类车体尾迹流动非定常特性
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摘要
通过与已发表的数据相比对,对大涡模拟方法的有效性进行验证.采用该数值方法对高雷诺数下25°后倾角Ahmed类车体背部斜面及尾部垂直面处尾迹区的流动进行解算.通过对背部斜面处分离泡、背部斜面侧边"C柱"处卷起的拖曳涡对及尾部垂直面处回流区流场信息的采用及相关频谱特性分析,研究并明确了尾迹区起主导作用的大尺度相干结构及运动的非定常特性.在流动的不同区域,类车体尾迹区流动的非定常特性不尽相同,主要体现为背部斜面分离泡的拍击振动具有绝对不稳定性特征,由KelvinHelmholtz(KH)不稳定性诱发的大尺度相干结构具有对流不稳定性特征;两侧"C柱"拖曳涡对在背部斜面上与展向涡相互耦合,具有较好的对称性;拖曳涡对在垂直面处回流区内与该区展向涡相互混掺,但无耦合作用且不具有对称性;垂直面处回流区内上、下侧剪切层卷起的展向涡以类似卡门涡街形式交替产生并脱落;高雷诺数时,整个尾迹区流动的特征频率趋于一致.
Compared with the published data,the validity of the large eddy simulation method is verified.The large eddy simulation method is applied to the study of the flow field around slanted surface and rear vertical base of the Ahmed body with a slant angle of 25°at high Reynolds number.This work aims to gain a relatively thorough understanding of the transient features of the predominant coherent structures around the Ahmed body by sampling flow field information and spectrum analyses of a recirculation bubble over the slanted surface,one pair of counter-rotating longitudinal C-pillar vortices originating from the two side edges of the rear window,and a recirculation torus behind the vertical base.In different regions of flow field,the transient features of the wake are different.The flapping motion of the recirculation bubble over the slanted surface corresponds to an absolute unsteadiness while the activity of the coherent structures is convective.The pair of C-pillar vortices coupled with the spanwise vortices over the slanted surface are symmetric.However,they only mix with the recirculation torus behind the vertical base resulting in asymmetric characteristic.The rolling up and shedding of the recirculation torus behind the vertical base are similar to Karman vortex street.At high Reynolds numbers,the characteristic frequency of different regions tends to be consistent.
Compared with the published data,the validity of the large eddy simulation method is verified.The large eddy simulation method is applied to the study of the flow field around slanted surface and rear vertical base of the Ahmed body with a slant angle of 25°at high Reynolds number.This work aims to gain a relatively thorough understanding of the transient features of the predominant coherent structures around the Ahmed body by sampling flow field information and spectrum analyses of a recirculation bubble over the slanted surface,one pair of counter-rotating longitudinal C-pillar vortices originating from the two side edges of the rear window,and a recirculation torus behind the vertical base.In different regions of flow field,the transient features of the wake are different.The flapping motion of the recirculation bubble over the slanted surface corresponds to an absolute unsteadiness while the activity of the coherent structures is convective.The pair of C-pillar vortices coupled with the spanwise vortices over the slanted surface are symmetric.However,they only mix with the recirculation torus behind the vertical base resulting in asymmetric characteristic.The rolling up and shedding of the recirculation torus behind the vertical base are similar to Karman vortex street.At high Reynolds numbers,the characteristic frequency of different regions tends to be consistent.
引文
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[2]KRENTEL D,MUMINOVIC R,BRUNN A,et al.Application of active flow control on generic 3D car models[C]//Active Flow Control II.Berlin:Springer,2010,108:223-239.
[3]PASTOOR M,HENNING L,NOACK BR,et al.Feedback shear layer control for bluff body drag reduction[J].Journal of Fluid Mechanics,2008,608:161.
[4]JOSEPH P,AMANDOLSE X,AIDER J L.Drag reduction on the 25°slant angle Ahmed reference body using pulsed jets[J].Experiments in Fluids,2012,52(5):1169.
[5]KRAJNOVIC S.Large eddy simulation exploration of passive flow control around an Ahmed body[J].Journal of Fluids Engineering,2014,136(16):121103.
[6]BEAUDOIN J F,AIDER J L.Drag and lift reduction of a 3Dbluff body using flaps[J].Experiments in Fluids,2008,44(4):491.
[7]MINGUEZ M,PASQUETTI R,SERRE E.High-order largeeddy simulation of flow over the“Ahmed body”car model[J].Physics of Fluids,2008,20(9):123.
[8]THACKER A,AUBRUN S,LEROY A,et al.Unsteady analyses of the flow separation on the rear window of a simplified ground vehicle model[C]//AIAA Applied Aerodynamics Conference.Chicago:AIAA,2010:2010-4569.
[9]THACKER A,AUBRUN S,LEROY A,et al.Experimental characterization of flow unsteadiness in the centerline plane of an Ahmed body rear slant[J].Experiments in Fluids,2013,54(3):1.
[10]HEFT A,INDINGER T,ADAMS N.Investigation of unsteady flow structures in the wake of a realistic generic car model[C]//AIAA Applied Aerodynamics Conference.Honolulu,AIAA,2011:2011-3669.
[11]ZHANG BF,ZHOU Y,TO S.Unsteady flow structures around a high-drag Ahmed body[J].Journal of Fluid Mechanics,2015,777:291.
[12]BEAUDOIN JF,AIDER JL.Drag and lift reduction of a 3Dbluff body using flaps[J].Experiments in Fluids,2008,44(4):491.
[13]GERMANO M,PIOMELLI U,MOIN P,et al.A dynamic subgrid-scale eddy viscosity model[J].Physics of Fluids,1998,3(3):1760.
[14]DK LILLY.A proposed modification of the Germano subgridscale closure method[J].Physics of Fluids A Fluid Dynamics,1998,4(4):633.
[15]LIENHART H,STOOTS C,BECKER S.Flow and turbulence structures in the wake of a simplified car model(Ahmed model)[C]//New Results in Numerical and Experimental Fluid Mechanics III.Stuttgart:Springer,2002,77:323-330.