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

In this paper, the instantaneous velocity component of turbulent boundary layers with a V-grooved surface and a smooth one has been finely measured by IFA300 constant temperature anemometer respectively in the low turbulent level wind tunnel.Time sequence of streamwise velocity and spanwise velocity component with different Reynold's numbers(free stream velocities) on turbulent boundary layer which is controlled by a V-grooved plate and a flat-plate placed side by side in the wind tunnel has been finely measured with resolution higher than the smallest turbulent time scale(Kolmogorov scale),with the double-hot-wire-sensor probe .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 turbulent skin friction drag and skin friction coefficient are measured with the method of mean velocity profile, by nonlinear iterating to fit the skin friction coefficient u? . Providing experimental foundations for acquainting the mechanism of riblet surface drag reduction more.It is employed that the coherent structures are identified by multi-scale coefficient of stream velocity component signal. The time scales of multi-scale turbulent structures are defined by self-correlation method. The conditional sampling technique and Phased-average technique are used to extract the Phased-average waveforms for streamwise and spanwise fluctuation velocity, as well as Reynolds stress of multi-scale coherent eddy structures in the boundary layer. We can see the differences and the relationship among the three. The dynamic course of the streamwise velocity component,spanwise velocity component and the Reynolds stress are studied when the multi-scale coherent eddy structures is burst.It is found that within the V-grooved surface, this method is introduced to change the Phased-average vaveforms,the energy distribution in turbulent boundary layer and the intermittence etc, then the multi-scale coherent structures in turbulent boundary layers can be manipulated, it is the drag reduction mechanism of the riblet surface.