Flowstructure in the turbulent boundary layer over directional riblets surfaces

The statistical characteristics and flow structures for turbulent flow over smooth, convergent and divergent riblets flat plate with zero pressure gradient are investigated with two-dimensional time-resolved particle image velocimetry (TRPIV). It is shown in the wall-normal planes of the convergent and divergent riblets flat plate that, compared to the smooth flat plate, the local boundary layer thickness, wall friction velocity, turbulent fluctuation and Reynolds stress are evidently reduced over the divergent surface. Furthermore, the effect of convergent riblets flat plate on turbulent boundary layer flow is different from the divergent one, which causes the near wall fluid move away and results in an increment of about 43% for turbulent boundary layer thickness. Meanwhile, the large scale coherent structures are more likely to be formed for flow over convergent riblets surface, this is not benefit for drag reduction. Besides, the population of the prograde vortices reaches a maximum value in the log region of turbulent boundary layer, and which appears much closer to the divergent riblets surface than the convergent one. The ejection and sweep induced by the prograde vortices make a great contribution to the mean shear in turbulent boundary layer, and it is the decrease of the prograde vortices which results in the reduction of the wall friction for the diverqent riblets surface.