as applied to the nonlinear delay differential eq...Presents information on a study which dealt with the error behavior and the stability analysis of a class of linear multistep methods with the Lagrangian interpolation as applied to the nonlinear delay differential equations. Methods and the basic lemmas; Analysis of convergence and stability.展开更多
This paper presents 3D （three-dimensional） CFD （computational fluid dynamic） simulation to analyse the FIR （flow-induced response） especially the yaw motion of a buoyancy can. The numerical cases are conducted w...This paper presents 3D （three-dimensional） CFD （computational fluid dynamic） simulation to analyse the FIR （flow-induced response） especially the yaw motion of a buoyancy can. The numerical cases are conducted with a buoyancy can under different reduced velocities utilizing our in-house code naoe-FOAM-SJTU, a solver based on the open source toolkit OpenFOAM. SST-DDES （shear-stress transport-delayed detached-eddy simulation） model is applied to handle the flowseparation and overset grid method is utilized to solve a large amplitude 6-DOF （6 degrees of freedom） motions. Free decay test and VIM （vortex-induced motion） test are built numerically. In VIM cases, the responses of trajectory, amplitude, frequency are calculated in a series of reduced velocities. With the increase of reduced velocity, yaw frequency is increased, which is similar to surge and sway frequency. And yaw frequency is equal to the sway frequency, which is consistent with experimental results. Furthermore, comparing two cases, one fixed in rotation and the other one free in rotation, it can be concluded that release in the degree of rotation can decrease the sway amplitude but make no difference in the surge amplitude.展开更多
基金supported by the National Natural Science Foundation of China (10502030 and 90505005)Innovation and Support Foundation of Chinese Astronautics
文摘Unsteady supersonic base flows around three afterbodies, cylindrical （Cy）, boattailed （BT） and three-step （MS）, are investigated in this paper. Reynolds-averaged Navier-Stokes （RANS） and two RANS/LES （large-eddy simulation） hybrid methods, detached eddy simulation （DES） and delayed-DES （DDES）, are used to predict the base flow characteristics around the baseline Cy afterbody. All the RANS and hybrid methods are based on the two-equation SST （shear-stress transport） model with compressible corrections （CC）. According to the comparison of measurements, both DES and DDES can produce more satisfactory results than RANS. RANS can only present the ＂stable＂ flow pat- terns, while the hybrid methods can demonstrate unsteady flow structures. DDES and DES results are little different from one another although the latter exhibits better agreement with the experiment. DES is taken to investigate the 5° BT and three-step afterbodies. The mean flow data and the instantaneous turbulent coherent structures are compared against available measurements.
基金supported by the National Natural Science Foundation of China (Nos.GZ 1280, 11722215 and 11721202)supported by the National Research Foundation of Korea (NRF) grant with funding from the Korean government (MSIT) (No.2011-0030013, No.2018R1A2B2007117)
文摘This research examines the vortex behaviors and aerodynamic forces in dynamic stall phenomena at a transitional Reynolds number(Re=90000)using experimental and numerical approaches.Periodic sinusoidal pitching motion at two different reduced frequencies is used to achieve the dynamic stall of a NACA 0012 airfoil.Several leading edge vortices form and detach in the dynamic stall stage.The flow then quickly transitions to a full separation zone in the stall stage when the angle of attack starts to decrease.There is discrepancy between the phaseaveraged and instantaneous flow field in that the small flow structures increased with angle of attack,which is a characteristic of the flow field at the transitional Reynolds number.The interaction between the streamwise vortices in the three-dimensional numerical results and the leading edge vortex are the main contribution to the turbulent flow.In addition,the leading edge vortex that supplies vortex lift is more stable at higher reduced frequency,which decreases the lift fluctuation in the dynamic stall stage.The leading edge vortex at higher reduced frequency is strong enough to stabilize the flow,even when the airfoil is in the down-stroke phase.
基金the National Natural Science Foundation of China (No.69974018).
