Laminar separation phenomenon is the characteristic feature of airfoils at low Reynolds number conditions. The laminar separation flow contains the complex flow structures of laminar separation, transition and reattachment. The formation and evolution of laminar separation flow are detrimental to the performance of airfoils. The Large Eddy Simulation (LES) method is utilized to predict the laminar separation flow over airfoils at different Reynolds numbers in the low range of Reynolds numbers. The effects of Reynolds number on aerodynamic characteristics of airfoils and the corresponding mechanism are studied. On the structurally patched mesh, the LES method adopt the implicit sub-grid-scale model, and utilize the AUSM+ scheme for spatial discretization and dual-time-step method for time marching. The correctness and reliability of the numerical method are proved by the validation example. The results indicate that the Reynolds number has a significant effect on the aerodynamic characteristics of airfoil. With the decrease of Reynolds numbers, the shape of the bubble increases, and the position moves towards the trailing edge, which leads to the increment of average drag coefficient. Besides, at the lower Reynolds numbers, the lift drag coefficient oscillates with time significantly. Further studies show that the instability and transition characteristics of the separated shear layer over the airfoil surface are responsible for the different time-average bubble configurations and aerodynamic characteristics. With the decrease of Reynolds numbers, the flow viscosity increases. So the velocity gradient of the shear layer decreases, and the positions of transition and reattachment move towards the trailing edge. At the lower Reynolds numbers, transition and reattachment do not occur over the airfoil.
ZHU Zhibin
,
SHANG Qing
,
BAI Peng
,
LIU Qiang
. Evolution of laminar separation phenomenon on low Reynolds number airfoil at different Reynolds numbers[J]. ACTA AERONAUTICAET ASTRONAUTICA SINICA, 2019
, 40(5)
: 122528
-122528
.
DOI: 10.7527/S1000-6893.2018.22528
[1] 李锋, 白鹏, 石文, 等. 微型飞行器低雷诺数空气动力学[J]. 力学进展, 2007, 37(2):257-268. LI F, BAI P, SHI W, et al. Micro air vehicle aerodynamics at low Reynolds number[J]. Advances in Mechanics, 2007, 37(2):257-268(in Chinese).
[2] 白鹏, 崔尔杰, 李锋, 等. 对称翼型低雷诺数小迎角升力系数非线性现象研究[J]. 力学学报, 2006, 38(1):1-8. BAI P, CUI E J, LI F, et al. Study of the non-linearl ift coefficient of the symmetric airfoil at low Reynolds number near the 0° angle of attack[J]. Chinese Journal of Theoretical and Applied Mechanics, 2006, 38(1):1-8(in Chinese).
[3] MUNDAY P M, TAIRA K, SUWA T, et al. Nonlinear lift on a triangular airfoil in low-Reynolds number compressible flow[J]. Journal of Aircraft, 2015, 52(3):924-931.
[4] MUELLER T J. The influence of laminar separation and transition on low Reynolds number airfoil hysteresis[J]. Journal of Aircraft, 2985, 22(9):763-770.
[5] YANG Z, IGARASHI H, MARTIN M, et al. An experimental investigation on aerodynamic hysteresis of a low Reynolds number airfoil:AIAA-2008-0315[R]. Reston, VA:AIAA, 2008.
[6] GASTER M. The structure and behavior of laminar separation bubbles:AGARD CP-4[R]. Paris:AGARD, 1966:813-854.
[7] HORTON H P. Laminar separation bubbles in two-and three-dimensional incompressible flow[D]. London:University of London, 1968:25-62.
[8] SELIG S M, GUGLIELMO J J, BROEREN A P, et al. Experiments on airfoils at low Reynolds numbers:AIAA-1996-0062[R]. Reston, VA:AIAA, 1996.
[9] BREHM C, MACH S, GROSS A, et al. Investigations of an airfoil at low Reynolds number conditions[C]//4th AIAA Flow Control Conference. Reston, VA:AIAA, 2008:3765.
[10] GROSS A, FASEL H F. Numerical investigation of separation for airfoils at low Reynolds numbers[C]//40th AIAA Fluid Dynamics Conference and Exhibit. Reston, VA:AIAA, 2010:4736.
[11] 白鹏, 李锋, 詹慧玲, 等. 翼型低Re数小攻角非线性非定常层流分离现象研究[J]. 中国科学:物理学力学天文学, 2015, 45(2):024703. BAI P, LI F, ZHAN H L, et al. Study about the non-linear and unsteady laminar separation phenomena around the airfoil at low Reynolds number with low icidence[J]. Scientia Sinica Physica, Mechanica & Astronomica, 2015, 45(2):024703(in Chinese).
[12] 刘强, 刘强, 白鹏, 等. 不同雷诺数下翼型气动特性及层流分离现象演化[J]. 航空学报, 2017, 38(4):120338. LIU Q, LIU Q, BAI P, et al. Aerodynamic characteristics of airfoil and evolution of laminar separation at different Reynolds numbers[J]. Acta Aeronautica et Astronautica Sinica, 2017, 38(4):120338(in Chinese).
[13] SHAN H, JIANG L, LIU C. Direct numerical simulation of flow separation around a NACA 0012 airfoil[J]. Computers and Fluids, 2005, 34(9):1096-1114.
[14] GALBRAITH C M, VISBAL R M. Implicit large eddy simulation of low Reynolds number flow past the SD7003 airfoil:AIAA-2008-0225[R]. Reston, VA:AIAA, 2008.
[15] KOJIMA R, NONOMURA T, OYAMA A, et al. Large-eddy simulation of low-reynolds-number flow over thick and thin NACA airfoils[J]. Journal of Aircraft, 2013, 50(1):187-196.
[16] XU C, CHEN L, LU X. Large-eddy simulation of the compressible flow past a wavy cylinder[J]. Journal of Fluid Mechanics, 2010, 665:238-273.
[17] BORIS J P, GRINSTEIN F F, ORAN E S, et al. New insights into large eddy simulation[J]. Fluid Dynamics Research, 1992, 10(4):199-228.
[18] GAMIER E, ADAMS N, SAGAUT P. Large eddy simulation for compressible flows[M]. Berlin:Springer, 2009:93.
[19] LIU M S. Progress towards an improved CFD method:AUSM+:AIAA-1995-1701[R]. Reston, VA:AIAA, 1995.
[20] LIU X, OSHER S, CHAN T. Weighted essentially non-oscillatory scheme[J]. Journal of Computational Physics, 1994, 115(1):200-212.
[21] JIANG G S, SHU C W. Efficient implementation of weighted ENO schemes[J]. Journal of Computational Physics, 1996, 126(1):202-228.
[22] ANTONY J. Time dependent calculations using multigrid with application to unsteady flows past airfoils and wings[C]//AIAA 10th Computational Fluid Dynamics Conference. Reston, VA:AIAA, 1991:1596.
[23] OL V M, MCAULIFFE R B, HANFF S E, et al. Comparison of laminar separation bubble measurements on a low Reynolds number airfoil in three facilities[C]//35th AIAA Fluid Dynamics Conference and Exhibit. Reston, VA:AIAA, 2005:5149.