TRPIV Measurement Of Drag-reduction In Turbulent Boundary Layer Over Riblet Surface

In this paper, the mechanism of drag reduction from the viewpoint of the multi-scale coherent structures manipulation in turbulent boundary layer(TBL) over aV-grooved riblets surface was experimentally investigated by a Time-ResolvedParticle Image Velocimetry(TR-PIV) system in a water channel. The2D-2Cinstantaneous time-series of the velocity field in turbulent boundary layer wasmeasured by TR-PIV measurement system.Based on the relationship among the mean velocity profile and the inner-scalephysical parameters, such as the skin-friction velocity and the kinematic viscosity inturbulent boundary layer over the V-grooved/smooth plate were calculated. Resultsillustrated that a drag-reduction of10.73percent was obtained. In addition, thephenomena that a thickened viscous sub-layer and an up-shift buffer/logarithmic layerin the near-wall region due to the existence of riblets were confirmed. Furthermore,the manipulation of the fluctuation velocities and Reynolds stresses associated withmulti-scale coherent structures in near-wall region are verified.Besides, the drag-reduction caused by riblets were discussed through thecomparison between the average velocity profile, turbulent intensity distribution, theReynolds stress, along with three and four order of turbulent fluctuation statistics. Andthe concept of multi-scale coherent structures was introduced to study the mechanismof drag-reduction. Wavelet transform of longitudinal instantaneous velocitycomponent were employed as an index to decompose the original velocity field intodifferent scales, and the two-dimensional topology of longitudinal and verticalfluctuating velocities, as well as spanwise vorticity and Reynolds shear stress in theTBL over riblets and smooth surface were extracted with the improvedquadrant-splitting method. The results indicated that: by contrast, the amplitudes oflongitudinal and vertical fluctuating velocity components, as well as spanwisevorticity and Reynolds shear stress were manipulated obviously both for ejection andsweeping events, among which the reduction of vertical fluctuation intensity was most,implying that the momentum exchanges of coherent structures in wall-normaldirection were reduced, so did the contribution to turbulence production.