Coherent Spanwise Structures in Turbulent BoundaryLayer over Drag-Reducing Riblets

• Published in: 天津大学学报：英文版 no. 4; pp. 317 - 323
• Main Author: Yang Shaoqiong Li Shan Tian Haiping Wang Qingyi Jiang Nan
• Format: Journal Article
• Language: English
• Published: 2015
• Subjects: boundary; coherentstructure; control; drag; flow; instability; Kelvin-Helmholtz-like; layer; particle; passive; reduction; riblet; turbulent; velocimetry
• ISSN: 1006-4982; 1995-8196

The time series of velocity vector fields and their statistics in the turbulent boundary layer（TBL）overriblets 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 thephysical vorticity for the coherent spanwise structures were detected and extracted at the fourth scale by utilizing animproved quadrant splitting method（IQSM）. Results revealed that nearly 6.17%, and 10.73%, of a drag reductionwas separately achieved over the riblets surface. Besides, it was visualized that the drag-reduction was acquired bythe 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 Kelvin-Helmholtz-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 coherentspanwise structures found at around or below y^＋ ≈20 in both cases of s^＋ ≈21 and s^＋ ≈28 and their topologicalchanges, were consequences or causes of the breakdown of the viscous regime. We prefer to suggest that the interactionsbetween those structures and the riblets, which contain the coherent spanwise structures extending towardthe wall and penetrating into the riblet grooves, are the root causes.