Spatial Distributions of Spanwise Vortices in a Turbulent Boundary Layer over a Micro-riblet Film

미세 리블렛 평판 상부 난류경계층 유동에서 횡방향 와의 공간적 분포특성

Turbulent boundary-layer over a micro-riblet film(MRF) was investigated experimentally. The MRF has sharp V-shaped micro scale grooves of $300{\mu}m$ in width and $176.8{\mu}m$ in height. Particle image velocimetry(PIV) system was employed to measure velocity fields of flow over the MRF coated plate. Flow over a smooth plate was also measured for comparison. The PIV measurements were taken in the streamwise wall-normal planes at Re$\theta$= 985 and 2342. Vortex structures of the flow were analyzed by extracting the swirling strength as an unambiguous vortex-identification criterion. As a result the number of spanwise vortices with clockwise(negative) rotation decreases rapidly in the near-wall region(y<0.2h), but decreases slowly in the outer region(0.2h