EXPERIMENTAL STUDY ON PROPERTIES OF TURBULENT/NON-TURBULENT INTERFACE OVER RIBLETS SURFACES AT LOW REYNOLDS NUMBERS1)

The turbulent/non-turbulent(T/NT) interface usually refers to the thin layer that separates turbulent flow from irrotational flow. The research of T/NT interface is of great importance to deepen the understanding of matter, momentum and energy transfer between the turbulent and irrotational flow. The geometrical and dynamical properties of the T/NT interface at different Reynolds numbers over smooth and riblets surfaces with zero-pressure-gradient at different Reynolds numbers are experimentally investigated using two-dimensional time-resolved particle image velocimetry (2D-TRPIV). The Reynolds number range is about $Re_{\tau } \approx $ 400 $\sim$ 1000 in the present experiment. The T/NT interface is detected with the turbulent kinetic energy criterion. The probability density function of the interface position, fractal characteristics and the conditional averaged velocity and vorticity near the interface are analyzed. It is shown that the mean heights of the interfaces are around 0.8 $\sim$ 0.9$\delta_{99} $ under different Reynolds numbers for both smooth and riblets surfaces. The probability density function of the interface position over drag reduction riblets surface is kept the same with the smooth surface, whose distribution agrees well with normal distribution. While for the drag increment riblets surface, the distribution of interface position deviates from the normal distribution and presents positive skewness. The fractal dimension of the interface and the velocity jump across the interface will gradually increase with the Reynolds number under the present experiment condition. Moreover, the dimensionless conditional averaged vorticities have similar distributions in the vicinity of the interface over both smooth and riblets surfaces when $Re_{\tau } $ is less than 1000, correspondingly, the maximum gradient of dimensionless conditional average streamwise velocity is approximately constant.