TRPIV Experimental Investigation On Drag-Reduction Mechanism Over 2D And 3D Riblets

Riblets are a kind of passive methods of turbulent boundary control that can reduce viscous drag,they have been studied for over 30 years.However,due to the complexity and diversity of the coherent structures in the turbulent flow,the whole understanding of drag reduction mechanism over riblets is still a challenge so far.In this thesis,time resolved particle image velocimetry(TRPIV)measurement system is used to obtain the streamwise-wall normal(x-y)and streamwise-spanwise(x-z)velocity vectors over smooth surface and riblets at different Reynolds numbers.The mechanism of drag reduction over 2D-riblets and 3D-riblets are investigated experimentally from the viewpoint of the coherent structures in turbulent boundary layer(TBL).Different from the usual PIV,the TRPIV can provide temporal information of the coherent structures'evolution in TBL.Over conventional 2D-riblets,mean velocity profile and the second-order statistics distribution show good accordance with the typical drag reduction surface.Proper orthogonal decomposition(POD)results present that riblets have marked effect on the gross features of large-scale structures,the sweep motions are suppressed over riblets.Hairpin-like vortices are identified over both surfaces using finite-time Lyapunov exponent(FTLE)method,both presenting an inclined streaky pattern in(x-y)plane;with their upstream leg portion extending from the wall;the downstream head portion rising into the outer area;meaning structural foundation of the coherent structures is not appreciably altered over riblets.Besides,the inclination angles of Lagrangian coherent structures(LCSs)calculated are significantly increased over riblets,especially in the buffer layer,which implies a noticed decrease of the contact area between the quasi-streamwise vortices and the surface.The evolution process of LCSs over the smooth surface illustrates the formation of a new hairpin-like structure,while no noticeable complete auto-generation process of the hairpin/hairpin packets are observed over riblets.The convection velocity U_c of typical LCSs obtained from the temporal-spatial analysis indicates that LCSs are advected much slower over riblets surface than the one over the smooth case,meaning the transport of momentum over riblets is reduced.Over 3D riblets,the log-law profiles with the same slope are displace upward over both kinds of riblets surfaces,reflecting the character of drag reduction near the wall,the drag reduction rate over sinusoidal-riblets(s-riblets)has a modest increase of compared to the 2D riblets.Amplitudes of turbulence fluctuation and Reynolds shear stress(RSS)also have significantly reduction,with the peaks of the distribution move farther away from the wall over riblets,indicating transport of momentum and energy is reduced.An extended quadrant analysis method is used to determine the fractional contributions to the intense RSS of each quadrant,with a parameters H is introduced as a filter.The results show that the ejection and sweep,Q2 and Q4,make the largest contributions to the RSS,among these two events,H has a larger influence on Q4compared to ejection motions,so sweep is more intense only in the near wall region.Based on correlation function and ?_ci criterion,structures over smooth surface and riblets are identified.Results show that the spatial scale of the coherent structures in the x and y direction are both reduced over riblets,the inclined angle with the mainstream is decreased,and the upward motion is suppressed.The inclination angle of the hairpin vortex over riblets are both reduced,especially over s-riblets,meaning the wall-normal motion of fluid is impeded,the intensity of eject motions induced by the head of a hairpin vortex is reduced,thus suppress the self-sustained mechanism of the coherent structures.Multi-scale study based on POD show that the spatial scale of the structures of each POD mode keep descaled with the increasing mode number.The distribution of the energy with the POD mode over s-riblets is more dispersively,so the transfer between different scale structures is slowing down.Study on the (x-z) plane show that the instantaneous velocity field presents different types of structures at different y positions.At the same Re,the spacing between the low speed streaks becomes wider with the increasing y⁺.While at the same y⁺,the spacing is much wider over riblets than that of the smooth surface,and the difference between these two kinds of riblets is small.The correlation analysis presents the statistical spatial pattern of the streak structure in the x-z plane,the same results are obtained.Multi-scale study based on POD show that the energy-contained mode is influenced by riblets.Vortices are generated by the interaction between the high-and low-speed streaks,enhance the spanwise motion of the streaks,and the latter become more instable.However,the distribution of streaks over riblets is well-aligned,indicating that the riblets restrain the spanwise movement of the streak structures,weaken the mix process of the high-and low-speed streaks in the near wall region,reduce the kinetic energy of momentum transfer.