文摘This paper deals with the asymptotic behavior of multistep Runge-Kutta methods for systems of delay differential equations (DDEs). With the help of K.J.in't Hout's analytic technique for the numerical stability of onestep Runge-Kutta methods, we obtain that a multistep Runge-Kutta method for DDEs is stable iff the corresponding methods for ODEs is A-stable under suitable interpolation conditions.
文摘通过采用三维瞬态DDES数值方法模拟强横风下,在路堤上运行的高速列车周围流场,对比3,6,9和12 m 4种路堤高度对高速列车瞬态气动性能的影响。研究结果表明:强横风下,随着路堤高度的增加,列车两侧压力差增大,并影响列车周围流速分布,使得流场情况更为复杂。瞬态流场结构显示,在路堤高度增加之后,车体背风侧的涡结构逐渐由体积较小、脉动频率较高、能量较小的分离状逐渐转变为融合度更高、体积更大、脉动频率较低、能量较大的涡结构,将会使得车体运动的稳定性受到更大影响,更容易发生倾覆危险。从气动力来看,随着路堤高度的增加,头车受到的气动载荷增加较大。
基金National Natural Science Foundation of China(No.51506107,No.51476082)
文摘With the increase of blade loading, the loss prediction model used in the design process needs refinement and improvement to meet the high-performance design. For the turbine design, most of existing profile loss models are developed for subsonic and transonic cases and their accuracy in high Mach number flow are limited. The primary research interest of this work is to study the flow mechanism of turbine cascade with high Mach number and the related profile loss. In this work, a transonic turbine cascade with strong shock wave is numerically studied with Reynolds Averaged Navier-Stokes(RANS). Also, to overcome the limitations of RANS modeling, Delayed Detached Eddy Simulation(DDES) type high-fidelity turbulence simulation is also conducted. Based on the numerical results, the primary loss sources, including the boundary layer loss, the trailing loss and the shock loss are analyzed and results from existing loss models are assessed. The results from current work may help to develop refinement profile loss model for the design of turbine cascade working in the high Mach number regime.
基金National Natural Science Foundation of China!No.69974018 Postdoctoral Science Foundation of China.
文摘Presents information on a study which dealt with the error behavior and the stability analysis of a class of linear multistep methods with the Lagrangian interpolation as applied to the nonlinear delay differential equations. Methods and the basic lemmas; Analysis of convergence and stability.
基金Acknowledgements This work is supported by the National Natural Science Foundation of China （51490675, 51379125, 11432009, 51579145）, Chang Jiang Scholars Program （T2014099）, Shanghai Excellent Academic Leaders Program （17XD1402300）, Shanghai Key Laboratory of Marine Engineering （K2015-11）, Program for Professor of Special Appointment （Eastern Scholar） at Shanghai Institutions of Higher Learning （2013022）, Innovative Special Project of Numerical Tank of Ministry of Industry and Information Technology of China （2016-23/09） and Lloyd＇s Register Foundation for doctoral student, to which the authors are most grateful.
文摘This paper presents 3D （three-dimensional） CFD （computational fluid dynamic） simulation to analyse the FIR （flow-induced response） especially the yaw motion of a buoyancy can. The numerical cases are conducted with a buoyancy can under different reduced velocities utilizing our in-house code naoe-FOAM-SJTU, a solver based on the open source toolkit OpenFOAM. SST-DDES （shear-stress transport-delayed detached-eddy simulation） model is applied to handle the flowseparation and overset grid method is utilized to solve a large amplitude 6-DOF （6 degrees of freedom） motions. Free decay test and VIM （vortex-induced motion） test are built numerically. In VIM cases, the responses of trajectory, amplitude, frequency are calculated in a series of reduced velocities. With the increase of reduced velocity, yaw frequency is increased, which is similar to surge and sway frequency. And yaw frequency is equal to the sway frequency, which is consistent with experimental results. Furthermore, comparing two cases, one fixed in rotation and the other one free in rotation, it can be concluded that release in the degree of rotation can decrease the sway amplitude but make no difference in the surge amplitude.
