EXPERIMENTAL STUDY ON THE TURBULENT BOUNDARY[1mm］ LAYER FLOW OVER RIBLETS SURFACE
沟槽面湍流边界层结构实验研究

it is always a Challenge for human beings to save energy resources. As we know, one of the main approaches is to reduce the surface friction in the design of the transportation tools. The study on drag reduction started in 1930s, and it was found that the micro--grooves along the streamwise direction may efficiently reduce the surface friction as reported by NASA Langley center in 1970's. In order to design the drag reduction surface efficiently, researchers have made afforts to understand the characteristics of turbulent boundary layer flow over riblets surface and drag reduction mechanism. In this paper, the Laser Doppler Velocimeter (LDV) and Hydrogen Bubble flow visualization techniques have been used to measure the characteristics of turbulent boundary layer flow over riblets surface and observe its coherent structure in the near wall region. The experiment was conducted in a glass-walled steel water channel with cross-section 18cmx22.5 cm. The riblets plate of L = 20 cm and L = 50 cm long was located at 68 cm away from the leading edge of the test plate respectively. An ordinary pencil used as tripping wire was attached on the smooth plate surface at 10 cm away from the leading edge of the test plate, so that the turbulent boundary layer can be achieved in front of the riblets plate. The incoming flow velocity was kept constant 17.5 cm/s in the whole experiment, and one-dimensional LDV with high spacial resolution made by Hydraulic Laboratory, Tsinghua University ensures the specious measurement. The experimental results indicate that, in comparison with the turbulent boundary layer flow over smooth surface, greater values are obtained for the thickness of viscous sublayer, the thickness of buffer region and the integral constant C in the log-law. Moreover, the non-dimensional distance of low-speed streak spacing also obeys the log-normal distribution, and its average value is reduced by 20%, implying that the turbulent drag reduction is connected with the reduction of the average value of non-dimensional low-speed streak spacing.