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PHASE DEPENDENCY OF NEAR-WALL STREAMWISE VORTICES AND ASSOCIATED REYNOLDS SHEAR STRESSES CLOSE TO SPANWISE OSCILLATING WALL

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Direct numerical simulation of fully-developed turbulent channel flow subjected to spanwise harmonic wall oscillation is carried out. The quantitative contributions of quadrant Reynolds shear stresses to the skin friction drag are calculated on the basis of so-called FIK identity [1]. It is found that the Q2 event characterized by upwelling of low-speed fluid away from the wall contributes to the skin friction drag reduction in all oscillation frequencies considered, whereas the Q4 event characterized by downwelling of high speed fluid toward the wall first contributes to drag reduction at small oscillation periods, and then to drag increase significantly with increasing the oscillation period. The conditional averaging technique is applied to near-wall streamwise vortices in order to explain different behavior of Q2 and Q4 with respect to the oscillation period.

Author(s):

Aiko Yakeno    
Japan Aeronautics of Exploration Agency (JAXA)
Japan

Yosuke Hasegawa    
University of Tokyo
Japan

Nobuhide Kasagi    
Japan Science and Technology Agency
Japan