TRPIV MEASUREMENT OF DRAG-REDUCTION IN THE TURBULENT BOUNDARY LAYER OVER RIBLETS PLATE

Time series of velocity vector field and statistics in the turbulent boundary layer(TBL) over the riblets surface and smooth one were measured utilizing the time-resolved particle image velocimetry (TRPIV); Several characteristic parameters in the TBL, such as the mean velocity profile, the Reynolds shear stress and the turbulent intensity, etc. were compared at the same free-stream velocity(0.19m/s) for the different surface plates. We firstly detected the coherent structures using multi-scale spatial locally-averaged structure function of the streamwise velocity component at different scales. Then, we utilized the conditional sampling and phase-average method to extract the spatial topologies of physical quantities, such as the velocity fluctuation and the spanwise vorticity, etc. based on the ejection or sweep events of coherent structures in the TBL. Results reveal that a drag-reduction of nearly 10.73 percent was acquired over the riblets surface when we compared the skin friction coe cient of such two Acrylic Plexiglas plates at the same free-stream velocity. In addition, that the streamwise turbulent intensity and the Reynolds shear stress of the TBL over riblets surface are both smaller than the ones over smooth surface at the same wall-normal position indicates riblets surface weaken the flow turbulence reducing the momentum exchange and energy loss in the flow. Lastly, we compared several statistic characteristics, mentioned above, based on the ejection and sweep events of coherent structures to clarify the effect of the drag-reduction over the riblets surface with triangle cross-section. More importantly, we find that the drag-reduction achieved by means of riblets surface suppressing the bursting events of coherent structures.