Experimental Investigation On Drag Reduction Mechanism Of Turbulent Boundary Layer With Grooved Surface In Low Velocity

In this paper, the measurement of constant temperature anemometry is used to do experimental study on turbulent multi-scale coherent structure and the mechanism of drag reduction in turbulent boundary layer with riblet surface under several free stream velocities without pressure gradient. Turbulent boundary layers of v-grooved surface and a smooth surface have been finely measured with IFA300 constant temperature anemometer with resolution higher than Kolmogorov scale, together with parallel-hot-wire-sensor probe, collecting the time sequence of instantaneous velocity component on the different normal positions under several Reynolds numbers. According to the relationship of skin friction velocity, fluid viscosity coefficient and other inner scale physics quantities, the turbulent skin friction and skin friction coefficient are measured, which is taken as one of the experimental foundations for researching the mechanism of riblet surface drag reduction .With the method of turbulent multi-scale coherent structure, the multi-scale coherent structure in the turbulent boundary layer is detected through taking multi-scale coefficient of stream velocity component signal as the characteristic quantity, The conditional sampling technique and phased-average technique are used to extract the phased-average waveforms of fluctuation velocity and stream velocity's gradient along the normal orientation of the riblet surface and the smooth surface, which is carried out during the eject and sweep process of coherent structure burst event, as well as the energy waveforms'development with scale and the waveforms of turbulent intensities are extracted by the conditional sampling technique.It is found that, on some flow conditions, riblet surface makes the amplitude or peak value of Phased-average waveforms of stream fluctuation velocity and stream velocity's gradient along the normal orientation decrease,the riblet surface has drag reduction effect. But the drag reduction effect changes along with the Reynolds number changing, the drag reduction effect of riblet surface gradually decreases.