Experimental studies on the near wall turbulent coherent structures over riblets surfaces

The hydrogen bubble flow visualization technique was used to study the low speed streaks in the near wall region for the turbulent boundary layer flow over riblets surfaces. By analyzing the flow visualization results, it is found that the low speed streaks appear to be flat and linear. This means that the streamwise riblets limit the flow motion in the spanwise direction, and the flow stability is enhanced. The statistical analysis indicates that, for the turbulent boundary layer flow over riblets surfaces, both the non-dimensional low speed streaks and the non-dimensional high speed streaks obey the normal-log distribution. For the same riblets plate, there is no obvious difference for the distributions of the low-and high-speed streaks. The average of the non-dimensional low speed streaks is influenced by the scale of riblets surfaces, and its value is less than the smooth wall results. In this experiments, the maximum decrement of the average of the non-dimensional low speed streaks is about 30% less than the smooth wall value. For the same riblets plate at a fixed position, the average of the non-dimensional low speed streaks decreases as the non-dimensional distance decreases, and the most probability increases. This implies that the riblets plate strongly influences flow in the near wall region. In addition, the relation between the low speed structures and the drag reduction property was also investigated, and it was obtained that the optimum design of the riblets plate is its non-dimensional spacing and height of 15-18.