Coherent Spanwise Structures in Turbulent Boundary Layer over Drag-Reducing Riblets  

作　　者：杨绍琼 李山 田海平 王清毅 姜楠 

机构地区：[1]School of Mechanical Engineering, Tianjin University [2]Tianjin Key Laboratory of Modern Engineering Mechanics [3]School of Mechanical and Aerospace Engineering, Nanyang Technological University

出　　处：《Transactions of Tianjin University》2015年第4期317-323,共7页天津大学学报（英文版）

基　　金：Supported by the National Natural Science Foundation of China(No.11332006,No.11272233 and No.11411130150);the foundation from the China Scholarship Council(CSC)(No.201306250092);the Foundation Project for Outstanding Doctoral Dissertations of Tianjin University

摘　　要：The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peakto-peak spacing being approximately s?≈21)and 0.19 m/s( s?≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified KelvinHelmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been discussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y?≈20 in both cases of s?≈21 and s?≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.The time series of velocity vector fields and their statistics in the turbulent boundary layer(TBL)over riblets and smooth plate were measured by utilizing a time-resolved particle image velocimetry(TR-PIV)system. The mean velocity profiles of the TBL were compared in the case of 0.13 m/s(the riblets with dimensionless peakto-peak spacing being approximately s?≈21)and 0.19 m/s( s?≈28)for these two kinds of plates, respectively. Two kinds of drag-reducing velocity profiles were illustrated and analyzed. Then the spatial topologies of the physical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing an improved quadrant splitting method(IQSM). Results revealed that nearly 6.17%, and 10.73%, of a drag reduction was separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired by the riblets influencing the bursting ejection(Q2)and sweep(Q4)events of the coherent spanwise vortex structures, the Q4 events in particular. Based on such two drag-reducing cases of the riblets, lastly, a simplified KelvinHelmholtz-like linear instability model proposed initially by García-Mayoral and Jiménez(2011)has been discussed. It is still difficult to establish with certainty whether the observed phenomena, the appearance of coherent spanwise structures found at around or below y?≈20 in both cases of s?≈21 and s?≈28 and their topological changes, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactions between those structures and the riblets, which contain the coherent spanwise structures extending toward the wall and penetrating into the riblet grooves, are the root causes.

关 键 词：TURBULENT boundary layer passive flow control drag reduction Kelvin-Helmholtz-like INSTABILITY coherentstructure riblet particle image VELOCIMETRY 

分 类 号：O357.5[理学—流体力学]