Experimental Investigation Of Drag Reduction Mechanism Over Riblet Surface In Turbulent Boundary Layer

In this paper, the mechanism of drag reduction in turbulent boundary layer with a V-groove riblets surface was studied experimentally. Time sequence of longitudinal, vertical and spanwise velocity component along different normal positions in turbulent boundary layer which is controlled by a V-grooved plate and a flat-plate with different Reynolds numbers has been finely measured by IFA300 constant temperature anemometer with TSI-1211-T1.5,TSI-1240-T1.5,TSI-1243-T1.5,TSI-1244-T1.5 four types probes.According to the relationship of the logarithmic law of mean velocity profile and skin friction velocity, fluid viscosity coefficient etc.,inner scale physics quantities the relationship of skin friction velocity and skin friction shear stress,nonlinear iterating was carried out to calculate the parameter of the logarithmic law and skin friction velocities for turbulent boundary layers over a V-grooved plate and a flat-plate respectively. The result verified the thickening of the viscous sublayer and the up-shift of logarithmic and buffer sublayer in the near wall region over V-grooved surface. It is confirmed that the V-grooved surface weakens the fluctuations of velocities and Reynolds stresses related to multi-scale coherent structures in buffer sublayer.It is employed the concept of multi-scale coherent structures to study the mechanism of riblet surface drag reduction in turbulent boundary layer. Wavelet coefficients of longitudinal velocity component are used as the index of detecting multi-scale coherent structures and extracting the phased-average waveforms of streamwise, vertical and spanwise fluctuating velocities, as well as Reynolds stress of in turbulent boundary layer over riblets and plate surface. Compared with smooth plate surface, both for eject and sweep process, the amplitudes of streamwise, vertical and spanwise fluctuating velocity components, as well as Reynolds stress , < u ' v'>,< u ' w'> are reduced clearly, progress tend to be laggardly, temporal scales of multi-scale coherent structures are becoming shorten. Among them, the reduction of spanwise fluctuation intensity of coherent structure over V-grooved surface is apparent, and spanwise momentum exchanges of coherent structures are reduced, so the contribution of coherent structures to turbulence production is decreased.A set of flow visualization experimental apparatus including of water channel, hydrogen bubble producing equipment and flow digital image acquisition and procession software is established. Using a'comparing-two-frame'quantitative analysis technique, the two-dimensional plane distributions of streamwise, spanwise velocity and vertical vorticity are obtained through processing and analyzing hydrogen bubble flow visualization images. The results indicate that the V-groove riblets surface weakens the streamwise, spanwise velocity and vorticity fluctuation through manipulating momentum exchanges of large-scale high and low-speed streaks in turbulent boundary layer.